Page:Great Neapolitan Earthquake of 1857.djvu/108

corresponding to the subabnormal sought; and the path of the latter will be found in a vertical plane passing through

$a'b'$. Join $op$, which is in the same vertical plane, and also in the plane of the fissures or fractures, $mp$, $qp$, and through the point of the quoin $e$ intersecting $a'b'$, draw $ab$ perpendicular to $op$; $ab$ is then the path of the subabnormal wave, emergent in the direction $a$ to $b$. This is tantamount to finding the resultant of all the parallel forces that resisted fracture, and of course assumes, that the masonry fractures equally readily everywhere.

This is practically sufficiently near the fact, except, perhaps, when the horizontal obliquity of the wavepath, or abnormal angle is very great; in that case one wall is broken by direct pull nearly, and the other nearly transversely, which may give rise to an unbalanced couple at $u$, in a line parallel to $qm$; and in that case the wedge-shaped mass, in place of simply sliding down or turning over, in a plane passing vertically through $a'b'$, and falling to pieces, at the base of the quoin, will have a small amount of rotation, either to the right or left of that plane; and the centre of gravity of the mass of debris, will be found correspondingly posited, to the right or left of the base of the quoin. No case has been observed