above it, or it may be at the side of the building, and
many other circumstances may occur to hinder us.

Sec. 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?

Sec. 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.

[Illustration: Plate 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.

Sec. 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.

Sec. 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