of the Mines of Cormvall and Devon. 261 



of several feet,* cannot extend its influence very far; and, 

 consequently, the observations made upon it will not, perhaps, 

 avail much in the present inquiry. 



No experiments have been made in Cornwall to determine 

 at what depths, in different soils and rocks, at various pei'iods 

 of the year, the effects of atmospheric temperature cease, and 

 those of subterranean heat commence. 



Mr R. W. Fox's observationst on the temperature at the 

 depth of a few feet, are the only ones here with which 1 am 

 acquainted ; and, because in many of the wells and shallower 

 parts of the mines which I have examined, I have obtained 

 the same average, viz. about 50 degrees, I concur with himf 

 in thinking that we may begin our computations at that tem- 

 perature ; and I have, accordingly, taken it as my point of de • 

 parture. 



* Professor Forbes, from observations made in tlie neighbourliood of 

 Edinburgh, states, on the average of three years' experiment, that the effects 

 of atmospheric temperature will become insensible at the following depths, 

 in diiferent substances : — 



Trap-rock, 55.5 feet. 



Sand, Co-8 ... 



Sandstone, 96.1 ... 



Reports of the Brit. Assoc. (1840), p. 435. 

 At Paris the effects of atmospheric temperature ceased at 25 feet; whilst 

 in various Prussian mining establishments it was found to extend to depths 

 varying from 27 to C3 feet. 



Professor Bischof, Ediii. New Phil. Jour, xxiii. (1837), p. 341. 



t The bulbs of the thermometers were sunk to a depth of three feet below 

 the surface, and the mean annual temperatures observed were, — 



At Wheal Gorland (granite) 48°.09 



... Dolcoath (slate) 49°.94 



... Falmouth (slate) 50'.C7 



Mean, 49' 86 



Mr R W. Fox, Com. Geo. Tram., iii. p. 326. 



Corn. Geo. Trans., iii p. 326, and Reports of Brit. As. (1840), p. 310. 



