Page:Great Neapolitan Earthquake of 1857.djvu/485

Rh that these balls, and more particularly that B, were projected by the transverse shock, the impressed movement being therefore in the same direction as that of the wave, but had their plane of projection altered by the immediately following main shock, which, acting on them by inertia, impressed a movement in the contrary direction to that of the wave. Unless this were understood the path obtained by the following method would appear erroneous.

We obtain the velocity of projection from the equation

This was the velocity in the plane of projection 64° W. of north, but this direction was not that of either shock but a resultant of both, the ball having necessarily received a certain amount of impulse from the first shock, and before it had completely parted hold from its support, been exposed to the impulse of the second. Now the direction of the second (i. e. the main) shock was 15° W. of north to south, and we have already ascertained at several points that its velocity was 12.97 feet per second. We have, therefore, two velocities, and the direction of the resultant, and of one component given, to find the direction of the other component.

Resolving, we find that the transverse shock made an angle with the primary or main one of 56° 40′, and that the wave-path or direction of the former was 41° 30′ E. of north to south, which is precisely the direction of a line drawn from the extremity of the mountain range northward and