Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/358

Rh to the rending, and probable filling with high-pressure steam, of a rent, commencing at or near the main focal point, and extending (as indicated by the wave-paths) about 7 geographical miles in length. We shall see reason to believe, however, that the extreme dimensions of the rent, exceeded this considerably. The production of the rent, was the originating impulse of the earthquake, its repletion with dense steam or a further extension of it, probably, that of the second shock.

Had we precise observation, of the exact time, during which the rushing sounds were heard at each, or at two or more stations, and knew the exact velocity of sound, in the various solid or other media, between the fracture and the hearers, we should obviously be in a condition, to calculate the length of the fissure itself, assuming the depth of focus, to be about its mean depth: and such exact observation, which may be readily made by a Brequet watch, or other chronoscope, may be, upon the grounds above indicated, commended to future observers of earthquakes.

It is obvious, also, that the conditions which have been pointed out, are sufficient to account for, most if not all, of the phenomena recorded, of the relative time of arrival and of duration, of the sound and of the shock, at given points; while the fact, that the sound was heard at all only, within a very limited area of the total surface convulsed, by this earthquake, (an area equal to not above th of the whole, within which the shock was distinctly felt by the unaided senses,) seems to point out, that the wave-pulses of audible sound, and those of shock, are different—differing not only in volume, but probably in velocity, and capable of propagation to very different distances, from the same origin.