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

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