I. We have seen in the last section how our means of vertical support may, for the sake of economy both of space and material, be gathered into piers or shafts, and directed to the sustaining of particular points. The next question is how to connect these points or tops of shafts with each other, so as to be able to lay on them a continuous roof. This the reader, as before, is to favor me by finding out for himself, under these following conditions.
Let s, s, Fig. XXIX. opposite, be two shafts, with their capitals ready prepared for their work; and a, b, b, and c, c, c, be six stones of different sizes, one very long and large, and two smaller, and three smaller still, of which the reader is to choose which he likes best, in order to connect the tops of the shafts.
I suppose he will first try if he can lift the great stone a, and if he can, he will put it very simply on the tops of the two pillars, as at A.
Very well indeed: he has done already what a number of Greek architects have been thought very clever for having done. But suppose he cannot lift the great stone a, or suppose I will not give it to him, but only the two smaller stones at b, b; he will doubtless try to put them up, tilted against each other, as at d. Very awkward this; worse than card-house building. But if he cuts off the corners of the stones, so as to make each of them of the form e, they will stand up very securely, as at B.
But suppose he cannot lift even these less stones, but can 123 raise those at c, c, c. Then, cutting each of them into the form at e, he will doubtless set them up as at f.
II. This last arrangement looks a little dangerous. Is there not a chance of the stone in the middle pushing the others out, or tilting them up and aside, and slipping down itself between them? There is such a chance: and if by somewhat altering the form of the stones, we can diminish this chance, all the better. I must say “we” now, for perhaps I may have to help the reader a little.
The danger is, observe, that the midmost stone at f pushes out the side ones: then if we can give the side ones such a shape as that, left to themselves, they would fall heavily forward, they will resist this push out by their weight, exactly in proportion to their own particular inclination or desire to tumble in. Take one of them separately, standing up as at g; it is just possible it may stand up as it is, like the Tower of Pisa: but we want it to fall forward. Suppose we cut away the parts that are shaded at h and leave it as at i, it is very certain it cannot stand alone now, but will fall forward to our entire satisfaction.
Farther: the midmost stone at f is likely to be troublesome chiefly by its weight, pushing down between the others; the more we lighten it the better: so we will cut it into exactly the same shape as the side ones, chiselling away the shaded parts, as at h. We shall then have all the three stones k, l, m, of the same shape; and now putting them together, we have, at C, what the reader, I doubt not, will perceive at once to be a much more satisfactory arrangement than that at f.
III. We have now got three arrangements; in one using only one piece of stone, in the second two, and in the third three. The first arrangement has no particular name, except the “horizontal:” but the single stone (or beam, it may be,) is called a lintel; the second arrangement is called a “Gable;” the third an “Arch.”
We might have used pieces of wood instead of stone in all these arrangements, with no difference in plan, so long as the beams were kept loose, like the stones; but as beams can be securely nailed together at the ends, we need not trouble ourselves so much about their shape or balance, and therefore the plan at f is a peculiarly wooden construction (the reader will doubtless recognise in it the profile of many a farm-house roof): and again, because beams are tough, and light, and long, as compared with stones, they are admirably adapted for the constructions at A and B, the plain lintel and gable, while that at C is, for the most part, left to brick and stone.
IV. But farther. The constructions, A, B, and C, though 125 very conveniently to be first considered as composed of one, two, and three pieces, are by no means necessarily so. When we have once cut the stones of the arch into a shape like that of k, l, and m, they will hold together, whatever their number, place, or size, as at n; and the great value of the arch is, that it permits small stones to be used with safety instead of large ones, which are not always to be had. Stones cut into the shape of k, l, and m, whether they be short or long (I have drawn them all sizes at n on purpose), are called Voussoirs; this is a hard, ugly French name; but the reader will perhaps be kind enough to recollect it; it will save us both some trouble: and to make amends for this infliction, I will relieve him of the term keystone. One voussoir is as much a keystone as another; only people usually call the stone which is last put in the keystone; and that one happens generally to be at the top or middle of the arch.
