Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/102

They appeared to have been formed at the same moment, or in immediate succession, by a force that had acted upon the tower, in a direction about from west to east; had split it through the weakest vertical lines, viz., through the heads of old apertures, &c., and that the S.W. half of the tower, had instantly separated again, by that fissure, and moved over towards the west. The direction of wave-path resulting from the whole was about 100° E. of north.

In the Magazino del Summano, a large storehouse of ponderous masonry (for grain, &c.), a wall running 30° W. of north has been fairly thrown to the S.W. in a direction 81° E. of north.

This wall near $a$, (Photog. No. 309, Coll. Roy. Soc, but not visible in it,) gave me a tolerable measure of velocity: it was not materially bound or constrained by other walls, or by timbers. Its whole height was thrown, moving upon the angle at ground level, its length was about 43 feet unobstructed, and its thickness at fracture was 2·0 feet. The masonry was of the calcareo-argillaceous stuff of which they form their rubble here, and applying the equation, of horizontal velocity for fracture,

$V_{f}=21.46\times {\frac {L\beta }{a^{2}}}$

solving, we have $V=4.61$ feet per second.

We must add to this the horizontal velocity for over throw from the equation

$v_{t}^{2}={\frac {4}{3}}g{\sqrt {a^{2}+\beta ^{2}}}\times \left({\frac {1-\cos \theta }{\cos ^{2}\theta }}\right)$

We have therefore

$V_{t}=6.665$ feet per second.

⁠VOL. II.⁠F