Page:Great Neapolitan Earthquake of 1857.djvu/484

388 moving, by its elasticity and inertia, towards the east, at the same time that the forward movement of the whole mass towards the west, by the wave of shock, projected the balls with a velocity equal the difference.

We shall take the velocity from the ball B, which fell unencumbered by its base, and whose mark where the sphere struck and fractured the tiling of the roof, at B, (Fig. 1, Diagram No. 238, and Fig. 2, Diagram No. 240,) left no doubt as to its precise range.

This ball was projected a horizontal range of 12 feet, along the plane of projection, and descended vertically 42 feet from the summit of the Campanile to the roof tiling.

We may assume the angle of emergence for this shock $$e = 20^\circ$$, the same as that of the primary shock (15° W. of north to south) here; because although coming from the limestone in which (at Padula) we found the emergence greater = 25° 30′, yet we are here some hundreds of feet lower, and the shock, in passing from the limestone into the clays and gravel, intervening before reaching the Certosa, must have suffered some refraction into a rarer medium, both tending to reduce the value of $$e$$.

Both these balls were attached to their pedestals or bases by a small wrought-iron dowal inserted into both, sufficient in strength to communicate its velocity from the base to the ball, but permitting easy separation of the two (i. e. ball and base) when not rusted into the sockets. It was so rusted, in the ball A which fell with its base, but in B, the dowal, which was about half an inch diameter, was found broken off, either previous to, or during the fall of the ball.

It must be borne in mind, in considering what follows,