Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/363

298 of the focal cavity; for example, let this, as in Fig. 350, be assumed an extensive lamelliform fissure, $$f\ f'$$ inclined to the seismic vertical $$o\ v$$, and sloped from the observer, at the point of the earth's surface $$p$$; let the rending of the fissure be supposed to commence, from an extremely small

Fig. 350.

cavity at the point $$c$$, and for simplicity, assume the medium homogeneous, and that waves of whatever amplitude, have the same rate of transit; then at the instant that the rent commences at $$c$$, tremulous waves, and those of sound, will begin to be transmitted, and will reach $$p$$ in the time $$c\ p$$, and, so far as the emergent angle of the former can be observed, will emerge with $$e=b\ p\ g$$.

Let us further suppose that until the fissure has enlarged to the depth $$h\ h$$, both ways from $$c$$, the originating impulse is not sufficient, to generate a wave with the amplitude necessary to produce the effects of shock; then the tremulous waves will have been transmitted to $$s$$; before the wave of shock will leave started; so that the shock will not reach the point $$p$$, until after the time $$c\ p + c\ s$$, and the angle of emergence $$b\ p\ g$$ will be nearly the same as that of the first tremors.

Moreover, if $$t$$ and $$t'$$equal the time of rending the fissure