Page:Great Neapolitan Earthquake of 1857.djvu/154

106 it falls, likewise, very commonly outwardly, but, by possibility, in the direction towards the floor also. This chain of events, one of the most potent in the destruction of domestic buildings in Italy by earthquakes, is illustrated in Figs. 76, 77.

When there are two or three floors, over each other, that thus carry away the walls, they all (walls and floors) go towards $$b$$, the walls wholly falling outwardly. It will readily be conceived, what an inextricable mass of confused ruin, houses thus thrown down present, for the side walls that run parallel with the joists (or beams), fissured before transversely to the line of wave transit, are always more or less shaken down also, by the tremendous descent of the floors and walls, upon them falling.

If the line of transit of the wave, be in an orthogonal direction however, parallel to the planking, and therefore transverse to the direction of the joists, these having hold of the walls through the intervention of their sockets, the opening of the fissures in the walls on which the joists rest (due to their own inertia), is augmented by the inertia of the floor. The joists, or some of them, in advance of the fissures towards the end $$b$$ in the side walls, going forward with the flooring and side walls, drag the planking from the remainder of the joists, tearing out or bending partially the spikes, or breaking short the trenails; and the main weight of the flooring, thus freed from constraint of the side walls, runs forward as before, and though to a less extent, induces the same train of events, that have already been described.

And when the line of transit of the wave is thus transverse to the direction of the joists, the same set of phenomena result from both the first and the second