CONSTRUCTION XI. THE ARCH MASONRY 165

§ 2. 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.

§ 3. What we have to say will apply to all arches, but the central pointed arch is the best for general illustration. Let a, Plate 3, 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

[Version 0.04: March 2008]