Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/375

310 of the focal cavity, by applying to its ascertained depth, the commonly reputed law of increase, of subterranean heat.

It may be interesting, however, to those who give more weight to the common doctrine of Hypogean temperature, to apply it to the case; the rather, as illustrative of the line of reasoning that we should be entitled to follow out, had we the requisite data certain.

There are two deep Artesian wells at Naples, the temperatures given by which, add to the discrepancy and doubt, before sufficient, as to the rate of increase with the depth.

One of these, is at the royal palace in the city; its depth is 1460 feet, and the temperature of the water is 68° Fahr., and taking the mean temperature of the surface soil to be 61° Fahr., this gives an increment, of only 1° Fahr. for 208 feet in depth.

The other one, is at the Largo Vittoria, also in the city; its depth is 909 feet, and its temperature is 71·6° Fahr., say 72°, or 1° for 82·64, or say 83 feet in depth.

These wells are not much more than a mile apart, yet the absolute temperatures differ, and the increments, by more than 2:1; both are much below the commonly received mean rate of increase, and these are wells, within ten miles of a volcanic active vent.

This low temperature in both, and great discrepancy between them respectively is traceable, I have no doubt, to the fact, that both are mainly, in tufa beds of vast depth, as porous as a filtering stone, and subjected to great and variable cooling influences, from the penetration of fresh and of sea water, while the adjacent sources of heat are variable also in a high degree.