On Underground Temperatures. 



33 



he took the large number of 415 observations made in ten Cornish 

 mines, and on reducing them to given mean depths, obtained the 

 following rate of increase of temperature with depth : — 





Average 



Mean temp. 



Rate of increase 





depth. 



at depth. 



per 1° Fahr. 



i. 



180 feet 



54-8° 



36 feet 



ii. 



432 „ 



60-8 



40 „ 



iii. 



762 „ 



67-4 



43'5*„ 



iv. 



1050 „ 



78-0 



40 „ 



He elsewhere gives the mean temperature for other depths : — 



Mean depth. Mean temp. Eate of increas 



672 feet .... 66'9° .... 40 feet 



1440 „ .... 85-5 .... 40-5 „ 



He noticed also that there was a difference between the tempera- 

 ture of the springs issuing from the granite and those from the slate 

 rocks. The mean of 134 observations made to a depth of 1200 feet 

 in Cornwall and Devon, gave the following thermometric gradients — 



In granite ...... 41 • 5 feet per 1° Fahr. 



In slate , 39*0 



Further, taking separately the springs issuing from veins of dif- 

 ferent characters, he arrived at the following results : — 



For cross-veins at a depth of 594 feet .... 40*0 feet per 1° 



For lodes generally „ 660 ,, .... 41"6 ,, 



For copper lodes , ; 840 „ 38'0 „ 



For tin lodes ,, 552 .... 51*5+ „ 



The result of these and various other calculations, is that for the 

 springs issuing from the rock, the mean thermometric gradient was 

 40*1 feet, and from the lodes 40"3 feet per degree. 



The early observations of Mr. Fox were made in the mine waters, 

 but his later experiments were made in the rocks, and he expressed 

 afterwards some doubts as to the value of his earlier experiments.^ 

 He considered that the rock observations were free from the direct 

 influence which the descent of the surface-waters exercises on the lodes 



* I am unable to explain this difference, The more rapid gradient in ISTo. 1 is 

 probably due to the heat resulting from the oxidation caused by the surface-waters 



t There would appear to be some mistake in this figure, as in another page he 

 states that the temperature in the tin and copper lodes conjointly at a mean depth 

 of 444 feet is 6r4°, which gives a rate of increase of 39 feet per degree. 



X Some were made in still water or sumps ; others in the air of the levels. 



VOL. XLI. D 



