s gradually downward, and this was done by
successive slopes or "set-offs," as they are termed, which assisted
(whether intentionally or not in the first instance) in further aiding
the correct architectural expression of the buttress. Then the
vaulting of the center aisle was carried so high and treated in so
bold a manner, with a progressive diminution of the wall piers (as the
taste for large traceried windows developed more and more), that a
flying buttress (see section, Fig. 110) was necessary to take the
thrust across to the exterior buttresses, and these again, under this
additional stress, were further increased in projection, and were at
the same time made narrower (to allow for all the window space that
was wanted between them), until the result was that the masses of
wall, which in the Romanesque building were placed longitudinally and
parallel to the axis of the building, have all turned about (Fig. 110,
plan) and placed themselves with their edges to the building to resist
the thrust of the roofing. The same amount of wall is there as in the
Romanesque building, but it is arranged in quite a new manner, in
order to meet the new constructive conditions of the complete Gothic
building.

[Illustration: Figs. 108-114.]

It will be seen thus how completely this important and characteristic
feature of Gothic architecture, the buttress, is the outcome of
practical conditions of construction. It is treated decoratively, but
it is itself a necessary engineering expedient in the construction.
The application of the same principle, and its effect upon
architectural expression, may be seen in some other examples besides
that of the buttress in its usual shape and position. The whole
arrangement and disposition of an arched building is affected by the
necessity of providing counterforts to resist the thrust of arches.
The position of the central tower, for instance, in so many cathedrals
and churches, at the intersection of the nave and transepts, is not
only the result of a feeling for architectural effect and the
centralizing of the composition, it is the position in which also the
tower has the cross walls of nave and transepts abutting against it in
all four directions: if the tower is to be placed over the central
roof at all, it could only be over this point of the plan. In the
Norman buildings, which in some respects were finer constructions than
those of later Gothic, the desire to provide a firm abutment for