before, but perhaps some cavity, at this mean depth and mean central point, which became extended under pressure, along the plane of greatest weakness, in the surrounding formations; we have no means of judging, beyond this, that the observed tremors indicate rending. Whether the cavity were pre-existent, or was then at the moment rent, the actual range of movement outwards, of its two great surfaces, (the opposite walls of the fissure,) cannot have exceeded a very small limit, as proved by the small amplitude of the wave of shock, at the places nearest and most exposed to it.
The dimensions thus assigned to the focal fissure, receive very interesting confirmation, from the actually observed duration, (so far as it was observed,) of the tremors that preceded the shock.
The length of the fissure was nine geographical miles (its extreme limits, assuming it rent) something more. If the fracture commenced, as we must suppose, at the centre of the length, then the distance it had to run either way, was 412 geographical miles, feet. If the rent occurred at the superior limit of , as already given, its rate, supposing it to have been all in Apennine limestone, would have been about 3640 feet per second. (See 'First Report on Facts of Earthquakes, Sec. 23rd, Reports Brit. Assoc. 1850.') The time of rending, then, would have been 712 seconds, and this would be the least period of time, during which the tremulous movements would be felt, previous to the arrival of the great shock, supposing the latter generated, at the moment the fissure attained its full dimensions. The least time actuaUy recorded is "six or eight seconds."
It is improbable, however, that the velocity of rending in
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