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<h1>Curiosities of the Sky</h1>
<h2>by Garrett Serviss</h2>
<hr>
<strong>Preface</strong>
<p>What Froude says of history is true also of astronomy: it is
the most impressive where it transcends explanation. It is not
the mathematics of astronomy, but the wonder and the mystery
that seize upon the imagination. The calculation of an eclipse
owes all its prestige to the sublimity of its data; the
operation, in itself, requires no more mental effort than the
preparation of a railway time-table.</p>
<p>The dominion which astronomy has always held over the minds
of men is akin to that of poetry; when the former becomes
merely instructive and the latter purely didactic, both lose
their power over the imagination. Astronomy is known as the
oldest of the sciences, and it will be the longest-lived
because it will always have arcana that have not been
penetrated.</p>
<p>Some of the things described in this book are little known
to the average reader, while others are well known; but all
possess the fascination of whatever is strange, marvelous,
obscure, or mysterious -- magnified, in this case, by the
portentous scale of the phenomena.</p>
<p>The idea of the author is to tell about these things in
plain language, but with as much scientific accuracy as plain
language will permit, showing the wonder that is in them
without getting away from the facts. Most of them have hitherto
been discussed only in technical form, and in treatises that
the general public seldom sees and never reads.</p>
<p>Among the topics touched upon are:</p>
<ul>
<li>The strange unfixedness of the ``fixed stars,'' the vast
migrations of the suns and worlds constituting the
universe.</li>
<li>The slow passing out of existence of those collocations
of stars which for thousands of years have formed famous
``constellations,'' preserving the memory of mythological
heroes and heroines, and perhaps of otherwise unrecorded
history.</li>
<li>The tendency of stars to assemble in immense clouds,
swarms, and clusters.</li>
<li>The existence in some of the richest regions of the
universe of absolutely black, starless gaps, deeps, or holes,
as if one were looking out of a window into the murkiest
night.</li>
<li>The marvelous phenomena of new, or temporary, stars,
which appear as suddenly as conflagrations, and often turn
into something else as eccentric as themselves.</li>
<li>The amazing forms of the ``whirlpool,'' ``spiral,''
``pinwheel,'' and ``lace,'' or ``tress,'' nebulæ.</li>
<li>The strange surroundings of the sun, only seen in
particular circumstances, but evidently playing a constant
part in the daily phenomena of the solar system.</li>
<li>The mystery of the Zodiacal Light and the
Gegenschein.</li>
<li>The extraordinary transformations undergone by comets and
their tails.</li>
<li>The prodigies of meteorites and masses of stone and metal
fallen from the sky.</li>
<li>The cataclysms that have wrecked the moon.</li>
<li>The problem of life and intelligence on the planet
Mars.</li>
<li>The problematical origin and fate of the asteroids.</li>
<li>The strange phenomena of the auroral lights.</li>
</ul>
<p>An attempt has been made to develop these topics in an
orderly way, showing their connection, so that the reader may
obtain a broad general view of the chief mysteries and problems
of astronomy, and an idea of the immense field of discovery
which still lies, almost unexplored, before it.</p>
<p><strong>The Windows of Absolute Night</strong></p>
<p>To most minds mystery is more fascinating than science. But
when science itself leads straight up to the borders of mystery
and there comes to a dead stop, saying, ``At present I can no
longer see my way,'' the force of the charm is redoubled. On
the other hand, the illimitable is no less potent in mystery
than the invisible, whence the dramatic effect of Keats'
``stout Cortez'' staring at the boundless Pacific while all his
men look at each other with a wild surmise, ``silent upon a
peak in Darien.'' It is with similar feelings that the
astronomer regards certain places where from the peaks of the
universe his vision seems to range out into endless empty
space. He sees there the shore of his little isthmus, and,
beyond, unexplored immensity.</p>
<p>The name, ``coal-sacks,'' given to these strange voids is
hardly descriptive. Rather they produce upon the mind the
effect of blank windows in a lonely house on a pitch-dark
night, which, when looked at from the brilliant interior,
become appalling in their rayless murk. Infinity seems to
acquire a new meaning in the presence of these black openings
in the sky, for as one continues to gaze it loses its purely
metaphysical quality and becomes a kind of entity, like the
ocean. The observer is conscious that he can actually
<em>see</em> the beginning of its ebon depths, in which the
visible universe appears to float like an enchanted island,
resplendent within with lights and life and gorgeous
spectacles, and encircled with screens of crowded stars, but
with its dazzling vistas ending at the fathomless sea of pure
darkness which encloses all.</p>
<p>The Galaxy, or Milky Way, surrounds the borders of our
island in space like a stellar garland, and when openings
appear in it they are, by contrast, far more impressive than
the general darkness of the interstellar expanse seen in other
directions. Yet even that expanse is not everywhere equally
dark, for it contains gloomy deeps discernable with careful
watching. Here, too, contrast plays an important part, though
less striking than within the galactic region. Some of Sir
William Herschel's observations appear to indicate an
association between these tenebrious spots and neighboring star
clouds and nebulæ. It is an illuminating bit of
astronomical history that when he was sweeping the then virgin
heavens with his great telescopes he was accustomed to say to
his sister who, note-book in hand, waited at his side to take
down his words, fresh with the inspiration of discovery:
``Prepare to write; the nebulæ are coming; here space is
vacant.''</p>
<p>The most famous of the ``coal-sacks,'' and the first to be
brought to general attention before astronomers had awakened to
the significance of such things, lies adjacent to the
``Southern Cross,'' and is truly an amazing phenomenon. It is
not alone the conspicuousness of this celestial vacancy,
opening suddenly in the midst of one of the richest parts of
the Galaxy, that has given it its fame, but quite as much the
superstitious awe with which it was regarded by the early
explorers of the South Seas. To them, as well as to those who
listened in rapt wonder to their tales, the ``Coal-sack''
seemed to possess some occult connection with the mystic
``Cross.'' In the eyes of the sailors it was not a vacancy so
much as a sable reality in the sky, and as, shuddering, they
stared at it, they piously crossed themselves. It was another
of the magical wonders of the unknown South, and as such it
formed the basis of many a ``wild surmise'' and many a
sea-dog's yarn. Scientific investigation has not diminished its
prestige, and today no traveler in the southern hemisphere is
indifferent to its fascinating strangeness, while some find it
the most impressive spectacle of the antarctic heavens.</p>
<p>All around, up to the very edge of the yawning gap, the
sheen of the Milky Way is surpassingly glorious; but there, as
if in obedience to an almighty edict, everything vanishes. A
single faint star is visible within the opening, producing a
curious effect upon the sensitive spectator, like the sight of
a tiny islet in the midst of a black, motionless, waveless
tarn. The dimensions of the lagoon of darkness, which is oval
or pear-shaped, are eight degrees by five, so that it occupies
a space in the sky about one hundred and thirty times greater
than the area of the full moon. It attracts attention as soon
as the eye is directed toward the quarter where it exists, and
by virtue of the rarity of such phenomena it appears a far
greater wonder than the drifts of stars that are heaped around
it. Now that observatories are multiplying in the southern
hemisphere, the great austral ``Coal-sack'' will, no doubt,
receive attention proportioned to its importance as one of the
most significant features of the sky. Already at the Sydney
Observatory photographs have shown that the southern portion of
this Dead Sea of Space is not quite ``bottomless,'' although
its northern part defies the longest sounding lines of the
astronomer.</p>
<p>There is a similar, but less perfect, ``coal-sack'' in the
northern hemisphere, in the constellation of ``The Swan,''
which, strange to say, also contains a well-marked figure of a
cross outlined by stars. This gap lies near the top of the
cross-shaped figure. It is best seen by averted vision, which
brings out the contrast with the Milky Way, which is quite
brilliant around it. It does not, however, exercise the same
weird attraction upon the eye as the southern ``Coal-sack,''
for instead of looking like an absolute void in the sky, it
rather appears as if a canopy of dark gauze had been drawn over
the stars. We shall see the possible significance of this
appearance later.</p>
<p>Just above the southern horizon of our northern middle
latitudes, in summer, where the Milky Way breaks up into vast
sheets of nebulous luminosity, lying over and between the
constellations Scorpio and Sagittarius, there is a remarkable
assemblage of ``coal-sacks,'' though none is of great size. One
of them, near a conspicuous star-cluster in Scorpio, M80, is
interesting for having been the first of these strange objects
noted by Herschel. Probably it was its nearness to M80 which
suggested to his mind the apparent connection of such vacancies
with star-clusters which we have already mentioned.</p>
<p>But the most marvelous of the ``coal-sacks'' are those that
have been found by photography in Sagittarius. One of Barnard's
earliest and most excellent photographs includes two of them,
both in the star-cluster M8. The larger, which is roughly
rectangular in outline, contains one little star, and its
smaller neighbor is lune-shaped -- surely a most singular form
for such an object. Both are associated with curious dark lanes
running through the clustered stars like trails in the woods.
Along the borders of these lanes the stars are ranked in
parallel rows, and what may be called the bottoms of the lanes
are not entirely dark, but pebbled with faint stellar points.
One of them which skirts the two dark gaps and traverses the
cluster along its greatest diameter is edged with lines of
stars, recalling the alignment of the trees bordering a French
highway. This <em>road of stars</em> cannot be less than many
billions of miles in length!</p>
<p>All about the cluster the bed of the Galaxy is strangely
disturbed, and in places nearly denuded, as if its contents had
been raked away to form the immense stack and the smaller
accumulations of stars around it. The well-known ``Trifid
Nebula'' is also included in the field of the photograph, which
covers a truly marvelous region, so intricate in its mingling
of nebulæ, star-clusters, star-swarms, star-streams, and
dark vacancies that no description can do it justice. Yet,
chaotic as it appears, there is an unmistakable suggestion of
unity about it, impressing the beholder with the idea that all
the different parts are in some way connected, and have not
been fortuitously thrown together. Miss Agnes M. Clerke made
the striking remark that the dusky lanes in M8 are exemplified
on the largest scale in the great rift dividing the Milky Way,
from Cygnus in the northern hemisphere all the way to the
``Cross'' in the southern. Similar lanes are found in many
other clusters, and they are generally associated with flanking
rows of stars, resembling in their arrangement the thick-set
houses and villas along the roadways that traverse the
approaches to a great city.</p>
<p>But to return to the black gaps. Are they really windows in
the star-walls of the universe? Some of them look rather as if
they had been made by a shell fired through a luminous target,
allowing the eye to range through the hole into the void space
beyond. If science is discretely silent about these things,
what can the more venturesome and less responsible imagination
suggest? Would a huge ``runaway sun,'' like Arcturus, for
instance, make such an opening if it should pass like a
projectile through the Milky Way? It is at least a stimulating
inquiry. Being probably many thousands of times more massive
than the galactic stars, such a stellar missile would not be
stopped by them, though its direction of flight might be
altered. It would drag the small stars lying close to its
course out of their spheres, but the ultimate tendency of its
attraction would be to sweep them round in its wake, thus
producing rather a star-swarm than a vacancy. Those that were
very close to it might be swept away in its rush and become its
satellites, careering away with it in its flight into outer
space; but those that were farther off, and they would, of
course, greatly outnumber the nearer ones, would tend inward
from all sides toward the line of flight, as dust and leaves
collect behind a speeding motor (though the forces operating
would be different), and would fill up the hole, if hole it
were. A swarm thus collected should be rounded in outline and
bordered with a relatively barren ring from which the stars had
been ``sucked'' away. In a general sense the M8 cluster answers
to this description, but even if we undertook to account for
its existence by a supposition like the above, the black gaps
would remain unexplained, unless one could make a further draft
on the imagination and suggest that the stars had been thrown
into a vast eddy, or system of eddies, whose vortices appear as
dark holes. Only a maelstrom-like motion could keep such a
funnel open, for without regard to the impulse derived from the
projectile, the proper motions of the stars themselves would
tend to fill it. Perhaps some other cause of the whirling
motion may be found. As we shall see when we come to the spiral
nebulæ, gyratory movements are exceedingly prevalent
throughout the universe, and the structure of the Milky Way is
everywhere suggestive of them. But this is hazardous sport even
for the imagination -- to play with <em>suns</em> as if they
were but thistle-down in the wind or corks in a mill-race.</p>
<p>Another question arises: What is the thickness of the hedge
of stars through which the holes penetrate? Is the depth of the
openings proportionate to their width? In other words, is the
Milky Way round in section like a rope, or flat and thin like a
ribbon? The answer is not obvious, for we have little or no
information concerning the relative distances of the faint
galactic stars. It would be easier, certainly, to conceive of
openings in a thin belt than in a massive ring, for in the
first case they would resemble mere rifts and breaks, while in
the second they would be like wells or bore-holes. Then, too,
the fact that the Milky Way is not a <em>continuous</em> body
but is made up of stars whose actual distances apart is great,
offers another quandary; persistent and sharply bordered
apertures in such an assemblage are <em>a priori</em> as
improbable, if not impossible, as straight, narrow holes
running through a swarm of bees.</p>
<p>The difficulty of these questions indicates one of the
reasons why it has been suggested that the seeming gaps, or
many of them, are not openings at all, but opaque screens
cutting off the light from stars behind them. That this is
quite possible in some cases is shown by Barnard's later
photographs, particularly those of the singular region around
the star Rho Ophiuchi. Here are to be seen somber lanes and
patches, apparently forming a connected system which covers an
immense space, and which their discoverer thinks may constitute
a ``dark nebula.'' This seems at first a startling suggestion;
but, after all, why should their not be dark nebulæ as
well as visible ones? In truth, it has troubled some
astronomers to explain the luminosity of the bright
nebulæ, since it is not to be supposed that matter in so
diffuse a state can be incandescent through heat, and
phosphorescent light is in itself a mystery. The supposition is
also in accord with what we know of the existence of dark solid
bodies in space. Many bright stars are accompanied by obscure
companions, sometimes as massive as themselves; the planets are
non-luminous; the same is true of meteors before they plunge
into the atmosphere and become heated by friction; and many
plausible reasons have been found for believing that space
contains as many obscure as shining bodies of great size. It is
not so difficult, after all, then, to believe that there are
immense collections of shadowy gases and meteoric dust whose
presence is only manifested when they intercept the light
coming from shining bodies behind them.</p>
<p>This would account for the apparent extinguishment of light
in open space, which is indicated by the falling off in
relative number of telescopic stars below the tenth magnitude.
Even as things are, the amount of light coming to us from stars
too faint to be seen with the naked eye is so great that the
statement of it generally surprises persons who are unfamiliar
with the inner facts of astronomy. It has been calculated that
on a clear night the total starlight from the entire celestial
sphere amounts to one-sixtieth of the light of the full moon;
but of this less than one-twenty-fifth is due to stars
separately distinguished by the eye. If there were no obscuring
medium in space, it is probable that the amount of starlight
would be noticeably and perhaps enormously increased.</p>
<p>But while it seems certain that some of the obscure spots in
the Milky Way are due to the presence of ``dark nebulæ,''
or concealing veils of one kind or another, it is equally
certain that there are many which are true apertures, however
they may have been formed, and by whatever forces they may be
maintained. These, then, are veritable windows of the Galaxy,
and when looking out of them one is face to face with the great
mystery of infinite space. <em>There</em> the known universe
visibly ends, but manifestly space itself does not end there.
It is not within the power of thought to conceive an end to
space, for the instant we think of a terminal point or line the
mind leaps forward to the <em>beyond.</em> There must be space
outside as well as inside. Eternity of time and infinity of
space are ideas that the intellect cannot fully grasp, but
neither can it grasp the idea of a limitation to either space
or time. The metaphysical conceptions of hypergeometry, or
fourth-dimensional space, do not aid us.</p>
<p>Having, then, discovered that the universe is a thing
<em>contained</em> in something indefinitely greater than
itself; having looked out of its windows and found only the
gloom of starless night outside -- what conclusions are we to
draw concerning the beyond? It <em>seems</em> as empty as a
vacuum, but is it really so? If it be, then our universe is a
single atom astray in the infinite; it is the only island in an
ocean without shores; it is the one oasis in an illimitable
desert. Then the Milky Way, with its wide-flung garland of
stars, is afloat like a tiny smoke-wreath amid a horror of
immeasurable vacancy, or it is an evanescent and solitary ring
of sparkling froth cast up for a moment on the viewless billows
of immensity. From such conclusions the mind instinctively
shrinks. It prefers to think that there is <em>something</em>
beyond, though we cannot see it. Even the universe could not
bear to be alone -- a Crusoe lost in the Cosmos! As the
inhabitants of the most elegant château, with its
gardens, parks, and crowds of attendants, would die of
loneliness if they did not know that they have neighbors,
though not seen, and that a living world of indefinite extent
surrounds them, so we, when we perceive that the universe has
limits, wish to feel that it is not solitary; that beyond the
hedges and the hills there are other centers of life and
activity. Could anything be more terrible than the thought of
an <em>isolated universe?</em> The greater the being, the
greater the aversion to seclusion. Only the infinite satisfies;
in that alone the mind finds rest.</p>
<p>We are driven, then, to believe that the universal night
which envelopes us is not tenantless; that as we stare out of
the star-framed windows of the Galaxy and see nothing but
uniform blackness, the fault is with our eyes or is due to an
obscuring medium. Since <em>our</em> universe is limited in
extent, there must be <em>other</em> universes beyond it on all
sides. Perhaps if we could carry our telescopes to the verge of
the great ``Coal-sack'' near the ``Cross,'' being then on the
frontier of our starry system, we could discern, sparkling afar
off in the vast night, some of the outer galaxies. They may be
grander than ours, just as many of the suns surrounding us are
immensely greater than ours. If we could take our stand
somewhere in the midst of immensity and, with vision of
infinite reach, look about us, we should perhaps see a
countless number of stellar systems, amid which ours would be
unnoticeable, like a single star among the multitude glittering
in the terrestial sky on a clear night. Some might be in the
form of a wreath, like our own; some might be globular, like
the great star-clusters in Hercules and Centaurus; some might
be glittering circles, or disks, or rings within rings. If we
could enter them we should probably find a vast variety of
composition, including elements unknown to terrestrial
chemistry; for while the <em>visible</em> universe appears to
contain few if any substances not existing on the earth or in
the sun, we have no warrant to assume that others may not exist
in infinite space.</p>
<p>And how as to gravitation? We do not <em>know</em> that
gravitation acts beyond the visible universe, but it is
reasonable to suppose that it does. At any rate, if we let go
<em>its</em> sustaining hand we are lost, and can only wander
hopelessly in our speculations, like children astray. If the
empire of gravitation is infinite, then the various outer
systems must have <em>some,</em> though measuring by our
standards an imperceptible, attractive influence upon each
other, for gravitation never lets go its hold, however great
the space over which it is required to act. Just as the stars
about us are all in motion, so the starry systems beyond our
sight may be in motion, and our system as a whole may be moving
in concert with them. If this be so, then after interminable
ages the aspect of the entire system of systems must change,
its various members assuming new positions with respect to one
another. In the course of time we may even suppose that our
universe will approach relatively close to one of the others;
and then, if men are yet living on the earth, they may glimpse
through the openings which reveal nothing to us now, the lights
of another nearing star system, like the signals of a strange
squadron, bringing them the assurance (which can be but an
inference at present) that the ocean of space has other
argosies venturing on its limitless expanse.</p>
<p>There remains the question of the luminiferous ether by
whose agency the waves of light are borne through space. The
ether is as mysterious as gravitation. With regard to ether we
only infer its existence from the effects which we ascribe to
it. Evidently the ether must extend as far as the most distant
visible stars. But does it continue on indefinitely in outer
space? If it does, then the invisibility of the other systems
must be due to their distance diminishing the quantity of light
that comes from them below the limit of perceptibility, or to
the interposition of absorbing media; if it does not, then the
reason why we cannot see them is owing to the absence of a
means of conveyance for the light waves, as the lack of an
interplanetary atmosphere prevents us from hearing the thunder
of sun-spots. (It is interesting to recall that Mr Edison was
once credited with the intention to construct a gigantic
microphone which should render the roar of sun-spots audible by
transforming the electric vibrations into sound-waves). On this
supposition each starry system would be enveloped in its own
globule of ether, and no light could cross from one to another.
But the probability is that both the ether and gravitation are
ubiquitous, and that all the stellar systems are immersed in
the former like clouds of phosphorescent organisms in the
sea.</p>
<p>So astronomy carries the mind from height to greater height.
Men were long in accepting the proofs of the relative
insignificance of the earth; they were more quickly convinced
of the comparative littleness of the solar system; and now the
evidence assails their reason that what they had regarded as
<em>the</em> universe is only one mote gleaming in the sunbeams
of Infinity.</p>
<p><strong>Star-Clouds, Star-Clusters, and
Star-Streams</strong></p>
<p>In the preceding chapter we have seen something of the
strangely complicated structure of the Galaxy, or Milky Way. We
now proceed to study more comprehensively that garlanded
``Pathway of the Gods.''</p>
<p>Judged by the eye alone, the Milky Way is one of the most
delicately beautiful phenomena in the entire realm of nature --
a shimmer of silvery gauze stretched across the sky; but
studied in the light of its revelations, it is the most
stupendous object presented to human ken. Let us consider,
first, its appearance to ordinary vision. Its apparent position
in the sky shifts according to the season. On a serene,
cloudless summer evening, in the absence of the moon, whose
light obscures it, one sees the Galaxy spanning the heavens
from north to southeast of the zenith like a phosphorescent
arch. In early spring it forms a similar but, upon the whole,
less brilliant arch west of the zenith. Between spring and
summer it lies like a long, faint, twilight band along the
northern horizon. At the beginning of winter it again forms an
arch, this time spanning the sky from east to west, a little
north of the zenith. These are its positions as viewed from the
mean latitude of the United States. Even the beginner in
star-gazing does not have to watch it throughout the year in
order to be convinced that it is, in reality, a great circle,
extending entirely around the celestial sphere. We appear to be
situated near its center, but its periphery is evidently far
away in the depths of space.</p>
<p>Although to the casual observer it seems but a delicate
scarf of light, brighter in some places than in others, but
hazy and indefinite at the best, such is not its appearance to
those who study it with care. They perceive that it is an
organic whole, though marvelously complex in detail. The
telescope shows that it consists of stars too faint and small
through excess of distance to be separately visible. Of the
hundred million suns which some estimates have fixed as the
probable population of the starry universe, the vast majority
(at least thirty to one) are included in this strange belt of
misty light. But they are not uniformly distributed in it; on
the contrary, they are arrayed in clusters, knots, bunches,
clouds, and streams. The appearance is somewhat as if the
Galaxy consisted of innumerable swarms of silver-winged bees,
more or less intermixed, some massed together, some crossing
the paths of others, but all governed by a single purpose which
leads them to encircle the region of space in which we are
situated.</p>
<p>From the beginning of the systematic study of the heavens,
the fact has been recognized that the form of the Milky Way
denotes the scheme of the sidereal system. At first it was
thought that the shape of the system was that of a vast round
disk, flat like a cheese, and filled with stars, our sun and
his relatively few neighbors being placed near the center.
According to this view, the galactic belt was an effect of
perspective; for when looking in the direction of the plane of
the disk, the eye ranged through an immense extension of stars
which blended into a glimmering blur, surrounding us like a
ring; while when looking out from the sides of the disk we saw
but few stars, and in those directions the heavens appeared
relatively blank. Finally it was recognized that this theory
did not correspond with the observed appearances, and it became
evident that the Milky Way was not a mere effect of
perspective, but an actual band of enormously distant stars,
forming a circle about the sphere, the central opening of the
ring (containing many scattered stars) being many times broader
than the width of the ring itself. Our sun is one of the
scattered stars in the central opening.</p>
<p>As already remarked, the ring of the Galaxy is very
irregular, and in places it is partly broken. With its sinuous
outline, its pendant sprays, its graceful and accordant curves,
its bunching of masses, its occasional interstices, and the
manifest order of a general plan governing the jumble of its
details, it bears a remarkable resemblance to a garland -- a
fact which appears the more wonderful when we recall its
composition. That an elm-tree should trace the lines of beauty
with its leafy and pendulous branches does not surprise us; but
we can only gaze with growing amazement when we behold <em>a
hundred million suns imitating the form of a chaplet!</em> And
then we have to remember that this form furnishes the
ground-plan of the universe.</p>
<p>As an indication of the extraordinary speculations to which
the mystery of the Milky Way has given rise, a theory recently
(1909) proposed by Prof. George C. Comstock may be mentioned.
Starting with the data (first) that the number of stars
increases as the Milky Way is approached, and reaches a maximum
in its plane, while on the other hand the number of
nebulæ is greatest outside the Milky Way and increases
with distance from it, and (second) that the Milky Way,
although a complete ring, is broad and diffuse on one side
through one-half its course -- that half alone containing
nebulæ -- and relatively narrow and well defined on the
opposite side, the author of this singular speculation avers
that these facts can best be explained by supposing that the
invisible universe consists of two interpenetrating parts, one
of which is a chaos of indefinite extent, strewn with stars and
nebulous dust, and the other a long, broad but comparatively
thin cluster of stars, including the sun as one of its central
members. This flat star-cluster is conceived to be moving
edgewise through the chaos, and, according to Professor
Comstock, it acts after the manner of a snow-plough sweeping
away the cosmic dust and piling it on either hand above and
below the plane of the moving cluster. It thus forms a
transparent rift, through which we see farther and command a
view of more stars than through the intensified dust-clouds on
either hand. This rift is the Milky Way. The dust thrown aside
toward the poles of the Milky Way is the substance of the
nebulæ which abound there. Ahead, where the front of the
star-plough is clearing the way, the chaos is nearer at hand,
and consequently there the rift subtends a broader angle, and
is filled with primordial dust, which, having been annexed by
the vanguard of the star-swarm, forms the nebulæ seen
only in that part of the Milky Way. But behind, the rift
appears narrow because there we look farther away between
dust-clouds produced ages ago by the front of the plough, and
no scattered dust remains in that part of the rift.</p>
<p>In quoting an outline of this strikingly original theory the
present writer should not be understood as assenting to it.
That it appears bizarre is not, in itself, a reason for
rejecting it, when we are dealing with so problematical and
enigmatical a subject as the Milky Way; but the serious
objection is that the theory does not sufficiently accord with
the observed phenomena. There is too much evidence that the
Milky Way is an organic system, however fantastic its form, to
permit the belief that it can only be a rift in chaotic clouds.
As with every organism, we find that its parts are more or less
clearly repeated in its ensemble. Among all the strange things
that the Milky Way contains there is nothing so extraordinary
as itself. Every astronomer must many times have found himself
marveling at it in those comparatively rare nights when it
shows all its beauty and all its strangeness. In its great
broken rifts, divisions, and spirals are found the gigantic
prototypes of similar forms in its star-clouds and clusters. As
we have said, it determines the general shape of the whole
sidereal system. Some of the brightest stars in the sky appear
to hang like jewels suspended at the ends of tassels dropped
from the Galaxy. Among these pendants are the Pleiades and the
Hyades. Orion, too, the ``Mighty Hunter,'' is caught in ``a
loop of light'' thrown out from it. The majority of the great
first-magnitude stars seem related to it, as if they formed an
inner ring inclined at an angle of some twenty degrees to its
plane. Many of the long curves that set off from it on both
sides are accompanied by corresponding curves of lucid stars.
In a word, it offers every appearance of structural connection
with the entire starry system. That the universe should have
assumed the form of a wreath is certainly a matter for
astonishment; but it would have been still more astonishing if
it had been a cube, a rhomboid, or a dodecahedron, for then we
should have had to suppose that something resembling the forces
that shape crystals had acted upon the stars, and the
difficulty of explaining the universe by the laws of
gravitation would have been increased.</p>
<p>From the Milky Way as a whole we pass to the vast clouds,
swarms, and clusters of stars of which it is made up. It may
be, as some astronomers hold, that most of the galactic stars
are much smaller than the sun, so that their faintness is not
due entirely to the effect of distance. Still, their intrinsic
brilliance attests their solar character, and considering their
remoteness, which has been estimated at not less than ten
thousand to twenty thousand light-years (a light-year is equal
to nearly six thousand thousand million miles) their actual
masses cannot be extremely small. The minutest of them are
entitled to be regarded as real suns, and they vary enormously
in magnitude. The effects of their attractions upon one another
can only be inferred from their clustering, because their
relative movements are not apparent on account of the brevity
of the observations that we can make. But imagine a being for
whom a million years would be but as a flitting moment; to him
the Milky Way would appear in a state of ceaseless agitation --
swirling with ``a fury of whirlpool motion.''</p>
<p>The cloud-like aspect of large parts of the Galaxy must
always have attracted attention, even from naked-eye observers,
but the true star-clouds were first satisfactorily represented
in Barnard's photographs. The resemblance to actual clouds is
often startling. Some are close-packed and dense, like cumuli;
some are wispy or mottled, like cirri. The rifts and
modulations, as well as the general outlines, are the same as
those of clouds of vapor or dust, and one notices also the
characteristic thinning out at the edges. But we must beware of
supposing that the component suns are thickly crowded as the
particles forming an ordinary cloud. They <em>look,</em>
indeed, as if they were matted together, because of the
irradiation of light, but in reality millions and billions of
miles separate each star from its neighbors. Nevertheless they
form real assemblages, whose members are far more closely
related to one another than is our sun to the stars around him,
and if we were in the Milky Way the aspect of the nocturnal sky
would be marvelously different from its present appearance.</p>
<p>Stellar clouds are characteristic of the Galaxy and are not
found beyond its borders, except in the ``Magellanic Clouds''
of the southern hemisphere, which resemble detached portions of
the Milky Way. These singular objects form as striking a
peculiarity of the austral heavens as does the great
``Coal-sack'' described in Chapter 1. But it is their isolation
that makes them so remarkable, for their composition is
essentially galactic, and if they were included within its
boundaries they would not appear more wonderful than many other
parts of the Milky Way. Placed where they are, they look like
masses fallen from the great stellar arch. They are full of
nebulæ and star-clusters, and show striking evidences of
spiral movement.</p>
<p>Star-swarms, which are also characteristic features of the
Galaxy, differ from star-clouds very much in the way that their
name would imply -- <em>i.e.,</em> their component stars are so
arranged, even when they are countless in number, that the idea
of an exceedingly numerous assemblage rather than that of a
cloud is impressed on the observer's mind. In a star-swarm the
separate members are distinguishable because they are either
larger or nearer than the stars composing a ``cloud.'' A
splendid example of a true star-swarm is furnished by Chi
Persei, in that part of the Milky Way which runs between the
constellations Perseus and Cassiopeia. This swarm is much
coarser than many others, and can be seen by the naked eye. In
a small telescope it appears double, as if the suns composing
it had divided into two parties which keep on their way side by
side, with some commingling of their members where the skirts
of the two companies come in contact.</p>
<p>Smaller than either star-clouds or star-swarms, and
differing from both in their organization, are star-clusters.
