Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/351



exact and careful examination of the sounds, audible in earthquakes, than has hitherto been accorded to them, will (as the deductions in the present instance to be made prove), be hereafter found a most valuable auxiliary, in deducing the nature of the focal cavity, and, probably, many other conditions of subterraneous action as yet little thought of.

The occurrence of sound at all, necessarily infers impulse at the focus, of the nature of a blow, or a succession of them, either due to, fracture of hard and elastic material, the sudden separation or rending open further, of existing fissures or cavities, or the sudden rush out, of highly elastic steam, or its as sudden production or condensation, so as to produce a musical note like those due to the impulse of wind in an organ pipe, or explosion, more or less sudden, as from the rush from the tail of a rocket to the explosion of a shell or mine.

The character of the sound heard in the shock of December, 1857, however various at different points, was everywhere made up of sudden explosive reports, (one, if not more than one,) variously contemporaneous with rushing and rolling sounds.