SUBTERRANEAN TEMPERATURES. 91 



A question of great interest, in a scientific point of 

 view, is the temperature of the centre of the earth. We 

 are, of course, without the means of solving this pro- 

 blem ; but we advance a little way onwards in the inquiry 

 by a careful examination of subterranean temperature at 

 such depths as the enterprise of man enables us to reach. 

 These researches show us, that where the mean tempera- 

 ture of the climate is 50, the temperature of the rock at 

 59 fathoms from the surface is 60 ; at 132 fathoms it is 

 70 ; at 239 fathoms it is 80 : being an increase of 10 

 at 59 fathoms deep, or 1 in 35'4 feet ; of 10 more at 

 73 fathoms deeper, or 1 in 43'8 feet ; and of 10 more 

 at 114 fathoms still deeper, or 1 in 64.2 feet.* 



Although this would indicate an increase to a certain 

 depth of about one degree in every fifty feet, yet it would 

 appear that the rate of increase diminishes with the 

 depth. It appears therefore probable, that the heat of 

 the earth, so far as man can examine it, is due to the 

 absorption of the solar rays by the surface. The 

 evidences of intense igneous action at a great depth 

 cannot be denied, but the doctrine of a cooling mass, and 

 of the existence of an incandescent mass, at the earth's 

 centre, remains but one of those guesses which active 

 minds delight in. The mean annual temperature of this 

 planet is subject to variations, which are probably 

 dependent upon some physical changes in the sun him- 

 self, or in the atmospheric envelope by which that orb is 

 surrounded. The variations over the earth's surface are 

 great. At the equator we may regard the temperature 



George Harvey, in the Encyclopaedia Metropolitana. These 

 chthonisothermal lines, as they are called, have been traced by 

 Humboldt and others over extensive districts. 



* 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 ; PhDosopliical Magazine, 1837, vol. ii. p. 520. 



