Page:Great Neapolitan Earthquake of 1857.djvu/89

Rh altitude, in most cases, indeed in every case of a normal or nearly normal wave, and that during its transit the whole building is simultaneously in motion.

The end wall begins to be affected by its own inertia at the moment that the forward phase of the wave $$\mathrm{A}$$ to $$\mathrm{C}$$ reaches it. The velocity of the vibrating mass increases to the maximum at the point $$p$$; when whatever fissure may take place occurs, and the centre of gravity of the mass begins to move in the direction from $$\mathrm{C}$$ to $$\mathrm{A}$$, the whole turning round the point $$x$$ at the base. This movement is continued, though with diminished energy, by the wave motion during the second half of its first semi-vibration, i.e., from $$p$$ to $$\mathrm{C}$$, when it passes through zero, and now, during the whole of the second semi-vibration from $$\mathrm{C}$$ to $$\mathrm{A}$$, passing through the second maximum at $$q$$, the motion of the earth is in a contrary sense to that of the wall, and of the wave transit.

It has set the detached mass in motion with a momentum = $$\mathrm{MV}$$, $$\mathrm{V}$$ being the velocity of first semiphase of the wave itself at its maximum. It tends to destroy this during the second half vibration, by a momentum = $$\mathrm{M}(\mathrm{V}-v)$$, $$v$$ being the difference of velocity in the semiphases. During the time of the second half, of the first semi-vibration from $$p$$ (when fracture occurs) to $$\mathrm{A}$$, the wall continues to fall or turn over outwards, and for a little beyond this; but this is now checked by the return or second semi-vibration, and unless the angular motion of the mass in the time from $$p$$ to $$\mathrm{A}$$ shall have carried its centre of gravity beyond the vertical passing through $$x$$, the wall shall not fall. If such be the case—i.e., if the wall do not fall—a contrary motion, more or less tending to restore its position,