§ 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.