Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/332



the nature, of the original impulse, if it were produced at a single point, or radiating from all points, of the surface of a spherical cavity, in a homogeneous medium, the isoseismals must be circles, and equal effort upon the surface, be transmitted at all equal distances, from the origin. The conditions are altered, however, when the focal cavity, from the parietes of which, the impulse, of whatever sort, is transmitted, has a different and less regular form. Every particular form, will give rise to a new modification of effects. It will be sufficient for illustration, to refer to one, and that the form, which no doubt, almost invariably occurs in nature, namely, where the focal cavity is a plate, or narrow fissure with considerable length and depth, and having some determinate position of its plane, with respect to the horizon, or surface. If any impulse, like that of suddenly accumulated, elastic pressure, be delivered forth from such a cavity, it must leave its walls, at all points, in directions normal to their surfaces. It is obvious that the impulse must produce its most effective effort, in the directions perpendicular to the general plane of the fissure, and in an