Page:Great Neapolitan Earthquake of 1857.djvu/473

Rh it momentarily rested, and will hence come to repose, after having twisted from left to right, or with the hands of a watch.

If the tilting up, had been produced by the second semiphase, of the same shock from east to west, then the pyramid would have risen upon the western edge of its base, and the same direction (north to south) of second shock, would have produced rotation upon that edge, but in a contrary direction to the preceding, or from right to left, or against the hands of a watch.

Again, if, on the first supposition, the first semiphase of the east to west shock, had tilted the pyramid upon its eastern edge of base, but the second shock had been from south to north, in place of the reverse as before, then the rotation would have been from right to left; and if tilted by the second semiphase on the western edge, the second shock, south to north, would produce rotation left to right.

It would therefore appear at first impossible, to determine the direction of motion in transit, of either shock, from such an observation: we can, however, generally discover upon which edge of the base any heavy body of stone or masonry has tilted, by the abrasion or splintering of the arris, and the rotation must have taken place round some point in that edge. If, therefore, we know the direction of either one of the two shocks, we can always discover that of the other, by the rotation observed; and if the time of oscillation of the body be ascertainable, we are enabled to calculate a major limit, for the interval of time that must have elapsed, between the arrival at the twisted body, of the first and of the second shock, when both the wave-paths are known.

With a single instance of such twisting, it may be