Page:Great Neapolitan Earthquake of 1857.djvu/151

Rh upon the direction of the line of wave transit, if very steeply emergent, and upon the form and relations, as to horizontal figure and height, of the building—or to the latter conditions only, if the transit were perfectly vertical.

The three cases are illustrated in Figs. 70, 71, and 72. In either of the two latter, large portions would become



detached, and would fall, leaving other portions, as in Fig. 73, still adherent and supported by the walls; while in the first case, if the width transverse to $$a$$—$$b$$ (Fig. 70) were sufficient, the segments detached by the parallel fractures, would break again transversely, by their own weight at or near the mid-length, and also close to the walls, and then fall.

So far, account has been taken only of direct seismic forces in the plane of the floors, but in the subnormal, subabnormal, and vertical waves, the inertia of the floor itself is brought into play transversely to its own plane, and all the displacements by fall just mentioned as due to gravity alone acting after fracture are produced upon an