Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/306

Rh, is not a mathematical point, but a subterraneous region, possessing determinate, and perhaps often, very large dimensions; and whatever be the nature of the impulsive force, or however it may operate, whether by producing the sudden rending of rock, or production of an enlargement of an existing cavity; the wave of impulse, as propagated outwards, passes simultaneously, or almost simultaneously, from many points about the actual focus, widely distant from each other. The direction of impulse, being everywhere necessarily normal to the surfaces of impact, the wave-path at the moment of starting at widely separated points, is in part determined, by the size and figure, or contour, of the focal cavity. Hence the wave-paths, on starting at such distant points of the focal cavity, if produced inwards, would not necessarily be found to meet or intersect in a single point, and hence no actual observations of wave-paths made upon the earth's surface could be found mathematically converging to a single point. The terms seismic vertical, and seismic focus, are strictly and alike applicable to every separate wave-path, each really having its own; but for our purpose, the terms really mean, the points upon and beneath the surface, through which a vertical passes that intersects the centre of gravity of all the partial foci referred to an horizontal plane, which is the focus of all the separate partial foci. Or, again, the collocation of all the partial seismic verticals, from the separate wave-paths, may be employed, to trace out the form and extent of the focal cavity, from which they unitedly spring, as we shall see further on.

A certain amount of divergence, then, from radiation from a single mathematical point of the wave-paths is to be