CENTIIAL HEAT* 



361 



heat is supported, is derived first, from the occurrence of volcanic 

 fires in almost every degree of latitude north or south : secondly, 

 from submarine volcanoes : thirdly, from the occurrence of numer- 

 ous thermal springs in countries remote from active volcanoes : lastly, 

 from direct experiments made on the temperature of the earth, at 

 various depths in mines, and by sinking and boring into the earth. 



Whether there exist a mass of heated matter under the whole sur- 

 face of the globe may be uncertain ; but that there is subterranean 

 fire, under a considerable extent of the surface, can scarcely be 

 doubted. The volcanoes that are thickly scattered over both the 

 northern and southern hemisphere, the long period of their activity, 

 and the connection that appears to subsist between the volcanoes in 

 distant districts (see Chap. XVIII.), prove the depth and extent of 

 the source of volcanic fire. The volcanoes that break out from un- 

 der the sea, and overcome the vast pressure of the incumbent ocean, 

 farther indicate, that the explosive force is situated at a great depth. 

 Thermal waters, prove the extensive effects of subterranean heat ; 

 for though many hot springs rise in volcanic districts, and are prop- 

 erly a part of volcanic phenomena, yet other thermal waters are far 

 removed from any active volcanoes. Some hot springs have flowed^ 

 without any known diminution of temperature, for nearly two thou- 

 sand years ; this is the case with the waters of Bath, which have no 

 volcanoes nearer to them, than those in Iceland and the south of 

 Italy. That thermal waters derive their temperature from a deep- 

 seated internal source of heat, and not from any local cause, or from 

 chemical changes near the surface, is rendered probable by various 

 circumstances. In many of these waters there is scarcely any ad- 

 mixture of saline or mineral matter, which there would be, were the 

 heat derived from chemical decomposition. Most warm springs are 

 situated near to crystallized primary rocks, or to basaltic rocks or 

 dykes, as I observed to be the case in the Alps. Hot springs often 

 rise among the loftiest mountain ranges in Asia and America. The 

 temperature of thermal waters in low situations, is frequently reduced 

 by admixture with cool springs near the surface, and this I believe to 

 be the principal cause why thermal waters so rarely rise in the upper 

 secondary strata, as I have more fully stated in an account of the 

 thermal waters of the Alps. (See Appendix.) It could scarcely 

 have been expected, that an enquiry relating to the temperature of 

 the central part of our planet, could be brought within the limits of 

 human observation and experiment, as the depth to which we can 

 explore by boring or by excavation, bears so inconsiderable a pro- 

 portion to the diameter of the earth ; yet from numerous observations 

 on the temperature of the earth in deep mines, and from experi- 

 ments on the temperature of water at different depths, it would ap- 

 pear, that this temperature increases in a very remarkable degree, 

 as we descend lower from the surface. In France, the subject has 

 been recently investigated with considerable activity, and the prac- 



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