These, unlike the others, are found outside as well as inside
the Milky Way, although they are more numerous inside its
boundaries than elsewhere. The term star-cluster is sometimes
applied, though improperly, to assemblages which are rather
groups, such, for instance, as the Pleiades. In their most
characteristic aspect star-clusters are of a globular shape --
globes of suns! A famous example of a globular star-cluster,
but one not included in the Milky Way, is the ``Great Cluster
in Hercules.'' This is barely visible to the naked eye, but a
small telescope shows its character, and in a large one it
presents a marvelous spectacle. Photographs of such clusters
are, perhaps, less effective than those of star-clouds, because
the central condensation of stars in them is so great that
their light becomes blended in an indistinguishable blur. The
beautiful effect of the incessant play of infinitesimal rays
over the apparently compact surface of the cluster, as if it
were a globe of the finest frosted silver shining in an
electric beam, is also lost in a photograph. Still, even to the
eye looking directly at the cluster through a powerful
telescope, the central part of the wonderful congregation seems
almost a solid mass in which the stars are packed like the ice
crystals in a snowball.</p>
<p>The same question rises to the lips of every observer: How
can they possibly have been brought into such a situation? The
marvel does not grow less when we know that, instead of being
closely compacted, the stars of the cluster are probably
separated by millions of miles; for we know that their
distances apart are slight as compared with their remoteness
from the Earth. Sir William Herschel estimated their number to
be about fourteen thousand, but in fact they are uncountable.
If we could view them from a point just within the edge of the
assemblage, they would offer the appearance of a hollow
hemisphere emblazoned with stars of astonishing brilliancy; the
near-by ones unparalleled in splendor by any celestial object
known to us, while the more distant ones would resemble
ordinary stars. An inhabitant of the cluster would not know,
except by a process of ratiocination, that he was dwelling in a
globular assemblage of suns; only from a point far outside
would their spherical arrangement become evident to the eye.
Imagine fourteen-thousand fire-balloons with an approach to
regularity in a spherical space -- say, ten miles in diameter;
there would be an average of less than thirty in every cubic
mile, and it would be necessary to go to a considerable
distance in order to see them as a globular aggregation; yet
from a point sufficiently far away they would blend into a
glowing ball.</p>
<p>Photographs show even better than the best telescopic views
that the great cluster is surrounded with a multitude of
dispersed stars, suggestively arrayed in more or less curving
lines, which radiate from the principle mass, with which their
connection is manifest. These stars, situated outside the
central sphere, look somewhat like vagrant bees buzzing round a
dense swarm where the queen bee is sitting. Yet while there is
so much to suggest the operation of central forces, bringing
and keeping the members of the cluster together, the attentive
observer is also impressed with the idea that the whole
wonderful phenomenon may be <em>the result of explosion.</em>
As soon as this thought seizes the mind, confirmation of it
seems to be found in the appearance of the outlying stars,
which could be as readily explained by the supposition that
they have been blown apart as that they have flocked together
toward a center. The probable fact that the stars constituting
the cluster are very much smaller than our sun might be
regarded as favoring the hypothesis of an explosion. Of their
real size we know nothing, but, on the basis of an uncertain
estimate of their parallax, it has been calculated that they
may average forty-five thousand miles in diameter -- something
more than half the diameter of the planet Jupiter. Assuming the
same mean density, fourteen thousand such stars might have been
formed by the explosion of a body about twice the size of the
sun. This recalls the theory of Olbers, which has never been
altogether abandoned or disproved, that the Asteroids were
formed by the explosion of a planet circulating between the
orbits of Mars and Jupiter. The Asteroids, whatever their
manner of origin, form a ring around the sun; but, of course,
the explosion of a great independent body, not originally
revolving about a superior center of gravitational force, would
not result in the formation of a ring of small bodies, but
rather of a dispersed mass of them. But back of any speculation
of this kind lies the problem, at present insoluble: How could
the explosion be produced? (See the question of explosions in
Chapters 6 and 14).</p>
<p>Then, on the other hand, we have the observation of
Herschel, since abundantly confirmed, that space is unusually
vacant in the immediate neighborhood of condensed star-clusters
and nebulæ, which, as far as it goes, might be taken as
an indication that the assembled stars had been drawn together
by their mutual attractions, and that the tendency to
aggregation is still bringing new members toward the cluster.
But in that case there must have been an original condensation
of stars at that point in space. This could probably have been
produced by the coagulation of a great nebula into stellar
nuclei, a process which seems now to be taking place in the
Orion Nebula.</p>
<p>A yet more remarkable globular star-cluster exists in the
southern hemisphere, Omega Centauri. In this case the central
condensation of stars presents an almost uniform blaze of
light. Like the Hercules cluster, that in Centaurus is
surrounded with stars scattered over a broad field and showing
an appearance of radial arrangement. In fact, except for its
greater richness, Omega Centauri is an exact duplicate of its
northern rival. Each appears to an imaginative spectator as a
veritable ``city of suns.'' Mathematics shrinks from the task
of disentangling the maze of motions in such an assemblage. It
would seem that the chance of collisions is not to be
neglected, and this idea finds a certain degree of confirmation
in the appearance of ``temporary stars'' which have more than
once blazed out in, or close by, globular star-clusters.</p>
<p>This leads up to the notable fact, first established by
Professor Bailey a few years ago, that such clusters are
populous with variable stars. Omega Centauri and the Hercules
cluster are especially remarkable in this respect. The
variables found in them are all of short period and the changes
of light show a noteworthy tendency to uniformity. The first
thought is that these phenomena must be due to collisions among
the crowded stars, but, if so, the encounters cannot be between
the stars themselves, but probably between stars and meteor
swarms revolving around them. Such periodic collisions might go
on for ages without the meteors being exhausted by
incorporation with the stars. This explanation appears all the
more probable because one would naturally expect that flocks of
meteors would abound in a close aggregation of stars. It is
also consistent with Perrine's discovery -- that the globular
star clusters are powdered with minute stars strewn thickly
among the brighter ones.</p>
<p>In speaking of Professor Comstock's extraordinary theory of
the Milky Way, the fact was mentioned that, broadly speaking,
the nebulæ are less numerous in the galactic belt than in
the comparatively open spaces on either side of it, but that
they are, nevertheless, abundant in the broader half of the
Milky Way which he designates as the front of the gigantic
``plough'' supposed to be forcing its way through the
enveloping chaos. In and around the Sagittarius region the
intermingling of nebulæ and galactic star clouds and
clusters is particularly remarkable. That there is a causal
connection no thoughtful person can doubt. We are unable to get
away from the evidence that a nebula is like a seed-ground from
which stars spring forth; or we may say that nebulæ
resemble clouds in whose bosom raindrops are forming. The
wonderful aspect of the admixtures of nebulæ and
star-clusters in Sagittarius has been described in Chapter 1.
We now come to a still more extraordinary phenomenon of this
kind -- the Pleiades nebulæ.</p>
<p>The group of the Pleiades, although lying outside the main
course of the Galaxy, is connected with it by a faint loop, and
is the scene of the most remarkable association of stars and
nebulous matter known in the visible universe. The naked eye is
unaware of the existence of nebulæ in the Pleiades, or,
at the best, merely suspects that there is something of the
kind there; and even the most powerful telescopes are far from
revealing the full wonder of the spectacle; but in photographs
which have been exposed for many hours consecutively, in order
to accumulate the impression of the actinic rays, the
revelation is stunning. The principle stars are seen surrounded
by, and, as it were, <em>drowned in,</em> dense nebulous clouds
of an unparalleled kind. The forms assumed by these clouds seem
at first sight inexplicable. They look like fleeces, or perhaps
more like splashes and daubs of luminous paint dashed
carelessly from a brush. But closer inspection shows that they
are, to a large extent, <em>woven</em> out of innumerable
threads of filmy texture, and there are many indications of
spiral tendencies. Each of the bright stars of the group --
Alcyone, Merope, Maia, Electra, Taygeta, Atlas -- is the focus
of a dense fog (totally invisible, remember, alike to the naked
eye and to the telescope), and these particular stars are
veiled from sight behind the strange mists. Running in all
directions across the relatively open spaces are nebulous wisps
and streaks of the most curious forms. On some of the nebular
lines, which are either straight throughout, or if they change
direction do so at an angle, little stars are strung like
beads. In one case seven or eight stars are thus aligned, and,
as if to emphasize their dependence upon the chain which
connects them, when it makes a slight bend the file of stars
turns the same way. Many other star rows in the group suggest
by their arrangement that they, too, were once strung upon
similar threads which have now disappeared, leaving the stars
spaced along their ancient tracks. We seem forced to the
conclusion that there was a time when the Pleiades were
embedded in a vast nebula resembling that of Orion, and that
the cloud has now become so rare by gradual condensation into
stars that the merest trace of it remains, and this would
probably have escaped detection but for the remarkable actinic
power of the radiant matter of which it consists. The richness
of many of these faint nebulous masses in ultra-violet
radiations, which are those that specifically affect the
photographic plate, is the cause of the marvelous revelatory
power of celestial photography. So the veritable unseen
universe, as distinguished from the ``unseen universe'' of
metaphysical speculation, is shown to us.</p>
<p>A different kind of association between stars and
nebulæ is shown in some surprising photographic objects
in the constellation Cygnus, where long, wispy nebulæ,
billions of miles in length, some of them looking like tresses
streaming in a breeze, lie amid fields of stars which seem
related to them. But the relation is of a most singular kind,
for notwithstanding the delicate structure of the long
nebulæ they appear to act as barriers, causing the stars
to heap themselves on one side. The stars are two, three, or
four times as numerous on one side of the nebulæ as on
the other. These nebulæ, as far as appearance goes, might
be likened to rail fences, or thin hedges, against which the
wind is driving drifts of powdery snow, which, while scattered
plentifully all around, tends to bank itself on the leeward
side of the obstruction. The imagination is at a loss to
account for these extraordinary phenomena; yet there they are,
faithfully giving us their images whenever the photographic
plate is exposed to their radiations.</p>
<p>Thus the more we see of the universe with improved methods
of observation, and the more we invent aids to human senses,
each enabling us to penetrate a little deeper into the unseen,
the greater becomes the mystery. The telescope carried us far,
photography is carrying us still farther; but what as yet
unimagined instrument will take us to the bottom, the top, and
the end? And then, what hitherto untried power of thought will
enable us to comprehend the meaning of it all?</p>
<p><strong>Stellar Migrations</strong></p>
<p>To the untrained eye the stars and the planets are not
distinguishable. It is customary to call them all alike
``stars.'' But since the planets more or less rapidly change
their places in the sky, in consequence of their revolution
about the sun, while the stars proper seem to remain always in
the same relative positions, the latter are spoken of as
``fixed stars.'' In the beginnings of astronomy it was not
known that the ``fixed stars'' had any motion independent of
their apparent annual revolution with the whole sky about the
earth as a seeming center. Now, however, we know that the term
``fixed stars'' is paradoxical, for there is not a single
really fixed object in the whole celestial sphere. The apparent
fixity in the positions of the stars is due to their immense
distance, combined with the shortness of the time during which
we are able to observe them. It is like viewing the plume of
smoke issuing from a steamer, hull down, at sea: if one does
not continue to watch it for a long time it appears to be
motionless, although in reality it may be traveling at great
speed across the line of sight. Even the planets seem fixed in
position if one watches them for a single night only, and the
more distant ones do not sensibly change their places, except
after many nights of observation. Neptune, for instance, moves
but little more than two degrees in the course of an entire
year, and in a month its change of place is only about
one-third of the diameter of the full moon.</p>
<p>Yet, fixed as they seem, the stars are actually moving with
a speed in comparison with which, in some cases, the planets
might almost be said to stand fast in their tracks. Jupiter's
speed in his orbit is about eight miles per second, Neptune's
is less than three and one-half miles, and the earth's is about
eighteen and one-half miles; while there are ``fixed stars''
which move two hundred or three hundred miles per second. They
do not all, however, move with so great a velocity, for some
appear to travel no faster than the planets. But in all cases,
notwithstanding their real speed, long-continued and
exceedingly careful observations are required to demonstrate
that they are moving at all. No more overwhelming impression of
the frightful depths of space in which the stars are buried can
be obtained than by reflecting upon the fact that a star whose
actual motion across the line of sight amounts to two hundred
miles per second does not change its apparent place in the sky,
in the course of a thousand years, sufficiently to be noticed
by the casual observer of the heavens!</p>
<p>There is one vast difference between the motions of the
stars and those of the planets to which attention should be at
once called: the planets, being under the control of a central
force emanating from their immediate master, the sun, all move
in the same direction and in orbits concentric about the sun;
the stars, on the other hand, move in every conceivable
direction and have no apparent center of motion, for all
efforts to discover such a center have failed. At one time,
when theology had finally to accept the facts of science, a
grandiose conception arose in some pious minds, according to
which the Throne of God was situated at the exact center of His
Creation, and, seated there, He watched the magnificent
spectacle of the starry systems obediently revolving around
Him. Astronomical discoveries and speculations seemed for a
time to afford some warrant for this view, which was, moreover,
an acceptable substitute for the abandoned geocentric theory in
minds that could only conceive of God as a superhuman
artificer, constantly admiring his own work. No longer ago than
the middle of the nineteenth century a German astronomer,
Maedler, believed that he had actually found the location of
the center about which the stellar universe revolved. He placed
it in the group of the Pleiades, and upon his authority an
extraordinary imaginative picture was sometimes drawn of the
star Alcyone, the brightest of the Pleiades, as the very seat
of the Almighty. This idea even seemed to gain a kind of
traditional support from the mystic significance, without known
historical origin, which has for many ages, and among widely
separated peoples, been attached to the remarkable group of
which Alcyone is the chief. But since Maedler's time it has
been demonstrated that the Pleiades cannot be the center of
revolution of the universe, and, as already remarked, all
attempts to find or fix such a center have proved abortive. Yet
so powerful was the hold that the theory took upon the popular
imagination, that even today astronomers are often asked if
Alcyone is not the probable site of ``Jerusalem the
Golden.''</p>
<p>If there were a discoverable center of predominant
gravitative power, to which the motions of all the stars could
be referred, those motions would appear less mysterious, and we
should then be able to conclude that the universe was, as a
whole, a prototype of the subsidiary systems of which it is
composed. We should look simply to the law of gravitation for
an explanation, and, naturally, the center would be placed
within the opening enclosed by the Milky Way. If it were there
the Milky Way itself should exhibit signs of revolution about
it, like a wheel turning upon its hub. No theory of the star
motions as a whole could stand which failed to take account of
the Milky Way as the basis of all. But the very form of that
divided wreath of stars forbids the assumption of its
revolution about a center. Even if it could be conceived as a
wheel having no material center it would not have the form
which it actually presents. As was shown in Chapter 2, there is
abundant evidence of motion in the Milky Way; but it is not
motion of the system as a whole, but motion affecting its
separate parts. Instead of all moving one way, the galactic
stars, as far as their movements can be inferred, are governed
by local influences and conditions. They appear to travel
crosswise and in contrary directions, and perhaps they eddy
around foci where great numbers have assembled; but of a
universal revolution involving the entire mass we have no
evidence.</p>
<p>Most of our knowledge of star motions, called ``proper
motions,'' relates to individual stars and to a few groups
which happen to be so near that the effects of their movements
are measurable. In some cases the motion is so rapid (not in
appearance, but in reality) that the chief difficulty is to
imagine how it can have been imparted, and what will eventually
become of the ``runaways.'' Without a collision, or a series of
very close approaches to great gravitational centers, a star
traveling through space at the rate of two hundred or three
hundred miles per second could not be arrested or turned into
an orbit which would keep it forever flying within the limits
of the visible universe. A famous example of these speeding
stars is ``1830 Groombridge,'' a star of only the sixth
magnitude, and consequently just visible to the naked eye,
whose motion across the line of sight is so rapid that it moves
upon the face of the sky a distance equal to the apparent
diameter of the moon every 280 years. The distance of this star
is at least 200,000,000,000,000 miles, and may be two or three
times greater, so that its actual speed cannot be less than two
hundred, and may be as much as four hundred, miles per second.
It could be turned into a new course by a close approach to a
great sun, but it could only be stopped by collision, head-on,
with a body of enormous mass. Barring such accidents it must,
as far as we can see, keep on until it has traversed our
stellar system, whence in may escape and pass out into space
beyond, to join, perhaps, one of those other universes of which
we have spoken. Arcturus, one of the greatest suns in the
universe, is also a runaway, whose speed of flight has been
estimated all the way from fifty to two hundred miles per
second. Arcturus, we have every reason to believe, possesses
hundreds of times the mass of our sun -- think, then, of the
prodigious momentum that its motion implies! Sirius moves more
moderately, its motion across the line of sight amounting to
only ten miles per second, but it is at the same time
approaching the sun at about the same speed, its actual
velocity in space being the resultant of the two
displacements.</p>
<p>What has been said about the motion of Sirius brings us to
another aspect of this subject. The fact is, that in every case
of stellar motion the displacement that we observe represents
only a part of the actual movement of the star concerned. There
are stars whose motion carries them straight toward or straight
away from the earth, and such stars, of course, show no cross
motion. But the vast majority are traveling in paths inclined
from a perpendicular to our line of sight. Taken as a whole,
the stars may be said to be flying about like the molecules in
a mass of gas. The discovery of the radial component in the
movements of the stars is due to the spectroscope. If a star is
approaching, its spectral lines are shifted toward the violet
end of the spectrum by an amount depending upon the velocity of
approach; if it is receding, the lines are correspondingly
shifted toward the red end. Spectroscopic observation, then,
combined with micrometric measurements of the cross motion,
enables us to detect the real movement of the star in space.
Sometimes it happens that a star's radial movement is
periodically reversed; first it approaches, and then it
recedes. This indicates that it is revolving around a near-by
companion, which is often invisible, and superposed upon this
motion is that of the two stars concerned, which together may
be approaching or receding or traveling across the line of
sight. Thus the complications involved in the stellar motions
are often exceedingly great and puzzling.</p>
<p>Yet another source of complication exists in the movement of
our own star, the sun. There is no more difficult problem in
astronomy than that of disentangling the effects of the solar
motion from those of the motions of the other stars. But the
problem, difficult as it is, has been solved, and upon its
solution depends our knowledge of the speed and direction of
the movement of the solar system through space, for of course
the sun carries its planets with it. One element of the
solution is found in the fact that, as a result of perspective,
the stars toward which we are going appear to move apart toward
all points of the compass, while those behind appear to close
up together. Then the spectroscopic principle already mentioned
is invoked for studying the shift of the lines, which is toward
the violet in the stars ahead of us and toward the red in those
that we are leaving behind. Of course the effects of the
independent motions of the stars must be carefully excluded.
The result of the studies devoted to this subject is to show
that we are traveling at a speed of twelve to fifteen miles per
second in a northerly direction, toward the border of the
constellations Hercules and Lyra. A curious fact is that the
more recent estimates show that the direction is not very much
out of a straight line drawn from the sun to the star Vega, one
of the most magnificent suns in the heavens. But it should not
be inferred from this that Vega is drawing us on; it is too
distant for its gravitation to have such an effect.</p>
<p>Many unaccustomed thoughts are suggested by this mighty
voyage of the solar system. Whence have we come, and whither do
we go? Every year of our lives we advance at least 375,000,000
miles. Since the traditional time of Adam the sun has led his
planets through the wastes of space no less than
225,000,000,000 miles, or more than 2400 times the distance
that separates him from the earth. Go back in imagination to
the geologic ages, and try to comprehend the distance over
which the earth has flown. Where was our little planet when it
emerged out of the clouds of chaos? Where was the sun when his
``thunder march'' began? What strange constellations shone down
upon our globe when its masters of life were the monstrous
beasts of the ``Age of Reptiles''? A million years is not much
of a span of time in geologic reckoning, yet a million years
ago the earth was farther from its present place in space than
any of the stars with a measurable parallax are now. It was
more than seven times as far as Sirius, nearly fourteen times
as far as Alpha Centauri, three times as far as Vega, and twice
as far as Arcturus. But some geologists demand two hundred,
three hundred, even one thousand million years to enable them
to account for the evolutionary development of the earth and
its inhabitants. In a thousand million years the earth would
have traveled farther than from the remotest conceivable depths
of the Milky Way!</p>
<p>Other curious reflections arise when we think of the form of
the earth's track as it follows the lead of the sun, in a
journey which has neither known beginning nor conceivable end.
There are probably many minds which have found a kind of
consolation in the thought that every year the globe returns to
the same place, on the same side of the sun. This idea may have
an occult connection with our traditional regard for
anniversaries. When that period of the year returns at which
any great event in our lives has occurred we have the feeling
that the earth, in its annual round, has, in a manner, brought
us back to the scene of that event. We think of the earth's
orbit as a well-worn path which we traverse many times in the
course of a lifetime. It seems familiar to us, and we grow to
have a sort of attachment to it. The sun we are accustomed to
regard as a fixed center in space, like the mill or pump around
which the harnessed patient mule makes his endless circuits.
But the real fact is that the earth never returns to the place
in space where it has once quitted. In consequence of the
motion of the sun carrying the earth and the other planets
along, the track pursued by our globe is a vast spiral in space
continually developing and never returning upon its course. It
is probable that the tracks of the sun and the others stars are
also irregular, and possibly spiral, although, as far as can be
at present determined, they appear to be practically straight.
Every star, wherever it may be situated, is attracted by its
fellow-stars from many sides at once, and although the force is
minimized by distance, yet in the course of many ages its
effects must become manifest.</p>
<p>Looked at from another side, is there not something
immensely stimulating and pleasing to the imagination in the
idea of so stupendous a journey, which makes all of us the
greatest of travelers? In the course of a long life a man is
transported through space thirty thousand million miles;
Halley's Comet does not travel one-quarter as far in making one
of its immense circuits. And there are adventures on this
voyage of which we are just beginning to learn to take account.
Space is full of strange things, and the earth must encounter
some of them as it advances through the unknown. Many singular
speculations have been indulged in by astronomers concerning
the possible effects upon the earth of the varying state of the
space that it traverses. Even the alternation of hot and
glacial periods has sometimes been ascribed to this source.
When tropical life flourished around the poles, as the remains
in the rocks assure us, the needed high temperature may, it has
been thought, have been derived from the presence of the earth
in a warm region of space. Then, too, there is a certain
interest for us in the thought of what our familiar planet has
passed through. We cannot but admire it for its long journeying
as we admire the traveler who comes to us from remote and
unexplored lands, or as we gaze with a glow of interest upon
the first locomotive that has crossed a continent, or a ship
that has visited the Arctic or Antarctic regions. If we may
trust the indications of the present course, the earth, piloted
by the sun, has come from the Milky Way in the far south and
may eventually rejoin that mighty band of stars in the far
north.</p>
<p>While the stars in general appear to travel independently of
one another, except when they are combined in binary or trinary
systems, there are notable exceptions to this rule. In some
quarters of the sky we behold veritable migrations of entire
groups of stars whose members are too widely separated to show
any indications of revolution about a common center of gravity.
This leads us back again to the wonderful group of the
Pleiades. All of the principle stars composing that group are
traveling in virtually parallel lines. Whatever force set them
going evidently acted upon all alike. This might be explained
by the assumption that when the original projective force acted
upon them they were more closely united than they are at
present, and that in drifting apart they have not lost the
impulse of the primal motion. Or it may be supposed that they
are carried along by some current in space, although it would
be exceedingly difficult, in the present state of our
knowledge, to explain the nature of such a current. Yet the
theory of a current has been proposed. As to an attractive
center around which they might revolve, none has been found.
Another instance of similar ``star-drift'' is furnished by five
of the seven stars constituting the figure of the ``Great
Dipper.'' In this case the stars concerned are separated very
widely, the two extreme ones by not less than fifteen degrees,
so that the idea of a common motion would never have been
suggested by their aspect in the sky; and the case becomes the
more remarkable from the fact that among and between them there
are other stars, some of the same magnitude, which do not share
their motion, but are traveling in other directions. Still
other examples of the same phenomenon are found in other parts
of the sky. Of course, in the case of compact star-clusters, it
is assumed that all the members share a like motion of
translation through space, and the same is probably true of
dense star-swarms and star-clouds.</p>
<p>The whole question of star-drift has lately assumed a new
phase, in consequence of the investigations of Kapteyn, Dyson,
and Eddington on the ``systematic motions of the stars.'' This
research will, it is hoped, lead to an understanding of the
general law governing the movements of the whole body of stars
constituting the visible universe. Taking about eleven hundred
stars whose proper motions have been ascertained with an
approach to certainty, and which are distributed in all parts
of the sky, it has been shown that there exists an apparent
double drift, in two independent streams, moving in different
and nearly opposed directions. The apex of the motion of what
is called ``Stream I'' is situated, according to Professor
Kapteyn, in right ascension 85°, declination south 11°,
which places it just south of the constellation Orion; while
the apex of ``Stream II'' is in right ascension 260°,
declination south 48°, placing it in the constellation Ara,
south of Scorpio. The two apices differ very nearly 180° in
right ascension and about 120° in declination. The
discovery of these vast star-streams, if they really exist, is
one of the most extraordinary in modern astronomy. It offers
the correlation of stellar movements needed as the basis of a
theory of those movements, but it seems far from revealing a
physical cause for them. As projected against the celestial
sphere the stars forming the two opposite streams appear
intermingled, some obeying one tendency and some the other. As
Professor Dyson has said, the hypothesis of this double
movement is of a revolutionary character, and calls for further
investigation. Indeed, it seems at first glance not less
surprising than would be the observation that in a snow-storm
the flakes over our heads were divided into two parties and
driving across each other's course in nearly opposite
directions, as if urged by interpenetrating winds.</p>
<p>But whatever explanation may eventually be found for the
motions of the stars, the knowledge of the existence of those
motions must always afford a new charm to the contemplative
observer of the heavens, for they impart a sense of life to the
starry system that would otherwise be lacking. A stagnant
universe, with every star fixed immovably in its place, would
not content the imagination or satisfy our longing for
ceaseless activity. The majestic grandeur of the evolutions of
the celestial hosts, the inconceivable vastness of the fields
of space in which they are executed, the countless numbers, the
immeasurable distances, the involved convolutions, the flocking
and the scattering, the interpenetrating marches and
countermarches, the strange community of impulsion affecting
stars that are wide apart in space and causing them to traverse
the general movement about them like aides and despatch-bearers
on a battle-field -- all these arouse an intensity of interest
which is heightened by the mystery behind them.</p>
<p><strong>The Passing of the Constellations</strong></p>
<p>From a historical and picturesque point of view, one of the
most striking results of the motions of the stars described in
the last chapter is their effect upon the forms of the
constellations, which have been watched and admired by mankind
from a period so early that the date of their invention is now
unknown. The constellations are formed by chance combinations
of conspicuous stars, like figures in a kaleidoscope, and if
our lives were commensurate with the æons of cosmic
existence we should perceive that the kaleidoscope of the
heavens was ceaselessly turning and throwing the stars into new
symmetries. Even if the stars stood fast, the motion of the
solar system would gradually alter the configurations, as the
elements of a landscape dissolve and recombine in fresh
groupings with the traveler's progress amid them. But with the
stars themselves all in motion at various speeds and in many
directions, the changes occur more rapidly. Of course,
``rapid'' is here understood in a relative sense; the wheel of
human history to an eye accustomed to the majestic progression
of the universe would appear to revolve with the velocity of a
whirling dynamo. Only the deliberation of geological movements
can be contrasted with the evolution and devolution of the
constellations.</p>
<p>And yet this secular fluctuation of the constellation
figures is not without keen interest for the meditative
observer. It is another reminder of the swift mutability of
terrestial affairs. To the passing glance, which is all that we
can bestow upon these figures, they appear so immutable that
they have been called into service to form the most lasting
records of ancient thought and imagination that we possess. In
the forms of the constellations, the most beautiful, and, in
imaginative quality, the finest, mythology that the world has
ever known has been perpetuated. Yet, in a broad sense, this
scroll of human thought imprinted on the heavens is as
evanescent as the summer clouds. Although more enduring than
parchment, tombs, pyramids, and temples, it is as far as they
from truly eternizing the memory of what man has fancied and
done.</p>
<p>Before studying the effects that the motions of the stars
have had and will have upon the constellations, it is worth
while to consider a little further the importance of the
stellar pictures as archives of history. To emphasize the
importance of these effects it is only necessary to recall that
the constellations register the oldest traditions of our race.
In the history of primeval religions they are the most valuable
of documents. Leaving out of account for the moment the more
familiar mythology of the Greeks, based on something older yet,
we may refer for illustration to that of the mysterious Maya
race of America. At Izamal, in Yucatan, says Mr Stansbury
Hagar, is a group of ruins perched, after the Mexican and
Central-American plan, on the summits of pyramidal mounds which
mark the site of an ancient theogonic center of the Mayas. Here
the temples all evidently refer to a cult based upon the
constellations as symbols. The figures and the names, of
course, were not the same as those that we have derived from
our Aryan ancestors, but the star groups were the same or
nearly so. For instance, the loftiest of the temples at Izamal
was connected with the sign of the constellation known to us as
Cancer, marking the place of the sun at the summer solstice, at
which period the sun was supposed to descend at noon like a
great bird of fire and consume the offerings left upon the
altar. Our Scorpio was known to the Mayas as a sign of the
``Death God.'' Our Libra, the ``Balance,'' with which the idea
of a divine weighing out of justice has always been connected,
seems to be identical with the Mayan constellation
Teoyaotlatohua, with which was associated a temple where dwelt
the priests whose special business it was to administer justice
and to foretell the future by means of information obtained
from the spirits of the dead. Orion, the ``Hunter'' of our
celestial mythology, was among the Mayas a ``Warrior,'' while
Sagittarius and others of our constellations were known to them
(under different names, of course), and all were endowed with a
religious symbolism. And the same star figures, having the same
significance, were familiar to the Peruvians, as shown by the
temples at Cuzco. Thus the imagination of ancient America
sought in the constellations symbols of the unchanging
gods.</p>
<p>But, in fact, there is no nation and no people that has not
recognized the constellations, and at one period or another in
its history employed them in some symbolic or representative
capacity. As handled by the Greeks from prehistoric times, the
constellation myths became the very soul of poetry. The
imagination of that wonderful race idealized the principal star
groups so effectively that the figures and traditions thus
attached to them have, for civilized mankind, displaced all
others, just as Greek art in its highest forms stands without
parallel and eclipses every rival. The Romans translated no
heroes and heroines of the mythical period of their history to
the sky, and the deified Cæsars never entered that lofty
company, but the heavens are filled with the early myths of the
Greeks. Herakles nightly resumes his mighty labors in the
stars; Zeus, in the form of the white ``Bull,'' Taurus, bears
the fair Europa on his back through the celestial waves;
Andromeda stretches forth her shackled arms in the star-gemmed
ether, beseeching aid; and Perseus, in a blaze of diamond
armor, revives his heroic deeds amid sparkling clouds of
stellar dust. There, too, sits Queen Cassiopeia in her dazzling
chair, while the Great King, Cepheus, towers gigantic over the
pole. Professor Young has significantly remarked that a great
number of the constellations are connected in some way or other
with the Argonautic Expedition -- that strangely fascinating
legend of earliest Greek story which has never lost its charm
for mankind. In view of all this, we may well congratulate
ourselves that the constellations will outlast our time and the
time of countless generations to follow us; and yet they are
very far from being eternal. Let us now study some of the
effects of the stellar motions upon them.</p>
<p>We begin with the familiar figure of the ``Great Dipper.''
