Page:Great Neapolitan Earthquake of 1857.djvu/73



an isolated wall (a parallelopiped) of masonry or brick, founded on level ground, be subjected to the transit of an earth wave, whose velocity is sufficient to affect the continuity of its parts, the resulting fractures will vary with the direction of the wave-path as respects the plane of the wall, and with the angle of emergence of the wave.

1st. If the wave-path be horizontal, or nearly so, and in the plan of the wall, the earth moving forward beneath the wall, tends to carry it forward by the grasp of its foundation and at its own velocity; but this is opposed by the wall's inertia. The material of the wall being, within narrow limits flexible and elastic, the tendency is to distort its figure, thus. The wave reaching the end $$a$$ (Fig. 14 bis) first, with a transit from $$a$$ towards $$b$$, the end $$a$$ first begins to assume the form $$e a$$, rapidly taken by the whole wall, if sufficiently high in relation to its length. The wave traverses beneath the whole length, the materials, in virtue of their elasticity, oscillate in the same direction between $$e$$ and $$f$$ throughout the whole mass, and if the wall be fissured,