Page:Great Neapolitan Earthquake of 1857.djvu/474

378 to decide, whether the twist has been due to one shock, (1st case) or to two shocks in succession, (2nd case); but when several bodies alike or dissimilar, at the same locality, are all found twisted in one direction, it is certain to have been the work of two distinct shocks, for it is beyond the reach of probability, that several bodies, should all happen to have their respective centres of adherence, at the same side of their respective centres of gravity, and unless they have, some will rotate in one, some in the other direction by any single shock; rotation thus produced, being always by the centre of gravity, moving contrary to the first or second semiphase of the wave, and carried round the centre of adherence, by the line joining them as a radius vector; the inertia of motion at the centre of gravity, and the resistance of the point of rotation in the edge of the base, or of the centre of adherence, forming in every case, the extremities of the dynamic couple.

All the effects of the double shock will be understood by examination of the Figures Nos. 235 and 236, in which



Fig. 235 shows the action of any double shock; Fig. 236 the variations of result produced, first, by rotation in the first