He who has not drunk inspiration from its celestial bowl is not
yet admitted to the circle of Olympus. This figure is made up
of seven conspicuous stars in the constellation Ursa Major, the
``Greater Bear.'' The handle of the ``Dipper'' corresponds to
the tail of the imaginary ``Bear,'' and the bowl lies upon his
flank. In fact, the figure of a dipper is so evident and that
of a bear so unevident, that to most persons the ``Great
Dipper'' is the only part of the constellation that is
recognizable. Of the seven stars mentioned, six are of nearly
equal brightness, ranking as of the second magnitude, while the
seventh is of only the third magnitude. The difference is very
striking, since every increase of one magnitude involves an
increase of two-and-a-half times in brightness. There appears
to be little doubt that the faint star, which is situated at
the junction of the bowl and the handle, is a variable of long
period, since three hundred years ago it was as bright as its
companions. But however that may be, its relative faintness at
the present time interferes but little with the perfection of
the ``Dipper's'' figure. In order the more readily to
understand the changes which are taking place, it will be well
to mention both the names and the Greek letters which are
attached to the seven stars. Beginning at the star in the upper
outer edge of the rim of the bowl and running in regular order
round the bottom and then out to the end of the handle, the
names and letters are as follows: Dubhe ({\alpha}), Merak
({\beta}), Phaed ({\gamma}), Megrez ({\delta}), Alioth
({\epsilon}), Mizar ({\zeta}), and Benetnasch ({\eta}). Megrez
is the faint star already mentioned at the junction of the bowl
and handle, and Mizar, in the middle of the handle, has a
close, naked-eye companion which is named Alcor. The Arabs
called this singular pair of stars ``The Horse and Rider.''
Merak and Duhbe are called ``The Pointers,'' because an
imaginary line drawn northward through them indicates the Pole
Star.</p>
<p>Now it has been found that five of these stars --
<em>viz.,</em> Merak, Phaed, Megrez, Alioth, and Mizar (with
its comrade) -- are moving with practically the same speed in
an easterly direction, while the other two, Dubhe and
Benetnasch, are simultaneously moving westward, the motions of
Benetnasch being apparently more rapid. The consequence of
these opposed motions is, of course, that the figure of the
``Dipper'' cannot always have existed and will not continue to
exist. In the accompanying diagrams it has been thought
interesting to show the relative positions of these seven
stars, as seen from the point which the earth now occupies,
both in the past and in the future. Arrows attached to the
stars in the figure representing the present appearance of the
``Dipper'' indicate the directions of the motions and the
distances over which they will carry the stars in a period of
about five hundred centuries. The time, no doubt, seems long,
but remember the vast stretch of ages through which the earth
has passed, and then reflect that no reason is apparent why our
globe should not continue to be a scene of animation for ten
thousand centuries yet to come. The fact that the little star
Alcor placed so close to Mizar should accompany the latter in
its flight is not surprising, but that two of the principal
stars of the group should be found moving in a direction
directly opposed to that pursued by the other five is
surprising in the highest degree; and it recalls the strange
theory of a double drift affecting all the stars, to which
attention was called in the preceding chapter. It would appear
that Benetnasch and Dubhe belong to one ``current,'' and Merak,
Phaed, Megrez, Alioth, and Mizar to the other. As far as is
known, the motion of the seven stars are not shared by the
smaller stars scattered about them, but on the theory of
currents there should be such a community of motion, and
further investigation may reveal it.</p>
<p>From the ``Great Dipper'' we turn to a constellation hardly
less conspicuous and situated at an equal distance from the
pole on the other side -- Cassiopeia. This famous star-group
commemorating the romantic Queen of Ethiopia whose vain
boasting of her beauty was punished by the exposure of her
daughter Andromeda to the ``Sea Monster,'' is well-marked by
five stars which form an irregular letter ``W'' with its open
side toward the pole. Three of these stars are usually ranked
as of the second magnitude, and two of the third; but to
ordinary observation they appear of nearly equal brightness,
and present a very striking picture. They mark out the chair
and a part of the figure of the beautiful queen. Beginning at
the right-hand, or western, end of the ``W,'' their Greek
letter designations are: Beta ({\beta}), Alpha ({\alpha}),
Gamma ({\gamma}), Delta ({\delta}), and Epsilon ({\epsilon}).
Four of them, Beta, Alpha, Delta, and Epsilon are traveling
eastwardly at various speeds, while the fifth, Gamma, moves in
a westerly direction. The motion of Beta is more rapid than
that of any of the others. It should be said, however, that no
little uncertainty attaches to the estimates of the rate of
motion of stars which are not going very rapidly, and different
observers often vary considerably in their results.</p>
<p>In the beautiful ``Northern Crown,'' one of the most perfect
and charming of all the figures to be found in the stars, the
alternate combining and scattering effects of the stellar
motions are shown by comparing the appearance which the
constellation must have had five hundred centuries ago with
that which it has at present and that which it will have in the
future. The seven principle stars of the asterism, forming a
surprisingly perfect coronet, have movements in three
directions at right angles to one another. That in these
circumstances they should ever have arrived at positions giving
them so striking an appearance of definite association is
certainly surprising; from its aspect one would have expected
to find a community of movement governing the brilliants of the
``Crown,'' but instead of that we find evidence that they will
inevitably drift apart and the beautiful figure will
dissolve.</p>
<p>A similar fate awaits such asterisms as the ``Northern
Cross'' in Cygnus; the ``Crow'' (Corvus), which stands on the
back of the great ``Sea Serpent,'' Hydra, and pecks at his
scales; ``Job's Coffin'' (Delphinus); the ``Great Square of
Pegasus''; the ``Twins'' (Gemini); the beautiful ``Sickle'' in
Leo; and the exquisite group of the Hyades in Taurus. In the
case of the Hyades, two controlling movements are manifest:
one, affecting five of the stars which form the well-known
figure of a letter ``V,'' is directed northerly; the other,
which controls the direction of two stars, has an easterly
trend. The chief star of the group, Aldebaran, one of the
finest of all stars both for its brilliance and its color, is
the most affected by the easterly motion. In time it will drift
entirely out of connection with its present neighbors. Although
the Hyades do not form so compact a group as the Pleiades in
the same constellation, yet their appearance of relationship is
sufficient to awaken a feeling of surprise over the fact that,
as with the stars of the ``Dipper,'' their association is only
temporary or apparent.</p>
<p>The great figure of Orion appears to be more lasting, not
because its stars are physically connected, but because of
their great distance, which renders their movements too
deliberate to be exactly ascertained. Two of the greatest of
its stars, Betelgeuse and Rigel, possess, as far as has been
ascertained, no perceptible motion across the line of sight,
but there is a little movement perceptible in the ``Belt.'' At
the present time this consists of an almost perfect straight
line, a row of second-magnitude stars about equally spaced and
of the most striking beauty. In the course of time, however,
the two right-hand stars, Mintaka and Alnilam (how fine are
these Arabic star names!) will approach each other and form a
naked-eye double, but the third, Alnita, will drift away
eastward, so that the ``Belt'' will no longer exist.</p>
<p>For one more example, let us go to the southern hemisphere,
whose most celebrated constellation, the ``Southern Cross,''
has found a place in all modern literatures, although it has no
claim to consideration on account of association with ancient
legends. This most attractive asterism, which has never ceased
to fascinate the imagination of Christendom since it was first
devoutly described by the early explorers of the South, is but
a passing collocation of brilliant stars. Yet even in its
transfigurations it has been for hundreds of centuries, and
will continue to be for hundreds of centuries to come, a most
striking object in the sky. Our figures show its appearance in
three successive phases: first, as it was fifty thousand years
ago (viewed from the earth's present location); second, as it
is in our day; and, third, as it will be an equal time in the
future. The nearness of these bright stars to one another --
the length of the longer beam of the ``Cross'' is only six
degrees -- makes this group very noticeable, whatever the
arrangement of its components may be. The largest star, at the
base of the ``Cross,'' is of the first magnitude, two of the
others are of the second magnitude, and the fourth is of the
third. Other stars, not represented in the figures, increase
the effect of a celestial blazonry, although they do not help
the resemblance to a cross.</p>
<p>But since the motion of the solar system itself will, in the
course of so long a period as fifty thousand years, produce a
great change in the perspective of the heavens as seen from the
earth, by carrying us nearly nineteen trillion miles from our
present place, why, it may be asked, seek to represent future
appearances of the constellations which we could not hope to
see, even if we could survive so long? The answer is: Because
these things aid the mind to form a picture of the effects of
the mobility of the starry universe. Only by showing the
changes from some definite point of view can we arrive at a due
comprehension of them. The constellations are more or less
familiar to everybody, so that impending changes of their forms
must at once strike the eye and the imagination, and make
clearer the significance of the movements of the stars. If the
future history of mankind is to resemble its past and if our
race is destined to survive yet a million years, then our
remote descendents will see a ``new heavens'' if not a ``new
earth,'' and will have to invent novel constellations to
perpetuate their legends and mythologies.</p>
<p>If our knowledge of the relative distances of the stars were
more complete, it would be an interesting exercise in celestial
geometry to project the constellations probably visible to the
inhabitants of worlds revolving around some of the other suns
of space. Our sun is too insignificant for us to think that he
can make a conspicuous appearance among them, except, perhaps,
in a few cases. As seen, for instance, from the nearest known
star, Alpha Centauri, the sun would appear of the average first
magnitude, and consequently from that standpoint he might be
the gem of some little constellation which had no Sirius, or
Arcturus, or Vega to eclipse him with its superior splendor.
But from the distance of the vast majority of the stars the sun
would probably be invisible to the naked eye, and as seen from
nearer systems could only rank as a fifth or sixth magnitude
star, unnoticed and unknown except by the star-charting
astronomer.</p>
<p><strong>Conflagrations in the Heavens</strong></p>
<p>Suppose it were possible for the world to take fire and burn
up -- as some pessimists think that it will do when the Divine
wrath shall have sufficiently accumulated against it -- nobody
out of our own little corner of space would ever be aware of
the catastrophe! With all their telescopes, the astronomers
living in the golden light of Arcturus or the diamond blaze of
Canopus would be unable to detect the least glimmer of the
conflagration that had destroyed the seat of Adam and his
descendents, just as now they are totally ignorant of its
existence.</p>
<p>But at least fifteen times in the course of recorded history
men looking out from the earth have beheld in the remote depths
of space great outbursts of fiery light, some of them more
splendidly luminous than anything else in the firmament except
the sun! If <em>they</em> were conflagrations, how many million
worlds like ours were required to feed their blaze?</p>
<p>It is probable that ``temporary'' or ``new'' stars, as these
wonderful apparitions are called, really are conflagrations;
not in the sense of a bonfire or a burning house or city, but
in that of a sudden eruption of inconceivable heat and light,
such as would result from the stripping off the shell of an
encrusted sun or the crashing together of two mighty orbs
flying through space with a hundred times the velocity of the
swiftest cannon-shot.</p>
<p>Temporary stars are the rarest and most erratic of
astronomical phenomena. The earliest records relating to them
are not very clear, and we cannot in every instance be certain
that it was one of these appearances that the ignorant and
superstitious old chroniclers are trying to describe. The first
temporary star that we are absolutely sure of appeared in 1572,
and is known as ``Tycho's Star,'' because the celebrated Danish
astronomer (whose remains, with his gold-and-silver artificial
nose -- made necessary by a duel -- still intact, were
disinterred and reburied in 1901) was the first to perceive it
in the sky, and the most assiduous and successful in his
studies of it. As the first fully accredited representative of
its class, this new star made its entry upon the scene with
becoming <em>éclat.</em> It is characteristic of these
phenomena that they burst into view with amazing suddenness,
and, of course, entirely unexpectedly. Tycho's star appeared in
the constellation Cassiopeia, near a now well-known and
much-watched little star named Kappa, on the evening of
November 11, 1572. The story has often been repeated, but it
never loses interest, how Tycho, going home that evening, saw
people in the street pointing and staring at the sky directly
over their heads, and following the direction of their hands
and eyes he was astonished to see, near the zenith, an unknown
star of surpassing brilliance. It outshone the planet Jupiter,
and was therefore far brighter than the first magnitude. There
was not another star in the heavens that could be compared with
it in splendor. Tycho was not in all respects free from the
superstitions of his time -- and who is? -- but he had the true
scientific instinct, and immediately he began to study the
stranger, and to record with the greatest care every change in
its aspect. First he determined as well as he could with the
imperfect instruments of his day, many of which he himself had
invented, the precise location of the phenomena in the sky.
Then he followed the changes that it underwent. At first it
brightened until its light equaled or exceeded that of the
planet Venus at her brightest, a statement which will be
appreciated at its full value by anyone who has ever watched
Venus when she plays her dazzling rôle of ``Evening
Star,'' flaring like an arc light in the sunset sky. It even
became so brilliant as to be visible in full daylight, since,
its position being circumpolar, it never set in the latitude of
Northern Europe. Finally it began to fade, turning red as it
did so, and in March, 1574, it disappeared from Tycho's
searching gaze, and has never been seen again from that day to
this. None of the astronomers of the time could make anything
of it. They had not yet as many bases of speculation as we
possess today.</p>
<p>Tycho's star has achieved a romantic reputation by being
fancifully identified with the ``Star of Bethlehem,'' said to
have led the wondering Magi from their eastern deserts to the
cradle-manger of the Savior in Palestine. Many attempts have
been made to connect this traditional ``star'' with some known
phenomenon of the heavens, and none seems more idle than this.
Yet it persistently survives, and no astronomer is free from
eager questions about it addressed by people whose imagination
has been excited by the legend. It is only necessary to say
that the supposition of a connection between the phenomenon of
the Magi and Tycho's star is without any scientific foundation.
It was originally based on an unwarranted assumption that the
star of Tycho was a variable of long period, appearing once
every three hundred and fifteen years, or thereabout. If that
were true there would have been an apparition somewhere near
the traditional date of the birth of Christ, a date which is
itself uncertain. But even the data on which the assumption was
based are inconsistent with the theory. Certain monkish records
speak of something wonderful appearing in the sky in the years
1264 and 945, and these were taken to have been outbursts of
Tycho's star. Investigation shows that the records more
probably refer to comets, but even if the objects seen were
temporary stars, their dates do not suit the hypothesis; from
945 to 1264 there is a gap of 319 years, and from 1264 to 1572
one of only 308 years; moreover 337 years have now (1909)
elapsed since Tycho saw the last glimmer of his star. Upon a
variability so irregular and uncertain as that, even if we felt
sure that it existed, no conclusion could be found concerning
an apparition occurring 2000 years ago.</p>
<p>In the year 1600 (the year in which Giordano Bruno was
burned at the stake for teaching that there is more than one
physical world), a temporary star of the third magnitude broke
out in the constellation Cygnus, and curiously enough,
considering the rarity of such phenomena, only four years later
another surprisingly brilliant one appeared in the
constellation Ophiuchus. This is often called ``Kepler's
star,'' because the great German astronomer devoted to it the
same attention that Tycho had given to the earlier phenomenon.
It, too, like Tycho's, was at first the brightest object in the
stellar heavens, although it seems never to have quite equaled
its famous predecessor in splendor. It disappeared after a
year, also turning of a red color as it became more faint. We
shall see the significance of this as we go on. Some of
Kepler's contemporaries suggested that the outburst of this
star was due to a meeting of atoms in space, and idea bearing a
striking resemblance to the modern theory of ``astronomical
collisions.''</p>
<p>In 1670, 1848, and 1860 temporary stars made their
appearance, but none of them was of great brilliance. In 1866
one of the second magnitude broke forth in the ``Northern
Crown'' and awoke much interest, because by that time the
spectroscope had begun to be employed in studying the
composition of the stars, and Huggins demonstrated that the new
star consisted largely of incandescent hydrogen. But this star,
apparently unlike the others mentioned, was not absolutely new.
Before its outburst it had shown as a star of the ninth
magnitude (entirely invisible, of course, to the naked eye),
and after about six weeks it faded to its original condition in
which it has ever since remained. In 1876 a temporary star
appeared in the constellation Cygnus, and attained at one time
the brightness of the second magnitude. Its spectrum and its
behavior resembled those of its immediate predecessor. In 1885,
astronomers were surprised to see a sixth-magnitude star
glimmering in the midst of the hazy cloud of the great
Andromeda Nebula. It soon absolutely disappeared. Its spectrum
was remarkable for being ``continuous,'' like that of the
nebula itself. A continuous spectrum is supposed to represent a
body, or a mass, which is either solid or liquid, or composed
of gas under great pressure. In January, 1892, a new star was
suddenly seen in the constellation Auriga. It never rose much
above the fourth magnitude, but it showed a peculiar spectrum
containing both bright and dark lines of hydrogen.</p>
<p>But a bewildering surprise was now in store; the world was
to behold at the opening of the twentieth century such a
celestial spectacle as had not been on view since the times of
Tycho and Kepler. Before daylight on the morning of February
22, 1901, the Rev. Doctor Anderson, of Edinburgh, an amateur
astronomer, who had also been the first to see the new star in
Auriga, beheld a strange object in the constellation Perseus
not far from the celebrated variable star Algol. He recognized
its character at once, and immediately telegraphed the news,
which awoke the startled attention of astronomers all over the
world. When first seen the new star was no brighter than Algol
(less than the second magnitude), but within twenty-four hours
it was ablaze, outshining even the brilliant Capella, and far
surpassing the first magnitude. At the spot in the sky where it
appeared nothing whatever was visible on the night before its
coming. This is known with certainty because a photograph had
been made of that very region on February 21, and this
photograph showed everything down to the twelfth magnitude, but
not a trace of the stranger which burst into view between the
21st and the 22nd like the explosion of a rocket.</p>
<p>Upon one who knew the stars the apparition of this intruder
in a well-known constellation had the effect of a sudden
invasion. The new star was not far west of the zenith in the
early evening, and in that position showed to the best
advantage. To see Capella, the hitherto unchallenged ruler of
that quarter of the sky, abased by comparison with this
stranger of alien aspect, for there was always an unfamiliar
look about the ``nova,'' was decidedly disconcerting. It seemed
to portend the beginning of a revolution in the heavens. One
could understand what the effect of such an apparition must
have been in the superstitious times of Tycho. The star of
Tycho had burst forth on the northern border of the Milky Way;
this one was on its southern border, some forty-five degrees
farther east.</p>
<p>Astronomers were well-prepared this time for the scientific
study of the new star, both astronomical photography and
spectroscopy having been perfected, and the results of their
investigations were calculated to increase the wonder with
which the phenomenon was regarded. The star remained at its
brightest only a few days; then, like a veritable
conflagration, it began to languish; and, like the reflection
of a dying fire, as it sank it began to glow with the red color
of embers. But its changes were spasmodic; once about every
three days it flared up only to die away again. During these
fluctuations its light varied alternately in the ratio of one
to six. Finally it took a permanent downward course, and after
a few months the naked eye could no longer perceive it; but it
remained visible with telescopes, gradually fading until it had
sunk to the ninth magnitude. Then another astonishing change
happened: in August photographs taken at the Yerkes Observatory
and at Heidelberg showed that the ``nova'' was <em>surrounded
by a spiral nebula!</em> The nebula had not been there before,
and no one could doubt that it represented a phase of the same
catastrophe that had produced the outburst of the new star. At
one time the star seemed virtually to have disappeared, as if
all its substance had been expanded into the nebulous cloud,
but always there remained a stellar nucleus about which the
misty spiral spread wider and ever wider, like a wave expanding
around a center of disturbance. The nebula too showed a
variability of brightness, and four condensations which formed
in it seemed to have a motion of revolution about the star. As
time went on the nebula continued to expand at a rate which was
computed to be not less than twenty thousand miles per second!
And now the star itself, showing indications of having turned
into a nebula, behaved in a most erratic manner, giving rise to
the suspicion that it was about to burst out again. But this
did not occur, and at length it sunk into a state of lethargy
from which it has to the present time not recovered. But the
nebulous spiral has disappeared, and the entire phenomena as it
now (1909) exists consists of a faint nebulous star of less
than the ninth magnitude.</p>
<p>The wonderful transformations just described had been
forecast in advance of the discovery of the nebulous spiral
encircling the star by the spectroscopic study of the latter.
At first there was no suggestion of a nebular constitution, but
within a month or two characteristic nebular lines began to
appear, and in less than six months the whole spectrum had been
transformed to the nebular type. In the mean time the shifting
of the spectral lines indicated a complication of rapid motions
in several directions simultaneously. These motions were
estimated to amount to from one hundred to five hundred miles
per second.</p>
<p>The human mind is so constituted that it feels forced to
seek an explanation of so marvelous a phenomenon as this, even
in the absence of the data needed for a sound conclusion. The
most natural hypothesis, perhaps, is that of a collision. Such
a catastrophe could certainly happen. It has been shown, for
instance, that in infinity of time the earth is sure to be hit
by a comet; in the same way it may be asserted that, if no time
limit is fixed, the sun is certain to run against some obstacle
in space, either another star, or a dense meteor swarm, or one
of the dark bodies which there is every reason to believe
abound around us. The consequences of such a collision are easy
to foretell, provided that we know the masses and the
velocities of the colliding bodies. In a preceding chapter we
have discussed the motions of the sun and stars, and have seen
that they are so swift that an encounter between any two of
them could not but be disastrous. But this is not all; for as
soon as two stars approached within a few million miles their
speed would be enormously increased by their reciprocal
attractions and, if their motion was directed radially with
respect to their centers, they would come together with a crash
that would reduce them both to nebulous clouds. It is true that
the chances of such a ``head-on'' collision are relatively very
small; two stars approaching each other would most probably
fall into closed orbits around their common center of gravity.
If there were a collision it would most likely be a grazing one
instead of a direct front-to-front encounter. But even a close
approach, without any actual collision, would probably prove
disastrous, owing to the tidal influence of each of the bodies
on the other. Suns, in consequence of their enormous masses and
dimensions and the peculiarities of their constitution, are
exceedingly dangerous to one another at close quarters.
Propinquity awakes in them a mutually destructive tendency.
Consisting of matter in the gaseous, or perhaps, in some cases,
liquid, state, their tidal pull upon each other if brought
close together might burst them asunder, and the photospheric
envelope being destroyed the internal incandescent mass would
gush out, bringing fiery death to any planets that were
revolving near. Without regard to the resulting disturbance of
the earth's orbit, the close approach of a great star to the
sun would be in the highest degree perilous to us. But this is
a danger which may properly be regarded as indefinitely remote,
since, at our present location in space, we are certainly far
from every star except the sun, and we may feel confident that
no great invisible body is near, for if there were one we
should be aware of its presence from the effects of its
attraction. As to dark nebulæ which may possibly lie in
the track that the solar system is pursuing at the rate of
375,000,000 miles per year, that is another question -- and
they, too, could be dangerous!</p>
<p>This brings us directly back to ``Nova Persei,'' for among
the many suggestions offered to explain its outburst, as well
as those of other temporary stars, one of the most fruitful is
that of a collision between a star and a vast invisible nebula.
Professor Seeliger, of Munich, first proposed this theory, but
it afterward underwent some modifications from others. Stated
in a general form, the idea is that a huge dark body, perhaps
an extinguished sun, encountered in its progress through space
a widespread flock of small meteors forming a dark nebula. As
it plunged into the swarm the friction of the innumerable
collisions with the meteors heated its surface to
incandescence, and being of vast size it then became visible to
us as a new star. Meanwhile the motion of the body through the
nebula, and its rotation upon itself, set up a gyration in the
blazing atmosphere formed around it by the vaporized meteors;
and as this atmosphere spread wider, under the laws of gyratory
motion a rotation in the opposite direction began in the
inflamed meteoric cloud outside the central part of the vortex.
Thus the spectral lines were caused to show motion in opposite
directions, a part of the incandescent mass approaching the
earth simultaneously with the retreat of another part. So the
curious spectroscopic observations before mentioned were
explained. This theory might also account for the appearance of
the nebulous spiral first seen some six months after the
original outburst. The sequent changes in the spectrum of the
``nova'' are accounted for by this theory on the assumption,
reasonable enough in itself, that at first the invading body
would be enveloped in a vaporized atmosphere of relatively
slight depth, producing by its absorption the fine dark lines
first observed; but that as time went on and the incessant
collisions continued, the blazing atmosphere would become very
deep and extensive, whereupon the appearance of the spectral
lines would change, and bright lines due to the light of the
incandescent meteors surrounding the nucleus at a great
distance would take the place of the original dark ones. The
vortex of meteors once formed would protect the flying body
within from further immediate collisions, the latter now
occurring mainly among the meteors themselves, and then the
central blaze would die down, and the original splendor of the
phenomenon would fade.</p>
<p>But the theories about Nova Persei have been almost as
numerous as the astronomers who have speculated about it. One
of the most startling of them assumed that the outburst was
caused by the running amuck of a dark star which had
encountered another star surrounded with planets, the renewed
outbreaks of light after the principal one had faded being due
to the successive running down of the unfortunate planets! Yet
another hypothesis is based on what we have already said of the
tidal influence that two close approaching suns would have upon
each other. Supposing two such bodies which had become
encrusted, but remained incandescent and fluid within, to
approach within almost striking distance; they would whirl each
other about their common center of gravity, and at the same
time their shells would burst under the tidal strain, and their
glowing nuclei being disclosed would produce a great outburst
of light. Applying this theory to a ``nova,'' like that of 1866
in the ``Northern Crown,'' which had been visible as a small
star before the outbreak, and which afterward resumed its
former aspect, we should have to assume that a yet shining sun
had been approached by a dark body whose attraction temporarily
burst open its photosphere. It might be supposed that in this
case the dark body was too far advanced in cooling to suffer
the same fate from the tidal pull of its victim. But a close
approach of that kind would be expected to result in the
formation of a binary system, with orbits of great
eccentricity, perhaps, and after the lapse of a certain time
the outburst should be renewed by another approximation of the
two bodies. A temporary star of that kind would rather be
ranked as a variable.</p>
<p>The celebrated French astronomer, Janssen, had a different
theory of Nova Persei, and of temporary stars in general.
According to his idea, such phenomena might be the result of
chemical changes taking place in a sun without interference by,
or collision with, another body. Janssen was engaged for many
years in trying to discover evidence of the existence of oxygen
in the sun, and he constructed his observatory on the summit of
Mount Blanc specially to pursue that research. He believed that
oxygen must surely exist in the sun since we find so many other
familiar elements included in the constitution of the solar
globe, and as he was unable to discover satisfactory evidence
of its presence he assumed that it existed in a form unknown on
the earth. If it were normally in the sun's chromosphere, or
coronal atmosphere, he said, it would combine with the hydrogen
which we know is there and form an obscuring envelope of water
vapor. It exists, then, in a special state, uncombined with
hydrogen; but let the temperature of the sun sink to a critical
point and the oxygen will assume its normal properties and
combine with the hydrogen, producing a mighty outburst of light
and heat. This, Janssen thought, might explain the phenomena of
the temporary stars. It would also, he suggested, account for
their brief career, because the combination of the elements
would be quickly accomplished, and then the resulting water
vapor would form an atmosphere cutting off the radiation from
the star within.</p>
<p>This theory may be said to have a livelier human interest
than some of the others, since, according to it, the sun may
carry in its very constitution a menace to mankind; one does
not like to think of it being suddenly transformed into a
gigantic laboratory for the explosive combination of oxygen and
hydrogen! But while Janssen's theory might do for some
temporary stars, it is inadequate to explain all the phenomena
of Nova Persei, and particularly the appearance of the great
spiral nebula that seemed to exhale from the heart of the star.
Upon the whole, the theory of an encounter between a star and a
dark nebula seems best to fit the observations. By that
hypothesis the expanding billow of light surrounding the core
of the conflagration is very well accounted for, and the
spectroscopic peculiarities are also explained.</p>
<p>Dr Gustov Le Bon offers a yet more alarming theory,
suggesting that temporary stars are the result of <em>atomic
explosion;</em> but we shall touch upon this more fully in
Chapter 14.</p>
<p>Twice in the course of this discussion we have called
attention to the change of color invariably undergone by
temporary stars in the later stages of their career. This was
conspicuous with Nova Persei which glowed more and more redly
as it faded, until the nebulous light began to overpower that
of the stellar nucleus. Nothing could be more suggestive of the
dying out of a great fire. Moreover, change of color from white
to red is characteristic of all variable stars of long period,
such as ``Mira'' in Cetus. It is also characteristic of stars
believed to be in the later stages of evolution, and
consequently approaching extinction, like Antares and
Betelgeuse, and still more notably certain small stars which
``gleam like rubies in the field of the telescope.'' These last
appear to be suns in the closing period of existence as
self-luminous bodies. Between the white stars, such as Sirius
and Rigel, and the red stars, such as Aldebaran and Alpha
Herculis, there is a progressive series of colors from golden
yellow through orange to deep red. The change is believed to be
due to the increase of absorbing vapors in the stellar
atmosphere as the body cools down. In the case of ordinary
stars these changes no doubt occupy many millions of years,
which represent the average duration of solar life; but the
temporary stars run through similar changes in a few months:
they resemble ephemeral insects -- born in the morning and
doomed to perish with the going down of the sun.</p>
<p><strong>Explosive and Whirling Nebulæ</strong></p>
<p>One of the most surprising triumphs of celestial photography
was Professor Keeler's discovery, in 1899, that the great
majority of the nebulæ have a distinctly spiral form.
This form, previously known in Lord Rosse's great ``Whirlpool
Nebula,'' had been supposed to be exceptional; now the
photographs, far excelling telescopic views in the revelation
of nebular forms, showed the spiral to be the typical shape.
Indeed, it is a question whether all nebulæ are not to
some extent spiral. The extreme importance of this discovery is
shown in the effect that it has had upon hitherto prevailing
views of solar and planetary evolution. For more than
three-quarters of a century Laplace's celebrated hypothesis of
the manner of origin of the solar system from a rotating and
contracting nebula surrounding the sun had guided speculation
on that subject, and had been tentatively extended to cover the
evolution of systems in general. The apparent forms of some of
the nebulæ which the telescope had revealed were
regarded, and by some are still regarded, as giving visual
evidence in favor of this theory. There is a ``ring nebula'' in
Lyra with a central star, and a ``planetary nebula'' in Gemini
bearing no little resemblance to the planet Saturn with its
rings, both of which appear to be practical realizations of
Laplace's idea, and the elliptical rings surrounding the
central condensation of the Andromeda Nebula may be cited for
the same kind of proof.</p>
<p>But since Keeler's discovery there has been a decided
turning away of speculation another way. The form of the spiral
nebulæ seems to be entirely inconsistent with the theory
of an originally globular or disk-shaped nebula condensing
around a sun and throwing or leaving off rings, to be
subsequently shaped into planets. Some astronomers, indeed, now
reject Laplace's hypothesis <em>in toto,</em> preferring to
think that even our solar system originated from a spiral
nebula. Since the spiral type prevails among the existing
nebulæ, we must make any mechanical theory of the
development of stars and planetary systems from them accord
with the requirements which that form imposes. A glance at the
extraordinary variations upon the spiral which Professor
Keeler's photographs reveal is sufficient to convince one of
the difficulty of the task of basing a general theory upon
them. In truth, it is much easier to criticize Laplace's
hypothesis than to invent a satisfactory substitute for it. If
the spiral nebulæ seem to oppose it there are other
nebulæ which appear to support it, and it may be that no
one fixed theory can account for all the forms of stellar
evolution in the universe. Our particular planetary system may
have originated very much as the great French mathematician
supposed, while others have undergone, or are now undergoing, a
different process of development. There is always a too strong
tendency to regard an important new discovery and the theories
and speculations based upon it as revolutionizing knowledge,
and displacing or overthrowing everything that went before.
