Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/336

Rh of the fissure. In the first case (Fig. 348) the wave-path in that axis, will be emergent at $$s$$, at the side of the more compressible medium, and in the second, at the side $$s$$ (Fig. 349) of the less compressible medium. The wave, in the harder and more elastic medium, to the side $$d$$ (Fig. 348), and $$s$$ (Fig. 349), will have a faster transit rate, and probably a greater velocity of the wave particle, than in the less hard and elastic medium; but the amplitude of the wave at starting, will be much greater in the latter, and will continue greater; and unless its velocity be very low, in comparison with that in the harder medium, that is, unless the difference in elasticity between the media, be very great, the wave of greater amplitude, will carry furthest, before sensible extinction. Observing in the two figures, the relative directions, in which the ellipsoidal wave-shells emerge, in both media; it is obvious that, taking the three elements, of seismal energy, or overthrowing power upon the surface, together; viz., wave amplitude, velocity of wave particle, and direction of wave-path, at emergence; the distance of equal effort, from the seismic vertical $$o\ v$$ will be greatest, in the direction of the more compressible medium, and the difference most, where the wave-path in the greatest axis, of the semi-ellipsoid of greatest amplitude, is emergent, as in Fig. 348, and vice versâ.

So that in the former case, (Fig 348,) the distances for equal effort of overthrow, from the seismic vertical, along the greatest axis, may be at $$q$$ and at $$s$$, and in the latter (Fig. 349) at $$q'$$ and $$r$$; the larger and fuller end of the isoseismal oval passing through $$s$$ and $$r$$, and the smaller and narrower through $$q$$and $$q'$$.

In the former case, (Fig. 348,) the difference between the observable angles of emergence, in the isoseismal curve,