I 



respecting the Temperature of Mines. 103 



fixing a limit as to the precise point below the surface, for 

 the commencement of augmented temperature : an examina- 

 tion of an experiment or two will prove the confidence we may 

 place in the conclusions of either." 



" Mr. Fox commences at 50 feet, and Dr. Forbes at 200 

 feet below the surface ; and from the extreme temperature ob- 

 served in our deepest mines, would deduct 6° for artificial and 

 extraneous causes of heat, thus reducing the actual degree at 

 about 1300 or 1400 feet to from 72° to 74°*; and after the 

 ratio of 1° for every 50 feet, it would be at the depth of 1044 

 feet, 68°. Now reverse the order of calculation, and we shall 

 find Mr. Fox to make it 69^°, and Dr. Forbes 66^° for the 

 same depth. This is the precise depth of the lowest of the 

 three levels driven under Trenoweth from Crenver, the tem- 

 perature of which is actually only 58°, although a spot not 

 in the course of working, yet has a distant communication 

 with the mine in general, and at a working spot on the same 

 level, the temperature is but 68°, after being exposed to all 

 the extraneous sources in common." 



Mr. Moyle next quotes an observation of Dr. Forbes, re- 

 specting the evidence of the natural high temperature of the 

 Cornish mines, which is afforded by that of extensive collec- 

 tions of water in abandoned mines and workings (for which 

 see our last vol. p. 446), and then inquires, " Will these gen- 

 tlemen still maintain the same sentiments ? If so, their theory 

 must fall to the ground, as we can now clearly prove that these 

 very collections of water possess even a less temperature than 

 the supposed mean of the climate ; e. g. Huel Ann, and the 

 third shaft in Herland; one 130, and the other 160 fathoms 

 in depth, Ding-Dong, Huel Rose, Huel Franchise, &c." 



* Taking, then, the mean results of my observations on the different 

 mines, as given in the last column of the table, it will be found that the 

 mean rate of increase is about one degree for every 50 or 60 feet." 



" This result comes very near that drawn from the observations of 

 Mr. Bald in the coal mines (see p. 105), and agrees with the deductions of 

 my friend Mr. Fox, which have been presented to the Society. Admitting 

 this as the true result of our observations, I should be disposed to deduct 

 six or seven degrees from the extreme amount in our deepest mines, as 

 attributable to the artificial and extraneous causes formerly detailed, and 

 would thus fix the actual temperature of the solid matter of our earth (in 

 Cornwall) at the depth of from 1300 to 1400 feet, at from 72° to 74° of 

 Fahrenheit." — U>: Forbes, Trans. Corn. Gcol. Soc. vol. ii. p. 208. 



"At what precise point below the surface the augmentation of tempera- 

 ture commences, I am unable to say with any degree of confidence ; but 

 from a consideration of the influence of extraneous causes in modifying the 

 temperature observed in the superior galleries of mines, and from some 

 particular observations made by myself, I am disposed to place this point 

 at about the depth of 200 feet from the surface." — 7W(/. p. 210. 



He 



