CONSTRUCTION XV. THE BUTTRESS 203

outside-the thrust from within, the prop without; or the crushing force of water on a ship’s side met by its cross timbers-the thrust here from without the wall, the prop within.

(2.) Moving weight may, of course, be resisted by the prop on the lee side 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.

(3.) 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.

§ 3. (1.) Buttresses for guard against moving weight, set, therefore, 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 harbour quays, and to the bases of lighthouses, 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 against the sweep of the tide, or

[Version 0.04: March 2008]