Page:Great Neapolitan Earthquake of 1857.djvu/58

22 then unless with a very great velocity, the effect of the wave in its first semiphase, however great its velocity, being merely to urge the whole solid against the ground in the direction $$b$$ to $$a$$; and if it stand free upon a surface with friction (as an article of furniture, a cabinet or press, for example), to cause it to slide in the direction $$n$$ to $$m$$ horizontally.

The initial velocity of a body projected by earthquake shock, or that of some point of one overturned, is equal to the maximum velocity of the earth wave; for upon the principle of the equality of action and reaction, the greatest effect produced must be due to the greatest applied velocity.

And so also of fractures; they are to be considered as due to a force $$\mathrm{M} \times \mathrm{V}$$, $$\mathrm{M}$$ being the mass of the fragment broken off, and $$\mathrm{V}$$ the velocity of its centre of gravity or of oscillation, and equal to the maximum velocity of the wave, at the instant of its passing through which, fracture occurs.

It will thus be apparent that the principal phenomena presented by the effects of earthquake shock upon the objects usually occurring upon the surface of the inhabited parts of the earth, resolve themselves into problems of three classes, and are all amenable to mechanical treatment, viz.—

in which last, as the velocity is small, and the mass usually