Upon the plea that ``Laplace only made a guess'' more recent
guesses have been driven to extremes and treated by injudicious
exponents as ``the solid facts at last.''</p>
<p>Before considering more recent theories than Laplace's, let
us see what the nature of the photographic revelations is. The
vast celestial maelstrom discovered by Lord Rosse in the
``Hunting Dogs'' may be taken as the leading type of the spiral
nebulæ, although there are less conspicuous objects of
the kind which, perhaps, better illustrate some of their
peculiarities. Lord Rosse's nebula appears far more wonderful
in the photographs than in his drawings made with the aid of
his giant reflecting telescope at Parsonstown, for the
photographic plate records details that no telescope is capable
of showing. Suppose we look at the photograph of this object as
any person of common sense would look at any great and strange
natural phenomenon. What is the first thing that strikes the
mind? It is certainly the appearance of violent whirling
motion. One would say that the whole glowing mass had been spun
about with tremendous velocity, or that it had been set
rotating so rapidly that it had become the victim of
``centrifugal force,'' one huge fragment having broken loose
and started to gyrate off into space. Closer inspection shows
that in addition to the principal focus there are various
smaller condensations scattered through the mass. These are
conspicuous in the spirals. Some of them are stellar points,
and but for the significance of their location we might suppose
them to be stars which happen to lie in a line between us and
the nebula. But when we observe how many of them follow most
faithfully the curves of the spirals we cannot but conclude
that they form an essential part of the phenomenon; it is not
possible to believe that their presence in such situations is
merely fortuitous. One of the outer spirals has at least a
dozen of these star-like points strung upon it; some of them
sharp, small, and distinct, others more blurred and nebulous,
suggesting different stages of condensation. Even the part
which seems to have been flung loose from the main mass has, in
addition to its central condensation, at least one stellar
point gleaming in the half-vanished spire attached to it. Some
of the more distant stars scattered around the ``whirlpool''
look as if they too had been shot out of the mighty vortex,
afterward condensing into unmistakable solar bodies. There are
at least two curved rows of minute stars a little beyond the
periphery of the luminous whirl which clearly follow lines
concentric with those of the nebulous spirals. Such facts are
simply dumbfounding for anyone who will bestow sufficient
thought upon them, for these are <em>suns,</em> though they may
be small ones; and what a birth is that for a sun!</p>
<p>Look now again at the glowing spirals. We observe that
hardly have they left the central mass before they begin to
coagulate. In some places they have a ``ropy'' aspect; or they
are like peascods filled with growing seeds, which eventually
will become stars. The great focus itself shows a similar
tendency, especially around its circumference. The sense that
it imparts of a tremendous shattering force at work is
overwhelming. There is probably more matter in that whirling
and bursting nebula than would suffice to make a hundred solar
systems! It must be confessed at once that there is no
confirmation of the Laplacean hypothesis here; but what
hypothesis will fit the facts? There is one which it has been
claimed does so, but we shall come to that later. In the
meanwhile, as a preparation, fix in the memory the appearance
of that second spiral mass spinning beside its master which
seems to have spurned it away.</p>
<p>For a second example of the spiral nebulæ look at the
one in the constellation Triangulum. <em>God, how hath the
imagination of puny man failed to comprehend Thee!</em> Here is
creation through destruction with a vengeance! The spiral form
of the nebula is unmistakable, but it is half obliterated amid
the turmoil of flying masses hurled away on all sides with
tornadic fury. The focus itself is splitting asunder under the
intolerable strain, and in a little while, as time is reckoned
in the Cosmos, it will be gyrating into stars. And then look at
the cyclonic rain of already finished stars whirling round the
outskirts of the storm. Observe how scores of them are yet
involved in the fading streams of the nebulous spirals; see how
they have been thrown into vast loops and curves, of a beauty
that half redeems the terror of the spectacle enclosed within
their lines -- like iridescent cirri hovering about the edges
of a hurricane. And so again are suns born!</p>
<p>Let us turn to the exquisite spiral in Ursa Major; how
different its aspect from that of the other! One would say that
if the terrific coil in Triangulum has all but destroyed itself
in its fury, this one on the contrary has just begun its
self-demolition. As one gazes one seems to see in it the
smooth, swift, accelerating motion that precedes catastrophe.
The central part is still intact, dense, and uniform in
texture. How graceful are the spirals that smoothly rise from
its oval rim and, gemmed with little stars, wind off into the
darkness until they have become as delicate as threads of
gossamer! But at bottom the story told here is the same --
creation by gyration!</p>
<p>Compare with the above the curious mass in Cetus. Here the
plane of the whirling nebula nearly coincides with our line of
sight and we see the object at a low angle. It is far advanced
and torn to shreds, and if we could look at it perpendicularly
to its plane it is evident that it would closely resemble the
spectacle in Triangulum.</p>
<p>Then take the famous Andromeda Nebula (see Frontispiece),
which is so vast that notwithstanding its immense distance even
the naked eye perceives it as an enigmatical wisp in the sky.
Its image on the sensitive plate is the masterpiece of
astronomical photography; for wild, incomprehensible beauty
there is nothing that can be compared with it. Here, if
anywhere, we look upon the spectacle of creation in one of its
earliest stages. The Andromeda Nebula is apparently less
advanced toward transformation into stellar bodies than is that
in Triangulum. The immense crowd of stars sprinkled over it and
its neighborhood seem in the main to lie this side of the
nebula, and consequently to have no connection with it. But
incipient stars (in some places clusters of them) are seen in
the nebulous rings, while one or two huge masses seem to give
promise of transformation into stellar bodies of unusual
magnitude. I say ``rings'' because although the loops
encompassing the Andromeda Nebula have been called spirals by
those who wish utterly to demolish Laplace's hypothesis, yet
they are not manifestly such, as can be seen on comparing them
with the undoubted spirals of the Lord Rosse Nebula. They look
quite as much like circles or ellipses seen at an angle of,
say, fifteen or twenty degrees to their plane. If they are
truly elliptical they accord fairly well with Laplace's idea,
except that the scale of magnitude is stupendous, and if the
Andromeda Nebula is to become a solar system it will surpass
ours in grandeur beyond all possibility of comparison.</p>
<p>There is one circumstance connected with the spiral
nebulæ, and conspicuous in the Andromeda Nebula on
account of its brightness, which makes the question of their
origin still more puzzling; they all show continuous spectra,
which, as we have before remarked, indicate that the mass from
which the light comes is either solid or liquid, or a gas under
heavy pressure. Thus nebulæ fall into two classes: the
``white'' nebulæ, giving a continuous spectrum; and the
``green'' nebulæ whose spectra are distinctly gaseous.
The Andromeda Nebula is the great representative of the former
class and the Orion Nebula of the latter. The spectrum of the
Andromeda Nebula has been interpreted to mean that it consists
not of luminous gas, but of a flock of stars so distant that
they are separately indistinguishable even with powerful
telescopes, just as the component stars of the Milky Way are
indistinguishable with the naked eye; and upon this has been
based the suggestion that what we see in Andromeda is an outer
universe whose stars form a series of elliptical garlands
surrounding a central mass of amazing richness. But this idea
is unacceptable if for no other reason than that, as just said,
all the spiral nebulæ possess the same kind of spectrum,
and probably no one would be disposed to regard them all as
outer universes. As we shall see later, the peculiarity of the
spectra of the spiral nebulæ is appealed to in support of
a modern substitute for Laplace's hypothesis.</p>
<p>Finally, without having by any means exhausted the variety
exhibited by the spiral nebulæ, let us turn to the great
representative of the other species, the Orion Nebula. In some
ways this is even more marvelous than the others. The early
drawings with the telescope failed to convey an adequate
conception either of its sublimity or of its complication of
structure. It exists in a nebulous region of space, since
photographs show that nearly the whole constellation is
interwoven with faintly luminous coils. To behold the entry of
the great nebula into the field even of a small telescope is a
startling experience which never loses its novelty. As shown by
the photographs, it is an inscrutable chaos of perfectly
amazing extent, where spiral bands, radiating streaks, dense
masses, and dark yawning gaps are strangely intermingled
without apparent order. In one place four conspicuous little
stars, better seen in a telescope than in the photograph on
account of the blurring produced by over-exposure, are
suggestively situated in the midst of a dark opening, and no
observer has ever felt any doubt that these stars have been
formed from the substance of the surrounding nebula. There are
many other stars scattered over its expanse which manifestly
owe their origin to the same source. But compare the general
appearance of this nebula with the others that we have studied,
and remark the difference. If the unmistakably spiral
nebulæ resemble bursting fly-wheels or grindstones from
whose perimeters torrents of sparks are flying, the Orion
Nebula rather recalls the aspect of a cloud of smoke and
fragments produced by the explosion of a shell. This idea is
enforced by the look of the outer portion farthest from the
bright half of the nebula, where sharply edged clouds with dark
spaces behind seem to be billowing away as if driven by a wind
blowing from the center.</p>
<p>Next let us consider what scientific speculation has done in
the effort to explain these mysteries. Laplace's hypothesis can
certainly find no standing ground either in the Orion Nebula or
in those of a spiral configuration, whatever may be its
situation with respect to the grand Nebula of Andromeda, or the
``ring'' and ``planetary'' nebulæ. Some other hypothesis
more consonant with the appearances must be found. Among the
many that have been proposed the most elaborate is the
``Planetesimal Hypothesis'' of Professors Chamberlin and
Moulton. It is to be remarked that it applies to the spiral
nebulæ distinctively, and not to an apparently chaotic
mass of gas like the vast luminous cloud in Orion. The gist of
the theory is that these curious objects are probably the
result of close approaches to each other of two independent
suns, reminding us of what was said on this subject when we
were dealing with temporary stars. Of the previous history of
these appulsing suns the theory gives us no account; they are
simply supposed to arrive within what may be called an
effective tide-producing distance, and then the drama begins.
Some of the probable consequences of such an approach have been
noticed in Chapter 5; let us now consider them a little more in
detail.</p>
<p>Tides always go in couples; if there is a tide on one side
of a globe there will be a corresponding tide on the other
side. The cause is to be found in the law that the force of
gravitation varies inversely as the square of the distance; the
attraction on the nearest surface of the body exercised by
another body is greater than on its center, and greater yet
than on its opposite surface. If two great globes attract each
other, each tends to draw the other out into an ellipsoidal
figure; they must be more rigid than steel to resist this --
and even then they cannot altogether resist. If they are liquid
or gaseous they will yield readily to the force of distortion,
the amount of which will depend upon their distance apart, for
the nearer they are the greater becomes the tidal strain. If
they are encrusted without and liquid or gaseous in the
interior, the internal mass will strive to assume the figure
demanded by the tidal force, and will, if it can, burst the
restraining envelope. Now this is virtually the predicament of
the body we call a sun when in the immediate presence of
another body of similarly great mass. Such a body is presumably
gaseous throughout, the component gases being held in a state
of rigidity by the compression produced by the tremendous
gravitational force of their own aggregate mass. At the surface
such a body is enveloped in a shell of relatively cool matter.
Now suppose a great attracting body, such as another sun, to
approach near enough for the difference in its attraction on
the two opposite sides of the body and on its center to become
very great; the consequence will be a tidal deformation of the
whole body, and it will lengthen out along the line of the
gravitational pull and draw in at the sides, and if its shell
offers considerable resistance, but not enough to exercise a
complete restraint, it will be violently burst apart, or blown
to atoms, and the internal mass will leap out on the two
opposite sides in great fiery spouts. In the case of a sun
further advanced in cooling than ours the interior might be
composed of molten matter while the exterior crust had become
rigid like the shell of an egg; then the force of the ``tidal
explosion'' produced by the appulse of another sun would be
more violent in consequence of the greater resistance overcome.
Such, then, is the mechanism of the first phase in the history
of a spiral nebula according to the Planetesimal Hypothesis.
Two suns, perhaps extinguished ones, have drawn near together,
and an explosive outburst has occured in one or both. The
second phase calls for a more agile exercise of the
imagination.</p>
<p>To simplify the case, let us suppose that only one of the
tugging suns is seriously affected by the strain. Its vast
wings produced by the outburst are twisted into spirals by
their rotation and the contending attractions exercised upon
them, as the two suns, like battleships in desperate conflict,
curve round each other, concentrating their destructive
energies. Then immense quantities of débris are
scattered about in which eddies are created, and finally, as
the sun that caused the damage goes on its way, leaving its
victim to repair its injuries as it may, the dispersed matter
cools, condenses, and turns into streams of solid particles
circling in elliptical paths about their parent sun. These
particles, or fragments, are the ``planetesimals'' of the
theory. In consequence of the inevitable intersection of the
orbits of the planetesimals, nodes are formed where the flying
particles meet, and at these nodes large masses are gradually
accumulated. The larger the mass the greater its attraction,
and at last the nodal points become the nuclei of great
aggregations from which planets are shaped.</p>
<p>This, in very brief form, is the Planetesimal Hypothesis
which we are asked to substitute for that based on Laplace's
suggestion as an explanation of the mode of origin of the solar
system; and the phenomena of the spiral nebulæ are
appealed to as offering evident support to the new hypothesis.
We are reminded that they are elliptical in outline, which
accords with the hypothesis; that their spectra are not
gaseous, which shows that they may be composed of solid
particles like the planetesimals; and that their central masses
present an oval form, which is what would result from the tidal
effects, as just described. We also remember that some of them,
like the Lord Rosse and the Andromeda nebulæ, are
visually double, and in these cases we might suppose that the
two masses represent the tide-burst suns that ventured into too
close proximity. It may be added that the authors of the theory
do not insist upon the appulse of two suns as the <em>only</em>
way in which the planetesimals may have originated, but it is
the only supposition that has been worked out.</p>
<p>But serious questions remain. It needs, for instance, but a
glance at the Triangulum monster to convince the observer that
it cannot be a solar system which is being evolved there, but
rather a swarm of stars. Many of the detached masses are too
vast to admit of the supposition that they are to be
transformed into planets, in our sense of planets, and the
distances of the stars which appear to have been originally
ejected from the focal masses are too great to allow us to
liken the assemblage that they form to a solar system. Then,
too, no nodes such as the hypothesis calls for are visible.
Moreover, in most of the spiral nebulæ the appearances
favor the view that the supposititious encountering suns have
not separated and gone each rejoicing on its way, after having
inflicted the maximum possible damage on its opponent, but
that, on the contrary, they remain in close association like
two wrestlers who cannot escape from each other's grasp. And
this is exactly what the law of gravitation demands; stars
cannot approach one another with impunity, with regard either
to their physical make-up or their future independence of
movement. The theory undertakes to avoid this difficulty by
assuming that in the case of our system the approach of the
foreign body to the sun was not a close one -- just close
enough to produce the tidal extrusion of the relatively
insignificant quantity of matter needed to form the planets.
But even then the effect of the appulse would be to change the
direction of flight, both of the sun and of its visitor, and
there is no known star in the sky which can be selected as the
sun's probable partner in their ancient <em>pas deux.</em> That
there are unconquered difficulties in Laplace's hypothesis no
one would deny, but in simplicity of conception it is
incomparably more satisfactory, and with proper modifications
could probably be made more consonant with existing facts in
our solar system than that which is offered to replace it. Even
as an explanation of the spiral nebulæ, not as solar
systems in process of formation, but as the birthplaces of
stellar clusters, the Planetesimal Hypothesis would be open to
many objections. Granting its assumptions, it has undoubtedly a
strong mathematical framework, but the trouble is not with the
mathematics but with the assumptions. Laplace was one of the
ablest mathematicians that ever lived, but he had never seen a
spiral nebula; if he had, he might have invented a hypothesis
to suit its phenomena. His actual hypothesis was intended only
for our solar system, and he left it in the form of a ``note''
for the consideration of his successors, with the hope that
they might be able to discover the full truth, which he
confessed was hidden from him. It cannot be said that that
truth has yet been found, and when it is found the chances are
that intuition and not logic will have led to it.</p>
<p>The spiral nebulæ, then, remain among the greatest
riddles of the universe, while the gaseous nebulæ, like
that of Orion, are no less mysterious, although it seems
impossible to doubt that both forms give birth to stars. It is
but natural to look to them for light on the question of the
origin of our planetary system; but we should not forget that
the scale of the phenomena in the two cases is vastly
different, and the forces in operation may be equally
different. A hill may have been built up by a glacier, while a
mountain may be the product of volcanic forces or of the
upheaval of the strata of the planet.</p>
<p><strong>The Banners of the Sun</strong></p>
<p>As all the world knows, the sun, a blinding globe pouring
forth an inconceivable quantity of light and heat, whose daily
passage through the sky is caused by the earth's rotation on
its axis, constitutes the most important phenomenon of
terrestial existence. Viewed with a dark glass to take off the
glare, or with a telescope, its rim is seen to be a sharp and
smooth circle, and nothing but dark sky is visible around it.
Except for the interference of the moon, we should probably
never have known that there is any more of the sun than our
eyes ordinarily see.</p>
<p>But when an eclipse of the sun occurs, caused by the
interposition of the opaque globe of the moon, we see its
immediate surroundings, which in some respects are more
wonderful than the glowing central orb. These surroundings,
although not in the sense in which we apply the term to the
gaseous envelope of the earth, may be called the sun's
atmosphere. They consist of two very different parts -- first,
the red ``prominences,'' which resemble tongues of flame
ascending thousands of miles above the sun's surface; and,
second, the ``corona,'' which extends to distances of millions
of miles from the sun, and shines with a soft, glowing light.
The two combined, when well seen, make a spectacle without
parallel among the marvels of the sky. Although many attempts
have been made to render the corona visible when there is no
eclipse, all have failed, and it is to the moon alone that we
owe its revelation. To cover the sun's disk with a circular
screen will not answer the purpose because of the illumination
of the air all about the observer. When the moon hides the sun,
on the other hand, the sunlight is withdrawn from a great
cylinder of air extending to the top of the atmosphere and
spreading many miles around the observer. There is then no
glare to interfere with the spectacle, and the corona appears
in all its surprising beauty. The prominences, however,
although they were discovered during an eclipse, can now, with
the aid of the spectroscope, be seen at any time. But the
prominences are rarely large enough to be noticed by the naked
eye, while the streamers of the corona, stretching far away in
space, like ghostly banners blown out from the black circle of
the obscuring moon, attract every eye, and to this weird
apparition much of the fear inspired by eclipses has been due.
But if the corona has been a cause of terror in the past it has
become a source of growing knowledge in our time.</p>
<p>The story of the first scientific observation of the corona
and the prominences is thrillingly interesting, and in fact
dramatic. The observation was made during the eclipse of 1842,
which fortunately was visible all over Central and Southern
Europe so that scores of astronomers saw it. The interest
centers in what happened at Pavia in Northern Italy, where the
English astronomer Francis Baily had set up his telescope. The
eclipse had begun and Bailey was busy at his telescope when, to
quote his own words in the account which he wrote for the
<em>Memoirs of the Royal Astronomical Society:</em></p>
<blockquote>
I was astounded by a tremendous burst of applause from the
streets below, and at the same moment was electrified by the
sight of one of the most brilliant and splendid phenomena
that can well be imagined; for at that instant the dark body
of the moon was suddenly surrounded with a corona, or kind of
bright glory, similar in shape and magnitude to that which
painters draw round the heads of saints...
<p>Pavia contains many thousand inhabitants, the major part
of whom were at this early hour walking about the streets and
squares or looking out of windows in order to witness this
long-talked-of phenomenon; and when the total obscuration
took place, which was <em>instantaneous,</em> there was a
universal shout from every observer which ``made the welkin
ring,'' and for the moment withdrew my attention from the
object with which I was immediately occupied. I had, indeed,
expected the appearance of a luminous circle round the moon
during the time of total obscurity; but I did not expect,
from any of the accounts of preceding eclipses that I had
read, to witness so magnificent an exhibition as that which
took place...</p>
<p>Splendid and astonishing, however, as this remarkable
phenomenon really was, and although it could not fail to call
forth the admiration and applause of every beholder, yet I
must confess that there was at the same time something in its
singular and wonderful appearance that was appalling...</p>
<p>But the most remarkable circumstance attending the
phenomenon was the appearance of <em>three large
protuberances</em> apparently emanating from the
circumference of the moon, but evidently forming a portion of
the corona. They had the appearance of mountains of a
prodigious elevation; their color was red tinged with lilac
or purple; perhaps the color of the peach-blossom would more
nearly represent it. They somewhat resembled the tops of the
snowy Alpine mountains when colored by the rising or the
setting sun. They resembled the Alpine mountains in another
respect, inasmuch as their light was perfectly steady, and
had none of that flickering or sparkling motion so visible in
other parts of the corona...</p>
<p>The whole of these protuberances were visible even to the
last moment of total obscuration, and when the first ray of
light was admitted from the sun they vanished, with the
corona, altogether, and daylight was instantly restored.</p>
</blockquote>
<p>I have quoted nearly all of this remarkable description not
alone for its intrinsic interest, but because it is the best
depiction that can be found of the general phenomena of a total
solar eclipse. Still, not every such eclipse offers an equally
magnificent spectacle. The eclipses of 1900 and 1905, for
instance, which were seen by the writer, the first in South
Carolina and the second in Spain, fell far short of that
described by Bailey in splendor and impressiveness. Of course,
something must be allowed for the effect of surprise; Bailey
had not expected to see what was so suddenly disclosed to him.
But both in 1900 and 1905 the amount of scattered light in the
sky was sufficient in itself to make the corona appear faint,
and there were no very conspicuous prominences visible. Yet on
both occasions there was manifest among the spectators that
mingling of admiration and awe of which Bailey speaks. The
South Carolinians gave a cheer and the ladies waved their
handkerchiefs when the corona, ineffably delicate of form and
texture, <em>melted</em> into sight and then in two minutes
melted away again. The Spaniards, crowded on the citadel hill
of Burgos, with their king and his royal retinue in their
midst, broke out with a great clapping of hands as the awaited
spectacle unfolded itself in the sky; and on both occasions,
before the applause began, after an awed silence a low murmur
ran through the crowds. At Burgos it is said many made the sign
of the cross.</p>
<p>It was not long before Bailey's idea that the prominences
were a part of the corona was abandoned, and it was perceived
that the two phenomena were to a great extent independent. At
the eclipse of 1868, which the astronomers, aroused by the
wonderful scene of 1842, and eager to test the powers of the
newly invented spectroscope, flocked to India to witness,
Janssen conceived the idea of employing the spectroscope to
render the prominences visible when there was no eclipse. He
succeeded the very next day, and these phenomena have been
studied in that way ever since.</p>
<p>There are recognized two kinds of prominences -- the
``erruptive'' and the ``quiescent.'' The latter, which are
cloud-like in form, may be seen almost anywhere along the edge
of the sun; but the former, which often shoot up as if hurled
from mighty volcanoes, appear to be associated with sun-spots,
and appear only above the zones where spots abound. Either of
them, when seen in projection against the brilliant solar disk,
appears white, not red, as against a background of sky. The
quiescent prominences, whose elevation is often from forty
thousand to sixty thousand miles, consist, as the spectroscope
shows, mainly of hydrogen and helium. The latter, it will be
remembered, is an element which was known to be in the sun many
years before the discovery that it also exists in small
quantities on the earth. A fact which may have a significance
which we cannot at present see is that the emanation from
radium gradually and spontaneously changes into helium, an
alchemistical feat of nature that has opened many curious
vistas to speculative thinkers. The eruptive prominences, which
do not spread horizontally like the others, but ascend with
marvelous velocity to elevations of half a million miles or
more, are apparently composed largely of metallic vapors --
<em>i.e.</em> metals which are usually solid on the earth, but
which at solar temperatures are kept in a volatilized state.
The velocity of their ascent occasionally amounts to three
hundred or four hundred miles per second. It is known from
mathematical considerations that the gravitation of the sun
would not be able to bring back any body that started from its
surface with a velocity exceeding three hundred and
eighty-three miles per second; so it is evident that some of
the matter hurled forth in eruptive prominences may escape from
solar control and go speeding out into space, cooling and
condensing into solid masses. There seems to be no reason why
some of the projectiles from the sun might not reach the
planets. Here, then, we have on a relatively small scale,
<em>explosions</em> recalling those which it has been imagined
may be the originating cause of some of the sudden phenomena of
the stellar heavens.</p>
<p>Of the sun-spots it is not our intention here specifically
to speak, but they evidently have an intimate connection with
eruptive prominences, as well as some relation, not yet fully
understood, with the corona. Of the real cause of sun-spots we
know virtually nothing, but recent studies by Professor Hale
and others have revealed a strange state of things in the
clouds of metallic vapors floating above them and their
surroundings. Evidences of a cyclonic tendency have been found,
and Professor Hale has proved that sun-spots are strong
magnetic fields, and consist of columns of ionized vapors
rotating in opposite directions in the two hemispheres. A fact
which may have the greatest significance is that titanium and
vanadium have been found both in sun-spots and in the
remarkable variable Mira Ceti, a star which every eleven
months, or thereabout, flames up with great brilliancy and then
sinks back to invisibility with the naked eye. It has been
suggested that sun-spots are indications of the beginning of a
process in the sun which will be intensified until it falls
into the state of such a star as Mira. Stars very far advanced
in evolution, without showing variability, also exhibit similar
spectra; so that there is much reason for regarding sunspots as
emblems of advancing age.</p>
<p>The association of the corona with sun-spots is less evident
than that of the eruptive prominences; still such an
association exists, for the form and extent of the corona vary
with the sun-spot period of which we shall presently speak. The
constitution of the corona remains to be discovered. It is
evidently in part gaseous, but it also probably contains matter
in the form of dust and small meteors. It includes one
substance altogether mysterious -- ``coronium.'' There are
reasons for thinking that this may be the lightest of all the
elements, and Professor Young, its discoverer, said that it was
``absolutely unique in nature; utterly distinct from any other
known form of matter, terrestial, solar, or cosmical.'' The
enormous extent of the corona is one of its riddles. Since the
development of the curious subject of the ``pressure of light''
it has been proposed to account for the sustentation of the
corona by supposing that it is borne upon the billows of light
continually poured out from the sun. Experiment has proved,
what mathematical considerations had previously pointed out as
probable, that the waves of light exert a pressure or driving
force, which becomes evident in its effects if the body acted
upon is sufficiently small. In that case the light pressure
will prevail over the attraction of gravitation, and propel the
attenuated matter away from the sun in the teeth of its
attraction. The earth itself would be driven away if, instead
of consisting of a solid globe of immense aggregate mass, it
were a cloud of microscopic particles. The reason is that the
pressure varies in proportion to the <em>surface</em> of the
body acted upon, while the gravitational attraction is
proportional to the <em>volume,</em> or the total amount of
matter in the body. But the surface of any body depends upon
the <em>square</em> of its diameter, while the volume depends
upon the <em>cube</em> of the diameter. If, for instance, the
diameter is represented by 4, the surface will be proportional
to 4 × 4, or 16, and the volume to 4 × 4 × 4,
or 64; but if the diameter is taken as 2, the surface will be 2
× 2, or 4, and the volume 2 × 2 × 2, or 8.
Now, the ratio of 4 to 8 is twice as great as that of 16 to 64.
If the diameter is still further decreased, the ratio of the
surface to the volume will proportionally grow larger; in other
words, the pressure will gain upon the attraction, and whatever
their original ratio may have been, a time will come, if the
diminution of size continues, when the pressure will become
more effective than the attraction, and the body will be driven
away. Supposing the particles of the corona to be below the
critical size for the attraction of a mass like that of the sun
to control them, they would be driven off into the surrounding
space and appear around the sun like the clouds of dust around
a mill. We shall return to this subject in connection with the
Zodiacal Light, the Aurora, and Comets.</p>
<p>On the other hand, there are parts of the corona which
suggest by their forms the play of electric or magnetic forces.
This is beautifully shown in some of the photographs that have
been made of the corona during recent eclipses. Take, for
instance, that of the eclipse of 1900. The sheaves of light
emanating from the poles look precisely like the ``lines of
force'' surrounding the poles of a magnet. It will be noticed
in this photograph that the corona appears to consist of two
portions: one comprising the polar rays just spoken of, and the
other consisting of the broader, longer, and less-defined
masses of light extending out from the equatorial and
middle-latitude zones. Yet even in this more diffuse part of
the phenomenon one can detect the presence of submerged curves
bearing more or less resemblance to those about the poles. Just
what part electricity or electro-magnetism plays in the
mechanism of the solar radiation it is impossible to say, but
on the assumption that it is a very important part is based the
hypothesis that there exists a direct solar influence not only
upon the magnetism, but upon the weather of the earth. This
hypothesis has been under discussion for half a century, and
still we do not know just how much truth it represents. It is
certain that the outbreak of great disturbances on the sun,
accompanied by the formation of sun-spots and the upshooting of
eruptive prominences (phenomena which we should naturally
expect to be attended by action), have been instantly followed
by corresponding ``magnetic storms'' on the earth and brilliant
displays of the auroral lights. There have been occasions when
the influence has manifested itself in the most startling ways,
a great solar outburst being followed by a mysterious gripping
of the cable and telegraph systems of the world, as if an
invisible and irresistible hand had seized them. Messages are
abruptly cut off, sparks leap from the telegraph instruments,
and the entire earth seems to have been thrown into a magnetic
flurry. These occurrences affect the mind with a deep
impression of the dependence of our planet on the sun, such as
we do not derive from the more familiar action of the sunlight
on the growth of plants and other phenomena of life depending
on solar influences.</p>
<p>Perhaps the theory of solar magnetic influence upon the
weather is best known in connection with the ``sun-spot
cycle.'' This, at any rate, is, as already remarked, closely
associated with the corona. Its existence was discovered in
1843 by the German astronomer Schwabe. It is a period of
variable length, averaging about eleven years, during which the
number of spots visible on the sun first increases to a
maximum, then diminishes to a minimum, and finally increases
again to a maximum. For unknown reasons the period is sometimes
two or three years longer than the average and sometimes as
much shorter. Nevertheless, the phenomena always recur in the
same order. Starting, for instance, with a time when the
observer can find few or no spots, they gradually increase in
number and size until a maximum, in both senses, is reached,
during which the spots are often of enormous size and
exceedingly active. After two or three years they begin to
diminish in number, magnitude, and activity until they almost
or quite disappear. A strange fact is that when a new period
opens, the spots appear first in high northern and southern
latitudes, far from the solar equator, and as the period
advances they not only increase in number and size, but break
out nearer and nearer to the equator, the last spots of a
vanishing period sometimes lingering in the equatorial region
after the advance-guard of its successor has made its
appearance in the high latitudes. Spots are never seen on the
equator nor near the poles. It was not very long after the
discovery of the sun-spot cycle that the curious observation
was made that a striking coincidence existed between the period
of the sun-spots and another period affecting the general
magnetic condition of the earth. When a curved line
representing the varying number of sun-spots was compared with
another curve showing the variations in the magnetic state of
the earth the two were seen to be in almost exact accord, a
rise in one curve corresponding to a rise in the other, and a
fall to a fall. Continued observation has proved that this is a
real coincidence and not an accidental one, so that the
connection, although as yet unexplained, is accepted as
established. But does the influence extend further, and
directly affect the weather and the seasons as well as the
magnetic elements of the earth? A final answer to this question
cannot yet be given, for the evidence is contradictory, and the
interpretations put upon it depend largely on the predilections
of the judges.</p>
<p>But, in a broad sense, the sun-spots and the phenomena
connected with them <em>must</em> have a relation to terrestial
meteorology, for they prove the sun to be a variable star.
