Page:Great Neapolitan Earthquake of 1857.djvu/52

16 or very nearly vertical, and that the maximum velocity of the wave does not exceed—

$$\mathrm{H}$$ being equal $$\lambda$$, the amplitude of the wave, the masses being in such case rapidly lifted up and let fall again without the withdrawal of the support of the base.

And generally, single objects situated upon the surface of the earth, in firm and rigid connection with it, or so circumstanced that the line of wave transit through the centre of gravity passes through the surfaces of repose and of attachment, move with the earth itself, and are seldom disturbed as to their former position. Thus also, flexible objects, rooted trees, flag staffs, telegraph posts, and the like, are bent by the transverse forces impressed, but return to their positions, leaving only perhaps traces in the earth disturbed at their bases, of the direction of movement.

A few examples may clear this part of the subject. In Fig. I let there be a large stone ball, adherent to a narrow base on top of its pedestal, which is fast in the



ground, and exposed to a shock, the direction of wave motion of which, in the first semiphase, is from $$a$$ to $$b$$, and with velocity sufficient to dislodge it. The ball, urged by inertia, and free to fall in any direction, will be projected in $$b$$ to $$a$$, contrary to the wave, and describing a trajectory