The Stones of Venice, Volume 1 (of 3)

§ 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 which may best fortify these visible lines against every temptation to lose their consistency.

Fig. XXX.

§ 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 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.

Fig. XXXI.

§ 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 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.

Fig. XXXII.

§ 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 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.

Fig. XXXIII.

§ 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 æsthetic 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 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.

CHAPTER XI.

THE ARCH MASONRY

§ I. On the subject of the stability of arches, volumes have been written and volumes more are required. The reader will not, therefore, expect from me any very complete explanation of its conditions within the limits of a single chapter. But that which is necessary for him to know is very simple and very easy; and yet, I believe, some part of it is very little known, or noticed.

We must first have a clear idea of what is meant by an arch. It is a curved shell of firm materials, on whose back a burden is to be laid of loose materials. So far as the materials above it are not loose, but themselves hold together, the opening below is not an arch, but an excavation. Note this difference very carefully. If the King of Sardinia tunnels through the Mont Cenis, as he proposes, he will not require to build a brick arch under his tunnel to carry the weight of the Mont Cenis: that would need scientific masonry indeed. The Mont Cenis will carry itself, by its own cohesion, and a succession of invisible granite arches, rather larger than the tunnel. But when Mr. Brunel tunnelled the Thames bottom, he needed to build a brick arch to carry the six or seven feet of mud and the weight of water above. That is a type of all arches proper.

§ II. Now arches, in practice, partake of the nature of the two. So far as their masonry above is Mont-Cenisian, that is to say, colossal in comparison of them, and granitic, so that the arch is a mere hole in the rock substance of it, the form of the arch is of no consequence whatever: it may be rounded, or lozenged, or ogee’d, or anything else; and in the noblest architecture there is always some character of this kind given to the masonry. It is independent enough not to care about the holes cut in it, and does not subside into them like sand. But the theory of arches does not presume on any such condition of things; it allows itself only the shell of the arch proper; the vertebræ, carrying their marrow of resistance; and, above this shell, it assumes the wall to be in a state of flux, bearing down on the arch, like water or sand, with its whole weight. And farther, the problem which is to be solved by the arch builder is not merely to carry this weight, but to carry it with the least thickness of shell. It is easy to carry it by continually thickening your voussoirs: if you have six feet depth of sand or gravel to carry, and you choose to employ granite voussoirs six feet thick, no question but your arch is safe enough. But it is perhaps somewhat too costly: the thing to be done is to carry the sand or gravel with brick voussoirs, six inches thick, or, at any rate, with the least thickness of voussoir which will be safe; and to do this requires peculiar arrangement of the lines of the arch. There are many arrangements, useful all in their way, but we have only to do, in the best architecture, with the simplest and most easily understood. We have first to note those which regard the actual shell of the arch, and then we shall give a few examples of the superseding of such expedients by Mont-Cenisian masonry.

§ III. What we have to say will apply to all arches, but the central pointed arch is the best for general illustration. Let a, Plate III., be the shell of a pointed arch with loose loading above; and suppose you find that shell not quite thick enough; and that the weight bears too heavily on the top of the arch, and is likely to break it in: you proceed to thicken your shell, but need you thicken it all equally? Not so; you would only waste your good voussoirs. If you have any common sense you will thicken it at the top, where a Mylodon’s skull is thickened for the same purpose (and some human skulls, I fancy), as at b. The pebbles and gravel above will now shoot off it right and left, as the bullets do off a cuirassier’s breastplate, and will have no chance of beating it in.

If still it be not strong enough, a farther addition may be made, as at c, now thickening the voussoirs a little at the base also. But as this may perhaps throw the arch inconveniently high, or occasion a waste of voussoirs at the top, we may employ another expedient.

