§ 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.
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.