V. Not only the arch, but even the lintel, may be built of many stones or bricks. The reader may see lintels built in this way over most of the windows of our brick London houses, and so also the gable: there are, therefore, two distinct questions respecting each arrangement;—First, what is the line or direction of it, which gives it its strength? and, secondly, what is the manner of masonry of it, which gives it its consistence? The first of these I shall consider in this Chapter under the head of the Arch Line, using the term arch as including all manner of construction (though we shall have no trouble except about curves); and in the next Chapter I shall consider the second, under the head, Arch Masonry.
VI. Now the arch line is the ghost or skeleton of the arch; or rather it is the spinal marrow of the arch, and the voussoirs are the vertebr�, which keep it safe and sound, and clothe it. This arch line the architect has first to conceive and shape in his mind, as opposed to, or having to bear, certain forces which will try to distort it this way and that; and against which he is first to direct and bend the line itself into as strong resistance as he may, and then, with his voussoirs and what else he can, to guard it, and help it, and keep it to its duty and in 126 its shape. So the arch line is the moral character of the arch, and the adverse forces are its temptations; and the voussoirs, and what else we may help it with, are its armor and its motives to good conduct.
VII. This moral character of the arch is called by architects its “Line of Resistance.” There is a great deal of nicety in calculating it with precision, just as there is sometimes in finding out very precisely what is a man’s true line of moral conduct; but this, in arch morality and in man morality, is a very simple and easily to be understood principle,—that if either arch or man expose themselves to their special temptations or adverse forces, outside of the voussoirs or proper and appointed armor, both will fall. An arch whose line of resistance is in the middle of its voussoirs is perfectly safe: in proportion as the said line runs near the edge of its voussoirs, the arch is in danger, as the man is who nears temptation; and the moment the line of resistance emerges out of the voussoirs the arch falls.
VIII. There are, therefore, properly speaking, two arch lines. One is the visible direction or curve of the arch, which may generally be considered as the under edge of its voussoirs, and which has often no more to do with the real stability of the arch, than a man’s apparent conduct has with his heart. The other line, which is the line of resistance, or line of good behavior, may or may not be consistent with the outward and apparent curves of the arch; but if not, then the security of the arch depends simply upon this, whether the voussoirs which assume or pretend to the one line are wide enough to include the other.
IX. Now when the reader is told that the line of resistance varies with every change either in place or quantity of the weight above the arch, he will see at once that we have no chance of arranging arches by their moral characters: we can only take the apparent arch line, or visible direction, as a ground of arrangement. We shall consider the possible or probable forms or contours of arches in the present Chapter, and in the succeeding one the forms of voussoir and other help 127 which may best fortify these visible lines against every temptation to lose their consistency.
X. Look back to Fig. XXIX. Evidently the abstract or ghost line of the arrangement at A is a plain horizontal line, as here at a, Fig. XXX. The abstract line of the arrangement at B, Fig. XXIX., is composed of two straight lines, set against each other, as here at b. The abstract line of C, Fig. XXIX., is a curve of some kind, not at present determined, suppose c, Fig. XXX. Then, as b is two of the straight lines at a, set up against each other, we may conceive an arrangement, d, made up of two of the curved lines at c, set against each other. This is called a pointed arch, which is a contradiction in terms: it ought to be called a curved gable; but it must keep the name it has got.
Now a, b, c, d, Fig. XXX., are the ghosts of the lintel, the gable, the arch, and the pointed arch. With the poor lintel ghost we need trouble ourselves no farther; there are no changes in him: but there is much variety in the other three, and the method of their variety will be best discerned by studying b and d, as subordinate to and connected with the simple arch at c.
XI. Many architects, especially the worst, have been very curious in designing out of the way arches,—elliptical arches, and four-centred arches, so called, and other singularities. The good architects have generally been content, and we for the present will be so, with God’s arch, the arch of the rainbow and of the apparent heaven, and which the sun shapes for us as it sets and rises. Let us watch the sun for a moment as it climbs: when it is a quarter up, it will give us the arch a, Fig. XXXI.; when it is half up, b, and when three quarters 128 up, c. There will be an infinite number of arches between these, but we will take these as sufficient representatives of all. Then a is the low arch, b the central or pure arch, c the high arch, and the rays of the sun would have drawn for us their voussoirs.
XII. We will take these several arches successively, and fixing the top of each accurately, draw two right lines thence to its base, d, e, f, Fig. XXXI. Then these lines give us the relative gables of each of the arches; d is the Italian or southern gable, e the central gable, f the Gothic gable.
