Page:Great Neapolitan Earthquake of 1857.djvu/155

Rh of the wave, and with effects proportionate to the velocities in each semiphase. The concrete and tiles of these floors adhere but very slightly to the planking on which they are laid, and the bond is so destroyed by the first slight movement, that the thick and heavy laminum often slides whole, or in large fragments, upon the planking, and batters the walls, already inclining outwards, independent of any constraint from the timbering.

When the wave is vertical, or of very steep emergence, the heavy tiled roofing, generally comes down upon the upper floor, almost at the instant that the inertia of each floor, acting at its centre of gravity in the opposite direction to the wave transit, tends to bring it down also. The impulse of the suddenly-imposed load of fallen roofing, conspiring with the effect of the shock, from which the resilience of the joists (if able to have done so at all), have not had time to recover, bring down the upper floor; the united load falls upon those beneath, and the whole are carried away in succession to the ground.

But, circumstances of building and of shock may be such, that the floor does not give way, but that, as an elastic plate or beam, supported and encastré at opposite ends (for the planking and concrete, we must bear in mind, have no insertion in the walls, and but slight connection with them), it merely bends downwards by its own inertia and that of whatever be upon it, under the upward stroke of the wave, as by a load suddenly applied over its whole surface. In this case, on the commencement of the second semi-vibration of the wave (or downward stroke), the bended timbers commence to straighten themselves again; their resilience, as a constant force acting through the versed sine of