Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/334

Rh distorted ellipses, the larger and fuller end of the oval, of each one, being found towards the end at which the wave-

path $$c\ s$$, in the line of the greatest axis of the ellipsoid, is emergent, and the smaller and narrower end, being opposite. And inasmuch as the direction of greatest effort at the side $$c\ d$$, plunges into the earth, and is never emergent at all, we may adapt the felicitous expression of Stokes, for the analogous case of sound, and call all the localities in such a case, existing (to the right in the Figure), in the direction of $$p,$$ from the seismic vertical, as places in earthquake shadow.

So far, we have viewed the medium as being the same and homogeneous, as to the opposite sides of the fissure. If this be not so, however, and that the plane of the fissure be situated, so as to have at one side of it, a dense, hard, highly elastic, but slightly compressible material; and at the other, a much less hard, but still elastic, and much more compressible, material; then a new cause will be added, to those preceding, for a still further distortion of the ellipsoidal wave-shells, and with them, of the form of the isoseismals.

This will produce its greatest effect, when the plane of the fissure is inclined to the horizon. There are two cases: the plane of the fissure may be inclined so as to slope, as in Fig. 348, towards the less compressible medium, lying to the