Page:Great Neapolitan Earthquake of 1857.djvu/99

Rh Referring again to Fig. 32, the effect of the vertical component of the subnormal wave, in its first semiphase, tends to drive the fractured mass at the end $$c$$ down upon its foundation, and to throw that at $$e$$ into the air.

Gravity, therefore at the former end, acts with the wave, in the first semiphase, but against it, at the latter end of the building, and vice versa. But the rhomboidal mass of wall between the fissures at the ends $$c$$ and $$e$$ is also acted on by gravity with the wave, and the result is frequently to force down great wedges, such as $$p n m$$, which close the fissure $$p$$ in such a way as to prevent a certain indication, for these wedges being detached all round, remain where they descended to last. This is, however, a result of less importance, because there never can be a mistake, as to the direction along the wave-path of steeply emergent subnormal shocks, in which the seismic vertical is to be found; it must lie to the side at which the wave-path dips below the horizon.

The overthrowing power, and, to a certain extent, the fracturing power of a subnormal wave differs in the first and second semiphase (without reference to the difference due to difference of velocity), being proportionate to the perpendiculars to the wave-path, let fall from the centres of oscillation to the fulcra round which the fractured masses turn.

This is greatest at the side $$e$$, towards which the wave travels, and the tendency of this is to equalize the widths of the fissures. Other consequences will be apparent to the mechanical reader on considering the conditions.

From observation of the effects of a subnormal wave, therefore, we may be enabled to arrive at conclusions as to—