Page:Great Neapolitan Earthquake of 1857.djvu/478



can calculate, therefore, from these data, to a good approximation, the interval of time that elapsed, between the arrival of the first and second shock, and thence the difference in transit velocity, of the two waves of shock; the first through the limestone, the second through the deep clays and gravels, &c.

The chimney may be regarded as a parallelopiped vibrating as a compound pendulum, upon $$b$$, and $$b_2$$ (Fig. 237) as points of suspension, whose centre of oscillation is in the plane of shock passing through the centre of gravity, and distant from $$b$$ or $$b_2$$ by two-thirds of the diagonal of the parallelopiped, in the same plane.

This distance = 4.33 feet is the length of the corresponding simple pendulum, with some small allowances for the hollowness and irregularity of form. The greatest possible are of vibration, is limited by that which would bring the centre of gravity, $$c$$, vertically over $$b$$ and $$b_2$$, Fig. 237, beyond which the mass must have fallen. This I found to be 21°, either side of the vertical through the centre of gravity, when the chimney was in its original undisturbed state.

The arc actually described, must have been less than this,