Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/379

314 shaft, at from 300 to 500 feet down, was sufficient to melt copper, or from 1900° to 2000° of Fahr. From the extremely bad conducting power, of the walls, of a volcanic shaft, there is scarcely any loss of heat, from any cause, except its enormous absorption, in the latent heat, of the prodigious volume of dry steam which is constantly being evolved. It is perfectly transparent for several yards above the orifice of the shaft, and is not only perfectly dry steam, but also super-heated; and although this steam, may be at the mouth very much below the highest temperature of the hottest point, the temperature of the shaft or duct that carries it off will be very nearly at all depths the same, to probably within a very short distance, of the point of greatest incandescence.

In the absence, at present, of better information, we may suppose the temperature of volcanic cavities in this region (where Vesuvius is the most "glaring instance") to be about 2000° Fahr. This would give a superior limit of temperature, for the interior of our seismic focal cavity, 2 times as great, as the maximum arrived at, by applying the supposed law of hypogeal increment, and would raise the tension, of the contained steam, (admitting that we know anything about the state of water, at such temperatures and pressures,) to much more than that due to fired gunpowder.

The capability of producing an earthquake impulse, depends greatly, however, upon the suddenness with which the steam is flashed off, and its tension brought to hear upon the walls of the cavity; and this is not most rapid, at the highest temperature, of the evaporating surface, unless, indeed, intense pressure, by bringing the fluid more