Reference was made, a few lines above, to the resemblance of
the spectra of sun-spots to those of certain stars which seem
to be failing through age. This in itself is extremely
suggestive; but if this resemblance had never been discovered,
we should have been justified in regarding the sun as variable
in its output of energy; and not only variable, but probably
increasingly so. The very inequalities in the sun-spot cycle
are suspicious. When the sun is most spotted its total light
may be reduced by one-thousandth part, although it is by no
means certain that its outgiving of thermal radiations is then
reduced. A loss of one-thousandth of its luminosity would
correspond to a decrease of .0025 of a stellar magnitude,
considering the sun as a star viewed from distant space. So
slight a change would not be perceptible; but it is not alone
sun-spots which obscure the solar surface, its entire globe is
enveloped with an obscuring veil. When studied with a powerful
telescope the sun's surface is seen to be thickly mottled with
relatively obscure specks, so numerous that it has been
estimated that they cut off from one-tenth to one-twentieth of
the light that we should receive from it if the whole surface
were as brilliant as its brightest parts. The condition of
other stars warrants the conclusion that this obscuring
envelope is the product of a process of refrigeration which
will gradually make the sun more and more variable until its
history ends in extinction. Looking backward, we see a time
when the sun must have been more brilliant than it is now. At
that time it probably shone with the blinding white splendor of
such stars as Sirius, Spica, and Vega; now it resembles the
relatively dull Procyon; in time it will turn ruddy and fall
into the closing cycle represented by Antares. Considering that
once it must have been more radiantly powerful than at present,
one is tempted to wonder if that could have been the time when
tropical life flourished within the earth's polar circles,
sustained by a vivific energy in the sun which it has now
lost.</p>
<p>The corona, as we have said, varies with the sun-spot cycle.
When the spots are abundant and active the corona rises strong
above the spotted zones, forming immense beams or streamers,
which on one occasion, at least, had an observed length of
<em>ten million miles.</em> At the time of a spot minimum the
corona is less brilliant and has a different outline. It is
then that the curved polar rays are most conspicuous. Thus the
vast banners of the sun, shaken out in the eclipse, are signals
to tell of its varying state, but it will probably be long
before we can read correctly their messages.</p>
<p><strong>The Zodiacal Light Mystery</strong></p>
<p>There is a singular phenomenon in the sky -- one of the most
puzzling of all -- which has long arrested the attention of
astronomers, defying their efforts at explanation, but which
probably not one in a hundred, and possibly not one in a
thousand, of the readers of this book has ever seen. Yet its
name is often spoken, and it is a conspicuous object if one
knows when and where to look for it, and when well seen it
exhibits a mystical beauty which at the same time charms and
awes the beholder. It is called ``The Zodiacal Light,'' because
it lies within the broad circle of the Zodiac, marking the
sun's apparent annual path through the stars. What it is nobody
has yet been able to find out with certainty, and books on
astronomy usually speak of it with singular reserve. But it has
given rise to many remarkable theories, and a true explanation
of it would probably throw light on a great many other
celestial mysteries. The Milky Way is a more wonderful object
to look upon, but its nature can be comprehended, while there
is a sort of uncanniness about the Zodiacal Light which
immediately impresses one upon seeing it, for its part in the
great scheme of extra-terrestrial affairs is not evident.</p>
<p>If you are out-of-doors soon after sunset -- say, on an
evening late in the month of February -- you may perceive, just
after the angry flush of the dying winter's day has faded from
the sky, a pale ghostly presence rising above the place where
the sun went down. The writer remembers from boyhood the first
time it was pointed out to him and the unearthly impression
that it made, so that he afterward avoided being out alone at
night, fearful of seeing the spectral thing again. The
phenomenon brightens slowly with the fading of the twilight,
and soon distinctly assumes the shape of an elongated pyramid
of pearly light, leaning toward the south if the place of
observation is in the northern hemisphere. It does not impress
the observer at all in the same manner as the Milky Way; that
looks far off and is clearly among the stars, but the Zodiacal
Light seems closer at hand, as if it were something more
intimately concerning the earth. To all it immediately suggests
a connection, also, with the sunken sun. If the night is clear
and the moon absent (and if you are in the country, for city
lights ruin the spectacles of the sky), you will be able to
watch the apparition for a long time. You will observe that the
light is brightest near the horizon, gradually fading as the
pyramidal beam mounts higher, but in favorable circumstances it
may be traced nearly to the meridian south of the zenith, where
its apex at last vanishes in the starlight. It continues
visible during the evenings of March and part of April, after
which, ordinarily, it is seen no more, or if seen is relatively
faint and unimpressive. But when autumn comes it appears again,
this time not like a wraith hovering above the westward tomb of
the day-god, but rather like a spirit of the morning announcing
his reincarnation in the east.</p>
<p>The reason why the Zodiacal Light is best seen in our
latitudes at the periods just mentioned is because at those
times the Zodiac is more nearly perpendicular to the horizon,
first in the west and then in the east; and, since the
phenomenon is confined within the borders of the Zodiac, it
cannot be favorably placed for observation when the zodiacal
plane is but slightly inclined to the horizon. Its faint light
requires the contrast of a background of dark sky in order to
be readily perceptible. But within the tropics, where the
Zodiac is always at a favorable angle, the mysterious light is
more constantly visible. Nearly all observant travelers in the
equatorial regions have taken particular note of this
phenomenon, for being so much more conspicuous there than in
the temperate zones it at once catches the eye and holds the
attention as a novelty. Humboldt mentions it many times in his
works, for his genius was always attracted by things out of the
ordinary and difficult of explanation, and he made many careful
observations on its shape, its brilliancy, and its variations;
for there can be no doubt that it does vary, and sometimes to
an astonishing degree. It is said that it once remained
practically invisible in Europe for several years in
succession. During a trip to South Africa in 1909 an English
astronomer, Mr E. W. Maunder, found a remarkable difference
between the appearance of the Zodiacal Light on his going and
coming voyages. In fact, when crossing the equator going south
he did not see it at all; but on returning he had, on March
6th, when one degree south of the equator, a memorable view of
it.</p>
<blockquote>
It was a bright, clear night, and the Zodiacal Light was
extraordinarily brilliant -- brighter than he had ever seen
it before. The Milky Way was not to be compared with it. The
brightest part extended 75° from the sun. There was a
faint and much narrower extension which they could just make
out beyond the Pleiades along the ecliptic, but the greater
part of the Zodiacal Light showed as a broad truncated
column, and it did not appear nearly as conical as he had
before seen it.
</blockquote>
<p>When out of the brief twilight of intertropical lands, where
the sun drops vertically to the horizon and night rushes on
like a wave of darkness, the Zodiacal Light shoots to the very
zenith, its color is described as a golden tint, entirely
different from the silvery sheen of the Milky Way. If I may
venture again to refer to personal experiences and impressions,
I will recall a view of the Zodiacal Light from the summit of
the cone of Mt Etna in the autumn of the year 1896 (more
briefly described in <em>Astronomy with the Naked Eye</em>).
There are few lofty mountains so favorably placed as Etna for
observations of this kind. It was once resorted to by Prof.
George E. Hale, in an attempt to see the solar corona without
an eclipse. Rising directly from sea-level to an elevation of
nearly eleven thousand feet, the observer on its summit at
night finds himself, as it were, lost in the midst of the sky.
But for the black flanks of the great cone on which he stands
he might fancy himself to be in a balloon. On the occasion to
which I refer the world beneath was virtually invisible in the
moonless night. The blaze of the constellations overhead was
astonishingly brilliant, yet amid all their magnificence my
attention was immediately drawn to a great tapering light that
sprang from the place on the horizon where the sun would rise
later, and that seemed to be blown out over the stars like a
long, luminous veil. It was the finest view of the Zodiacal
light that I had ever enjoyed -- thrilling in its strangeness
-- but I was almost disheartened by the indifference of my
guide, to whom it was only a light and nothing more. If he had
no science, he had less poetry -- rather a remarkable thing, I
thought, for a child of his clime. The Light appeared to me to
be distinctly brighter than the visible part of the Milky Way
which included the brilliant stretches in Auriga and Perseus,
and its color, if one may speak of color in connection with
such an object, seemed richer than that of the galactic band;
but I did not think of it as yellow, although Humboldt has
described it as resembling a golden curtain drawn over the
stars, and Du Chaillu in Equatorial Africa found it of a bright
yellow color. It may vary in color as in conspicuousness. The
fascination of that extraordinary sight has never faded from my
memory. I turned to regard it again and again, although I had
never seen the stellar heavens so brilliant, and it was one of
the last things I looked for when the morning glow began softly
to mount in the east, and Sicily and the Mediterranean slowly
emerged from the profound shadow beneath us.</p>
<p>The Zodiacal Light seems never to have attracted from
astronomers in general the amount of careful attention that it
deserves; perhaps because so little can really be made of it as
far as explanation is concerned. I have referred to the
restraint that scientific writers apparently feel in speaking
of it. The grounds for speculation that it affords may be too
scanty to lead to long discussions, yet it piques curiosity,
and as we shall see in a moment has finally led to a most
interesting theory. Once it was the subject of an elaborate
series of studies which carried the observer all round the
world. That was in 1845--46, during the United States Exploring
Expedition that visited the then little known Japan. The
chaplain of the fleet, the Rev. Mr Jones, went out prepared to
study the mysterious light in all its phases. He saw it from
many latitudes on both sides of the equator, and the
imagination cannot but follow him with keen interest in his
world-circling tour, keeping his eyes every night fixed upon
the phantasm overhead, whose position shifted with that of the
hidden sun. He demonstrated that the flow extends at times
completely across the celestial dome, although it is relatively
faint directly behind the earth. On his return the government
published a large volume of his observations, in which he
undertook to show that the phenomenon was due to the reflection
of sunlight from a ring of meteoric bodies encircling the
earth. But, after all, this elaborate investigation settled
nothing.</p>
<p>Prof. E. E. Barnard has more recently devoted much attention
to the Zodiacal Light, as well as to a strange attendant
phenomenon called the ``Gegenschein,'' or Counterglow, because
it always appears at that point in the sky which is exactly
opposite the sun. The Gegenschein is an extremely elusive
phenomenon, suitable only for eyes that have been specially
trained to see it. Professor Newcomb has cautiously remarked
that</p>
<blockquote>
it is said that in that point of the heavens directly
opposite the sun there is an elliptical patch of light...
This phenomenon is so difficult to account for that its
existence is sometimes doubted; yet the testimony in its
favor is difficult to set aside.
</blockquote>
<p>It certainly cannot be set aside at all since the
observations of Barnard. I recall an attempt to see it under
his guidance during a visit to Mount Hamilton, when he was
occupied there with the Lick telescope. Of course, both the
Gegenschein and the Zodiacal Light are too diffuse to be
studied with telescopes, which, so to speak, magnify them out
of existence. They can only be successfully studied with the
naked eye, since every faintest glimmer that they afford must
be utilized. This is especially true of the Gegenschein. At
Mount Hamilton, Mr Barnard pointed out to me its location with
reference to certain stars, but with all my gazing I could not
be sure that I saw it. To him, on the contrary, it was obvious;
he had studied it for months, and was able to indicate its
shape, its boundaries, its diameter, and the declination of its
center with regard to the ecliptic. There is not, of course,
the shadow of a doubt of the existence of the Gegenschein, and
yet I question if one person in a million has ever seen or ever
will see it. The Zodiacal Light, on the other hand, is plain
enough, provided that the time and the circumstances of the
observation are properly chosen.</p>
<p>In the attempts to explain the Zodiacal Light, the favorite
hypothesis has been that it is an appendage of the sun --
perhaps simply an extension of the corona in the plane of the
ecliptic, which is not very far from coinciding with that of
the sun's equator. This idea is quite a natural one, because of
the evident relation of the light to the position of the sun.
The vast extension of the equatorial wings of the corona in
1878 gave apparent support to this hypothesis; if the substance
of the corona could extend ten million miles from the sun, why
might it not extend even one hundred million, gradually fading
out beyond the orbit of the earth? A variation of this
hypothesis assumes that the reflection is due to swarms of
meteors circling about the sun, in the plane of its equator,
all the way from its immediate neighborhood to a distance
exceeding that of the earth. But in neither form is the
hypothesis satisfactory; there is nothing in the appearance of
the corona to indicate that it extends even as far as the
planet Mercury, while as to meteors, the orbits of the known
swarms do not accord with the hypothesis, and we have no reason
to believe that clouds of others exist traveling in the part of
space where they would have to be in order to answer the
requirements of the theory. The extension of the corona in 1878
did not resemble in its texture the Zodiacal Light.</p>
<p>Now, it has so often happened in the history of science that
an important discovery in one branch has thrown unexpected but
most welcome light upon some pending problem in some other
branch, that a strong argument might be based upon that fact
alone against the too exclusive devotion of many investigators
to the narrow lines of their own particular specialty; and the
Zodiacal Light affords a case in point, when it is considered
in connection with recent discoveries in chemistry and physics.
From the fact that atoms are compound bodies made up of
corpuscles at least a thousand times smaller than the smallest
known atom -- a fact which astounded most men of science when
it was announced a few years ago -- a new hypothesis has been
developed concerning the nature of the Zodiacal Light (as well
as other astronomical riddles), and this hypothesis comes not
from an astronomer, but from a chemist and physicist, the
Swede, Svante Arrhenius. In considering an outline of this new
hypothesis we need neither accept nor reject it; it is a case
rather for suspension of judgment.</p>
<p>To begin with, it carries us back to the ``pressure of
light'' mentioned in the preceding chapter. The manner in which
this pressure is believed generally to act was there
sufficiently explained, and it only remains to see how it is
theoretically extended to the particles of matter supposed to
constitute the Zodiacal Light. We know that corpuscles, or
``fragments of atoms'' negatively electrified, are discharged
from hot bodies. Streams of these ``ions'' pour from many
flames and from molten metals; and the impact of the cathode
and ultra-violet rays causes them to gush even from cold
bodies. In the vast laboratory of the sun it is but reasonable
to suppose that similar processes are taking place. ``As a very
hot metal emits these corpuscles,'' says Prof. J. J. Thomson,
``it does not seem an improbable hypothesis that they are
emitted by that very hot body, the sun.'' Let it be assumed,
then, that the sun does emit them; what happens next?
Negatively charged corpuscles, it is known, serve as nuclei to
which particles of matter in the ordinary state are attracted,
and it is probable that those emitted from the sun immediately
pick up loads in this manner and so grow in bulk. If they grow
large enough the gravitation of the sun draws them back, and
they produce a negative charge in the solar atmosphere. But it
is probable that many of the particles do not attain the
critical size which, according to the principles before
explained, would enable the gravitation of the sun to retain
them in opposition to the pressure of the waves of light, and
with these particles the light pressure is dominant. Clouds of
them may be supposed to be continually swept away from the sun
into surrounding space, moving mostly in or near the plane of
the solar equator, where the greatest activity, as indicated by
sunspots and related phenomena, is taking place. As they pass
outward into space many of them encounter the earth. If the
earth, like the moon, had no atmosphere the particles would
impinge directly on its surface, giving it a negative electric
charge. But the presence of the atmosphere changes all that,
for the first of the flying particles that encounter it impart
to it their negative electricity, and then, since like electric
charges repel like, the storm of particles following will be
sheered off from the earth, and will stream around it in a maze
of hyperbolic paths. Those that continue on into space beyond
the earth may be expected to continue picking up wandering
particles of matter until their bulk has become so great that
the solar attraction prevails again over the light pressure
acting upon them, and they turn again sunward. Passing the
earth on their return they will increase the amount of
dust-clouds careering round it; and these will be further
increased by the action of the ultra-violet rays of the
sunlight causing particles to shoot radially away from the
earth when the negative charge of the upper atmosphere has
reached a certain amount, which particles, although starting
sunward, will be swept back to the earth with the oncoming
streams. As the final result of all this accumulation of flying
and gyrating particles in the earth's neighborhood, we are told
that the latter must be transformed into the semblance of a
gigantic solid-headed comet provided with streaming tails, the
longest of them stretching away from the direction of the sun,
while another shorter one extends toward the sun. This shorter
tail is due to the particles that we have just spoken of as
being driven sunward from the earth by the action of
ultra-violet light. No doubt this whole subject is too
technical for popular statement; but at any rate the general
reader can understand the picturesque side of the theory, for
its advocates assure us that if we were on the moon we would
doubtless be able to see the comet-like tails of the earth, and
then we could appreciate the part that they play in producing
the phenomenon of the Zodiacal Light.</p>
<p>That the Light as we see it could be produced by the
reflection of sunlight from swarms of particles careering round
the earth in the manner supposed by Arrhenius' hypothesis is
evident enough; and it will be observed that the new theory,
after all, is only another variant of the older one which
attributes the Zodiacal Light to an extension of the solar
corona. But it differs from the older theory in offering an
explanation of the manner in which the extension is effected,
and it differentiates between the corona proper and the streams
of negative particles shot away from the sun. In its details
the hypothesis of Arrhenius also affords an explanation of many
peculiarities of the Zodiacal Light, such as that it is
confined to the neighborhood of the ecliptic, and that it is
stronger on the side of the earth which is just turning away
from a position under the sun than on the other side; but it
would carry us beyond our limits to go into these particulars.
The Gegenschein, according to this theory, is a part of the
same phenomenon as the Zodiacal Light, for by the laws of
perspective it is evident that the reflection from the streams
of particles situated at a point directly opposite to the sun
would be at a maximum, and this is the place which the
Gegenschein occupies. Apart from its geometrical relations to
the position of the sun, the variability of the Zodiacal Light
appears to affirm its solar dependence, and this too would be
accounted for by Arrhenius' hypothesis better than by the old
theory of coronal extension. The amount of corpuscular
discharge from the sun must naturally be governed by the state
of relative activity or inactivity of the latter, and this
could not but be reflected in the varying splendor of the
Zodiacal Light. But much more extended study than has yet been
given to the subject will be required before we can feel that
we know with reasonable certainty what this mysterious
phenomenon really is. By the hypothesis of Arrhenius every
planet that has an atmosphere must have a Zodiacal Light
attending it, but the phenomenon is too faint for us to be able
to see it in the case, for instance, of Venus, whose atmosphere
is very abundant. The moon has no corresponding ``comet's
tail'' because, as already explained, of the lack of a lunar
atmosphere to repel the streams by becoming itself electrified;
but if there were a lunar Zodiacal Light, no doubt we could see
it because of the relative nearness of our satellite.</p>
<p><strong>Marvels of the Aurora</strong></p>
<p>One of the most vivid recollections of my early boyhood is
that of seeing my father return hastily into the house one
evening and call out to the family: ``Come outside and look at
the sky!'' Ours was a country house situated on a commanding
site, and as we all emerged from the doorway we were
dumbfounded to see the heavens filled with pale flames which
ran licking and quivering over the stars. Instantly there
sprang into my terrified mind the recollection of an awful
description of ``the Day of Judgment'' (the <em>Dies
Iræ</em>), which I had heard with much perturbation of
spirit in the Dutch Reformed church from the lips of a tall,
dark-browed, dreadfully-in-earnest preacher of the
old-fashioned type. My heart literally sank at sight of the
spectacle, for it recalled the preacher's very words; it was
just as he had said it would be, and it needed the assured
bearing of my elders finally to convince me that</p>
<blockquote>
<br/>
That Day of Wrath, O dreadful day,<br/>
When Heaven and Earth shall pass away,<br/>
As David and the Sibyl say<br/>
<br/>
</blockquote>
<p>had not actually come upon us. And even the older members of
the household were not untouched with misgivings when menacing
spots of crimson appeared, breaking out now here, now there, in
the shuddering sky. Toward the north the spectacle was
appalling. A huge arch spanned an unnaturally dark segment
resting on the horizon, and above this arch sprang up beams and
streamers in a state of incessant agitation, sometimes shooting
up to the zenith with a velocity that took one's breath, and
sometimes suddenly falling into long ranks, and <em>marching,
marching, marching,</em> like an endless phalanx of fiery
specters, and moving, as I remember, always from east to west.
The absolute silence with which these mysterious evolutions
were performed and the quavering reflections which were thrown
upon the ground increased the awfulness of the exhibition.
Occasionally enormous curtains of lambent flame rolled and
unrolled with a majestic motion, or were shaken to and fro as
if by a mighty, noiseless wind. At times, too, a sudden
billowing rush would be made toward the zenith, and for a
minute the sky overhead would glow so brightly that the stars
seemed to have been consumed. The spectacle continued with
varying intensity for hours.</p>
<p>This exhibition occurred in Central New York, a latitude in
which the Aurora Borealis is seldom seen with so much splendor.
I remember another similar one seen from the city of New York
in November, 1882. On this last occasion some observers saw a
great upright beam of light which majestically moved across the
heavens, stalking like an apparition in the midst of the
auroral pageant, of whose general movements it seemed to be
independent, maintaining always its upright posture, and
following a magnetic parallel from east to west. This
mysterious beam was seen by no less than twenty-six observers
in different parts of the country, and a comparison of their
observations led to a curious calculation indicating that the
apparition was about <em>one hundred and thirty-three miles
tall</em> and moved at the speed of ten miles per second!</p>
<p>But, as everybody knows, it is in the Arctic regions that
the Aurora, or the ``Northern Lights,'' can best be seen.
There, in the long polar night, when for months together the
sun does not rise, the strange coruscations in the sky often
afford a kind of spectral daylight in unison with the weird
scenery of the world of ice. The pages in the narratives of
Arctic exploration that are devoted to descriptions of the
wonderful effects of the Northern Lights are second to none
that man has ever penned in their fascination. The lights, as I
have already intimated, display astonishing colors,
particularly shades of red and green, as they flit from place
to place in the sky. The discovery that the magnetic needle is
affected by the Aurora, quivering and darting about in a state
of extraordinary excitement when the lights are playing in the
sky, only added to the mystery of the phenomenon until its
electro-magnetic nature had been established. This became
evident as soon as it was known that the focus of the displays
was the magnetic pole; and when the far South was visited the
Aurora Australis was found, having its center at the South
Magnetic Pole. Then, if not before, it was clear that the earth
was a great globular magnet, having its poles of opposite
magnetism, and that the auroral lights, whatever their precise
cause might be, were manifestations of the magnetic activity of
our planet. After the invention of magnetic telegraphy it was
found that whenever a great Aurora occurred the telegraph lines
were interrupted in their operation, and the ocean cables
ceased to work. Such a phenomenon is called a ``magnetic
storm.''</p>
<p>The interest excited by the Aurora in scientific circles was
greatly stimulated when, in the last half of the nineteenth
century, it was discovered that it is a phenomenon intimately
associated with disturbances on the sun. The ancient ``Zurich
Chronicles,'' extending from the year 1000 to the year 1800, in
which both sun-spots visible to the naked eye and great
displays of the auroral lights were recorded, first set Rudolf
Wolf on the track of this discovery. The first notable proof of
the suspected connection was furnished with dramatic emphasis
by an occurrence which happened on September 1, 1859. Near noon
on that day two intensely brilliant points suddenly broke out
in a group of sun-spots which were under observation by Mr R.
C. Carrington at his observatory at Redhill, England. The
points remained visible for not more than five minutes, during
which interval they moved <em>thirty-five thousand miles</em>
across the solar disk. Mr R. Hodgson happened to see the same
phenomenon at his observatory at Highgate, and thus all
possibility of deception was removed. But neither of the
startled observers could have anticipated what was to follow,
and, indeed, it was an occurrence which has never been
precisely duplicated. I quote the eloquent account given by
Miss Clerke in her <em>History of Astronomy During the
Nineteenth Century.</em></p>
<blockquote>
<p>This unique phenomenon seemed as if specially designed to
accentuate the inference of a sympathetic relation between
the earth and the sun. From August 28 to September 4, 1859, a
magnetic storm of unparalleled intensity, extent, and
duration was in progress over the entire globe. Telegraphic
communication was everywhere interrupted -- except, indeed,
that it was in some cases found practicable to work the lines
<em>without batteries</em> by the agency of the
earth-currents alone; sparks issued from the wires; gorgeous
auroras draped the skies in solemn crimson over both
hemispheres, and even in the tropics; the magnetic needle
lost all trace of continuity in its movements and darted to
and fro as if stricken with inexplicable panic. The
coincidence was even closer. <em>At the very instant</em> of
the solar outburst witnessed by Carrington and Hodgson the
photographic apparatus at Kew registered a marked disturbance
of all the three magnetic elements; while shortly after the
ensuing midnight the electric agitation culminated, thrilling
the whole earth with subtle vibrations, and lighting up the
atmosphere from pole to pole with coruscating splendors which
perhaps dimly recall the times when our ancient planet itself
shone as a star.</p>
</blockquote>
<p>If this amazing occurrence stood alone, and as I have
already said it has never been exactly duplicated, doubt might
be felt concerning some of the inferences drawn from it; but in
varying forms it has been repeated many times, so that now
hardly anyone questions the reality of the assumed connection
between solar outbursts and magnetic storms accompanied by
auroral displays on the earth. It is true that the late Lord
Kelvin raised difficulties in the way of the hypothesis of a
direct magnetic action of the sun upon the earth, because it
seemed to him that an inadmissible quantity of energy was
demanded to account for such action. But no calculation like
that which he made is final, since all calculations depend upon
the validity of the data; and no authority is unshakable in
science, because no man can possess omniscience. It was Lord
Kelvin who, but a few years before the thing was actually
accomplished, declared that aerial navigation was an
impracticable dream, and demonstrated its impracticability by
calculation. However the connection may be brought about, it is
as certain as evidence can make it that solar outbursts are
coincident with terrestial magnetic disturbances, and
coincident in such a way as to make the inference of a causal
connection irresistible. The sun is only a little more than a
hundred times its own diameter away from the earth. Why, then,
with the subtle connection between them afforded by the ether
which conveys to us the blinding solar light and the
life-sustaining solar heat, should it be so difficult to
believe that the sun's enormous electric energies find a way to
us also? No doubt the impulse coming from the sun acts upon the
earth after the manner of a touch upon a trigger, releasing
energies which are already stored up in our planet.</p>
<p>But besides the evidence afforded by such occurrences as
have been related of an intimate connection between solar
outbreaks and terrestial magnetic flurries, attended by
magnificent auroral displays, there is another line of proof
pointing in the same direction. Thus, it is known that the
sun-spot period, as remarked in a preceding chapter, coincides
in a most remarkable manner with the periodic fluctuations in
the magnetic state of the earth. This coincidence runs into the
most astonishing details. For instance, when the sun-spot
period shortens, the auroral period shortens to precisely the
same extent; as the short sun-spot periods usually bring the
most intense outbreaks of solar activity, so the corresponding
short auroral periods are attended by the most violent magnetic
storms; a secular period of about two hundred and twenty-two
years affecting sun-spots is said to have its auroral
duplicate; a shorter period of fifty-five and a half years,
which some observers believe that they have discovered appears
also to be common to the two phenomena; and yet another
``superposed'' period of about thirty-five years, which some
investigators aver exists, affects sun-spots and aurora alike.
In short, the coincidences are so numerous and significant that
one would have to throw the doctrine of probability to the
winds in order to be able to reject the conclusion to which
they so plainly lead.</p>
<p>But still the question recurs: How is the influence
transmitted? Here Arrhenius comes once more with his hypothesis
of negative corpuscles, or ions, driven away from the sun by
light-pressure -- a hypothesis which seems to explain so many
things -- and offers it also as an explanation of the way in
which the sun creates the Aurora. He would give the Aurora the
same lineage with the Zodiacal Light. To understand the
application of this theory we must first recall the fact that
the earth is a great magnet having its two opposite poles of
magnetism, one near the Arctic and the other near the Antarctic
Circle. Like all magnets, the earth is surrounded with ``lines
of force,'' which, after the manner of the curved rays we saw
in the photograph of a solar eclipse, start from a pole, rising
at first nearly vertically, then bend gradually over, passing
high above the equator, and finally descending in converging
sheaves to the opposite pole. Now the axis of the earth is so
placed in space that it lies at nearly a right angle to the
direction of the sun, and as the streams of negatively charged
particles come pouring on from the sun (see the last preceding
chapter), they arrive in the greatest numbers over the earth's
equatorial regions. There they encounter the lines of magnetic
force at the place where the latter have their greatest
elevation above the earth, and where their direction is
horizontal to the earth's surface. Obeying a law which has been
demonstrated in the laboratory, the particles then follow the
lines of force toward the poles. While they are above the
equatorial regions they do not become luminescent, because at
the great elevation that they there occupy there is virtually
no atmosphere; but as they pass on toward the north and the
south they begin to descend with the lines of force, curving
down to meet at the poles; and, encountering a part of the
atmosphere comparable in density with what remains in an
exhausted Crookes tube, they produce a glow of cathode rays.
This glow is conceived to represent the Aurora, which may
consequently be likened to a gigantic exhibition of vacuum-tube
lights. Anybody who recalls his student days in the college
laboratory and who has witnessed a display of Northern Lights
will at once recognize the resemblance between them in colors,
forms, and behavior. This resemblance had often been noted
before Arrhenius elaborated his hypothesis.</p>
<p>Without intending to treat his interesting theory as more
than a possibly correct explanation of the phenomena of the
Aurora, we may call attention to some apparently confirmatory
facts. One of the most striking of these relates to a seasonal
variation in the average number of auroræ. It has been
observed that there are more in March and September than at any
other time of the year, and fewer in June and December;
moreover (and this is a delicate test as applied to the
theory), they are slightly rarer in June than in December. Now
all these facts seem to find a ready explanation in the
hypothesis of Arrhenius, thus: (1) The particles issuing from
the sun are supposed to come principally from the regions whose
excitement is indicated by the presence of sun-spots (which
accords with Hale's observation that sun-spots are columns of
ionized vapors), and these regions have a definite location on
either side of the solar equator, seldom approaching it nearer
than within 5° or 10° north or south, and never
extending much beyond 35° toward either pole; (2) The
equator of the sun is inclined about 7° to the plane of the
earth's orbit, from which it results that twice in a year --
<em>viz.,</em> in June and December -- the earth is directly
over the solar equator, and twice a year -- <em>viz.,</em> in
March and September -- when it is farthest north or south of
the solar equator, it is over the inner edge of the sun-spot
belts. Since the corpuscles must be supposed to be propelled
radially from the sun, few will reach the earth when the latter
is over the solar equator in June and December, but when it is
over, or nearly over, the spot belts, in March and September,
it will be in the line of fire of the more active parts of the
solar surface, and relatively rich streams of particles will
reach it. This, as will be seen from what has been said above,
is in strict accord with the observed variations in the
frequency of auroræ. Even the fact that somewhat fewer
auroræ are seen in June than in December also finds its
explanation in the known fact that the earth is about three
million miles nearer the sun in the winter than in the summer,
and the number of particles reaching it will vary, like the
intensity of light, inversely as the square of the distance.