§ IV. I imagine the reader’s common sense, if not his previous knowledge, will enable him to understand that if the arch at a, Plate III., burst in at the top, it must burst out at the sides. Set up two pieces of pasteboard, edge to edge, and press them down with your hand, and you will see them bend out at the sides. Therefore, if you can keep the arch from starting out at the points p, p, it cannot curve in at the top, put what weight on it you will, unless by sheer crushing of the stones to fragments.

§ V. Now you may keep the arch from starting out at p by loading it at p, putting more weight upon it and against it at that point; and this, in practice, is the way it is usually done. But we assume at present that the weight above is sand or water, quite unmanageable, not to be directed to the points we choose; and in practice, it may sometimes happen that we cannot put weight upon the arch at p. We may perhaps want an opening above it, or it may be at the side of the building, and many other circumstances may occur to hinder us.

§ VI. But if we are not sure that we can put weight above it, we are perfectly sure that we can hang weight under it. You may always thicken your shell inside, and put the weight upon it as at x x, in d, Plate III. Not much chance of its bursting out at p, now, is there?

§ VII. Whenever, therefore, an arch has to bear vertical pressure, it will bear it better when its shell is shaped as at b or d, than as at a: b and d are, therefore, the types of arches built to resist vertical pressure, all over the world, and from the beginning of architecture to its end. None others can be compared with them: all are imperfect except these.

III.

ARCH MASONRY.

The added projections at x x, in d, are called Cusps, and they are the very soul and life of the best northern Gothic; yet never thoroughly understood nor found in perfection, except in Italy, the northern builders working often, even in the best times, with the vulgar form at a.

The form at b is rarely found in the north: its perfection is in the Lombardic Gothic; and branches of it, good and bad according to their use, occur in Saracenic work.

§ VIII. The true and perfect cusp is single only. But it was probably invented (by the Arabs?) not as a constructive, but a decorative feature, in pure fantasy; and in early northern work it is only the application to the arch of the foliation, so called, of penetrated spaces in stone surfaces, already enough explained in the “Seven Lamps,” Chap. III., p. 85 et seq. It is degraded in dignity, and loses its usefulness, exactly in proportion to its multiplication on the arch. In later architecture, especially English Tudor, it is sunk into dotage, and becomes a simple excrescence, a bit of stone pinched up out of the arch, as a cook pinches the paste at the edge of a pie.

§ IX. The depth and place of the cusp, that is to say, its exact application to the shoulder of the curve of the arch, varies with the direction of the weight to be sustained. I have spent more than a month, and that in hard work too, in merely trying to get the forms of cusps into perfect order: whereby the reader may guess that I have not space to go into the subject now; but I shall hereafter give a few of the leading and most perfect examples, with their measures and masonry.

§ X. The reader now understands all that he need about the shell of the arch, considered as an united piece of stone.

He has next to consider the shape of the voussoirs. This, as much as is required, he will be able best to comprehend by a few examples; by which I shall be able also to illustrate, or rather which will force me to illustrate, some of the methods of Mont-Cenisian masonry, which were to be the second part of our subject.

§ XI. 1 and 2, Plate IV., are two cornices; 1 from St. Antonio, Padua; 2, from the Cathedral of Sens. I want them for cornices; but I have put them in this plate because, though their arches are filled up behind, and are in fact mere blocks of stone with arches cut into their faces, they illustrate the constant masonry of small arches, both in Italian and Northern Romanesque, but especially Italian, each arch being cut out of its own proper block of stone: this is Mont-Cenisian enough, on a small scale.

3 is a window from Carnarvon Castle, and very primitive and interesting in manner,—one of its arches being of one stone, the other of two. And here we have an instance of a form of arch which would be barbarous enough on a large scale, and of many pieces; but quaint and agreeable thus massively built.

4 is from a little belfry in a Swiss village above Vevay; one fancies the window of an absurd form, seen in the distance, but one is pleased with it on seeing its masonry. It could hardly be stronger.

