Page:Great Neapolitan Earthquake of 1857.djvu/120

78 round in small circles in the directions of the bent arrows 1 and 2 in Fig. 58. In vibrating elastic masses having special and pendulous vibrations of their own, when set in motion by the wave, the axis $$c d$$ may rapidly, though with a much slower relative movement, rotate in the direction of the external arrows or in the reverse one. The formidable torsional and wrenching strains which are known to arise from vertical shocks, are most probably thus produced.

It must be remarked, however, that these torsional strains—"Vorticosi" of the Italians and Mexicans—must not be supposed capable of producing those twistings of objects upon their bases, such as vases, chimneys, obelisks, &c., of which we shall record many examples, but which are due to other circumstances first explained by myself several years since.

A continuous jarring movement, consisting of a rapidly arriving series of waves, moving in a horizontal plane, as in Fig. 57, often occurs, and in lofty buildings, such as churches or towers, when the time of torsion vibration of the building itself (once set in motion), happens to be isochronous with that of the wave vibration, twisting strains of enormous violence result.

The effect upon the walls, then, of the vertical wave is chiefly to produce fractures which are transverse to the lines of twisting distortion. As the twist is alternately in opposite directions, these fractures cross each other, the opposite contained angles being double, those of the lines of maximum torsion strain with the vertical. These motions being accompanied by rapid up-and-down ones of much