Page:Great Neapolitan Earthquake of 1857.djvu/527

broke a good deal of the connection. This wall was cracked nearly horizontally, about a foot above its base, and leaned over to the north about 1 1/2 inch. It had no batter: its thickness was 2.75 feet, and its height 20 feet. We can calculate the horizontal velocity necessary to have fractured its base, without overthrowing it, from the equation―

$V={\tfrac {2}{3}}g\times {\frac {b\mathrm {L} }{a^{2}}}$

here b = 2.75 feet; and to allow for the wall being 12° out of square to the wave-path, we shall take it = 2.8 feet; a = 20 feet, L = 2 × 52 feet. The coefficient for rubble limestone masonry of this highly indurated mortar being fully double that of ordinary work. Then

$V=21.46\times {\frac {104\times 2.8}{400}}=15.623$ feet per second.

I have no measure of the value of e, for Saponars, except those of the Castello, the maximum of which was 16°. We may assume it about the same as at Sarconi,= 16° 25′; and taking the velocity of the wave itself (in its normal direction) to be the same as at Polla = 12.97 feet per second, we have its horizontal velocity = 12.97/sec e = 12.45 ft. per second; deducting which from the preceding, we obtain 3.173 feet per second as the velocity of oscillation of the hill of Saponara itself, which may be considered as having been horizontal in direction.

The extreme limit of total velocity then, at Saponara, cannot have exceeded about sixteen per second, only about three feet per second greater than at Polla, Padula, &c.—a striking proof how small an increment of velocity, is sufficient to sweep all before it.