emselves beyond a certain height, but will need cross bars to steady
them. Cross bars of stone are, therefore, to be introduced at necessary
intervals, not to divide the glass, but to support the upright stone
bars. The glass is always to be divided longitudinally as far as
possible, and the upright bars which divide it supported at proper
intervals. However high the window, it is almost impossible that it
should require more than two cross bars.

Sec. VIII. It may sometimes happen that when tall windows are placed very
close to each other for the sake of more light, the masonry between them
may stand in need, or at least be the better of, some additional
support. The cross bars of the windows may then be thickened, in order
to bond the intermediate piers more strongly together, and if this
thickness appear ungainly, it may be modified by decoration.

Sec. IX. We have thus arrived at the idea of a vertical frame work of
subordinated bars, supported by cross bars at the necessary intervals,
and the only remaining question is the method of insertion into the
aperture. Whatever its form, if we merely let the ends of the bars into
the voussoirs of its heading, the least settlement of the masonry would
distort the arch, or push up some of its voussoirs, or break the window
bars, or push them aside. Evidently our object should be to connect the
window bars among themselves, so framing them together that they may
give the utmost possible degree of support to the whole window head in
case of any settlement. But we know how to do this already: our window
bars are nothing but small shafts. Capital them; throw small arches
across between the smaller bars, large arches over them between the
larger bars, one comprehensive arch over the whole, or else a horizontal
lintel, if the window have a flat head; and we have a complete system of
mutual support, independent of the aperture head, and yet assisting to
sustain it, if need be. But we want the spandrils of this arch system to
be themselves as light, and to let as much light through them, as
possible: and we know already how to pierce them (Chap. XII. Sec. VII.).
We pierce them with circles; and we have, if the circles are small and the
stonework strong, the traceries of Giotto and the Pisan school; if the
circles are as large as possible and the bars slender, those which I
have already figured and described as the only perfect traceries of the
Northern Gothic.[58] The varieties of t