Correspondence. 573 



Fox* observed in a mine in Cornwall that thermometers sunk into a me- 

 talliferous vein, stood in general 2-25 higher than when placed in holes 

 bored in the rock, particularly granite, and that tin veins usually shew- 

 ed themselves colder than copper. From observations made in the 

 Prussian Mines, Von Dichenf has proved that the increase of tempera- 

 ture is, in general, much more rapid in metalliferous mines, but whe- 

 ther this is owing to the decomposition of the iron pyrites and coal, or 

 the locality of the mine, he does not decide. Condier also mentions, in 

 his observations on mines, the great influence of a metalliferous zone on 

 the subterranean temperature, owing to its superior conductibility of heat. 

 Professor Forbes has long been engaged in Scotland in carrying on a 

 series of experiments, in order to shew the relative powers of conduc- 

 tions of each rock. It has also engaged the attention of the British 

 Association,! and Arago§ considers this phenomenon of such import- 

 ance, as to state that its further investigation would not only serve to 

 prove the existence of a superior temperature in the interior of rocks 

 and metalliferous veins, but would afford an additional objection to the 

 hypothesis supported by several philosophers, that the high tempera- 

 ture in mines is the result of the action of the air upon substances, par- 

 ticularly the pyrites, on the walls of the galleries. To prove that the 

 observation alluded to indicates a more important source of heat in the 

 interior, he adds two mathematical formulae from Fowrier's Theorie de la 

 Chaleur, from which it follows that, ceteris paribus, the temperature of the 

 mass of a lode which is in contact with the atmosphere is greater, the more 

 easily heat is conveyed into the interior of the mass. There is however a 

 certain depth throughout the whole earth, varying from 40 to 100 feet, 

 where the temperature is invariable at all times || and seasons, and which 

 differs but little from the mean annual temperature of the place above. 

 Thus it has been remarked that the temperature of the earth at a depth 

 of 90 feet in the caves of the observatory at Paris, has never in the 

 course of half a century been above or below 53° Faht. which is only 20° 

 above the mean annual temperature of the city. "This zone," remarks 

 Mrs. Somerville,H "unaffected by the sun's rays from above, or by the 

 internal heat from below, serves as an origin, whence the effects of the 

 external heat are estimated on one side, and the internal temperature 

 of the earth on the other." By experiment it has been proved, that for 



* Annal de Chimic de Phys. vol. xvi. p. 80. 



t Poggend, Annal. vol. xxii. p. 497. 



t Mem. de l'Acad. Royal des Sciences des Paris, vol. vii. p. 473. 



§ Bischoff. loco citato, p. 126. 



II Transactions of the Association. 



V Phillip's Geology, vol. ii. p. 231, and Mrs. Somerville on the Physical Sciences, p. 266. 



