Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/406

Rh as great as that with which the body of one who should leap from the top of the Duke of York's column at London, would strike the pavement; and taking the greatest velocity that we have ascertained for this earthquake at 15 feet per second, this maximum velocity is $$= 5.33$$ times greater, than the velocity of our Neapolitan shock.

Admitting, as we must, the identity of the originating cause, (whatever it be,) of the volcano and the earthquake, it should follow that the relative intensities of earthquakes in different and wide-apart regions, must be proportionate directly to the altitudes above the sea level of the volcanic vents adjacent to or situated in each respectively—to the altitudes, and not to the mass of the volcanic cones, because the volcanic effort of ejection producing these, is counter-balanced and therefore measured, by the height of the column of liquid lava, sustained in the volcanic shaft to overflow at its crater lips; as the mercury of the barometer measures the atmospheric pressure, or a steam-boiler gauge, the pressure within it; and the projectile power for stones and lapilli, is subject to the same measure. But the mass, or cube of the altitude, is the proper measure of the energy and time of its operation, at the spot, taken together, or of the antiquity of the volcano, for equal energy.

The altitude of Vesuvius, which is the "pressure gauge" for the Neapolitan earthquake region, has varied within a few hundred years, from 4000 feet, down to 3500, in round numbers.

Now the wave velocity, as ascertained for this earthquake, should bear to the altitude of Vesuvius, the same relation that the velocity of the wave at Riobamba, did to the known altitudes, of the volcanic vents of the Andes.