constant member of cornices, corresponds to the under stone
_c_, in Fig. II., which is a constant member of bases. The reader has no
idea at present of the enormous importance of these members; but as we
shall have to refer to them perpetually, I must ask him to compare them,
and fix their relations well in his mind: and, for convenience, I shall
call the bevelled or sloping stone, X, and the upright edged stone, Y.
The reader may remember easily which is which; for X is an intersection
of two slopes, and may therefore properly mean either of the two sloping
stones; and Y is a figure with a perpendicular line and two slopes, and
may therefore fitly stand for the upright stone in relation to each of
the sloping ones; and as we shall have to say much more about cornices
than about bases, let X and Y stand for the stones of the cornice, and
Xb and Yb for those of the base, when distinction is needed.

[Illustration: Fig. V.]

Sec. IV. Now the form at _d_, Fig. IV., is the great root and primal type
of all cornices whatsoever. In order to see what forms may be developed
from it, let us take its profile a little larger--_a_, Fig. V., with X
and Y duly marked. Now this form, being the root of all cornices, may
either have to finish the wall and so keep off rain; or, as so often
stated, to carry weight. If the former, it is evident that, in its
present profile, the rain will run back down the slope of X; and if the
latter, that the sharp angle or edge of X, at _k_, may be a little too
weak for its work, and run a chance of giving way. To avoid the evil in
the first case, suppose we hollow the slope of X inwards, as at _b_; and
to avoid it in the second case, suppose we strengthen X by letting it
bulge outwards, as at c.

Sec. V. These (_b_ and _c_) are the profiles of two vast families of
cornices, springing from the same root, which, with a third arising
from their combination (owing its origin to aesthetic considerations, and
inclining sometimes to the one, sometimes to the other), have been
employed, each on its third part of the architecture of the whole world
throughout all ages, and must continue to be so employed through such
time as is yet to come. We do not at present speak of the third or
combined group; but the relation of the two main branches to each other,
and to the line of origin, is given at _e_, Fig. V.; where the dotted
lines are the representatives of the two families, and the straight line
of the root.