Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/365

of impulse $c$, (Fig. 351,) due to the compression of the walls, (of whatever form,) of a focal cavity, in an elastic medium, (which for simplicity we may suppose homogeneous); then besides the primary wave of shock of large amplitude, as in the continuous spherical or ellipsoidal shells, $s\ s'\ s''$, &c., there will be a reflected wave, from the continuous and indefinitely extended mass, of elastic and

Fig. 351.

resistant material, at $l\ o$, and below it, given back by the primary or previous compression. This will be delivered, vertically upwards, in the general direction of the seismic vertical $o\ v$, and divergently upon the surface around it, in the spherical shells, $r\ r'\ r''$, &c., in the dotted lines; and as these emerge at the surface, they will generate, a smaller transversal wave, $t\ t'\ t''$, &c., which will be transmitted upon the surface, with an apparent emergence at every point nearly vertical, as at $p\ t$. And these small transversal waves, may reach a distant point of observation $p$, coincident with the shock, or before it, or after it, according to varying conditions, as to depth $c\ r$, to distance $r\ p$, and to the form of the focal cavity when not spherical, as supposed in the Figure.