Page:Great Neapolitan Earthquake of 1857.djvu/54

18 the wave transit was made, whether from $$a$$ to $$b$$ or the contrary. We might ascertain the path of the wave, or rather the azimuth in which it lay, but no more.

If at two distant localities we can obtain even that much information, we can assign the place of the seismic vertical. For if (Fig. 3) by one shock the ball $$\mathrm{B}$$ be projected in the wave-path $$a'$$ to $$b'$$, in either direction, suppose contrary to the wave transit, and also the ball $$\mathrm{A}$$ at a distant place not in the same right line, also projected, say either as the



former or in the same direction as the wave transit $$a$$ to $$b$$, then we obtain two azimuths, which can have but one point of intersection in $$o$$, which is that through which the seismic vertical passes.

Where the body projected (Fig. 4) is circumstanced so as to retain the position in which it alighted upon the ground, so that we can measure the vertical and horizontal axes $$b\ c$$, $$b\ d$$, then knowing the maximum velocity of the wave, and which is equal that of projection, we can find the angle of emergence in the, plane of projection whose azimuth is observed, and vice versâ.

And if we have two closely adjacent objects projected in the same locality, and the above conditions observed, we can calculate both the angle of emergence and the velocity.