of the focal cavity; for example, let this, as in Fig. 350, be assumed an extensive lamelliform fissure, inclined to the seismic vertical , and sloped from the observer, at the point of the earth's surface ; let the rending of the fissure be supposed to commence, from an extremely small
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Fig. 350.
cavity at the point , 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 , tremulous waves, and those of sound, will begin to be transmitted, and will reach in the time , and, so far as the emergent angle of the former can be observed, will emerge with .
Let us further suppose that until the fissure has enlarged to the depth , both ways from , 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 ; before the wave of shock will leave started; so that the shock will not reach the point , until after the time , and the angle of emergence will be nearly the same as that of the first tremors.
Moreover, if and equal the time of rending the fissure