Page:Great Neapolitan Earthquake of 1857.djvu/118

76 be forward and backward in a right line or in an elliptic or some other closed curve, in a vertical plane, neglecting the transversal movement (which is small) for the present. This, although perhaps not physically true, was sufficient for our purpose.

Were we at liberty to consider the vibration of the wave with vertical emergence performed in like manner, the movement of any point of matter upon the surface due to it would be limited to two motions—one directly up and down, due to the amplitude, and the other forward and backward horizontally, due to its altitude and to the senses. The movement would thus be similar to that of a normal or subnormal wave. The universal testimony, however, of those who have experienced these vertically-arriving shocks is of a twisting and wriggling motion in different planes, violent in its changes of direction, and attended by a movement up and down of much greater range, to which the word "sussultatore" is often applied.



It seems highly probable that the path of a wave particle moving normally or nearly so, may be elliptic, as in Fig. 52, in a vertical