Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/376

Rh Let us assume, however, that at the focal cavity, the increment is the usual mean, of 1° for each 60 feet of descent; then, at the upper, mean, and lowest levels, of the cavity, we have the corresponding temperatures thus:

assuming, as before, the mean temperature of the surface soil, = 61° Fahr.

If the cavity were filled with dense steam, at these respective temperatures, the corresponding pressure upon its walls, would be, from the formula,

$$t$$, being reckoned, above 212° Fahr.

In accordance with the law, that vapour can exist, in a vessel whose walls are at different temperatures, at the tension only, due to the least temperature of any part; it might be concluded, that the greatest possible pressure within the cavity, would be that due to the temperature, of its uppermost and coolest part; but as the steam must be supposed suddenly admitted to the cavity, in this instance, and may be supposed unlimited in supply; we may conclude that for the instant, it would reach any higher tension, up to the limit of the maximum temperature.