24 6
Width of nave 32 8

The shafts average 5 feet 4 inches in circumference, as near the base as
they can be got at, being covered with wood; and the broadest sides of
the main piers are 4 feet 7 inches wide, their narrowest sides 3 feet 6
inches. The distance _a c_ from the outmost angle of these piers to the
beginning of the curve of the apse is 25 feet, and from that point the
apse is nearly semicircular, but it is so encumbered with renaissance
fittings that its form cannot be ascertained with perfect accuracy. It
is roofed by a concha, or semi-dome; and the external arrangement of its
walls provides for the security of this dome by what is, in fact, a
system of buttresses as effective and definite as that of any of the
northern churches, although the buttresses are obtained entirely by
adaptations of the Roman shaft and arch, the lower story being formed by
a thick mass of wall lightened by ordinary semicircular round-headed
niches, like those used so extensively afterwards in renaissance
architecture, each niche flanked by a pair of shafts standing clear of
the wall, and bearing deeply moulded arches thrown over the niche. The
wall with its pillars thus forms a series of massy buttresses (as seen
in the ground plan), on the top of which is an open gallery, backed by a
thinner wall, and roofed by arches whose shafts are set above the pairs
of shafts below. On the heads of these arches rests the roof. We have,
therefore, externally a heptagonal apse, chiefly of rough and common
brick, only with marble shafts and a few marble ornaments; but for that
very reason all the more interesting, because it shows us what may be
done, and what was done, with materials such as are now at our own
command; and because in its proportions, and in the use of the few
ornaments it possesses, it displays a delicacy of feeling rendered
doubly notable by the roughness of the work in which laws so subtle are
observed, and with which so thoughtful ornamentation is associated.

Sec. XV. First, for its proportions: I shall have occasion in Chapter V.
to dwell at some length on the peculiar subtlety of the early Venetian
perception for ratios of magnitude; the relations of the sides of this
heptagonal apse supply one of the first and most curious instances of
it. The proportions above given of the nave and aisles might have been
dictated by a mere love of mathematical precision; but those of the apse
could