Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/362

Rh waves of small amplitude, although recognizable as producing sensible movement, and those of very much smaller amplitude, which are only capable of recognition by the ear, have the same rate of transit. The wave of the great shock is the subsequent one of great amplitude, (in proportion,) and which cannot be generated with an amplitude sufficient, to produce the effect of earthquake shock, until after the focal cavity has already been enlarged to a certain amount.

These waves, therefore, start in succession, the tremulous waves first, then the shock wave, of large amplitude, and, lastly, the concluding tremulous waves, the sound waves probably accompanying all; and if the velocity of inceptive rending be sufficient, the sound waves setting out the earliest of all: so that at a distant point of surface, the observer shall hear the mutterings of the earthquake first, shall then perceive the tremors before the shock, then the great shove of the shock itself, and, lastly, the tremors with which it departs along with the sound. If Mr. Earnshaw's mathematical views of the progression of sound, be adopted, ('Trans. Brit. Ass. 1858',) it would follow, that the transit rate for the wave of large amplitude, (the shock,) must be greater than that for either the tremors or the sounds; and if so, an additional reason is afforded in explanation of the commonly observed fact that the duration of the tremors before and after the shock is often unequal. Mr. Earnshaw's conclusions as to the necessary formation of "breaking waves" where the transit velocities are unequal, have extremely interesting relations with this part of our subject.

The relative times of arrival, at a distant point of observation, however, must be also largely affected, by the form