Page:A Treatise on Geology, volume 2.djvu/281

 CHAP. IX. the earth at depths of 1, 2, 4, 8 feet, their stems rising above the surface, so as to be easily inspected, we find that the variations of temperature continually diminish downwards;—at 1 foot, the extreme monthly differences corresponding to summer and winter were 21° and 19.6°; at 2 feet, 16.5° and 16.3°; at 4 feet, 12.8° and 11.5°; at 8 feet, 8.0° and 8.2.°

2. The epochs of highest and lowest temperature continually differ more and more from the summer and winter solstices, according as the depth in the earth is greater; or, in other words, the time taken by the sun's rays to penetrate and warm the ground augments with the depth.

Thus, at 1 foot from the surface, January is the coldest and July the hottest month; at 8 feet from the surface, February and March are the coldest months, and September the hottest.

3. The average mean temperature of the year augments from the surface downwards; but does not reach the average of the air temperature, in the range of these experiments.

These results have been more than confirmed—they have been enlarged—by the experiments of Arago in Paris, Quetelet in Brussels, and Forbes in Edinburgh, and extended to the depth of 25 feet. M. Quetelet has founded on the experiments at Brussels a mathematical investigation of the highest interest.

Among the data for computation employed by M. Quetelet are experiments analogous to those of Leslie, made in 1762 at Zurich by M. Ott; a series made at Strasburg by Herrenschneider, in 1821, 1822, and 1823; another at Heidelberg by M. Muncke; one made at Upsal in 1832-3, by M. Rudberg; others at the observatories at Paris and Brussels descending to 25 feet. The original memoir must be consulted for the mathematical part of the subject; but we shall present the conclusions which the investigation has established.