Page:Great Neapolitan Earthquake of 1857.djvu/476

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The complex forms of these objects, which rendered the ascertainment of the positions of their centres of oscillation on the edges of their bases, difficult and uncertain, unless by experiment, prevents any calculation of a precise character, from their movements, as to the velocity of either shock, nor do we require it.

They give us other valuable information, however. In the case of the parallelopipedal chimney, (F. Fig. 1, Diagram Nos. 238–240, and Fig. 4, same diagram), twisted upon its base, it had rotated upon a point in the western edge of its base at b, Fig. 237. We know already that the direction (generally) of the first shock, was from some points east or N. E. towards the west or S. W., the second being from 15° W. of north to south. The chimney stalk had therefore made, one semi-oscillation, and one complete oscillation; that is, it was being acted on by the second semiphase of the wave of the first shock, at the moment when the second shock arrived at it, as in Fig. 237.

1857 Earthquake fig. 237.png

The centre of oscillation of the chimney above b thus tilted was, as nearly as could be ascertained, 4.33 ft distant from the edges of the base upon which it tilted b and b_2. The first shock, east to west, fractured the chimney from its base, and produced in the detached chimney, one semi-oscillation eastward (\mathrm{A}, Fig. 237). The chimney then