earth. The geological reasons which have been advanced in support of this theory, may be referred to two general divisions.
In the first place, it has been maintained, and long and persevering efforts have been made to demonstrate the fact, that the heat of the earth increases in proportion as we descend below the surface. These efforts have consisted chiefly in various series of experiments, to ascertain the temperature of mines and other excavations at different depths; but the results hitherto obtained by such experiments, appear to have been contradictory and entirely unsatisfactory. For several years past, but little attention has been given to this class of experiments, as may be seen by examining the back numbers of Silliman's Journal. A very thorough examination of this subject may be found in a treatise on geology, by Dr. Thompson, Professor of Chemistry and Geology in the University of Glasgow. In the work referred to, the author has collected and presented tables of all the most important series of experiments. The author is disposed to favor the theory of an intense central heat, and hence would be naturally solicitous to confirm that theory by the experiments in question, if it could fairly be done. But after a very full and thorough examination, he arrives at the conclusion that, "to whatever cause we are to ascribe the augmented temperature which is sometimes found in mines and other excavations, the facts are incompatible with the notion that it is owing to a central fire." (Thompson's Geology, vol. 2, page 29.)
Another argument in support of this theory, has been derived from the frequent occurrence of earthquakes, and the existence of an immense number of volcanoes, on different parts of the Globe. The reader will of course remember that many of the best geologists, among whom is Prof. Lyell, (see his Geology vol. 1. p. 452, et Seq.) have thought it most reasonable to ascribe these phenomena to electrical and chemical agencies. But even supposing them to be oc-
