Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/360



The shock, properly so called, that which shakes down buildings, &c., and the precedent and succedent tremors, are all waves of the same order, differing only in dimensions, and more or less in direction; the shock, being a wave of large amplitude, the tremors of small or very small; the latter also, in part, mixed with small transversals.

Whatever be the form, of the focal cavity, whether spherical, linear, or lamellar, we must assume it, suddenly enlarged in dimensions; either by its walls giving way at once, to a previously steady, or to a slowly increasing pressure from within, or by a sudden increase of that pressure, brought upon its parietes.

In either case, if the cavity itself is enlarged in volume, by rending or fracture of the material, composing its walls, the velocity with which a rent or fracture can be propagated through any material substance, however great may be the force producing it, can never exceed that expressed by the equation

in which $$d$$, and $$h$$, are the density of mercury at 0°, and