of the Mines of Oornivall and Devon. 255 



nature of the veins ; but, on the other hand, it must not be 

 forgotten, that this porous structure also affords a readier 

 passage for the descending water. 



(4). The working miners of Cornwall have long known that 

 the lodes containing tin-ores are, at equal depths, colder than 

 those in which copper-ores occur. This has been noticed by 

 Mr M. P. Moyle f but no attempt has, until now, been made 

 to determine the exact difference between them. 



Table IV. clearly proves the truth of this popular opinion ; 

 and indicates that the tin-lodes possess the lowest tempera- 

 ture,t and copper-lodes the highest ; whilst the lodes in which 

 the ores of both metals are mixed hold, in this respect, an 

 intermediate position. 



(5). The general fact of a progressive elevation of tem- 

 perature as we penetrate farther into the crust of the earth, 

 naturally leads us to inquire the ratio of its increase. 



The manner in which Table V. is constructed has been 

 already described ; and as the ratios presented by the different 

 varieties of rocks and veins are there exhibited connectedly 

 and at one view, it is needless to dilate on them. 



From the surface to 150 fathoms deep, the rise of tempera- 

 ture, for equal increments of depth, seems to be in a dimi- 

 nishing ratio,— a fact previously known.J But deeper obser- 

 vations disclose the curious, and, as it would seem, almost 

 anomalous circumstance, that at more than 150 fathoms deep, 

 the progression again becomes more rapid ; and that the ratio 

 at about 150 fathoms in depth is at a minimum, and increases, 

 both at greater and smaller depths. 



Whether further experiments may confirm or disprove the 

 generality of this fact, I do not pretend even to conjecture ; 



* Mr B. W. Fox, Corn. Geo. Trans., ii. p. 414. 



t The same general fact is also known in Germany; for Professor Bis- 

 chof says -" The tin-mines of Sauberg at Ehrenfriedersdorf shew a re- 

 markably'low temperature: indeed, it is a prevailing opinion there, that 

 stanniferous mountains arc colder than others. . . • The low tempera- 

 ture of Ileinrichssohle, in the Altenberg district in the Erzgebirge, was also 

 ascTibable to the rocks of tin-stone." Edin. New FIUl. Journal, xxiv. (1838), 



^'' { Mr R. W. Fox, Eeports of the British Association (1840), p. 315. 



