Page:Great Neapolitan Earthquake of 1857.djvu/50

14 With nearly the same rapidity the velocity increases in the opposite direction from zero to the maximum, and back to zero again. The wave has then passed the given point, its whole phase or entire vibration has been completed, and it has produced its effects. The movement applied, is opposed by the inertia of the body moved, whose motions and final displacement depend upon the direction of the wave-path with regard to the centre of the body, its form, and the position of its base, or points of adherence or of support, and to the maximum velocity of the wave's proper motion. The applied velocity acts at the centre of gravity and in the direction of the wavepath, and the body, if free, apparently moves in the opposite direction to the wave in its first semiphase in consequence of its inertia. The force of displacement, with a given maximum velocity of vibration is therefore always proportionate to M, the mass, so that a heavy body, in the same shape and conditions is as easily upset as a light one. If the body be not free, if the line of wave transit passing through its centre of gravity pass within the base or through any other support, it does not move in the first semiphase of the wave; but if it be free in the opposite direction, it will be displaced in the second semiphase of the wave; but as the wave movement is now reverse to that of its transit, the inertia of the body acting still contrary to the applied velocity, now impels the body in the same direction as the wave transit.

In either case, and in either semiphase of the wave; the movement impressed, may be one of mere overthrow or upsetting, or it may be one of actual projection, or of both