[61] _On the causes of the temperature of Hot and Thermal Springs; and on the bearings of this subject as connected with the general question regarding the internal temperature of the Earth_: by Professor Gustav Bischoff, of Bonn.--Edinburgh New Philosophical Journal, vol. xx. p. 376; vol. xxiii. p. 330. Some interesting information on the temperature of the ground will be found in Erman’s _Travels in Siberia_, translated by W. D. Cooley, vol. i. p. 339; vol. ii. p. 366. _Sur la Profondeur à laquelle se trouve la couche de Température invariable entre les Tropiques_, by Boussingault: Annales de Chimie et de Physique, 1833, p. 225. Reference may also be made to Humboldt’s _Cosmos_, Otto’s translation; and to the excellent article on _Meteorology_, by George Harvey, in the Encyclopædia Metropolitana. These chthonisothermal lines, as they are called, have been traced by Humboldt and others over extensive districts. [62] These results are obtained from the valuable observations of Robert Were Fox, Esq., made with great care by that gentleman in several of the Cornish mines: _Report on some observations on Subterranean Temperature_.--British Association Reports, vol. ix. p. 309; Philosophical Magazine, 1837, vol. ii. p. 520. [63] From his experiments, the following conclusions were arrived at by M. Delaroche:-- 1. Invisible radiant heat may, in some circumstances, pass directly through glass. 2. The quantity of radiant heat which passes directly through glass is so much greater, relative to the whole heat emitted in the same direction, as the temperature of the source of heat is more elevated. 3. The calorific rays which have already passed through a screen of glass, experience, in passing through a second glass screen of a similar nature, a much smaller diminution of their intensity than they did in passing through the first screen. 4. The rays emitted by a hot body differ from each other in their faculty to pass through glass. 5. A thick glass, though as much or more permeable to light than a thin glass of worse quality, allows a much smaller quantity of radiant heat to pass. The difference is so much the less as the temperature of the radiating source is more elevated. 6. The quantity of heat which a hot body yields in a given time, by radiation to a cold body situate at a distance, increases, _cæteris paribus_, in a greater ratio than the excess of temperature of the first body above the second.--Journal de Physique, vol. lxxv.