Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/337

272 will be greater, (for equal overthrow,) between $$q$$ and $$s'$$ than between $$q'$$ and $$r$$; and the difference, both in distance and angle, greater, as the difference in compressibility of the two media, is greater, and the elasticity of the less compressible, less perfect.

The phenomena may be illustrated to the senses thus:—

If two large blocks—one of marble, and the other of caoutchouc—be laid in mutual contact; and in a cavity,formed equally, in the centre of the plane of contact of both materials, a small charge of gunpowder be exploded, its impulse will be almost entirely expended, in propagating a wave of movement through the india-rubber, the range of movement in which, will be far greater, than in the marble, although the rate of its propagation may be slower; but a loose body, laid upon the upper surface of either block, will be disturbed by the shock transmitted at a far greater distance, upon the caoutchouc, than upon the marble, &c., &c.

I have shown elsewhere ('Fourth Report Facts of Earthquakes, Rep. Brit. Assoc, 1858'), that in perfectly homogeneous media, the meizoseismal zone, or curve, depends upon the emergence of the wave-path at the surface being at the angle where the horizontal component is a maximum. The angle $$e$$ has a meizoseismal value of 54° 44' 9", upon one assumption as to the law of decay of the wave, and of 45° upon another and the more probable assumption, that the decay is directly as the distance from the original; the radius of the meizoseismal zone (circle) being, in the former case, equal to the side of a square, whose diagonal is the depth of the focal point from the surface, and in the latter equal to this depth. What has preceded proves, that this does not hold good for unequal dimensions of focal cavity nor for heterogeneous media.