These coincidences are certainly very striking, and they have a
cumulative force. If we accept the theory, it would appear that
we ought to congratulate ourselves that the inclination of the
sun's equator is so slight, for as things stand the earth is
never directly over the most active regions of the sun-spots,
and consequently never suffers from the maximum bombardment of
charged particles of which the sun is capable. Incessant
auroral displays, with their undulating draperies, flitting
colors, and marching columns might not be objectionable from
the point of view of picturesqueness, but one magnetic storm of
extreme intensity following closely upon the heels of another,
for months on end, crazing the magnetic needle and continually
putting the telegraph and cable lines out of commission, to say
nothing of their effect upon ``wireless telegraphy'', would
hardly add to the charms of terrestrial existence.</p>
<p>One or two other curious points in connection with
Arrhenius' hypothesis may be mentioned. First, the number of
auroræ, according to his explanation, ought to be
greatest in the daytime, when the face of the earth on the
sunward side is directly exposed to the atomic bombardment. Of
course visual observation can give us no information about
this, since the light of the Aurora is never sufficiently
intense to be visible in the presence of daylight, but the
records of the magnetic observatories can be, and have been,
appealed to for information, and they indicate that the facts
actually accord with the theory. Behind the veil of sunlight in
the middle of the afternoon, there is good reason to believe,
auroral exhibitions often take place which would eclipse in
magnificence those seen at night if we could behold them.
Observation shows, too, that auroræ are more frequent
before than after midnight, which is just what we should expect
if they originate in the way that Arrhenius supposes. Second,
the theory offers an explanation of the alleged fact that the
formation of clouds in the upper air is more frequent in years
when auroræ are most abundant, because clouds are the
result of the condensation of moisture upon floating particles
in the atmosphere (in an absolutely dustless atmosphere there
would be no clouds), and it has been proved that negative ions
like those supposed to come from the sun play a master part in
the phenomena of cloud formation.</p>
<p>Yet another singular fact, almost mystical in its
suggestions, may be mentioned. It seems that the dance of the
auroral lights occurs most frequently during the absence of the
moon from the hemisphere in which they appear, and that they
flee, in greater part, to the opposite hemisphere when the
moon's revolution in an orbit considerably inclined to the
earth's equator brings her into that where they have been
performing. Arrhenius himself discovered this curious relation
of auroral frequency to the position of the moon north or south
of the equator, and he explains it in this way. The moon, like
the earth, is exposed to the influx of the ions from the sun;
but having no atmosphere, or almost none, to interfere with
them, they descend directly upon her surface and charge her
with an electric negative potential to a very high degree. In
consequence of this she affects the electric state of the upper
parts of the earth's atmosphere where they lie most directly
beneath her, and thus prevents, to a large extent, the negative
discharges to which the appearance of the Aurora is due. And so
``the extravagant and erring spirit'' of the Aurora avoids the
moon as Hamlet's ghost fled at the voice of the cock announcing
the awakening of the god of day.</p>
<p>There are even other apparent confirmations of the
hypothesis, but we need not go into them. We shall, however,
find one more application of it in the next chapter, for it
appears to be a kind of cure-all for astronomical troubles; at
any rate it offers a conceivable solution of the question, How
does the sun manage to transmit its electric influence to the
earth? And this solution is so grandiose in conception, and so
novel in the mental pictures that it offers, that its
acceptance would not in the least detract from the impression
that the Aurora makes upon the imagination.</p>
<p><strong>Strange Adventures of Comets</strong></p>
<p>The fears and legends of ancient times before Science was
born, and the superstitions of the Dark Ages, sedulously
cultivated for theological purposes by monks and priests, have
so colored our ideas of the influence that comets have had upon
the human mind that many readers may be surprised to learn that
it was the apparition of a wonderful comet, that of 1843, which
led to the foundation of our greatest astronomical institution,
the Harvard College Observatory. No doubt the comet
superstition existed half a century ago, as, indeed, it exists
yet today, but in this case the marvelous spectacle in the sky
proved less effective in inspiring terror than in awakening a
desire for knowledge. Even in the sixteenth century the views
that enlightened minds took of comets tended powerfully to
inspire popular confidence in science, and Halley's prediction,
after seeing and studying the motion of the comet which
appeared in 1682, that it would prove to be a regular member of
the sun's family and would be seen returning after a period of
about seventy-six years, together with the fulfillment of that
prediction, produced a revulsion from the superstitious notions
which had so long prevailed.</p>
<p>Then the facts were made plain that comets are subject to
the law of gravitation equally with the planets; that there are
many which regularly return to the neighborhood of the sun
(perihelion); and that these travel in orbits differing from
those of the planets only in their greater eccentricity,
although they have the peculiarity that they do not, like the
planets, all go round the sun in the same direction, and do not
keep within the general plane of the planetary system, but
traverse it sometimes from above and sometimes from below.
Other comets, including most of the ``great'' ones, appear to
travel in parabolic or, in a few cases, hyperbolic orbits,
which, not being closed curves, never bring them back again.
But it is not certain that these orbits may not be extremely
eccentric ellipses, and that after the lapse of hundreds, or
thousands, of years the comets that follow them may not
reappear. The question is an interesting one, because if all
orbits are really ellipses, then all comets must be permanent
members of the solar system, while in the contrary case many of
them are simply visitors, seen once and never to be seen again.
The hypothesis that comets are originally interlopers might
seem to derive some support from the fact that the certainly
periodic ones are associated, in groups, with the great outer
planets, whose attraction appears to have served as a trap for
them by turning them into elliptical orbits and thus making
them prisoners in the solar system. Jupiter, owing to his great
mass and his commanding situation in the system, is the chief
``comet-catcher;'' but he catches them not for himself, but for
the sun. Yet if comets do come originally from without the
borders of the planetary system, it does not, by any means,
follow that they were wanderers at large in space before they
yielded to the overmastering attraction of the sun.
Investigation of the known cometary orbits, combined with
theoretical considerations, has led some astronomers to the
conclusion that as the sun travels onward through space he
``picks up <em>en route</em>'' cometary masses which, without
belonging strictly to his empire, are borne along in the same
vast ``cosmical current'' that carries the solar system.</p>
<p>But while no intelligent person any longer thinks that the
appearance of a great comet is a token from the heavenly powers
of the approaching death of a mighty ruler, or the outbreak of
a devastating war, or the infliction of a terrible plague upon
wicked mankind, science itself has discovered mysteries about
comets which are not less fascinating because they are more
intellectual than the irrational fancies that they have
displaced. To bring the subject properly before the mind, let
us see what the principal phenomena connected with a comet
are.</p>
<p>At the present day comets are ordinarily ``picked up'' with
the telescope or the photographic plate before any one except
their discoverer is aware of their existence, and usually they
remain so insignificant in appearance that only astronomers
ever see them. Yet so great is the prestige of the word
``comet'' that the discovery of one of these inconspicuous
wanderers, and its subsequent movements, become items of the
day's news which everybody reads with the feeling, perhaps,
that at least he knows what is going on in the universe even if
he doesn't understand it. But a truly great comet presents
quite a different proposition. It, too, is apt to be detected
coming out of the depths of space before the world at large can
get a glimpse of it, but as it approaches the sun its aspect
undergoes a marvelous change. Agitated apparently by solar
influence, it throws out a long streaming tail of nebulous
light, directed away from the sun and looking as if blown out
like a pennon by a powerful wind. Whatever may be the position
of the comet with regard to the sun, as it circles round him it
continually keeps its tail on the off side. This, as we shall
soon see, is a fact of capital importance in relation to the
probable nature of comets' tails. Almost at the same time that
the formation of the tail is observed a remarkable change takes
place in the comet's head, which, by the way, is invariably and
not merely occasionally its most important part. On approaching
the sun the head usually contracts. Coincidently with this
contraction a nucleus generally makes its appearance. This is a
bright, star-like point in the head, and it probably represents
the totality of solid matter that the comet possesses. But it
is regarded as extremely unlikely that even the nucleus
consists of a uniformly solid mass. If it were such, comets
would be far more formidable visitors when they pass near the
planets than they have been found to be. The diameter of the
nucleus may vary from a few hundred up to several thousand
miles; the heads, on the average, are from twenty-five thousand
to one hundred thousand miles in diameter, although a few have
greatly exceeded these dimensions; that of the comet of 1811,
one of the most stupendous ever seen, was a million and a
quarter miles in diameter! As to the tails, not withstanding
their enormous length -- some have been more than a hundred
million miles long -- there is reason to believe that they are
of extreme tenuity, ``as rare as vacuum.'' The smallest stars
have been seen shining through their most brilliant portions
with undiminished luster.</p>
<p>After the nucleus has been formed it begins to throw out
bright jets directed toward the sun. A stream, and sometimes
several streams, of light also project sunward from the
nucleus, occasionally appearing like a stunted tail directed
oppositely to the real tail. Symmetrical envelopes which, seen
in section, appear as half circles or parabolas, rise sunward
from the nucleus, forming a concentric series. The ends of
these stream backward into the tail, to which they seem to
supply material. Ordinarily the formation of these ejections
and envelopes is attended by intense agitation of the nucleus,
which twists and turns, swinging and gyrating with an
appearance of the greatest violence. Sometimes the nucleus is
seen to break up into several parts. The entire heads of some
comets have been split asunder in passing close around the sun;
The comet of 1882 retreated into space after its perihelion
passage with <em>five heads</em> instead of the one that it had
originally, and each of these heads had its own tail!</p>
<p>The possession of the spectroscope has enabled astronomers
during later years to study the chemical composition of comets
by analyzing their light. At first the only substances thus
discovered in them were hydro-carbon compounds, due evidently
to the gaseous envelopes in which some combination of hydrogen
with carbon existed. Behind this gaseous spectrum was found a
faint continuous spectrum ascribed to the nucleus, which
apparently both reflects the sunlight and gives forth the light
of a glowing solid or liquid. Subsequently sodium and iron
lines were found in cometary spectra. The presence of iron
would seem to indicate that some of these bodies may be much
more massive than observations on their attractive effects have
indicated. In some recent comets, such as Morehouse's, in 1908,
several lines have been found, the origin of which is
unknown.</p>
<p>Without going back of the nineteenth century we may find
records of some of the most extraordinary comets that man has
ever looked upon. In 1811, still spoken of as ``the year of the
comet,'' because of the wonderful vintage ascribed to the skyey
visitor, a comet shaped like a gigantic sword amazed the whole
world, and, as it remained visible for seventeen months, was
regarded by superstitious persons as a symbol of the fearful
happenings of Napoleon's Russian campaign. This comet, the
extraordinary size of whose head, greatly exceeding that of the
sun itself, has already been mentioned, was also remarkable for
exhibiting so great a brilliancy without approaching even to
the earth's distance from the sun. But there was once a comet
(and only once -- in the year 1729) which never got nearer to
the sun than four times the distance of the earth and yet
appeared as a formidable object in the sky. As Professor Young
has remarked, ``it must have been an enormous comet to be
visible from such a distance.'' And we are to remember that
there were no great telescopes in the year 1729. That comet
affects the imagination like a phantom of space peering into
the solar system, displaying its enormous train afar off
(which, if it had approached as near as other comets, would
probably have become <em>the</em> celestial wonder of all human
memory), and then turning away and vanishing in the depths of
immensity.</p>
<p>In 1843 a comet appeared which was so brilliant that it
could be seen in broad day close beside the sun! This was the
first authenticated instance of that kind, but the occurrence
was to be repeated, as we shall see in a moment, less than
forty years later.</p>
<p>The splendid comet of 1858, usually called Donati's, is
remembered by many persons yet living. It was, perhaps, both as
seen by the naked eye and with the telescope, the most
beautiful comet of which we have any record. It too marked a
rich vintage year, still remembered in the vineyards of France,
where there is a popular belief that a great comet ripens the
grape and imparts to the wine a flavor not attainable by the
mere skill of the cultivator. There are ``comet wines,''
carefully treasured in certain cellars, and brought forth only
when their owner wishes to treat his guests to a sip from
paradise.</p>
<p>The year 1861 saw another very remarkable comet, of an
aspect strangely vast and diffuse, which is believed to have
swept the earth with its immense tail when it passed between us
and the sun on the night of June 30th, an event which produced
no other known effect than the appearance of an unwonted amount
of scattered light in the sky.</p>
<p>The next very notable comet was the ``Great Southern Comet''
of 1880, which was not seen from the northern hemisphere. It
mimicked the aspect of the famous comet of 1843, and to the
great surprise of astronomers appeared to be traveling in the
same path. This proved to be the rising of the curtain for an
astronomical sensation unparalleled in its kind; for two years
later another brilliant comet appeared, first in the southern
hemisphere, <em>and it too followed the same track.</em> The
startling suggestion was now made that this comet was identical
with those of 1843 and 1880, its return having been hastened by
the resistance experienced in passing twice through the coronal
envelope, and there were some who thought that it would now
swing swiftly round and then plunge straight into the sun, with
consequences that might be disastrous to us on account of the
``flash of heat'' that would be produced by the impact. Nervous
people were frightened, but observation soon proved that the
danger was imaginary, for although the comet almost grazed the
sun, and must have rushed through two or three million miles of
the coronal region, no retardation of its immense velocity was
perceptible, and it finally passed away in a damaged condition,
as before remarked, and has never since appeared.</p>
<p>Then the probable truth was perceived -- <em>viz.,</em> that
the three comets (1843, 1880, and 1882) were not one identical
body, but three separate ones all traveling in the same orbit.
It was found, too, that a comet seen in 1668 bore similar
insignia of relationship. The natural inference was that these
four bodies had once formed a single mass which had been split
apart by the disruptive action of the sun. Strength was lent to
this hypothesis by the fact that the comet of 1882 was
apparently torn asunder during its perihelion passage,
retreating into space in a dissevered state. But Prof. George
Forbes has a theory that the splitting of the original cometary
mass was effected by an unknown planet, probably greater than
Jupiter, situated at a hundred times the earth's distance from
the sun, and revolving in a period of a thousand years. He
supposes that the original comet was not that of 1668, but one
seen in 1556, which has since been ``missing,'' and that its
disruption occurred from an encounter with the supposititious
planet about the year 1700. Truly from every point of view
comets are the most extraordinary of adventurers!</p>
<p>The comet of 1882 was likewise remarkable for being visible,
like its predecessor of 1843, in full daylight in close
proximity to the sun. The story of its detection when almost in
contact with the solar disk is dramatic. It had been discovered
in the southern hemisphere only a couple of weeks before its
perihelion, which occurred on September 17th, and on the
forenoon of that day it was seen by Doctor Common in England,
and by Doctor Elkin and Mr Finlay at the Cape of Good Hope,
almost touching the sun. It looked like a dazzling white bird
with outspread wings. The southern observers watched it go
<em>right into the sun,</em> when it instantly disappeared.
What had happened was that the comet in passing its perihelion
point had swung exactly between the earth and the sun. On the
following morning it was seen from all parts of the world close
by the sun on the opposite side, and it remained thus visible
for three days, gradually receding from the solar disk. It then
became visible for northern observers in the morning sky before
sunrise, brandishing a portentous sword-shaped tail which, if
it had been in the evening sky, would have excited the wonder
of hundreds of millions, but situated where it was,
comparatively few ever saw it.</p>
<p>The application of photography to the study of comets has
revealed many curious details which might otherwise have
escaped detection, or at best have remained subject to doubt.
It has in particular shown not only the precise form of the
tails, but the remarkable vicissitudes that they undergo.
Professor Barnard's photographs of Brooks' comet in 1893
suggested, by the extraordinary changes in the form of the tail
which they revealed, that the comet was encountering a series
of obstructions in space which bent and twisted its tail into
fantastic shapes. The reader will observe the strange form into
which the tail was thrown on the night of October 21st. A cloud
of meteors through which the comet was passing might have
produced such deformations of its tail. In the photograph of
Daniels' comet of 1907, a curious striping of the tail will be
noticed. The short bright streaks seen in the photograph, it
may be explained, are the images of stars which are drawn out
into lines in consequence of the fact that the photographic
telescope was adjusted to follow the motion of the comet while
the stars remained at rest.</p>
<p>But the adventures of comets are not confined to possible
encounters with unknown obstacles. We have referred to the fact
that the great planets, and especially Jupiter, frequently
interfere with the motions of comets. This interference is not
limited to the original alteration of their orbits from
possible parabolas to ellipses, but is sometimes exercised
again and again, turning the bewildered comets into elliptical
paths of all degrees of eccentricity. A famous example of this
kind of planetary horse-play is furnished by the story of
Lexell's missing comet. This comet was first seen in 1770.
Investigation showed that it was moving in an orbit which
should bring it back to perihelion every five and a half years;
yet it had never been seen before and, although often searched
for, has never been seen since. Laplace and Leverrier proved
mathematically that in 1767 it had approached so close to
Jupiter as to be involved among the orbits of his satellites.
What its track had been before is not known, but on that
occasion the giant planet seized the interloper, threw it into
a short elliptic orbit and sent it, like an arrested vagrant,
to receive sentence at the bar of the sun. On this journey it
passed within less than 1,500,000 miles of the earth. The form
of orbit which Jupiter had impressed required, as we have said,
its return in about five and a half years; but soon after 1770
it had the misfortune a second time to encounter Jupiter at
close range, and he, as if dissatisfied with the leniency of
the sun, or indignant at the stranger's familiarity, seized the
comet and hurled it out of the system, or at any rate so far
away that it has never since been able to rejoin the family
circle that basks in the immediate rays of the solar hearth.
Nor is this the only instance in which Jupiter has dealt
summarily with small comets that have approached him with too
little deference.</p>
<p>The function which Jupiter so conspicuously fulfills as
master of the hounds to the sun is worth considering a little
more in detail. To change the figure, imagine the sun in its
voyage through space to be like a majestic battleship
surrounded by its scouts. Small vessels (the comets, as they
are overhauled by the squadron, are taken in charge by the
scouts, with Jupiter for their chief, and are forced to
accompany the fleet, but not all are impressed. If a strange
comet undertakes to run across Jupiter's bows the latter brings
it to, and makes prize of it by throwing it into a relatively
small ellipse with the sun for its focus. Thenceforth, unless,
as happened to the unhappy comet of Lexell, it encounters
Jupiter again in such a way as to be diverted by him into a
more distant orbit, it can never get away. About thirty comets
are now known to have thus been captured by the great planet,
and they are called ``Jupiter's Comet Family.'' But, on the
other hand, if a wandering comet crosses the wake of the chief
planetary scout the latter simply drives it away by
accelerating its motion and compels it to steer off into open
space. The transformation of comets into meteors will be
considered in the next chapter, but here, in passing, mention
may be made of the strange fate of one member of Jupiter's
family, Biela's comet, which, having become over bold in its
advances to its captor, was, after a few revolutions in is
impressed orbit, torn to pieces and turned into a flock of
meteors.</p>
<p>And now let us return to the mystery of comets' tails. That
we are fully justified in speaking of the tails of comets as
mysterious is proved by the declaration of Sir John Herschel,
who averred, in so many words, that ``there is some profound
secret and mystery of nature concerned in this phenomenon,''
and this profound secret and mystery has not yet been
altogether cleared up. Nevertheless, the all-explaining
hypothesis of Arrhenius offers us once more a certain amount of
aid. Comets' tails, Arrhenius assures us, are but another
result of the pressure of light. The reader will recall the
applications of this theory to the Zodiacal Light and the
Aurora. In the form in which we now have to deal with it, the
supposition is made that as a comet approaches the sun
eruptions of vapor, due to the solar heat, occur in its
nucleus. These are naturally most active on the side which is
directly exposed to the sun, whence the appearance of the
immense glowing envelopes that surround the nucleus on the
sunward side. Among the particles of hydro-carbon, and perhaps
solid carbon in the state of fine dust, which are thus set free
there will be many whose size is within the critical limit
which enables the light-waves from the sun to drive them away.
Clouds of such particles, then, will stream off behind the
advancing comet, producing the appearance of a tail. This
accounts for the fact that the tails of comets are always
directed away from the sun, and it also explains the varying
forms of the tails and the extraordinary changes that they
undergo. The speed of the particles driven before the
light-waves must depend upon their size and weight, the
lightest of a given size traveling the most swiftly. By
accretion certain particles might grow, thus losing velocity
and producing the appearance of bunches in the tail, such as
have been observed. The hypothesis also falls in with the
researches of Bredichin, who has divided the tails of comets
into three principal classes -- <em>viz.:</em> (1) Those which
appear as long, straight rays; (2) Those which have the form of
curved plumes or scimitars; (3) Those which are short, brushy,
and curved sharply backward along the comet's path. In the
first type he calculates the repulsive force at from twelve to
fifteen times the force of gravity; in the second at from two
to four times; and in the third at about one and a half times.
The straight tails he ascribes to hydrogen because the hydrogen
atom is the lightest known; the sword-shaped tails to
hydro-carbons; and the stumpy tails to vaporized iron. It will
be seen that, if the force driving off the tails is that which
Arrhenius assumes it to be, the forms of those appendages would
accord with those that Bredichin's theory calls for. At the
same time we have an explanation of the multiple tails with
which some comets have adorned themselves. The comet of 1744,
for instance, had at one time no less than seven tails spread
in a wide curved brush behind it. Donati's comet of 1858 also
had at least two tails, the principal one sword-shaped and the
other long, narrow, and as straight as a rule. According to
Bredichin, the straight tail must have been composed of
hydrogen, and the other of some form of hydro-carbon whose
atoms are heavier than those of hydrogen, and, consequently,
when swept away by the storm of light-waves, followed a
curvature depending upon the resultant of the forces operating
upon them. The seven tails of the comet of 1744 presented a
kind of diagram graphically exhibiting its complex composition,
and, if we knew a little more about the constituents of a
comet, we might be able to say from the amount of curvature of
the different tails just what were the seven substances of
which that comet consisted.</p>
<p>If these theories seem to the reader fantastic, at any rate
they are no more fantastic than the phenomena that they seek to
explain.</p>
<p><strong>Meteors, Fire-Balls, and Meteorites</strong></p>
<p>One of the most terrorizing spectacles with which the
heavens have ever caused the hearts of men to quake occurred on
the night of November 13, 1833. On that night North America,
which faced the storm, was under a continual rain of fire from
about ten o'clock in the evening until daybreak.</p>
<p><em>The fragments of a comet had struck the earth.</em></p>
<p>But the meaning of what had happened was not discovered
until long afterward. To the astronomers who, with astonishment
not less than that of other people, watched the wonderful
scene, it was an unparalleled ``shower of meteors.'' They did
not then suspect that those meteors had once formed the head of
a comet. Light dawned when, a year later, Prof. Denison
Olmsted, of Yale College, demonstrated that the meteors had all
moved in parallel orbits around the sun, and that these orbits
intersected that of the earth at the point where our planet
happened to be on the memorable night of November 13th.
Professor Olmsted even went so far as to suggest that the cloud
of meteors that had encountered the earth might form a diffuse
comet; but full recognition of the fact that they were cometary
débris came later, as the result of further
investigation. The key to the secret was plainly displayed in
the spectacle itself, and was noticed without being understood
by thousands of the terror-stricken beholders. It was <em>an
umbrella of fire</em> that had opened overhead and covered the
heavens; in other words, the meteors all radiated from a
particular point in the constellation Leo, and, being countless
as the snowflakes in a winter tempest, they ribbed the sky with
fiery streaks. Professor Olmsted showed that the radiation of
the meteors from a fixed point was an effect of perspective,
and in itself a proof that they were moving in parallel paths
when they encountered the earth. The fact was noted that there
had been a similar, but incomparably less brilliant, display of
meteors on the same day of November, 1832, and it was rightly
concluded that these had belonged to the same stream, although
the true relationship of the phenomena was not immediately
apprehended. Olmsted ascribed to the meteors a revolution about
the sun once in every six months, bringing them to the
intersection of their orbit with that of the earth every
November 13th; but later investigators found that the real
period was about thirty-three and one-quarter years, so that
the great displays were due three times in a century, and their
return was confidently predicted for the year 1866. The
appearance of the meteors in 1832, a year before the great
display, was ascribed to the great length of the stream which
they formed in space -- so great that they required more than
two years to cross the earth's orbit. In 1832 the earth had
encountered a relatively rare part of the stream, but in 1833,
on returning to the crossing-place, it found there the richest
part of the stream pouring across its orbit. This explanation
also proved to be correct, and the predicted return in 1866 was
duly witnessed, although the display was much less brilliant
than in 1833. It was followed by another in 1867.</p>
<p>In the mean time Olmsted's idea of a cometary relationship
of the meteors was demonstrated to be correct by the researches
of Schiaparelli and others, who showed that not only the
November meteors, but those of August, which are seen more or
less abundantly every year, traveled in the tracks of
well-known comets, and had undoubtedly an identical origin with
those comets. In other words the comets and the meteor-swarms
were both remnants of original masses which had probably been
split up by the action of the sun, or of some planet to which
they had made close approaches. The annual periodicity of the
August meteors was ascribed to the fact that the separation had
taken place so long ago that the meteors had become distributed
all around the orbit, in consequence of which the earth
encountered some of them every year when it arrived at the
crossing-point. Then Leverrier showed that the original comet
associated with the November meteors was probably brought into
the system by the influence of the planet Uranus in the year
126 of the Christian era. Afterward Alexander Herschel
identified the tracks of no less than seventy-six meteor-swarms
(most of them inconspicuous) with those of comets. The still
more recent researches of Mr W. F. Denning make it probable
that there are no meteors which do not belong to a flock or
system probably formed by the disintegration of a cometary
mass; even the apparently sporadic ones which shoot across the
sky, ``lost souls in the night,'' being members of flocks which
have become so widely scattered that the earth sometimes takes
weeks to pass through the region of space where their paths
lie.</p>
<p>The November meteors should have exhibited another pair of
spectacles in 1899 and 1900, and their failure to do so caused
at first much disappointment, until it was made plain that a
good reason existed for their absence. It was found that after
their last appearance, in 1867, they had been disturbed in
their movements by the planets Jupiter and Saturn, whose
attractions had so shifted the position of their orbit that it
no longer intersected that of the earth, as it did before.
Whether another planetary interference will sometime bring the
principal mass of the November meteors back to the former point
of intersection with the earth's orbit is a question for the
future to decide. It would seem that there may be several
parallel streams of the November meteors, and that some of
them, like those of August, are distributed entirely around the
orbit, so that every mid-November we see a few of them.</p>
<p>We come now to a very remarkable example of the
disintegration of a comet and the formation of a meteor-stream.
In 1826 Biela, of Josephstadt, Austria, discovered a comet to
which his name was given. Calculation showed that it had an
orbital period of about six and a half years, belonging to
Jupiter's ``family.'' On one of its returns, in 1846, it
astonished its watchers by suddenly splitting in two. The two
comets thus formed out of one separated to a distance of about
one hundred and sixty thousand miles, and then raced side by
side, sometimes with a curious ligature connecting them, like
Siamese twins, until they disappeared together in
interplanetary space. In 1852 they came back, still nearly side
by side, but now the distance between them had increased to a
million and a quarter of miles. After that, at every recurrence
of their period, astronomers looked for them in vain, until
1872, when an amazing thing happened. On the night of November
28th, when the earth was crossing the plane of the orbit of the
missing comet, a brilliant shower of meteors burst from the
northern sky, traveling nearly in the track which the comet
should have pursued. The astronomers were electrified.
Klinkerfues, of Göttingen, telegraphed to Pogson, of
Madras: <em>``Biela touched earth; search near Theta
Centauri.''</em> Pogson searched in the place indicated and saw
a cometary mass retreating into the southern heavens, where it
was soon swallowed from sight!</p>
<p>Since then the Biela meteors have been among the recognized
periodic spectacles of the sky, and few if any doubt that they
represent a portion of the missing comet whose disintegration
began with the separation into two parts in 1846. The comet
itself has never since been seen. The first display of these
meteors, sometimes called the ``Andromedes,'' because they
radiate from the constellation Andromeda, was remarkable for
the great brilliancy of many of the fire-balls that shot among
the shower of smaller sparks, some of which were described as
equaling the full moon in size. None of them is known to have
reached the earth, but during the display of the same meteors
in 1885 a meteoric mass fell at Mazapil in Northern Mexico (it
is now in the Museum at Vienna), which many have thought may
actually be a piece of the original comet of Biela. This brings
us to the second branch of our subject.</p>
<p>More rare than meteors or falling stars, and more startling,
except that they never appear in showers, are the huge balls of
fire which occasionally dart through the sky, lighting up the
landscapes beneath with their glare, leaving trains of sparks
behind them, often producing peals of thunder when they
explode, and in many cases falling upon the earth and burying
themselves from a few inches to several feet in the soil, from
which, more than once, they have been picked up while yet hot
and fuming. These balls are sometimes called bolides. They are
not really round in shape, although they often look so while
traversing the sky, but their forms are fragmentary, and
occasionally fantastic. It has been supposed that their origin
is different from that of the true meteors; it has even been
conjectured that they may have originated from the giant
volcanoes of the moon or have been shot out from the sun during
some of the tremendous explosions that accompany the formation
of eruptive prominences. By the same reasoning some of them
might be supposed to have come from some distant star. Others
have conjectured that they are wanderers in space, of unknown
origin, which the earth encounters as it journeys on, and Lord
Kelvin made a suggestion which has become classic because of
its imaginative reach -- <em>viz.,</em> that the first germs of
life may have been brought to the earth by one of these bodies,
``a fragment of an exploded world.''</p>
<p>It is a singular fact that astronomers and scientific men in
general were among the last to admit the possibility of solid
masses falling from the sky. The people had believed in the
reality of such phenomena from the earliest times, but the
savants shook their heads and talked of superstition. This was
the less surprising because no scientifically authenticated
instance of such an occurrence was known, and the stones
popularly believed to have fallen from the sky had become the
objects of worship or superstitious reverence, a fact not
calculated to recommend them to scientific credence. The
celebrated ``black stone'' suspended in the Kaaba at Mecca is
one of these reputed gifts from heaven; the ``Palladium'' of
ancient Troy was another; and a stone which fell near
Ensisheim, in Germany, was placed in a church as an object to
be religiously venerated. Many legends of falling stones
existed in antiquity, some of them curiously transfigured by
the imagination, like the ``Lion of the Peloponnesus,'' which
was said to have sprung down from the sky upon the Isthmus of
Corinth. But near the beginning of the nineteenth century, in
1803, a veritable shower of falling stones occurred at L'Aigle,
in Northern France, and this time astronomers took note of the
phenomenon and scientifically investigated it. Thousands of the
strange projectiles came from the sky on this occasion, and
were scattered over a wide area of country, and some buildings
were hit. Four years later another shower of stones occurred at
Weston, Conn., numbering thousands of individuals. The local
alarm created in both cases was great, as well it might be, for
what could be more intimidating than to find the blue vault of
heaven suddenly hurling solid missiles at the homes of men?
