Page:Great Neapolitan Earthquake of 1857.djvu/94



proceed next to consider the effects of a Subnormal wave, or one whose transit is emergent at an angle to the horizon, and in a vertical plane, parallel to two of the walls of a rectangular building.

If the angle of emergence—viz. that contained between the line of transit of the wave and the horizon be small, not exceeding 10° or thereabouts—the effects, when producing fissures or overthrow, can seldom be distinguished alone from those of a normal wave. Fissures are produced near the quoins, at the tops of the walls, and if overthrow takes place the end walls and detached portions of the sides, are thrown outwards as already described. In accordance with the general law, the fractures tend to place themselves at right angles to the direction of wave transit. Thus referring to Fig. 31, where the angle of emergence (of the wave whose direction of transit is $$a$$ to $$b$$) is $$hia$$, the end wall $$c$$ is thrown back by inertia, and that at $$e$$ projected forward, with a difference of velocity = $$v$$, as before. As the joints of the masonry or brickwork, which the fractures follow, upon the whole run vertically and horizontally, and as the fracturing force is