Page:Great Neapolitan Earthquake of 1857.djvu/152

104 exaggerated scale by the introduction of this force conspiring with inertia at the moment of shock vertically downwards.

Now these are, in fact, precisely the forms of fracture and destruction, observable in these heavy floors of concrete and tiles, so far as they are left free, in a limited degree, by the constraint of the planking and joists beneath.

The joists and planking, are as one mass, and move together, and parallel to their respective lengths, under forces parallel to either, whether horizontal or slightly emergent. The union, however, is not sufficiently complete, and the jointing of the planking is too rough and open, to prevent "racking" by diagonal forces, as in Figs. 74, 75; and as the several quoins give out unequally, in the case of diagonal wave transit, the buildings are no longer truly rectangular in plan, and so in this the floors follow the walls. If the wave be normal or subnormal, and the joists lie parallel to its line of transit, the wall at the end first reached by the wave draws off from the ends of the joists embedded in it. The floor itself, more or less as a whole, follows the wall, and the other ends of the joists draw from the opposite wall, all during the first semi-vibration of the wave. During the second semi-vibration, the whole floor returns by inertia, in the opposite direction to its first movement, and following in the direction of transit of the wave, thrusts back again the previously drawn ends of the joists, into their sockets in the wall $$b$$ (Fig. 76), and the mass is stopped by coming into contact with the interior face of this wall, against which, it strikes with enormous violence, the edge of the planking and of the concrete striking the whole length of the wall, almost at the