§ XII. These then are arches cut of one block. The next step is to form them of two pieces, set together at the head of the arch. 6, from the Eremitani, Padua, is very quaint and primitive in manner: it is a curious church altogether, and has some strange traceries cut out of single blocks. One is given in the “Seven Lamps,” Plate VII., in the left-hand corner at the bottom.

7, from the Frari, Venice, very firm and fine, and admirably decorated, as we shall see hereafter. 5, the simple two-pieced construction, wrought with the most exquisite proportion and precision of workmanship, as is everything else in the glorious church to which it belongs, San Fermo of Verona. The addition of the top piece, which completes the circle, does not affect the plan of the beautiful arches, with their simple and perfect cusps; but it is highly curious, and serves to show how the idea of the cusp rose out of mere foliation. The whole of the architecture of this church may be characterised as exhibiting the maxima of simplicity in construction, and perfection in workmanship,—a rare unison: for, in general, simple designs are rudely worked, and as the builder perfects his execution, he complicates his plan. Nearly all the arches of San Fermo are two-pieced.

IV.

ARCH MASONRY.

§ XIII. We have seen the construction with one and two pieces: a and b, Fig. 8, Plate IV., are the general types of the construction with three pieces, uncusped and cusped; c and d with five pieces, uncusped and cusped. Of these the three-pieced construction is of enormous importance, and must detain us some time. The five-pieced is the three-pieced with a joint added on each side, and is also of great importance. The four-pieced, which is the two-pieced with added joints, rarely occurs, and need not detain us.

§ XIV. It will be remembered that in first working out the principle of the arch, we composed the arch of three pieces. Three is the smallest number which can exhibit the real principle of arch masonry, and it may be considered as representative of all arches built on that principle; the one and two-pieced arches being microscopic Mont-Cenisian, mere caves in blocks of stone, or gaps between two rocks leaning together.

But the three-pieced arch is properly representative of all; and the larger and more complicated constructions are merely produced by keeping the central piece for what is called a keystone, and putting additional joints at the sides. Now so long as an arch is pure circular or pointed, it does not matter how many joints or voussoirs you have, nor where the joints are; nay, you may joint your keystone itself, and make it two-pieced. But if the arch be of any bizarre form, especially ogee, the joints must be in particular places, and the masonry simple, or it will not be thoroughly good and secure; and the fine schools of the ogee arch have only arisen in countries where it was the custom to build arches of few pieces.

§ XV. The typical pure pointed arch of Venice is a five-pieced arch, with its stones in three orders of magnitude, the longest being the lowest, as at b2, Plate III. If the arch be very large, a fourth order of magnitude is added, as at a2. The portals of the palaces of Venice have one or other of these masonries, almost without exception. Now, as one piece is added to make a larger door, one piece is taken away to make a smaller one, or a window, and the masonry type of the Venetian Gothic window is consequently three-pieced, c2.

§ XVI. The reader knows already where a cusp is useful. It is wanted, he will remember, to give weight to those side stones, and draw them inwards against the thrust of the top stone. Take one of the side stones of c2 out for a moment, as at d. Now the proper place of the cusp upon it varies with the weight which it bears or requires; but in practice this nicety is rarely observed; the place of the cusp is almost always determined by æsthetic considerations, and it is evident that the variations in its place may be infinite. Consider the cusp as a wave passing up the side stone from its bottom to its top; then you will have the succession of forms from e to g (Plate III.), with infinite degrees of transition from each to each; but of which you may take e, f, and g, as representing three great families of cusped arches. Use e for your side stones, and you have an arch as that at h below, which may be called a down-cusped arch. Use f for the side stone, and you have i, which may be called a mid-cusped arch. Use g, and you have k, an up-cusped arch.

§ XVII. The reader will observe that I call the arch mid-cusped, not when the cusped point is in the middle of the curve of the arch, but when it is in the middle of the side piece, and also that where the side pieces join the keystone there will be a change, perhaps somewhat abrupt, in the curvature.