^X-Xi 



I, 



•ustof 

 ion of 



^*V 



n. 



-solid 

 '1 lead 

 'lid to 

 rtiund 



natter 



)pki]is 

 j"con- 

 !ii,we 



)ftlie 

 :litlie 



Vortli 



r still 



r 



actual 



J sup- 

 do 



of 

 ?00 



or 



»s 



re 



thit. 



r 



the 



;. 



f 





P 



ot 



i 



t^ 



f 



^ 



o«- 



Ch. XXXIL] 



THEOEY OF CENTRAL HEAT. 



205 



the rocks, and on tlie water issuing from tliem. The mean 

 rate of increase, calculated from the most careful experiments 

 yet made in 2 shafts, one near Durham, and another near 

 Manchester, each of them 2,000 feet deep, is 1"^ Fahrenheit 



65 to 70 feet, a rate of 



from 



om 



same 



This rate, however, agrees 



ide in several of 



m 



verj nearly with previous observations 

 the principal lead and silver mines m Saxony, which gave 

 1° Fahr. for every 65 feet. In this case, the bulb of the 

 thermometer was introduced into cavities purposely cut in 

 the solid rock at depths varying from 200 to about 900 

 feet. But in other mines of the same country, it was 

 necessary to descend thrice as far for each degree of tem- 

 perature, t 



A thermometer was fixed in the 



mine, in Cornwall, by Mr. Fox, at the great depth of 1,380 

 feet, and frequently observed during 18 months ; the mean 

 temperature was 68"^ Fahr., that of the surface being 50"", 

 which gives 1° for every 75 feet. 



KupfFer, after an extensive comparison of the results in 

 different countries, makes the increase 1° Fahr. for about 



rock of the Dolcoath 



t M 



temperat 



^ — ' 



the interior of tlie earth, that the heat increases rapidly with 

 the depth; but the increase does not follow the same law 

 over the whole earth, being twice or three times as much in 

 one country as in another, and these differences are not in 

 constant relation either with the latitudes or longitudes of 

 places. He is of opinion, however, that the increase would 

 not^be overstated at 1° Cent, for every 25 metres, or about 



^ery 45 feet.§ The experimental well bored at 

 Grenelle, near Paris, gave, as b(jfore stated (Vol. I. p. 390), an 

 increase of about 1° Fahr. for every 60 English feet to the 

 depth of 1,800 feet. 



^- These observations were made by Temperature of the Interior of the 



1 



Professor Philips. 



t Cordier, M^m. de Tlnstit. tom. vii. 



I Pog. Ann. tom. xv. p. 159. 



'^ See M. Cordier's Memoir on the 



Earth, June, 1827. Mem. de I'lnstit. 

 tom. vii., and Edin. New Phil. Journal, 

 No. viii. p. 273. 



li 



