Page:Great Neapolitan Earthquake of 1857.djvu/53

Rh in the plane of the wave-path, will fall to $$c$$, and if its velocity in the horizontal axis be not wholly destroyed (as by falling on soft ground), it may roll, and so cease to give any indication of subsequent value. We are not now concerned with what is the trajectory, or with its modifying conditions. It is obvious that these might be such that the ball shall drop, nearly plumb to the ground, but always in a direction contrary to that of the wave transit.

If a similar ball however, and exposed to a like shock, have a support—such as a wall, for example, at the side a (Fig. 2), which is not overthrown by the shock, but carried along with the wave in the forward movement of its first semiphase—then the ball, although pressed by inertia against the adjacent face of the wall, is prevented falling in that direction. It also, therefore, has impressed upon it, the maximum velocity of the wave in its first semiphase. When, therefore, the wave itself arrives at its maximum velocity in the contrary direction—viz., in its second semiphase—the ball, by its inertia of motion impressed in the previous semiphase, is then thrown in the same direction as the wave transit $$a$$ to $$b$$; and projected to the ground at $$d$$, as before.

It might happen that the wall might be so related in dimensions, &c., to the velocity and direction of the wave, that it should remain standing long enough to produce the effects described upon the ball, but should immediately afterwards begin to fall, fracturing and turning over upon the point $$f$$ in a direction contrary to that of the wave transit; and in such a ease there might remain no evidence to show, from either the ball or the wall, in which direction