ide of
the wall, but is often more effectually met, on the side which is
attacked, by buttresses of peculiar forms, cunning buttresses, which do
not attempt to sustain the weight, but _parry_ it, and throw it off in
directions clear of the wall.

Thirdly: concussions and vibratory motion, though in reality only
supported by the prop buttress, must be provided for by buttresses on
both sides of the wall, as their direction cannot be foreseen, and is
continually changing.

We shall briefly glance at these three systems of buttressing; but the
two latter being of small importance to our present purpose, may as well
be dismissed first.

Sec. III. 1. Buttresses for guard against moving weight and set towards
the weight they resist.

The most familiar instance of this kind of buttress we have in the sharp
piers of a bridge, in the centre of a powerful stream, which divide the
current on their edges, and throw it to each side under the arches. A
ship's bow is a buttress of the same kind, and so also the ridge of a
breastplate, both adding to the strength of it in resisting a cross
blow, and giving a better chance of a bullet glancing aside. In
Switzerland, projecting buttresses of this kind are often built round
churches, heading up hill, to divide and throw off the avalanches. The
various forms given to piers and harbor quays, and to the bases of
light-houses, in order to meet the force of the waves, are all
conditions of this kind of buttress. But in works of ornamental
architecture such buttresses are of rare occurrence; and I merely name
them in order to mark their place in our architectural system, since in
the investigation of our present subject we shall not meet with a single
example of them, unless sometimes the angle of the foundation of a
palace set against the sweep of the tide, or the wooden piers of some
canal bridge quivering in its current.

Sec. IV. 2. Buttresses for guard against vibratory motion.

The whole formation of this kind of buttress resolves itself into mere
expansion of the base of the wall, so as to make it stand steadier, as a
man stands with his feet apart when he is likely to lose his balance.
This approach to a pyramidal form is also of great use as a guard
against the action of artillery; that if a stone or tier of stones be
battered out of the lower portions of the wall, the whole upper part may
not topple over or crumble down at once. Various forms of this buttress,
sometimes applie