There were various conveniences in this way of working, one of the principal being that the side pieces with their cusps were always cut to their complete form, and that no part of the cusp was carried out into the keystone, which followed the curve of the outer arch itself. The ornaments of the cusp might thus be worked without any troublesome reference to the rest of the arch.
§ XIX. Now let us take a pair of side pieces, made to order, like that at l, and see what we can make of them. We will try to fit them first with a keystone which continues the curve of the outer arch, as at m. This the reader assuredly thinks an ugly arch. There are a great many of them in Venice, the ugliest things there, and the Venetian builders quickly began to feel them so. What could they do to better them? The arch at m has a central piece of the form r. Substitute for it a piece of the form s, and we have the arch at n.
§ XX. This arch at n is not so strong as that at m; but, built of good marble, and with its pieces of proper thickness, it is quite strong enough for all practical purposes on a small scale. I have examined at least two thousand windows of this kind and of the other Venetian ogees, of which that at y (in which the plain side-piece d is used instead of the cusped one) is the simplest; and I never found one, even in the most ruinous palaces (in which they had had to sustain the distorted weight of falling walls) in which the central piece was fissured; and this is the only danger to which the window is exposed; in other respects it is as strong an arch as can be built.
It is not to be supposed that the change from the r keystone to the s keystone was instantaneous. It was a change wrought out by many curious experiments, which we shall have to trace hereafter, and to throw the resultant varieties of form into their proper groups.
§ XXI. One step more: I take a mid-cusped side piece in its block form at t, with the bricks which load the back of it. Now, as these bricks support it behind, and since, as far as the use of the cusp is concerned, it matters not whether its weight be in marble or bricks, there is nothing to hinder us from cutting out some of the marble, as at u, and filling up the space with bricks. (Why we should take a fancy to do this, I do not pretend to guess at present; all I have to assert is, that, if the fancy should strike us, there would be no harm in it). Substituting this side piece for the other in the window n, we have that at w, which may, perhaps, be of some service to us afterwards; here we have nothing more to do with it than to note that, thus built, and properly backed by brickwork, it is just as strong and safe a form as that at n; but that this, as well as every variety of ogee arch, depends entirely for its safety, fitness, and beauty, on the masonry which we have just analysed; and that, built on a large scale, and with many voussoirs, all such arches would be unsafe and absurd in general architecture. Yet they may be used occasionally for the sake of the exquisite beauty of which their rich and fantastic varieties admit, and sometimes for the sake of another merit, exactly the opposite of the constructional ones we are at present examining, that they seem to stand by enchantment.
| V. |
| Arch Masonry. BRULETTO OF COMO. |
§ XXII. In the above reasonings, the inclination of the joints of the voussoirs to the curves of the arch has not been considered. It is a question of much nicety, and which I have not been able as yet fully to investigate: but the natural idea of the arrangement of these lines (which in round arches are of course perpendicular to the curve) would be that every voussoir should have the lengths of its outer and inner arched surface in the same proportion to each other. Either this actual law, or a close approximation to it, is assuredly enforced in the best Gothic buildings.
§ XXIII. I may sum up all that it is necessary for the reader to keep in mind of the general laws connected with this subject, by giving him an example of each of the two forms of the perfect Gothic arch, uncusped and cusped, treated with the most simple and magnificent masonry, and partly, in both cases, Mont-Cenisian.
The first, [Plate V.], is a window from the Broletto of Como. It shows, in its filling, first, the single-pieced arch, carried on groups of four shafts, and a single slab of marble filling the space above, and pierced with a quatrefoil (Mont-Cenisian, this), while the mouldings above are each constructed with a separate system of voussoirs, all of them shaped, I think, on the principle above stated, § XXII., in alternate serpentine and marble; the outer arch being a noble example of the pure uncusped Gothic construction, b of [Plate III.]
| Fig. XXXIV. |