Note 90 — Chapter XIV · SUBTERRANEAN HEAT—ITS EXISTENCE DEMONSTRATED BY FACTS.

It would be more strictly correct to say that the rate of increase varies considerably in different places, though the main fact is everywhere palpably apparent that the deeper we descend into the Earth the higher the temperature becomes. Sir Charles Lyell records a number of careful experiments made in England, France, Germany, and Italy, which seem to show that an increase of one degree Fahrenheit for every sixty-five feet of descent would represent pretty correctly the general average. See his Principles of Geology, vol. ii., pp. 205, 206.