34 



Prof. J. Prestwich. 



and cross-veins — of which the effects may either be to raise the tem- 

 perature of the underground springs when chemical decomposition is 

 going on in the upper part of the lode, or to lower the temperature 

 when the surf ace- waters are abundant and infiltrate rapidly. 



The following observations made in the rock appear to be amongst 

 the most reliable of the results obtained, the temperature having 

 been taken in holes 2 to 3 feet deep, and the stations having been 

 selected at places where no working had recently been going on, and 

 as distant as possible from the shaft : — 



1 



~No. in 

 Table I. 



Name of mine. 



Rock. 



Depth. 



Temp, 

 of rock. 



Eate 

 of increase 

 for 1° F. 



Observer. 









feet. 





feet. 





19 





Granite .... 



1350 



78° 



48 



Fox. 







Granite and 



1530 



87 











slate 





85 



| 43-7 



>> 



12 





Granite and 



1128 



79 



40 



)> 







greenstone 











80 



Fowey Consoli- 

















Slate 



1728 



93 



41 





11 



Par Consols . . . 





1248 



84 



38 



» 

 >> 



40 



Consolidated . . 



Slate 



1740 



85-3 



49 



Henwood. 



122a 



Tresavean .... 



Granite .... 



2130 



5)9 



43-5 



Hunt. 



18 



Tresavean .... 





1572 



82-5 



48-5 



Fox. 









Mean . . 44 feet 





This gives a mean thermometric gradient of just 44 feet per 1° F., 

 or combining the results obtained by observations in springs with those 

 in rocks, we get an average gradient of 43'2 feet per degree. 



Mean of observations in springs 42*4 feefc 



„ „ rock 44-0 „ 



Mean.. 43'2 „ 



Foreign Mines. — We know too little of all the conditions which 

 obtain in foreign mines to draw any definite conclusion as to the rate 

 of increase of temperature. Possibly the observations may prove 

 available when we have more certain information respecting the 

 mean annual temperature at each place, the height of the mine above 

 the sea, and the position, especially in mountainous districts, of each 

 station with reference to its depth beneath the surface. 



This latter element is one, which the coal-mines observations pre- 

 viously given, clearly show should be taken into account. In Cornwall 

 it is not of much importance, as the elevations are small, and the 