After these occurrences it was impossible for the most
skeptical to doubt any longer, and the regular study of
``aerolites,'' or ``meteorites,'' began.</p>
<p>One of the first things recognized was the fact that
fire-balls are solid meteorites in flight, and not gaseous
exhalations in the air, as some had assumed. They burn in the
air during their flight, and sometimes, perhaps, are entirely
consumed before reaching the ground. Their velocity before
entering the earth's atmosphere is equal to that of the planets
in their orbits -- <em>viz.,</em> from twenty to thirty miles
per second -- a fact which proves that the sun is the seat of
the central force governing them. Their burning in the air is
not difficult to explain; it is the heat of friction which so
quickly brings them to incandescence. Calculation shows that a
body moving through the air at a velocity of about a mile per
second will be brought, superficially, to the temperature of
``red heat'' by friction with the atmosphere. If its velocity
is twenty miles per second the temperature will become
thousands of degrees. This is the state of affairs with a
meteorite rushing into the earth's atmosphere; its surface is
liquefied within a few seconds after the friction begins to
act, and the melted and vaporized portion of its mass is swept
backward, forming the train of sparks that follows every great
fire-ball. However, there is one phenomenon connected with the
trains of meteorites which has never been satisfactorily
explained: they often persist for long periods of time,
drifting and turning with the wind, but not ceasing to glow
with a phosphorescent luminosity. The question is, Whence comes
this light? It must be light without heat, since the fine dust
or vapor of which the train can only consist would not retain
sufficient heat to render it luminous for so long a time. An
extremely remarkable incident of this kind occurred on February
22, 1909, when an immense fire-ball that passed over southern
England left a train that remained visible during two hours,
assuming many curious shapes as it was drifted about by
currents in the air.</p>
<p>But notwithstanding the enormous velocity with which
meteorites enter the air they are soon slowed down to
comparatively moderate speed, so that when they disappear they
are usually traveling not faster than a mile a second. The
courses of many have been traced by observers situated along
their track at various points, and thus a knowledge has been
obtained of their height above the ground during their flight
and of the length of their visible courses. They generally
appear at an elevation of eighty or a hundred miles, and are
seldom visible after having descended to within five miles of
the ground, unless the observer happens to be near the
striking-point, when he may actually witness the fall.
Frequently they burst while high in the air and their fragments
are scattered like shrapnel over the surface of the ground,
sometimes covering an area of several square miles, but of
course not thickly; different fragments of the same meteorite
may reach the ground at points several miles apart. The
observed length of their courses in the atmosphere varies from
fifty to five hundred miles. If they continued a long time in
flight after entering the air, even the largest of them would
probably be consumed to the last scrap, but their fiery career
is so short on account of their great speed that the heat does
not have time to penetrate very deeply, and some that have been
picked up immediately after their fall have been found cold as
ice within. Their size after reaching the ground is variable
within wide limits; some are known which weigh several tons,
but the great majority weigh only a few pounds and many only a
few ounces.</p>
<p>Meteorites are of two kinds: <em>stony</em> meteorites and
<em>iron</em> meteorites. The former outnumber the latter
twenty to one; but many stone meteorites contain grains of
iron. Nickel is commonly found in iron meteorites, so that it
might be said that that redoubtable alloy nickel-steel is of
cosmical invention. Some twenty-five chemical elements have
been found in meteorites, including carbon and the
``sun-metal,'' helium. The presence of the latter is certainly
highly suggestive in connection with the question of the origin
of meteorites. The iron meteorites, besides metallic iron and
nickel, of which they are almost entirely composed, contain
hydrogen, helium, and carbonic oxide, and about the only
imaginable way in which these gases could have become absorbed
in the iron would be through the immersion of the latter while
in a molten or vaporized state in a hot and dense atmosphere
composed of them, a condition which we know to exist only in
the envelopes of the sun and the stars.</p>
<p>The existence of carbon in the Canyon Diablo iron meteorites
is attended by a circumstance of the most singular character --
a very ``fairy tale of science.'' In some cases <em>the carbon
has become diamond!</em> These meteoric diamonds are very
small; nevertheless, they are true diamonds, resembling in many
ways the little black gems produced by Moissan's method with
the aid of the electric furnace. The fact that they are found
embedded in these iron meteorites is another argument in favor
of the hypothesis of the solar or stellar origin of the latter.
To appreciate this it is necessary to recall the way in which
Moissan made his diamonds. It was by a combination of the
effects of great heat, great pressure, and sudden or rapid
superficial cooling on a mass of iron containing carbon. When
he finally broke open his iron he found it a pudding stuffed
with miniature black diamonds. When a fragment of the Canyon
Diablo meteoric iron was polished in Philadelphia over fifteen
years ago it cut the emery-wheel to pieces, and examination
showed that the damage had been effected by microscopic
diamonds peppered through the mass. How were those diamonds
formed? If the sun or Sirius was the laboratory that prepared
them, we can get a glimpse at the process of their formation.
There is plenty of heat, plenty of pressure, and an abundance
of vaporized iron in the sun and the stars. When a great solar
eruption takes place, masses of iron which have absorbed carbon
may be shot out with a velocity which forbids their return.
Plunged into the frightful cold of space, their surfaces are
quickly cooled, as Moissan cooled his prepared iron by throwing
it into water, and thus the requisite stress is set up within,
and, as the iron solidifies, the included carbon crystallizes
into diamonds. Whether this explanation has a germ of truth in
it or not, at any rate it is evident that iron meteorites were
not created in the form in which they come to us; they must
once have been parts of immeasurably more massive bodies than
themselves.</p>
<p>The fall of meteorites offers an appreciable, though
numerically insignificant, peril to the inhabitants of the
earth. Historical records show perhaps three or four instances
of people being killed by these bodies. But for the protection
afforded by the atmosphere, which acts as a very effective
shield, the danger would doubtless be very much greater. In the
absence of an atmosphere not only would more meteorites reach
the ground, but their striking force would be incomparably
greater, since, as we have seen, the larger part of their
original velocity is destroyed by the resistance of the air. A
meteorite weighing many tons and striking the earth with a
velocity of twenty or thirty miles per second, would probably
cause frightful havoc.</p>
<p>It is a singular fact that recent investigations seem to
have proved that an event of this kind actually happened in
North America -- perhaps not longer than a thousand or two
thousand years ago. The scene of the supposed catastrophe is in
northern central Arizona, at Coon Butte, where there is a
nearly circular crater in the middle of a circular elevation or
small mountain. The crater is somewhat over four thousand feet
in diameter, and the surrounding rim, formed of upturned strata
and ejected rock fragments, rises at its highest point one
hundred and sixty feet above the plain. The crater is about six
hundred feet in depth -- that is, from the rim to the visible
floor or bottom of the crater. There is no evidence that
volcanic action has ever taken place in the immediate
neighborhood of Coon Butte. The rock in which the crater has
been made is composed of horizontal sandstone and limestone
strata. Between three hundred and four hundred million tons of
rock fragments have been detached, and a large portion hurled
by some cause out of the crater. These fragments lie
concentrically distributed around the crater, and in large
measure form the elevation known as Coon Butte. The region has
been famous for nearly twenty years on account of the masses of
meteoric iron found scattered about and known as the ``Canyon
Diablo'' meteorites. It was one of these masses, which consist
of nickel-iron containing a small quantity of platinum, and of
which in all some ten tons have been recovered for sale to the
various collectors throughout the world, that as before
mentioned destroyed the grinding-tool at Philadelphia through
the cutting power of its embedded diamonds. These meteoric
irons are scattered about the crater-hill, in concentric
distribution, to a maximum distance of about five miles. When
the suggestion was first made in 1896 that a monster meteorite
might have created by its fall this singular lone crater <em>in
stratified rocks,</em> it was greeted with incredulous smiles;
but since then the matter has assumed a different aspect. The
Standard Iron Company, formed by Messrs. D. M. Barringer, B. C.
Tilghman, E. J. Bennitt, and S. J. Holsinger, having become, in
1903, the owner of this freak of nature, sunk shafts and bored
holes to a great depth in the interior of the crater, and also
trenched the slopes of the mountain, and the result of their
investigations has proved that the meteoric hypothesis of
origin is correct. (See the papers published in the
<em>Proceedings of the Academy of Natural Sciences of
Philadelphia,</em> December, 1905, wherein it is proved that
the United States Geological Survey was wrong in believing this
crater to have been due to a steam explosion. Since that date
there has been discovered a great amount of additional
confirmatory proof). Material of unmistakably meteoric origin
was found by means of the drills, mixed with crushed rock, to a
depth of six hundred to seven hundred feet below the floor of
the crater, and a great deal of it has been found admixed with
the ejected rock fragments on the outer slopes of the mountain,
absolutely proving synchronism between the two events, the
formation of this great crater and the falling of the meteoric
iron out of the sky. The drill located in the bottom of the
crater was sent, in a number of cases, much deeper (over one
thousand feet) into unaltered horizontal red sandstone strata,
but no meteoric material was found below this depth (seven
hundred feet, or between eleven and twelve hundred feet below
the level of the surrounding plain), which has been assumed as
being about the limit of penetration. It is not possible to
sink a shaft at present, owing to the water which has drained
into the crater, and which forms, with the finely pulverized
sandstone, a very troublesome quicksand encountered at about
two hundred feet below the visible floor of the crater. As soon
as this water is removed by pumping it will be easy to explore
the depths of the crater by means of shafts and drifts. The
rock strata (sandstone and limestone) of which the walls
consist present every appearance of having been violently
upturned by a huge body penetrating the earth like a
cannon-ball. The general aspect of the crater strikingly
resembles the impression made by a steel projectile shot into
an armor-plate. Mr Tilghman has estimated that a meteorite
about five hundred feet in diameter and moving with a velocity
of about five miles per second would have made just such a
perforation upon striking rocks of the character of those found
at this place. There was some fusion of the colliding masses,
and the heat produced some steam from the small amount of water
in the rocks. As a result there has been found at depth a
considerable amount of fused quartz (original sandstone), and
with it innumerable particles or sparks of fused nickel-iron
(original meteorite). A projectile of that size penetrating
eleven to twelve hundred feet into the rocky shell of the globe
must have produced a shock which was perceptible several
hundred miles away.</p>
<p>The great velocity ascribed to the supposed meteorite at the
moment of striking could be accounted for by the fact that it
probably plunged nearly vertically downward, for it formed a
circular crater in the rocky crust of the earth. In that case
it would have been less retarded by the resistance of the
atmosphere than are meteorites which enter the air at a lower
angle and shoot ahead hundreds of miles until friction has
nearly destroyed their original motion when they drop upon the
earth. Some meteoric masses of great size, such as Peary's iron
meteorite found at Cape York, Greenland, and the almost equally
large mass discovered at Bacubirito, Mexico, appear to have
penetrated but slightly on striking the earth. This may be
explained by supposing that they pursued a long, horizontal
course through the air before falling. The result would be
that, their original velocity having been practically
destroyed, they would drop to the ground with a velocity nearly
corresponding to that which gravity would impart within the
perpendicular distance of their final fall. A
six-hundred-and-sixty-pound meteorite, which fell at Knyahinya,
Hungary, striking at an angle of 27° from the vertical,
penetrated the ground to a depth of eleven feet.</p>
<p>It has been remarked that the Coon Butte meteorite may have
fallen not longer ago than a few thousand years. This is based
upon the fact that the geological indications favor the
supposition that the event did not occur more than five
thousand years ago, while on the other hand the rings of growth
in the cedar-trees growing on the slopes of the crater show
that they have existed there about seven hundred years. Prof.
William H. Pickering has recently correlated this with an
ancient chronicle which states that at Cairo, Egypt, in the
year 1029, ``many stars passed with a great noise.'' He remarks
that Cairo is about 100°, by great circle, from Coon Butte,
so that if the meteorite that made the crater was a member of a
flock of similar bodies which encountered the earth moving in
parallel lines, some of them might have traversed the sky
tangent to the earth's surface at Cairo. That the spectacle
spoken of in the chronicle was caused by meteorites he deems
exceedingly probable because of what is said about ``a great
noise;'' meteorites are the only celestial phenomena attended
with perceptible sounds. Professor Pickering conjectures that
this supposed flock of great meteorites may have formed the
nucleus of a comet which struck the earth, and he finds
confirmation of the idea in the fact that out of the ten
largest meteorites known, no less than seven were found within
nine hundred miles of Coon Butte. It would be interesting if we
could trace back the history of that comet, and find out what
malicious planet caught it up in its innocent wanderings and
hurled it with so true an aim at the earth! This remarkable
crater is one of the most interesting places in the world, for
there is absolutely no record of such a mass, possibly an
iron-headed comet, from outer space having come into collision
with our earth. The results of the future exploration of the
depths of the crater will be awaited with much interest.</p>
<p><strong>The Wrecking of the Moon</strong></p>
<p>There are sympathetic moods under whose influence one gazes
with a certain poignant tenderness at the worn face of the
moon; that little ``fossil world'' (the child of our mother
earth, too) bears such terrible scars of its brief convulsive
life that a sense of pity is awakened by the sight. The moon is
the wonder-land of the telescope. Those towering mountains,
whose ``proud aspiring peaks'' cast silhouettes of shadow that
seem drawn with india-ink; those vast plains, enchained with
gentle winding hills and bordered with giant ranges; those oval
``oceans,'' where one looks expectant for the flash of
wind-whipped waves; those enchanting ``bays'' and recesses at
the seaward feet of the Alps; those broad straits passing
between guardian heights incomparably mightier than Gibraltar;
those locket-like valleys as secluded among their mountains as
the Vale of Cashmere; those colossal craters that make us smile
at the pretensions of Vesuvius, Etna, and Cotopaxi; those
strange white ways which pass with the unconcern of Roman roads
across mountain, gorge, and valley -- all these give the
beholder an irresistible impression that it is truly a world
into which he is looking, a world akin to ours, and yet no more
like our world than Pompeii is like Naples. Its air, its
waters, its clouds, its life are gone, and only a skeleton
remains -- a mute but eloquent witness to a cosmical tragedy
without parallel in the range of human knowledge.</p>
<p>One cannot but regret that the moon, if it ever was the seat
of intelligent life, has not remained so until our time. Think
what the consequences would have been if this other world at
our very door had been found to be both habitable and
inhabited! We talk rather airily of communicating with Mars by
signals; but Mars never approaches nearer than 35,000,000
miles, while the moon when nearest is only a little more than
220,000 miles away. Given an effective magnifying power of five
thousand diameters, which will perhaps be possible at the
mountain observatories as telescopes improve, and we should be
able to bring the moon within an apparent distance of about
forty miles, while the corresponding distance for Mars would be
more than seven thousand miles. But even with existing
telescopic powers we can see details on the moon no larger than
some artificial constructions on the earth. St Peter's at Rome,
with the Vatican palace and the great piazza, if existing on
the moon, would unquestionably be recognizable as something
else than a freak of nature. Large cities, with their radiating
lines of communication, would at once betray their real
character. Cultivated tracts, and the changes produced by the
interference of intelligent beings, would be clearly
recognizable. The electric illumination of a large town at
night would probably be markedly visible. Gleams of reflected
sunlight would come to us from the surfaces of the lakes and
oceans, and a huge ``liner'' traversing a lunar sea could
probably be followed by its trail of smoke. As to
communications by ``wireless'' signals, which certain
enthusiasts have thought of in connection with Mars, in the
case of the moon they should be a relatively simple matter, and
the feat might actually be accomplished. Think what a
literature would grow up about the moon if it were a living
world! Its very differences from the earth would only
accentuate its interest for us. Night and day on the moon are
each two weeks in length; how interesting it would be to watch
the manner in which the lunarians dealt with such a situation
as that. Lunar and terrestrial history would keep step with
each other, and we should record them both. Truly one might
well wish to have a neighbor world to study; one would feel so
much the less alone in space.</p>
<p>It is not impossible that the moon did at one time have
inhabitants of some kind. But, if so, they vanished with the
disappearance of its atmosphere and seas, or with the advent of
its cataclysmic age. At the best, its career as a living world
must have been brief. If the water and air were gradually
absorbed, as some have conjectured, by its cooling interior
rocks, its surface might, nevertheless, have retained them for
long ages; but if, as others think, their disappearance was due
to the escape of their gaseous molecules in consequence of the
inability of the relatively small lunar gravitation to retain
them, then the final catastrophe must have been as swift as it
was inevitable. Accepting Darwin's hypothesis, that the moon
was separated from the earth by tidal action while both were
yet plastic or nebulous, we may reasonably conclude that it
began its career with a good supply of both water and air, but
did not possess sufficient mass to hold them permanently. Yet
it may have retained them long enough for life to develop in
many forms upon its surface; in fact, there are so many
indications that air and water have not always been lacking to
the lunar world that we are driven to invent theories to
explain both their former presence and their present
absence.</p>
<p>But whatever the former condition of the moon may have been,
its existing appearance gives it a resistless fascination, and
it bears so clearly the story of a vast catastrophe sculptured
on its rocky face that the thoughtful observer cannot look upon
it without a feeling of awe. The gigantic character of the
lunar features impresses the beholder not less than the
universality of the play of destructive forces which they
attest. Let us make a few comparisons. Take the lunar crater
called ``Tycho'', which is a typical example of its kind. In
the telescope Tycho appears as a perfect ring surrounding a
circular depression, in the center of which rises a group of
mountains. Its superficial resemblance to some terrestrial
volcanic craters is very striking. Vesuvius, seen from a point
vertically above, would no doubt look something like that (the
resemblance would have been greater when the Monte del Cavallo
formed a more complete circuit about the crater cone). But
compare the dimensions. The remains of the outer crater ring of
Vesuvius are perhaps half a mile in diameter, while the active
crater itself is only two or three hundred feet across at the
most; Tycho has a diameter of fifty-four miles! The group of
relatively insignificant peaks in the center of the crater
floor of Tycho is far more massive than the entire mountain
that we call Vesuvius. The largest known volcanic crater on the
earth, Aso San, in Japan, has a diameter of seven miles; it
would take <em>sixty</em> craters like Aso San to equal Tycho
in area! And Tycho, though one of the most perfect, is by no
means the largest crater on the moon. Another, called
``Theophilus,'' has a diameter of sixty-four miles, and is
eighteen thousand feet deep. There are hundreds from ten to
forty miles in diameter, and thousands from one to ten miles.
They are so numerous in many places that they break into one
another, like the cells of a crushed honeycomb.</p>
<p>The lunar craters differ from those of the earth more
fundamentally than in the matter of mere size; <em>they are not
situated on the tops of mountains.</em> If they were, and if
all the proportions were the same, a crater like Tycho might
crown a conical peak fifty or one hundred miles high! Instead
of being cavities in the summits of mountains, the lunar
craters are rather gigantic sink-holes whose bottoms in many
cases lie two or three miles below the general surface of the
lunar world. Around their rims the rocks are piled up to a
height of from a few hundred to two or three thousand feet,
with a comparatively gentle inclination, but on the inner side
they fall away in gigantic broken precipices which make the
dizzy cliffs of the Matterhorn seem but ``lover's leaps.'' Down
they drop, ridge below ridge, crag under crag, tottering wall
beneath wall, until, in a crater named ``Newton,'' near the
south lunar pole, they attain a depth where the rays of the sun
never reach. Nothing more frightful than the spectacle which
many of these terrible chasms present can be pictured by the
imagination. As the lazy lunar day slowly advances, the
sunshine, unmitigated by clouds or atmospheric veil of any
kind, creeps across their rims and begins to descend the
opposite walls. Presently it strikes the ragged crest of a
ridge which had lain hidden in such darkness as we never know
on the earth, and runs along it like a line of kindling fire.
Rocky pinnacles and needles shoot up into the sunlight out of
the black depths. Down sinks the line of light, mile after
mile, and continually new precipices and cliffs are brought
into view, until at last the vast floor is attained and begins
to be illuminated. In the meanwhile the sun's rays, darting
across the gulf, have touched the summits of the central peaks,
twenty or thirty miles from the crater's inmost edge, and they
immediately kindle and blaze like huge stars amid the darkness.
So profound are some of these awful craters that days pass
before the sun has risen high enough above them to chase the
last shadows from their depths.</p>
<p>Although several long ranges of mountains resembling those
of the earth exist on the moon, the great majority of its
elevations assume the crateriform aspect. Sometimes, instead of
a crater, we find an immense mountain ring whose form and
aspect hardly suggest volcanic action. But everywhere the true
craters are in evidence, even on the sea-beds, although they
attain their greatest number and size on those parts of the
moon -- covering sixty per cent of its visible surface -- which
are distinctly mountainous in character and which constitute
its most brilliant portions. Broadly speaking, the southwestern
half of the moon is the most mountainous and broken, and the
northeastern half the least so. Right down through the center,
from pole to pole, runs a wonderful line of craters and
crateriform valleys of a magnitude stupendous even for the
moon. Another similar line follows the western edge. Three or
four ``seas'' are thrust between these mountainous belts. By
the effects of ``libration'' parts of the opposite hemisphere
of the moon which is turned away from the earth are from time
to time brought into view, and their aspect indicates that that
hemisphere resembles in its surface features the one which
faces the earth. There are many things about the craters which
seem to give some warrant for the hypothesis which has been
particularly urged by Mr G. K. Gilbert, that they were formed
by the impact of meteors; but there are also many things which
militate against that idea, and, upon the whole, the volcanic
theory of their origin is to be preferred.</p>
<p>The enormous size of the lunar volcanoes is not so difficult
to account for when we remember how slight is the force of
lunar gravity as compared with that of the earth. With equal
size and density, bodies on the moon weigh only one-sixth as
much as on the earth. Impelled by the same force, a projectile
that would go ten miles on the earth would go sixty miles on
the moon. A lunar giant thirty-five feet tall would weigh no
more than an ordinary son of Adam weighs on his greater planet.
To shoot a body from the earth so that it would not drop back
again, we should have to start it with a velocity of seven
miles per second; a mile and a half per second would serve on
the moon. It is by no means difficult to believe, then, that a
lunar volcano might form a crater ring eight or ten times
broader than the greatest to be found on the earth, especially
when we reflect that in addition to the relatively slight force
of gravity, the materials of the lunar crust are probably
lighter than those of our terrestrial rocks.</p>
<p>For similar reasons it seems not impossible that the theory
mentioned in a former chapter -- that some of the meteorites
that have fallen upon the earth originated from the lunar
volcanoes -- is well founded. This would apply especially to
the stony meteorites, for it is hardly to be supposed that the
moon, at least in its superficial parts, contains much iron. It
is surely a scene most strange that is thus presented to the
mind's eye -- that little attendant of the earth's (the moon
has only one-fiftieth of the volume, and only one-eightieth of
the mass of the earth) firing great stones back at its parent
planet! And what can have been the cause of this furious
outbreak of volcanic forces on the moon? Evidently it was but a
passing stage in its history; it had enjoyed more quiet times
before. As it cooled down from the plastic state in which it
parted from the earth, it became incrusted after the normal
manner of a planet, and then oceans were formed, its atmosphere
being sufficiently dense to prevent the water from evaporating
and the would-be oceans from disappearing continually in mist.
This, if any, must have been the period of life in the lunar
world. As we look upon the vestiges of that ancient world
buried in the wreck that now covers so much of its surface, it
is difficult to restrain the imagination from picturing the
scenes which were once presented there; and, in such a case,
should the imagination be fettered? We give it free rein in
terrestrial life, and it rewards us with some of our greatest
intellectual pleasures. The wonderful landscapes of the moon
offer it an ideal field with just enough half-hidden
suggestions of facts to stimulate its powers.</p>
<p>The great plains of the <em>Mare Imbrium</em> and the
<em>Mare Serenitatis</em> (the ``Sea of Showers'' and the ``Sea
of Serenity''), bordered in part by lofty mountain ranges
precisely like terrestrial mountains, scalloped along their
shores with beautiful bays curving back into the adjoining
highlands, and united by a great strait passing between the
nearly abutting ends of the ``Lunar Apennines'' and the ``Lunar
Caucasus,'' offer the elements of a scene of world beauty such
as it would be difficult to match upon our planet. Look at the
finely modulated bottom of the ancient sea in Mr Ritchey's
exquisite photograph of the western part of the <em>Mare
Serenitatis,</em> where one seems to see the play of the watery
currents heaping the ocean sands in waving lines, making
shallows, bars, and deeps for the mariner to avoid or seek, and
affording a playground for the creatures of the main. What
geologist would not wish to try his hammer on those rocks with
their stony pages of fossilized history? There is in us an
instinct which forbids us to think that there was never any
life there. If we could visit the moon, there is not among us a
person so prosaic and unimaginative that he would not, the very
first thing, begin to search for traces of its inhabitants. We
would look for them in the deposits on the sea bottoms; we
would examine the shores wherever the configuration seemed
favorable for harbors and the sites of maritime cities --
forgetting that it may be a little ridiculous to ascribe to the
ancient lunarians the same ideas that have governed the
development of our race; we would search through the valleys
and along the seeming courses of vanished streams; we would
explore the mountains, not the terrible craters, but the
pinnacled chains that recall our own Alps and Rockies; seeking
everywhere some vestige of the transforming presence of
intelligent life. Perhaps we should find such traces, and
perhaps, with all our searching, we should find nothing to
suggest that life had ever existed amid that universal
ruin.</p>
<p>Look again at the border of the ``Sea of Serenity'' -- what
a name for such a scene! -- and observe how it has been rent
with almost inconceivable violence, the wall of the colossal
crater Posidonius dropping vertically upon the ancient shore
and obliterating it, while its giant neighbor, Le Monnier,
opens a yawning mouth as if to swallow the sea itself. A scene
like this makes one question whether, after all, those may not
be right who have imagined that the so-called sea bottoms are
really vast plains of frozen lava which gushed up in floods so
extensive that even the mighty volcanoes were half drowned in
the fiery sea. This suggestion becomes even stronger when we
turn to another of the photographs of Mr Ritchey's wonderful
series, showing a part of the <em>Mare Tranquilitatis</em>
(``Sea of Tranquility''!). Notice how near the center of the
picture the outline of a huge ring with radiating ridges shows
through the sea bottom; a fossil volcano submerged in a
petrified ocean! This is by no means the only instance in which
a buried world shows itself under the great lunar plains. Yet,
as the newer craters in the sea itself prove, the volcanic
activity survived this other catastrophe, or broke out again
subsequently, bringing more ruin to pile upon ruin.</p>
<p>Yet notwithstanding the evidence which we have just been
considering in support of the hypothesis that the ``seas'' are
lava floods, Messrs. Loewy and Puiseux, the selenographers of
the Paris Observatory, are convinced that these great plains
bear characteristic marks of the former presence of immense
bodies of water. In that case we should be forced to conclude
that the later oceans of the moon lay upon vast sheets of
solidified lava; and thus the catastrophe of the lunar world
assumes a double aspect, the earliest oceans being swallowed up
in molten floods issuing from the interior, while the lands
were reduced to chaos by a universal eruption of tremendous
volcanoes; and then a period of comparative quiet followed,
during which new seas were formed, and new life perhaps began
to flourish in the lunar world, only to end in another
cataclysm, which finally put a term to the existence of the
moon as a life-supporting world.</p>
<p>Suppose we examine two more of Mr Ritchey's illuminating
photographs, and, first, the one showing the crater Theophilus
and its surroundings. We have spoken of Theophilus before,
citing the facts that it is sixty-four miles in diameter and
eighteen thousand feet deep. It will be noticed that it has two
brother giants -- Cyrillus the nearer, and Catharina the more
distant; but Theophilus is plainly the youngest of the trio.
