which perhaps goes far to
account for the never-failing delightfulness of this zigzag decoration.
The diminution of the zigzag, as it gradually shares the defeat of the
voussoir, and is at last overwhelmed by the complicated, railroad-like
fluency of the later Gothic mouldings, is to me one of the saddest
sights in the drama of architecture.

Sec. XV. One farther circumstance is deserving of especial note in Plate
V., the greater depth of the voussoirs at the top of the arch. This has
been above alluded to as a feature of good construction, Chap. XI., Sec.
III.; it is to be noted now as one still more valuable in decoration:
for when we arrive at the deep succession of concentric archivolts, with
which northern portals, and many of the associated windows, are headed,
we immediately find a difficulty in reconciling the outer curve with the
inner. If, as is sometimes the case, the width of the group of
archivolts be twice or three times that of the inner aperture, the inner
arch may be distinctly pointed, and the outer one, if drawn with
concentric arcs, approximate very nearly to a round arch. This is
actually the case in the later Gothic of Verona; the outer line of the
archivolt having a hardly perceptible point, and every inner arch of
course forming the point more distinctly, till the innermost becomes a
lancet. By far the nobler method, however, is that of the pure early
Italian Gothic; to make every outer arch a _magnified fac-simile_ of the
innermost one, every arc including the same number of degrees, but
degrees of a larger circle. The result is the condition represented in
Plate V., often found in far bolder development; exquisitely springy and
elastic in its expression, and entirely free from the heaviness and
monotony of the deep northern archivolts.

Sec. XVI. We have not spoken of the intermediate form, _b_, of Fig. LXIX.
(which its convenience for admission of light has rendered common in
nearly all architectures), because it has no transitions peculiar to
itself: in the north it sometimes shares the fate of the outer
architrave, and is channelled into longitudinal mouldings; sometimes
remains smooth and massy, as in military architecture, or in the simpler
forms of domestic and ecclesiastical. In Italy it receives surface
decoration like the architrave, but has, perhaps, something of peculiar
expression in being placed between the tracery of the window within, and
its shafts and tabernacle work without,