Page:Great Neapolitan Earthquake of 1857.djvu/56

20, overpasses it, and after a succession of decreasing oscillations remains vertical as at first.

In so resuming a position of rest, it may be so circumstanced as to the nature of its base and arrises of oscillation, as to twist considerably from its first position, round one or more vertical axes.

This is a condition of things that very rarely occurs, except with small objects, like vases, statues, or pinnacles consisting of a single block. There are few masses actually found sufficiently hard, when of large size, to prevent the arris $$f$$ splintering or crushing at the first movement to such an extent as to destroy all chance of restoration of position, even if the mass held together as one block; but in walls, towers, campaniles, or other compound masses, made up of blocks more or less firmly united, dislocation at several points takes place from the outset. Such masses being more or less flexible and elastic, bend first, break at the moment of maximum velocity of the wave, and then topple over piecemeal.

With the same velocity of wave, very different effects are produced, with regard to overthrow as the angle of emergence varies, and as the form of the body is different. Thus (Figs. 6 and 7), in the first, the wave emergent in the



direction $$a$$ to $$b$$, through the centre of gravity produces no disturbance of position in the "boulder stone," the extreme point $$s$$ of the bed preventing rotation in the first semiphase of the wave, by a force measured by $$d c$$, and the