REFRIGERA'TION o• TITF1 EARTH. That the earth is at present losing heat, is an immediate consequence of the observed fact, that the temperature of its crust increases as we descend; for, in any conducting body, the flux of heat is always from warmer to colder parts; and the rate at which heat is thus lost can be easily calculated if we know the conducting power (for heat) of the rocks forming the crust, and the rate at which the temperature increases with the depth under time surface; for the conductivity may be measured by the quantity of heat which, in unity of time, passes (per square foot of sur face) through a layer of rock of 1 foot thickness, whose upper and lower surfacer are main tained at temperatures differing by 1° Fahr. Hence, if k be the conductivity of the crust, and if the temperature increases by 1° Fahr. every x feet of descent, the quantity of heat lost in unit of time from each square foot of surface, is measured by -. k and x can be determined by experiment for any particular locality, and thus the loss may be deter mined. These quantities vary very much in value in different localities, thus x is some times as great as 110. sometimes as small as 15. The value 50 is generally supposed to give a fair average—that is, for every 50 ft. of descent the temperature increases by Fahr. Hence the stifling heat experienced in deep mines. At the depth of a mile the tempera ture would on this estimate exceed that of the surface by more than 100° Fahr. Beds of coal at such a depth could not be wrought, as the temperature would far exceed that of tropical climates.
Three methods of accounting for this increase of temperature toward the interior of the milli have been proposed: 1. That the earth was originally molten, either through out or for a considerable depth over the whole surface; 2. That the internal heat is due to chemical combination; 3. That the earth, ages ago, passed through a region of space where the temperature was far above that of its present envelope.
Of (1) it is sufficient to say that such a the necessary consequence of impact, if the earth was formed by the aggregation of cosmical masses due to their mutual gravitation. It is scarcely doubted now that this is the origin of solar and stellar heat; and the fact of the moon's turning always the same face to the earth (see ROTATION), is most easily explained on the hypothesis of her original fluidity. The figure of The earth (see EARTH) is also a strong argument in favor of this hypothesis. This explanation of the origin of the earth's internal beat is obviously consistent with the increase of tempera ture as we descend below the surface—for a spherical mass of molten rock will evidently soon cool externally, while its low conductivity (rendered still lower by the high tem perature) will prevent the interior from supplying anything at all equivalent to the loss at the surface. On this hypothesis the rate of loss of heat must constantly become smaller and smaller. but very slowly; and it is possible that a considerable portion of the
earth's mass may still he in a melted stare.
The second hypothesis is perfectly sufficient to account for observed 'facts, but is apparently unnecessary, since (1) has been shown to be, in the universe, a vera eausa. It is only alluded to here because Lyell and other distinguished geologists have endeav ored to show from it that the earth need not be losing beat on the whole, a result per fectly untenable. They suppose the internal bent to be generated by chemical combina Lion, and then that the compounds so formed are again decomposed by electric currents produced by the heat ,(see Timitmo-ELEurniciTv), and are thus prepared to combine again and reproduce the heat. Were this the ease we should have a perpetual motion (q.v.), and, in the present state of science, this is known to be impossible.
The third hypothesis, proposed by Poisson, is easily shown to be inconsistent with known facts; for, if the passage through the warm region he supposed to have taken place from 1250 to 5,000 years ago, the temperature at the earth's surface must have been from 25° to 50" Fahr. above the present mean temperature, which is inconsistent with history. If it took place 20,000 years ago, the mean temperature must have been 100' Fahr. above its present value. Geology shows that this cannot be And, if it be supposed to have taken place more than 20,000 years ago, the requisite temperature have been incompatible with the existence of animal or vegetable life.
From the above argument, which is taken from a paper by sir William Thomson in the Transactions of the Royal Society of Edinburgh (1862), it is obvious that the first hypothe sis is that which we must, in the present state of our knowledge, adopt.
Supposing time temperature of melting rock to be from 7,000' to 10,000° Fahr. (and experiments seem to show that it lies somewhere between these limits), the present state of temperature of the crust indicates that the earth became solid somewhere between 100,000,000 and 200,000,000 years ago. These estimates are based on the known laws of conduction of heat discovered by Fourier, and the conductivity of rocks and soils, deduced by principal Forbes (q.v.)from observations made in the neighborhood of Edin burgh. But as these observations refer to conductivity at very moderate temperatures only, and as Forbes has shown that conductivity is in general lowered by heating, the lower limit above may possibly be reduced to twenty million years.
In conclusion, we may mention, to show how little the internal heat of the earth has to do with surface temperature, that Thomson has shown (Proc. R. S. E., 1863-64)that if we accept the estimate of 1° Fahr. of increase of temperature for 50 ft. of descent. the earth's surface is heated (by conduction of heat from within) only of a degree Fahrenheit.