Centuries, and perhaps thousands of years, must have elapsed
between the periods of their upheaval, for the two older
craters are partly filled with débris, while it is
manifest at a glance that when the south eastern wall of
Theophilus was formed, it broke away and destroyed a part of
the more ancient ring of Cyrillus. There is no more tremendous
scene on the moon than this; viewed with a powerful telescope,
it is absolutely appalling.</p>
<p>The next photograph shows, if possible, a still wilder
region. It is the part of the moon lying between Tycho and the
south pole. Tycho is seen in the lower left-hand part of the
picture. To the right, at the edge of the illuminated portion
of the moon, are the crater-rings, Longomontanus and Wilhelm I,
the former being the larger. Between them are to be seen the
ruins of two or three more ancient craters which, together with
portions of the walls of Wilhelm I and Longomontanus, have been
honeycombed with smaller craters. The vast crateriform
depression above the center of the picture is Clavius, an
unrivaled wonder of lunar scenery, a hundred and forty-two
miles in its greatest length, while its whole immense floor has
sunk two miles below the general surface of the moon outside
the ring. The monstrous shadow-filled cavity above Clavius
toward the right is Blancanus, whose aspect here gives a good
idea of the appearance of these chasms when only their rims are
in the sunlight. But observe the indescribable savagery of the
entire scene. It looks as though the spirit of destruction had
gone mad in this spot. The mighty craters have broken forth one
after another, each rending its predecessor; and when their
work was finished, a minor but yet tremendous outbreak
occurred, and the face of the moon was gored and punctured with
thousands of smaller craters. These relatively small craters
(small, however, only in a lunar sense, for many of them would
appear gigantic on the earth) recall once more the theory of
meteoric impact. It does not seem impossible that some of them
may have been formed by such an agency.</p>
<p>One would not wish for our planet such a fate as that which
has overtaken the moon, but we cannot be absolutely sure that
something of the kind may not be in store for it. We really
know nothing of the ultimate causes of volcanic activity, and
some have suggested that the internal energies of the earth may
be accumulating instead of dying out, and may never yet have
exhibited their utmost destructive power. Perhaps the best
assurance that we can find that the earth will escape the
catastrophe that has overtaken its satellite is to be found in
the relatively great force of its gravitation. The moon has
been the victim of its weakness; given equal forces, and the
earth would be the better able to withstand them. It is
significant, in connection with these considerations, that the
little planet Mercury, which seems also to have parted with its
air and water, shows to the telescope some indications that it
is pitted with craters resembling those that have torn to
pieces the face of the moon.</p>
<p>Upon the whole, after studying the dreadful lunar
landscapes, one cannot feel a very enthusiastic sympathy with
those who are seeking indications of the continued existence of
some kind of life on the moon; such a world is better without
inhabitants. It has met its fate; let it go! Fortunately, it is
not so near that it cannot hide its scars and appear beautiful
-- except when curiosity impels us to look with the penetrating
eyes of the astronomer.</p>
<p><strong>The Great Mars Problem</strong></p>
<p>Let any thoughtful person who is acquainted with the general
facts of astronomy look up at the heavens some night when they
appear in their greatest splendor, and ask himself what is the
strongest impression that they make upon his mind. He may not
find it easy to frame an answer, but when he has succeeded it
will probably be to the effect that the stars give him an
impression of the universality of intelligence; they make him
feel, as the sun and the moon cannot do, that his world is not
alone; that all this was not made simply to form a gorgeous
canopy over the tents of men. If he is of a devout turn of
mind, he thinks, as he gazes into those fathomless deeps and
among those bewildering hosts, of the infinite multitude of
created beings that the Almighty has taken under his care. The
narrow ideas of the old geocentric theology, which made the
earth God's especial footstool, and man his only rational
creature, fall away from him like a veil that had obscured his
vision; they are impossible in the presence of what he sees
above. Thus the natural tendency, in the light of modern
progress, is to regard the universe as everywhere filled with
life.</p>
<p>But science, which is responsible for this broadening of
men's thoughts concerning the universality of life, itself
proceeds to set limits. Of spiritual existences it pretends to
know nothing, but as to physical beings, it declares that it
can only entertain the supposition of their existence where it
finds evidence of an environment suited to their needs, and
such environment may not everywhere exist. Science, though
repelled by the antiquated theological conception of the
supreme isolation of man among created beings, regards with
complacency the probability that there are regions in the
universe where no organic life exists, stars which shine upon
no inhabited worlds, and planets which nourish no animate
creatures. The astronomical view of the universe is that it
consists of matter in every stage of evolution: some nebulous
and chaotic; some just condensing into stars (suns) of every
magnitude and order; some shaped into finished solar bodies
surrounded by dependent planets; some forming stars that
perhaps have no planets, and will have none; some constituting
suns that are already aging, and will soon lose their radiant
energy and disappear; and some aggregated into masses that long
ago became inert, cold, and rayless, and that can only be
revivified by means about which we can form conjectures, but of
which we actually know nothing.</p>
<p>As with the stars, so with the planets, which are the
satellites of stars. All investigations unite to tell us that
the planets are not all in the same state of development. As
some are large and some small, so some are, in an evolutionary
sense, young, and some old. As they depend upon the suns around
which they revolve for their light, heat, and other forms of
radiant energy, so their condition varies with their distance
from those suns. Many may never arrive at a state suitable for
the maintenance of life upon their surfaces; some which are not
at present in such a state may attain it later; and the forms
of life themselves may vary with the peculiar environment that
different planets afford. Thus we see that we are not
scientifically justified in affirming that life is ubiquitous,
although we are thus justified in saying that it must be, in a
general sense, universal. We might liken the universe to a
garden known to contain every variety of plant. If on entering
it we see no flowers, we examine the species before us and find
that they are not of those which bloom at this particular
season, or perhaps they are such as never bear flowers. Yet we
feel no doubt that we shall find flowers somewhere in the
garden, because there <em>are</em> species which bloom at this
season, and the garden contains <em>all</em> varieties.</p>
<p>While it is tacitly assumed that there are planets revolving
around other stars than the sun, it would be impossible for us
to see them with any telescope yet invented, and no instrument
now in the possession of astronomers could assure us of their
existence; so the only planetary system of which we have visual
knowledge is our own. Excluding the asteroids, which could not
from any point of view be considered as habitable, we have in
the solar system eight planets of various sizes and situated at
various distances from the sun. Of these eight we know that
one, the earth, is inhabited. The question, then, arises: Are
there any of the others which are inhabited or habitable? Since
it is our intention to discuss the habitability of only one of
the seven to which the question applies, the rest may be
dismissed in a few words. The smallest of them, and the nearest
to the sun, is Mercury, which is regarded as uninhabitable
because it has no perceptible supply of water and air, and
because, owing to the extraordinary eccentricity of its orbit,
it is subjected to excessive and very rapid alterations in the
amount of solar heat and light poured upon its surface, such
alterations being inconsistent with the supposition that it can
support living beings. Even its average temperature is more
than six and a half times that prevailing on the earth! Another
circumstance which militates against its habitability is that,
according to the results of the best telescopic studies, it
always keeps the same face toward the sun, so that one half of
the planet is perpetually exposed to the fierce solar rays, and
the other half faces the unmitigated cold of open space. Venus,
the next in distance from the sun, is almost the exact twin of
the earth in size, and many arguments may be urged in favor of
its habitability, although it is suspected of possessing the
same peculiarity as Mercury, in always keeping the same side
sunward. Unfortunately its atmosphere appears to be so dense
that no permanent markings on its surface are certainly
visible, and the question of its actual condition must, for the
present, be left in abeyance. Mars, the first planet more
distant from the sun than the earth, is the special subject of
this chapter, and will be described and discussed a few lines
further on. Jupiter, Saturn, Uranus, and Neptune, the four
giant planets, all more distant than Mars, and each more
distant than the other in the order named, are all regarded as
uninhabitable because none of them appears to possess any
degree of solidity. They may have solid or liquid nuclei, but
exteriorly they seem to be mere balls of cloud. Of course, one
can imagine what he pleases about the existence of creatures
suited to the physical constitution of such planets as these,
but they must be excluded from the category of habitable worlds
in the ordinary sense of the term. We go back, then, to
Mars.</p>
<p>It will be best to begin with a description of the planet.
Mars is 4230 miles in diameter; its surface is not much more
than one-quarter as extensive as that of the earth (.285). Its
mean distance from the sun is 141,500,000 miles, 48,500,000
miles greater than that of the earth. Since radiant energy
varies inversely as the square of distance, Mars receives less
than half as much solar light and heat as the earth gets. Mars'
year (period of revolution round the sun) is 687 days. Its mean
density is 71 per cent of the earth's, and the force of gravity
on its surface is 38 per cent of that on the surface of the
earth; <em>i.e.,</em> a body weighing one hundred pounds on the
earth would, if transported to Mars, weigh but thirty-eight
pounds. The inclination of its equator to the plane of its
orbit differs very little from that of the earth's equator, and
its axial rotation occupies 24 hours 37 minutes. so that the
length of day and night, and the extent of the seasonal changes
on Mars, are almost precisely the same as on the earth. But
owing to the greater length of its year, the seasons of Mars,
while occurring in the same order, are almost twice as long as
ours. The surface of the planet is manifestly solid, like that
of our globe, and the telescope reveals many permanent markings
on it, recalling the appearance of a globe on which
geographical features have been represented in reddish and
dusky tints. Around the poles are plainly to be seen rounded
white areas, which vary in extent with the Martian seasons,
nearly vanishing in summer and extending widely in winter. The
most recent spectroscopic determinations indicate that Mars has
an atmosphere perhaps as dense as that to be found on our
loftiest mountain peaks, and there is a perceptible amount of
watery vapor in this atmosphere. The surface of the planet
appears to be remarkably level, and it has no mountain ranges.
No evidences of volcanic action have been discovered on Mars.
The dusky and reddish areas were regarded by the early
observers as respectively seas and lands, but at present it is
not believed that there are any bodies of water on the planet.
There has never been much doubt expressed that the white areas
about the poles represent snow.</p>
<p>It will be seen from this brief description that many
remarkable resemblances exist between Mars and the earth, and
there is nothing wonderful in the fact that the question of the
habitability of the former has become one of extreme and
wide-spread interest, giving rise to the most diverse views, to
many extraordinary speculations, and sometimes to regrettably
heated controversy. The first champion of the habitability of
Mars was Sir William Herschel, although even before his time
the idea had been suggested. He was convinced by the
revelations of his telescopes, continually increasing in power,
that Mars was more like the earth than any other planet. He
could not resist the testimony of the polar snows, whose
suggestive conduct was in such striking accord with what occurs
upon the earth. Gradually, as telescopes improved and observers
increased in number, the principal features of the planet were
disclosed and charted, and ``areography,'' as the geography of
Mars was called, took its place among the recognized branches
of astronomical study. But it was not before 1877 that a
fundamentally new discovery in areography gave a truly
sensational turn to speculation about life on ``the red
planet.'' In that year Mars made one of its nearest approaches
to the earth, and was so situated in its orbit that it could be
observed to great advantage from the northern hemisphere of the
earth. The celebrated Italian astronomer, Schiaparelli, took
advantage of this opportunity to make a trigonometrical survey
of the surface of Mars -- as coolly and confidently as if he
were not taking his sights across a thirty-five-million-mile
gulf of empty space -- and in the course of this survey he was
astonished to perceive that the reddish areas, then called
continents, were crossed in many directions by narrow, dusky
lines, to which he gave the suggestive name of ``canals.'' Thus
a kind of firebrand was cast into the field of astronomical
speculation, which has ever since produced disputes that have
sometimes approached the violence of political faction. At
first the accuracy of Schiaparelli's observations was
contested; it required a powerful telescope, and the most
excellent ``seeing,'' to render the enigmatical lines visible
at all, and many searchers were unable to detect them. But
Schiaparelli continued his studies in the serene sky of Italy,
and produced charts of the gridironed face of Mars containing
so much astonishing detail that one had either to reject them
<em>in toto</em> or to confess that Schiaparelli was right. As
subsequent favorable oppositions of Mars occurred, other
observers began to see the ``canals'' and to confirm the
substantial accuracy of the Italian astronomer's work, and
finally few were found who would venture to affirm that the
``canals'' did not exist, whatever their meaning might be.</p>
<p>When Schiaparelli began his observations it was generally
believed, as we have said, that the dusky areas on Mars were
seas, and since Schiaparelli thought that the ``canals''
invariably began and ended at the shores of the ``seas,'' the
appropriateness of the title given to the lines seemed
apparent. Their artificial character was immediately assumed by
many, because they were too straight and too suggestively
geometrical in their arrangement to permit the conclusion that
they were natural watercourses. A most surprising circumstance
noted by Schiaparelli was that the ``canals'' made their
appearance <em>after</em> the melting of the polar snow in the
corresponding hemisphere had begun, and that they grew darker,
longer, and more numerous in proportion as the polar
liquidation proceeded; another very puzzling observation was
that many of them became double as the season advanced; close
beside an already existing ``canal,'' and in perfect
parallelism with it, another would gradually make its
appearance. That these phenomena actually existed and were not
illusions was proved by later observations, and today they are
seen whenever Mars is favorably situated for observation.</p>
<p>In the closing decade of the nineteenth century, Mr Percival
Lowell took up the work where Schiaparelli had virtually
dropped it, and soon added a great number of ``canals'' to
those previously known, so that in his charts the surface of
the wonderful little planet appears covered as with a spider's
web, the dusky lines criss-crossing in every direction, with
conspicuous knots wherever a number of them come together. Mr
Lowell has demonstrated that the areas originally called seas,
and thus named on the earlier charts, are not bodies of water,
whatever else they may be. He has also found that the
mysterious lines do not, as Schiaparelli supposed, begin and
end at the edges of the dusky regions, but often continue on
across them, reaching in some cases far up into the polar
regions. But Schiaparelli was right in his observation that the
appearance of the ``canals'' is synchronous with the gradual
disappearance of the polar snows, and this fact has become the
basis of the most extraordinary theory that the subject of life
in other worlds has ever given birth to.</p>
<p>Now, the effect of such discoveries, as we have related,
depends upon the type of mind to whose attention they are
called. Many are content to accept them as strange and
inexplicable at present, and to wait for further light upon
them; others insist upon an immediate inquiry concerning their
probable nature and meaning. Such an inquiry can only be based
upon inference proceeding from analogy. Mars, say Mr Lowell and
those who are of his opinion, is manifestly a solidly incrusted
planet like the earth; it has an atmosphere, though one of
great rarity; it has water vapor, as the snows in themselves
prove; it has the alternation of day and night, and a
succession of seasons closely resembling those of the earth;
its surface is suggestively divided into regions of contrasting
colors and appearance, and upon that surface we see an immense
number of lines geometrically arranged, with a system of
symmetrical intersections where the lines expand into circular
and oval areas -- and all connected with the annual melting of
the polar snows in a way which irresistibly suggests the
interference of intelligence directed to a definite end. Why,
with so many concurrent circumstances to support the
hypothesis, should we not regard Mars as an inhabited
globe?</p>
<p>But the differences between Mars and the earth are in many
ways as striking as their resemblances. Mars is relatively
small; it gets less than half as much light and heat as we
receive; its atmosphere is so rare that it would be distressing
to us, even if we could survive in it at all; it has no lakes,
rivers, or seas; its surface is an endless prairie. and its
``canals'' are phenomena utterly unlike anything on the earth.
Yet it is precisely upon these divergences between the earth
and Mars, this repudiation of terrestrial standards, that the
theory of ``life on Mars,'' for which Mr Lowell is mainly
responsible, is based. Because Mars is smaller than the earth,
we are told it must necessarily be more advanced in planetary
evolution, the underlying cause of which is the gradual cooling
and contraction of the planet's mass. Mars has parted with its
internal heat more rapidly than the earth; consequently its
waters and its atmosphere have been mostly withdrawn by
chemical combinations, but enough of both yet remain to render
life still possible on its surface. As the globe of Mars is
evolutionally older than that of the earth, so its forms of
organic life may be proportionally further advanced, and its
inhabitants may have attained a degree of cultivated
intelligence much superior to what at present exists upon the
earth. Understanding the nature and the causes of the
desiccation of their planet, and possessing engineering science
and capabilities far in advance of ours, they may be conceived
to have grappled with the stupendous problem of keeping their
world in a habitable condition as long as possible. Supposing
them to have become accustomed to live in their rarefied
atmosphere (a thing not inconceivable, since men can live for a
time at least in air hardly less rare), the most pressing
problem for them is that of a water-supply, without which plant
life cannot exist, while animal life in turn depends for its
existence upon vegetation. The only direction in which they can
seek water is that of the polar regions, where it is
alternately condensed into snow and released in the liquid form
by the effect of the seasonal changes. It is, then, to the
annual melting of the polar snow-fields that the Martian
engineers are supposed to have recourse in supplying the needs
of their planet, and thus providing the means of prolonging
their own existence. It is imagined that they have for this
purpose constructed a stupendous system of irrigation extending
over the temperate and equatorial regions of the planet. The
``canals'' represent the lines of irrigation, but the narrow
streaks that we see are not the canals themselves, but the
irrigated bands covered by them. Their dark hue, and their
gradual appearance after the polar melting has begun, are due
to the growth of vegetation stimulated by the water. The
rounded areas visible where several ``canals'' meet and cross
are called by Mr Lowell ``oases.'' These are supposed to be the
principal centers of population and industry. It must be
confessed that some of them, with their complicated systems of
radiating lines, appear to answer very well to such a theory.
No attempt to explain them by analogy with natural phenomena on
the earth has proved successful.</p>
<p>But a great difficulty yet remains: How to explain the
seemingly miraculous powers of the supposed engineers? Here
recourse is had once more to the relative smallness of the
planet. We have remarked that the force of gravity on Mars is
only thirty-eight per cent of that on the earth. A steam-shovel
driven by a certain horse-power would be nearly three times as
effective there as here. A man of our stature on Mars would
find his effective strength increased in the same proportion.
But just because of the slight force of gravity there, a
Martian might attain to the traditional stature of Goliath
without finding his own weight an encumbrance to his activity,
while at the same time his huge muscles would come into
unimpeded play, enabling him single-handed to perform labors
that would be impossible to a whole gang of terrestrial
workmen. The effective powers of huge machines would be
increased in the same way; and to all this must be added the
fact that the mean density of the materials of which Mars is
composed is much less than that of the constituents of the
earth. Combining all these considerations, it becomes much less
difficult to conceive that public works might be successfully
undertaken on Mars which would be hopelessly beyond the limits
of human accomplishment.</p>
<p>Certain other difficulties have also to be met; as, for
instance, the relative coldness of the climate of Mars. At its
distance it gets considerably less than half as much light and
heat as we receive. In addition to this, the rarity of its
atmosphere would naturally be expected to decrease the
effective temperature at the planet's surface, since an
atmosphere acts somewhat like the glass cover of a hot-house in
retaining the solar heat which has penetrated it. It has been
calculated that, unless there are mitigating circumstances of
which we know nothing, the average temperature at the surface
of Mars must be far below the freezing-point of water. To this
it is replied that the possible mitigating circumstances spoken
of evidently exist in fact, because we can <em>see</em> that
the watery vapor condenses into snow around the poles in
winter, but melts again when summer comes. The mitigating agent
may be supposed to exist in the atmosphere where the presence
of certain gases would completely alter the temperature
gradients.</p>
<p>It might also be objected that it is inconceivable that the
Martian engineers, however great may be their physical powers,
and however gigantic the mechanical energies under their
control, could force water in large quantities from the poles
to the equator. This is an achievement that measures up to the
cosmical standard. It is admitted by the champions of the
theory that the difficulty is a formidable one; but they call
attention to the singular fact that on Mars there can be found
no chains of mountains, and it is even doubtful if ranges of
hills exist there. The entire surface of the planet appears to
be almost ``as smooth as a billiard ball,'' and even the broad
regions which were once supposed to be seas apparently lie at
practically the same level as the other parts, since the
``canals'' in many cases run uninterruptedly across them.
Lowell's idea is that these sombre areas may be expanses of
vegetation covering ground of a more or less marshy character,
for while the largest of them appear to be permanent, there are
some which vary coincidently with the variations of the
canals.</p>
<p>As to the kind of machinery employed to force the water from
the poles, it has been conjectured that it may have taken the
form of a gigantic system of pumps and conduits; and since the
Martians are assumed to be so far in advance of us in their
mastery of scientific principles, the hypothesis will at least
not be harmed by supposing that they have learned to harness
forces of nature whose very existence in a manageable form is
yet unrecognized on the earth. If we wish to let the
imagination loose, we may conjecture that they have conquered
the secret of those intra-atomic forces whose resistless energy
is beginning to become evident to us, but the possibility of
whose utilization remains a dream, the fulfillment of which
nobody dares to predict.</p>
<p>Such, in very brief form, is the celebrated theory of Mars
as an inhabited world. It certainly captivates the imagination,
and if we believe it to represent the facts, we cannot but
watch with the deepest sympathy this gallant struggle of an
intellectual race to preserve its planet from the effects of
advancing age and death. We may, indeed, wonder whether our own
humanity, confronted by such a calamity, could be counted on to
meet the emergency with equal stoutness of heart and
inexhaustibleness of resource. Up to the present time we
certainly have shown no capacity to confront Nature toe to toe,
and to seize her by the shoulders and turn her round when she
refuses to go our way. If we could get into wireless telephonic
communication with the Martians we might learn from their own
lips the secret of their more than ``Roman recovery.''</p>
<p><strong>The Riddle of the Asteroids</strong></p>
<p>Between the orbits of Mars and Jupiter revolves the most
remarkable system of little bodies with which we are acquainted
-- the Asteroids, or Minor Planets. Some six hundred are now
known, and they may actually number thousands. They form
virtually a ring about the sun. The most striking general fact
about them is that they occupy the place in the sky which
should be occupied, according to Bode's Law, by a single large
planet. This fact, as we shall see, has led to the invention of
one of the most extraordinary theories in astronomy --
<em>viz.,</em> that of the explosion of a world!</p>
<p>Bode's Law, so-called, is only an empiric formula, but until
the discovery of Neptune it accorded so well with the distances
of the planets that astronomers were disposed to look upon it
as really representing some underlying principle of planetary
distribution. They were puzzled by the absence of a planet in
the space between Mars and Jupiter, where the ``law'' demanded
that there should be one, and an association of astronomers was
formed to search for it. There was a decided sensation when, in
1801, Piazzi, of Palermo, announced that he had found a little
planet which apparently occupied the place in the system which
belonged to the missing body. He named it Ceres, and it was the
first of the Asteroids. The next year Olbers, of Bremen, while
looking for Ceres with his telescope, stumbled upon another
small planet which he named Pallas. Immediately he was inspired
with the idea that these two planets were fragments of a larger
one which had formerly occupied the vacant place in the
planetary ranks, and he predicted that others would be found by
searching in the neighborhood of the intersection of the orbits
of the two already discovered. This bold prediction was
brilliantly fulfilled by the finding of two more -- Juno in
1804, and Vesta in 1807. Olbers would seem to have been led to
the invention of his hypothesis of a planetary explosion by the
faith which astronomers at that time had in Bode's Law. They
appear to have thought that several planets revolving in the
gap where the ``law'' called for but one could only be
accounted for upon the theory that the original <em>one</em>
had been broken up to form the several. Gravitation demanded
that the remnants of a planet blown to pieces, no matter how
their orbits might otherwise differ, should all return at
stated periods to the point where the explosion had occurred;
hence Olbers' prediction that any asteroids that might
subsequently be discovered would be found to have a common
point of orbital intersection. And curiously enough all of the
first asteroids found practically answered to this requirement.
Olbers' theory seemed to be established.</p>
<p>After the first four, no more asteroids were found until
1845, when one was discovered; then, in 1847, three more were
added to the list; and after that searchers began to pick them
up with such rapidity that by the close of the century hundreds
were known, and it had become almost impossible to keep track
of them. The first four are by far the largest members of the
group, but their actual sizes remained unknown until less than
twenty years ago. It was long supposed that Vesta was the
largest, because it shines more brightly than any of the
others; but finally, in 1895, Barnard, with the Lick telescope,
definitely measured their diameters, and proved to everybody's
surprise that Ceres is really the chief, and Vesta only the
third in rank. His measures are as follows: Ceres, 477 miles;
Pallas, 304 miles; Vesta, 239 miles; and Juno, 120 miles. They
differ greatly in the reflective power of their surfaces, a
fact of much significance in connection with the question of
their origin. Vesta is, surface for surface, rather more than
three times as brilliant as Ceres, whence the original mistake
about its magnitude.</p>
<p>Nowadays new asteroids are found frequently by photography,
but physically they are most insignificant bodies, their
average diameter probably not exceeding twenty miles, and some
are believed not to exceed ten. On a planet only ten miles in
diameter, assuming the same mean density as the earth's, which
is undoubtedly too much, the force of gravity would be so
slight that an average man would not weigh more than three
ounces, and could jump off into space whenever he liked.</p>
<p>Although the asteroids all revolve around the sun in the
same direction as that pursued by the major planets, their
orbits are inclined at a great variety of angles to the general
plane of the planetary system, and some of them are very
eccentric -- almost as much so as the orbits of many of the
periodic comets. It has even been conjectured that the two tiny
moons of Mars and the four smaller satellites of Jupiter may be
asteroids gone astray and captured by those planets. Two of the
asteroids are exceedingly remarkable for the shapes and
positions of their orbits; these are Eros, discovered in 1898,
and T. G., 1906, found eight years later. The latter has a mean
distance from the sun slightly greater than that of Jupiter,
while the mean distance of Eros is less than that of Mars. The
orbit of Eros is so eccentric that at times it approaches
within 15,000,000 miles of the earth, nearer than any other
regular member of the solar system except the moon, thus
affording an unrivaled means of measuring the solar parallax.
But for our present purpose the chief interest of Eros lies in
its extraordinary changes of light.</p>
<p>These changes, although irregular, have been observed and
photographed many times, and there seems to be no doubt of
their reality. Their significance consists in their possible
connection with the form of the little planet, whose diameter
is generally estimated at not more than twenty miles. Von
Oppolzer found, in 1901, that Eros lost three-fourths of its
brilliancy once in every two hours and thirty-eight minutes.
Other observers have found slightly different periods of
variability, but none as long as three hours. The most
interesting interpretation that has been offered of this
phenomenon is that it is due to a great irregularity of figure,
recalling at once Olbers' hypothesis. According to some, Eros
may be double, the two bodies composing it revolving around
each other at very close quarters; but a more striking, and it
may be said probable, suggestion is that Eros has a form not
unlike that of a dumb-bell, or hour-glass, turning rapidly end
over end so that the area of illuminated surface presented to
our eyes continually changes, reaching at certain times a
minimum when the amount of light that it reflects toward the
earth is reduced to a quarter of its maximum value. Various
other bizarre shapes have been ascribed to Eros, such, for
instance, as that of a flat stone revolving about one of its
longer axes, so that sometimes we see its face and sometimes
its edge.</p>
<p>All of these explanations proceed upon the assumption that
Eros cannot have a simple globular figure like that of a
typical planet, a figure which is prescribed by the law of
gravitation, but that its shape is what may be called
accidental; in a word, it is a <em>fragment,</em> for it seems
impossible to believe that a body formed in interplanetary
space, either through nebular condensation or through the
aggregation of particles drawn together by their mutual
attractions, should not be practically spherical in shape. Nor
is Eros the only asteroid that gives evidence by variations of
brilliancy that there is something abnormal in its
constitution; several others present the same phenomenon in
varying degrees. Even Vesta was regarded by Olbers as
sufficiently variable in its light to warrant the conclusion
that it was an angular mass instead of a globe. Some of the
smaller ones show very notable variations, and all in short
periods, of three or four hours, suggesting that in turning
about one of their axes they present a surface of variable
extent toward the sun and the earth.</p>
<p>The theory which some have preferred -- that the variability
of light is due to the differences of reflective power on
different parts of the surface -- would, if accepted, be hardly
less suggestive of the origin of these little bodies by the
breaking up of a larger one, because the most natural
explanation of such differences would seem to be that they
arose from variations in the roughness or smoothness of the
reflecting surface, which would be characteristic of
fragmentary bodies. In the case of a large planet alternating
expanses of land and water, or of vegetation and desert, would
produce a notable variation in the amount of reflection, but on
bodies of the size of the asteroids neither water nor
vegetation could exist, and an atmosphere would be equally
impossible.</p>
<p>One of the strongest objections to Olbers' hypothesis is
that only a few of the first asteroids discovered travel in
orbits which measurably satisfy the requirement that they
should all intersect at the point where the explosion occurred.
To this it was at first replied that the perturbations of the
asteroidal orbits, by the attractions of the major planets,
would soon displace them in such a manner that they would cease
to intersect. One of the first investigations undertaken by the
late Prof. Simon Newcomb was directed to the solution of this
question, and he arrived at the conclusion that the planetary
perturbations could not explain the actual situation of the
asteroidal orbits. But afterward it was pointed out that the
difficulty could be avoided by supposing that not one but a
series of explosions had produced the asteroids as they now
are. After the primary disruption the fragments themselves,
according to this suggestion, may have exploded, and then the
resulting orbits would be as ``tangled'' as the heart could
wish. This has so far rehabilitated the explosion theory that
it has never been entirely abandoned, and the evidence which we
have just cited of the probably abnormal shapes of Eros and
other asteroids has lately given it renewed life. It is a
subject that needs a thorough rediscussion.</p>
<p>We must not fail to mention, however, that there is a rival
hypothesis which commends itself to many astronomers --
<em>viz.,</em> that the asteroids were formed out of a
relatively scant ring of matter, situated between Mars and
Jupiter and resembling in composition the immensely more
massive rings from which, according to Laplace's hypothesis,
the planets were born. It is held by the supporters of this
theory that the attraction of the giant Jupiter was sufficient
to prevent the small, nebulous ring that gave birth to the
asteroids from condensing like the others into a single
planet.</p>
<p>But if we accept the explosion theory, with its corollary
that minor explosions followed the principal one, we have still
an unanswered question before us: What caused the explosions?
The idea of <em>a world blowing up</em> is too Titanic to be
shocking; it rather amuses the imagination than seriously
impresses it; in a word, it seems essentially chimerical. We
can by no appeal to experience form a mental picture of such an
occurrence. Even the moon did not blow up when it was wrecked
by volcanoes. The explosive nebulæ and new stars are far
away in space, and suggest no connection with such a
catastrophe as the bursting of a planet into hundreds of
pieces. We cannot conceive of a great globe thousands of miles
in diameter resembling a pellet of gunpowder only awaiting the
touch of a match to cause its sudden disruption. Somehow the
thought of human agency obtrudes itself in connection with the
word ``explosion,'' and we smile at the idea that giant powder
or nitro-glycerine could blow up a planet. Yet it would only
need <em>enough</em> of them to do it.</p>
<p>After all, we may deceive ourselves in thinking, as we are
apt to do, that explosive energies lock themselves up only in
small masses of matter. There are many causes producing
explosions in nature, every volcanic eruption manifests the
activity of some of them. Think of the giant power of confined
steam; if enough steam could be suddenly generated in the
center of the earth by a downpour of all the waters of the
oceans, what might not the consequences be for our globe? In a
smaller globe, and it has never been estimated that the
original asteroid was even as large as the moon, such a
catastrophe would, perhaps, be more easily conceivable; but
since we are compelled in this case to assume that there was a
series of successive explosions, steam would hardly answer the
purpose; it would be more reasonable to suppose that the cause
of the explosion was some kind of chemical reaction, or
something affecting the atoms composing the exploding body.
Here Dr Gustav Le Bon comes to our aid with a most startling
suggestion, based on his theory of the dissipation of
intra-atomic energy. It will be best to quote him at some
length from his book on <em>The Evolution of Forces.</em></p>
<p>``It does not seem at first sight,'' says Doctor Le Bon,</p>
<blockquote>
very comprehensible that worlds which appear more and more
stable as they cool could become so unstable as to afterward
dissociate entirely. To explain this phenomenon, we will
inquire whether astronomical observations do not allow us to
witness this dissociation.
<p>We know that the stability of a body in motion, such as a
top or a bicycle, ceases to be possible when its velocity of
rotation descends below a certain limit. Once this limit is
reached it loses its stability and falls to the ground. Prof.
J. J. Thomson even interprets radio-activity in this manner,
and points out that when the speed of the elements composing
the atoms descends below a certain limit they become unstable
and tend to lose their equilibria. There would result from
this a commencement of dissociation, with diminution of their
potential energy and a corresponding increase of their
kinetic energy sufficient to launch into space the products
of intra-atomic disintegration.</p>
<p>It must not be forgotten that the atom being an enormous
reservoir of energy is by this very fact comparable with
explosive bodies. These last remain inert so long as their
internal equilibria are undisturbed. So soon as some cause or
other modifies these, they explode and smash everything
around them after being themselves broken to pieces.</p>
<p>Atoms, therefore, which grow old in consequence of the
diminution of a part of their intra-atomic energy gradually
lose their stability. A moment, then, arrives when this
stability is so weak that the matter disappears by a sort of
explosion more or less rapid. The bodies of the radium group
offer an image of this phenomenon -- a rather faint image,
however, because the atoms of this body have only reached a
period of instability when the dissociation is rather slow.
It probably precedes another and more rapid period of
dissociation capable of producing their final explosion.
Bodies such as radium, thorium, etc., represent, no doubt, a
state of old age at which all bodies must some day arrive,
and which they already begin to manifest in our universe,
since all matter is slightly radio-active. It would suffice
for the dissociation to be fairly general and fairly rapid
for an explosion to occur in a world where it was
manifested.</p>
<p>These theoretical considerations find a solid support in
the sudden appearances and disappearances of stars. The
explosions of a world which produce them reveal to us,
perhaps, how the universes perish when they become old.</p>
<p>As astronomical observations show the relative frequency
of these rapid destructions, we may ask ourselves whether the
end of a universe by a sudden explosion after a long period
of old age does not represent its most general ending.</p>
</blockquote>
<p>Here, perhaps, it will be well to stop, since, entrancing as
the subject may be, we know very little about it, and Doctor Le
Bon's theory affords a limitless field for the reader's
imagination.</p>
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