ranged up and down the slope of a steep hill, who fire in file, commencing at the centre, or standing at the same distance from one extremity of the line. In the former case, we shall hear the whole sound, in less time by nearly 12 : 1 than in the latter; while the explosive-like character and loudness will be nearly as 2 : 1, twice as much sound impinging on the ear in the same time.
It should be observed, that the same sort of continuous sound, variable in duration at different stations, could be produced, by a single sound or explosion, occurring simultaneously along the whole length, of a line such as that of our fissure; the sound from its different points, arriving at the ear in succession, as they came from a greater distance; the case being then analogous to the rumbling of thunder, simultaneously produced, along the whole length of the flash, so far as our senses can take note of it.
But to this view there are great objections—
1st. The sound in that case, could be nowhere "sospiranti;" it would be everywhere rumbling, and only vary in loudness, and in length of duration.
2nd. Bearing in mind the extreme velocity of propagation of sound in solids, as compared with its velocity in air, the assumption of a single explosion, must involve, the existence of a line of simultaneous rupture, of enormous and improbable length, to account sufficiently for the prolongation of the sound.
3rd. The physical conception, of any such simultaneous fracture, or of any force, or mode of application of forces, capable of producing it, is difficult, if not impossible.
It may therefore be concluded, that the sounds heard, in the earthquake of December 16th, 1857, were due
