Page:Great Neapolitan Earthquake of 1857.djvu/74

34 it will be by a nearly vertical crack, widest open at top, and extending more or less down towards the base. If the wall were of absolutely uniform cohesion, there would be two such fissures near either end, or only one in the mid length, dependent upon the density, cohesion, and rate of force transmission of its materials, and the velocity of the wave movement: practically, such a wall is usually fissured in the weakest place.

2nd. If the wave transit be horizontal, or nearly so, and oblique to the plane of the wall, the latter either falls prostrate wholly, or a triangular fragment is thrown off from the end last reached by the wave, and in the direction contrary to its transit, or the wall is fissured, as in the first case only, dependent chiefly upon the greater or less obliquity of the line of transit to the plane of the wall.

Isolated walls, exposed to oblique or to directly transverse action, thus when tolerably thick, may sometimes be twisted considerably out of plumb without losing equilibrium or complete cohesion.

3rd. If the wave emerge with a steep angle to the horizon, the distortion is that of compression in the diagonal of the wall's plane, nearest parallel to the line of wave transit; and the fissures, if they occur, are also diagonal to the horizon, and approximate to directions perpendicular to the lines of pressure, i.e., to the line of wave transit. If the velocity of the wave be sufficient, in relation to the density and cohesion of the wall, a triangular mass may be projected from the end at which the wave passes out from it.

Reference will frequently occur to the directions in azimuth and emergence of the earth wave, relative to those