heir design arise partly from the
different size of window and consequent number of bars; partly from the
different heights of their pointed arches, as well as the various
positions of the window head in relation to the roof, rendering one or
another arrangement better for dividing the light, and partly from
aesthetic and expressional requirements, which, within certain limits,
may be allowed a very important influence: for the strength of the bars
is ordinarily so much greater than is absolutely necessary, that some
portion of it may be gracefully sacrificed to the attainment of variety
in the plans of tracery--a variety which, even within its severest
limits, is perfectly endless; more especially in the pointed arch, the
proportion of the tracery being in the round arch necessarily more
fixed.

Sec. X. The circular window furnishes an exception to the common law, that
the bars shall be vertical through the greater part of their length: for
if they were so, they could neither have secure perpendicular footing,
nor secure heading, their thrust being perpendicular to the curve of the
voussoirs only in the centre of the window; therefore, a small circle,
like the axle of a wheel, is put into the centre of the window, large
enough to give footing to the necessary number of radiating bars; and
the bars are arranged as spokes, being all of course properly capitaled
and arch-headed. This is the best form of tracery for circular windows,
naturally enough called wheel windows when so filled.

Sec. XI. Now, I wish the reader especially to observe that we have arrived
at these forms of perfect Gothic tracery without the smallest reference
to any practice of any school, or to any law of authority whatever. They
are forms having essentially nothing whatever to do either with Goths or
Greeks. They are eternal forms, based on laws of gravity and cohesion;
and no better, nor any others so good, will ever be invented, so long as
the present laws of gravity and cohesion subsist.

Sec. XII. It does not at all follow that this group of forms owes its
origin to any such course of reasoning as that which has now led us to
it. On the contrary, there is not the smallest doubt that tracery began,
partly, in the grouping of windows together (subsequently enclosed
within a large arch[59]), and partly in the fantastic penetrations of a
single slab of stones under the arch, as the circle in Plate V. above.
The perfect form seems to have been acc