f Pisa. We
must either mend it, therefore, or understand it in some broader sense.

[Illustration: Fig. VIII.]

Sec. LXXXV. And now, if the reader will look back to the fifth paragraph
of Chap. III. Vol. I., he will find that I carefully extended my
definition of a roof so as to include more than is usually understood by
the term. It was there said to be the covering of a space, _narrow or
wide_. It does not in the least signify, with respect to the real nature
of the covering, whether the space protected be two feet wide, or ten;
though in the one case we call the protection an arch, in the other a
vault or roof. But the real point to be considered is, the manner in which
this protection stands, and not whether it is narrow or broad. We call the
vaulting of a bridge "an arch," because it is narrow with respect to the
river it crosses; but if it were built above us on the ground, we should
call it a waggon vault, because then we should feel the breadth of it.
The real question is the nature of the curve, not the extent of space
over which it is carried: and this is more the case with respect to
Gothic than to any other architecture; for, in the greater number of
instances, the form of the roof is entirely dependent on the ribs; the
domical shells being constructed in all kinds of inclinations, quite
undeterminable by the eye, and all that is definite in their character
being fixed by the curves of the ribs.

[Illustration: Fig. IX.]

Sec. LXXXVI. Let us then consider our definition as including the
narrowest arch, or tracery bar, as well as the broadest roof, and it will
be nearly a perfect one. For the fact is, that all good Gothic is nothing
more than the developement, in various ways, and on every conceivable
scale, of the group formed by the _pointed arch for the bearing line_
below, and _the gable for the protecting line_ above; and from the huge,
gray, shaly slope of the cathedral roof, with its elastic pointed vaults
beneath, to the slight crown-like points that enrich the smallest niche
of its doorway, one law and one expression will be found in all. The
modes of support and of decoration are infinitely various, but the real
character of the building, in all good Gothic, depends upon the single
lines of the gable over the pointed arch, Fig. IX., endlessly rearranged
or repeated. The larger woodcut, Fig. X., represents three
characteristic conditions of the treatment of the group: _a_, from a
tomb at Verona (