Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/388

sponding decrease in the velocity of the wave particle takes place. As the velocity is proportionate to the amplitude of the wave, for a given depth, and the amplitude depends upon the depth, the analogy is with the wave of light, rather than with that of sound or shock. The similarity, however, is seen in the general equation of elastic wave motion.

$v\ =\ a\cos \left({\frac {2\pi }{\lambda }}(x-a\ t)\right)$

where $a$, the transit velocity, depends upon the elastic modulus, and may differ to any extent from $v$; being, in fact, connected only through $a^{2}$, the intensity. Geologists must not be surprised, therefore, at finding, (now that the velocity of transit and the velocity of the wave particle at its maximum have been both for the first time measured, and for the same shock,) that they differ enormously from each other; that the velocity of transit is about half that of a cannon shot; but that the velocity of the wave particle (which does the mischief) is not as great as that with which a man reaches the ground when he jumps off a table; and yet that this small velocity is competent to produce all the violent and formidable effects of earthquake, no longer admits of doubt. On the other hand, let any one apply his common sense, to consider what should be the effect, if the velocity impressed on solid bodies by the wave, were that of its transit. Take, for instance, the balls of limestone of 1 1/2 foot diameter, projected from the Campanile of Padula monastery (Certosa). If, in place of with a velocity of 11 or 12 feet per second, they had been put in motion with one of 700 or 800 feet per second, which is equal to the velocity of a 13-inch shell on leaving the mortar, in

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