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. § 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 applied to particular points of the wall, sometimes forming a great sloping rampart along its base, are frequent in buildings of countries exposed to earthquake. They give a peculiarly heavy outline to much of the architecture of the kingdom of Naples, and they are of the form in which strength and solidity are first naturally sought, in the slope of the Egyptian wall. The base of Guy's Tower at Warwick is a singularly bold example of their military use; and so, in general, bastion and rampart profiles, where, however, the object of stability against a shock is complicated with that of sustaining weight of earth in the rampart behind. § V. 3. Prop buttresses against dead weight. This is the group with which we have principally to do; and a buttress of this kind acts in two ways, partly by its weight and partly by its strength. It acts by its weight when its mass is so great that the weight it sustains cannot stir it, but is lost upon it, buried in it, and annihilated: neither the shape of such a buttress nor the cohesion of its materials are of much consequence; a heap of stones or sandbags, laid up against the wall, will answer as well as a built and cemented mass.