XIII. We will again take the three arches with their gables in succession, and on each of the sides of the gable, between it and the arch, we will describe another arch, as at g, h, i. Then the curves so described give the pointed arches belonging to each of the round arches; g, the flat pointed arch, h, the central pointed arch, and i, the lancet pointed arch.
XIV. If the radius with which these intermediate curves are drawn be the base of f, the last is the equilateral pointed 129 arch, one of great importance in Gothic work. But between the gable and circle, in all the three figures, there are an infinite number of pointed arches, describable with different radii; and the three round arches, be it remembered, are themselves representatives of an infinite number, passing from the flattest conceivable curve, through the semicircle and horseshoe, up to the full circle.
The central and the last group are the most important. The central round, or semicircle, is the Roman, the Byzantine, and Norman arch; and its relative pointed includes one wide branch of Gothic. The horseshoe round is the Arabic and Moorish arch, and its relative pointed includes the whole range of Arabic and lancet, or Early English and French Gothics. I mean of course by the relative pointed, the entire group of which the equilateral arch is the representative. Between it and the outer horseshoe, as this latter rises higher, the reader will find, on experiment, the great families of what may be called the horseshoe pointed,—curves of the highest importance, but which are all included, with English lancet, under the term, relative pointed of the horseshoe arch.
XV. The groups above described are all formed of circular arcs, and include all truly useful and beautiful arches for ordinary work. I believe that singular and complicated curves are made use of in modern engineering, but with these the general reader can have no concern: the Ponte della Trinita at Florence is the most graceful instance I know of such structure; the arch made use of being very subtle, and approximating to the low ellipse; for which, in common work, a barbarous pointed arch, called four-centred, and composed of bits of circles, is substituted by the English builders. The high ellipse, I believe, exists in eastern architecture. I have never myself met with it on a large scale; but it occurs in the niches of the later portions of the Ducal palace at Venice, together with a singular hyperbolic arch, a in Fig. XXXIII., to be described hereafter: with such caprices we are not here concerned.
XVI. We are, however, concerned to notice the absurdity 130 of another form of arch, which, with the four-centred, belongs to the English perpendicular Gothic.
Taking the gable of any of the groups in Fig. XXXI. (suppose the equilateral), here at b, in Fig. XXXIII., the dotted line representing the relative pointed arch, we may evidently conceive an arch formed by reversed curves on the inside of the gable, as here shown by the inner curved lines. I imagine the reader by this time knows enough of the nature of arches to understand that, whatever strength or stability was gained by the curve on the outside of the gable, exactly so much is lost by curves on the inside. The natural tendency of such an arch to dissolution by its own mere weight renders it a feature of detestable ugliness, wherever it occurs on a large scale. It is eminently characteristic of Tudor work, and it is the profile of the Chinese roof (I say on a large scale, because this as well as all other capricious arches, may be made secure by their masonry when small, but not otherwise). Some allowable modifications of it will be noticed in the chapter on Roofs.
XVII. There is only one more form of arch which we have to notice. When the last described arch is used, not as the principal arrangement, but as a mere heading to a common pointed arch, we have the form c, Fig. XXXIII. Now this is better than the entirely reversed arch for two reasons; first, less of the line is weakened by reversing; secondly, the double curve has a very high �sthetic value, not existing in the mere segments of circles. For these reasons arches of this kind are not only admissible, but even of great desirableness, when their scale and masonry render them secure, but above a certain scale they are altogether barbarous; and, with the reversed 131 Tudor arch, wantonly employed, are the characteristics of the worst and meanest schools of architecture, past or present.
This double curve is called the Ogee; it is the profile of many German leaden roofs, of many Turkish domes (there more excusable, because associated and in sympathy with exquisitely managed arches of the same line in the walls below), of Tudor turrets, as in Henry the Seventh’s Chapel, and it is at the bottom or top of sundry other blunders all over the world.
XVIII. The varieties of the ogee curve are infinite, as the reversed portion of it may be engrafted on every other form of arch, horseshoe, round, or pointed. Whatever is generally worthy of note in these varieties, and in other arches of caprice, we shall best discover by examining their masonry; for it is by their good masonry only that they are rendered either stable or beautiful. To this question, then, let us address ourselves.