Page:Great Neapolitan Earthquake of 1857.djvu/55

Rh It will most generally happen that a regular solid (such as an obelisk, &c.) will fall prostrate whenever the maximum velocity of the wave is such as to produce in it oscillation sufficient to destroy statical equilibrium; and as the arc of oscillation due to a given velocity may be assigned, if we know the angle of emergence of the wave, so as to arrive at its horizontal component of velocity, we can always assign an inferior limit to the maximum velocity of the wave that overthrew the object whose dimensions, &c., we have observed. And if any other regular solid, although dissimilar in form, can be found at the same locality, which has not been overthrown, we may obtain from it a superior limit of such velocity.

It is possible, however, that oscillation may occur to the limit of equilibrium, or even somewhat beyond it, without involving the fall of the body; for the relation may possibly be such between the time of oscillation of the body (Fig. 5) upon one of its edges or arrises $$f$$, and the time of a complete phase of the wave, that the equilibrium may be restored by the movement impressed in the second semiphase of the wave in the contrary direction to that first communicated, and before the body has had time to fall over, beyond the limit of such restoration; the adherence or friction of the arris $$f$$, with the base, producing the necessary hold, by which the wave so acts upon the body during its second semiphase, in the direction $$a'$$ to $$b'$$.

If this be sufficient to bring back the centre of gravity through the horizontal distance between the verticals $$c$$ and $$f$$ during the time of the second semiphase of the wave, the body does not fall, but on the cessation of earth movement, topples back to its original position of c 2