32 



Prof. J. Prestwich. 



observations taken in rock, and on the other to those in water, Cornish 

 geologists have been divided in opinion as to the best plan to adopt. 

 The two great authorities on the subject, Mr. R. Were Fox and 

 Mr. W. J. Henwood, respectively gave preference, — the one to rock 

 and the other to water (or rather springs), — and we may feel sure that 

 the subject was well weighed and considered by both. Either system 

 is in fact open to some objections, and neither can be employed with- 

 out recourse to those safeguards and precautions, which no one knew 

 better how to use than these two experienced observers. 



Mr. Henwood gives as his reason for preferring springs, — that the 

 rocks forming the sides of the shaft and levels must, to a certain 

 extent, partake of the temperature of the air circulating through them, 

 and that this air could not ' escape the influence of heat-producing 

 causes at one time, and at another of the cooling effects of the intake- 

 air. For the same reason he objected to the use for temperature pur- 

 poses of the water standing in the levels, or sumps. He found it 

 difficult to select stations at which the influence of those conditions 

 would be always alike, and he was led to confine his observations as 

 much as possible to the temperature of the streams of water imme- 

 diately as they issue from fresh opened unbroken rock, — before they 

 could be affected by the temperature of the levels, — as the places 

 which would give the most correct temperature readings. 



The following observations appear to me amongst the most reliable 

 of those obtained in springs. The deepest seated springs are probably 

 the most free from interference by the surface temperature and other 

 disturbing causes. 



No. in 

 Table. 



Mine. 



Depth. 



Temp. 



Therm, 

 gradient. 



Spring in 

 rock or lode. 







feet. 



Eahr. 



feet. 





52. 





810 



71° 



41 



Rock. 



49/. 





660 



66 



44 



Lode. 



49#. 





834 



71 



40 



Lode. 



49a. 



East Wheal Crofty. . 



810 



70-7 



39 



Rock. 



59. 





2112 



93-5 



48'5 



Mock. 



2*. 





1440 



82 



45 



Lode. 



49S. 





1764 



92-5 



41 -5 



Lode. 



49c. 





1080 



74 



45 



Lode. 



49Z. 



East Wheal Virgin 



1722 



94-5 



39 



Rock. 



51. 



Devonshire Wheal Friendship. 



810 



69-5 



41 -5 



Lode. 









Mean. . 42 *4 





Instead of a few selected observations, Mr. Henwood took a larger 

 number and a wider range, though to this there is the objection that 

 he thereby includes a number of extreme and doubtful cases. Thus, 



