Page:Great Neapolitan Earthquake of 1857.djvu/51

Rh combined, depending upon the special conditions of the body and its supports, &c.

Where the body is projected from a base or support with which it has had friction or adherence, and that the line of wave transit through its centre of gravity does not also pass through the centre of adherence (that is, the point of the base, or between it and supports, in which all the resisting forces, of adherence, &c., may be supposed concentrated), then, besides projection, a movement round a centre of spontaneous rotation within the body will also be impressed. Where this is due to adherence at the base, the rotation is generally in a vertical plane, and does not seriously disturb the plane of projection from that of the wave-path, i.e., of a vertical plane passing through the seismic focus and the body displaced; but when also due to lateral adherence, or other still more complex conditions, the body is flung forward and whirls round on inclined axes, and finally comes to rest in some position quite abnormal to its original status, giving rise occasionally to complex phenomena from which nothing can be inferred. Where the body is large, such as a house or church of masonry, or even a single wall exposed to shock in the plane of its length, overthrow may be impossible with given dimensions and given angle of emergence of the wave; but in such case dislocation or fissuring occurs, and the severed parts may or may not be overthrown, dependent upon the amount of the applied velocity consumed in producing fracture only.

There may be no displacement whatever of loose objects, nor any dislocation of large masses, such as churches, &c., though exposed to violent shock, if its emergence be quite