CONSTRUCTION XI. THE ARCH MASONRY 171

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 ęsthetic 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 3), 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.

§ 17. The reader will observe that I call the arch midcusped, not when the cusp 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.

I have preferred to call the arch mid-cusped with respect to its side piece than with respect to its own curve, because the most beautiful Gothic arches in the world, those of the Lombard Gothic, have, in all the instances I have examined, a form more or less approximating to this mid-cusped one at i (Plate 3), but having the curvature of the cusp carried up into the keystone, as we shall see presently: where, however, the arch is built of many voussoirs, a mid-cusped arch will mean one which has the point of the cusp midway between its own base and apex.

The Gothic arch of Venice is almost invariably up-cusped, as at k. The reader may note that, in both down-cusped and up-cusped arches, the piece of stone, added to form the cusp, is of the shape of a scymitar, held down in the one case and up in the other.

[Version 0.04: March 2008]