Page:Great Neapolitan Earthquake of 1857.djvu/472

376 directly westward. All those that are twisted are turned from left to right.

Now there are two distinct trains of earthquake causation, by either of which bodies may be twisted on their bases. 1st. By the action of a single shock, when the centre of adherence of the base of the object, lies to one side or other of the vertical plane passing through the centre of gravity, and the line of the wave-path. 2nd. By the conjoiutconjoint [sic] action of two closely successive shocks. By the first shock, the body is tilted up from its base, but not overthrown, so that for a time greater or less, it rests wholly upon one edge of its base; while thus poised, if another shock bear upon it, in any direction transverse to the first, it acts as usual at the centre of gravity of the body, to displace it by inertia, in the contrary direction to the wave transit; but the body is held more or less, by friction at the edge momentarily in contact with its support, and there only; but this edge must always lie to one side of the vertical plane passing through the centre of gravity, in the direction of the wave-path: hence the tilted body, while relapsing upon its base also rotates, round some point situated in the edge of its base upon which it had been tilted, and thus it comes to rest in a new position, having twisted more or less round a vertical axis.

If the observer look due south at a square pyramid, for example, whose sides stood cardinal, and it be tilted by the first semiphase of a shock from east to west, the pyramid will tilt or rise upon the eastern edge of its base; and if, before it has had time to fall back, it be acted on by another shock from north to south, the pyramid will rotate, upon the bisection or on some other point, of the edge on which