Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/364

Rh from $$c$$ to $$f$$ and $$f'$$, the tremulous waves, will continue to start from successive points along the fissure, from $$c$$ to those limits, and up to the limits of time of $$f\ p+t$$, and $$f'\ p +t'$$, and will continue emerging, for the time $$s\ r$$, after the shock, with constantly varying angles between $$a\ p\ g$$, and $$e\ p\ g$$, up to the completion of the cycle of phenomena.

Such were precisely, the circumstances described to me, by all the observers present, at the shock of December, 1857, as those which then occurred. It began everywhere, with tremors; the sounds generally, arrived at the same time; the apparent direction of movement, of the tremulous oscillations, appeared rapidly to change, and still more rapidly to increase in amplitude; then the great shove of the destructive shock arrived, in some places rather before, in some a little after, the moment of loudest sound, and it died away suddenly, (i.e. with extreme rapidity,) into tremors again, but differing in direction from that of the great shock itself. (Compare Part II. in locis.)

To this train of phenomena the exceptions observed, were only at Naples, and in the Terra di Lavoro, where the direction of movement of all the oscillations, (the whole being small,) did not change, and were all horizontal, or apparently so; but the exception here, is confirmatory of the truth of the explanation, because, as already explained, the earthquake at Naples, was one only of reflected and refracted waves, transmitted horizontally, or nearly so, from an origin of a totally different character from that of a focal cavity, namely, the axial line, of the St. Angelo range of mountains. Another condition productive of tremulous waves, remains however, to be noticed.

In any case in which a shock is transmitted, from a centre