of the Mhics of Cornwall and Devon. 253 



A detailed table, which I have prepared, presents many 

 examples of different temperatures at equal depths ; not only 

 in the same mines, but even in various parts of the streams of 

 water flowing out of the very same crevices and apertures, with- 

 in a few feet, or even inches, of each other. It need, there- 

 fore, excite no surprise that there is not a perfect uniformity 

 in the temperatures of tracts miles distant from each other. 



(2). No attempt to determine the difference between the 

 mean temperature of the granite and slate, at various depths, 

 was made before this inquiry had been considerably ad- 

 vanced ;* although the fact had been alluded to,t and had 

 been, from time immemorial, known to practical miners.;]: 



This difference is exhibited in Table II., and is far more 

 conspicuous in the deeper than in the shallower levels. 



The granitic rocks may be more exposed to the cooling 

 influences of descending streams, as their structure permits 

 the percolation of water more readily than that of the slate- 

 series. 



(3). The general impression has been, that the temperature 

 of the rocks is lower than that of the veins :§ the results of 

 my inquiries, however, conclusively demonstrate, that, at all 

 depths, the rocks are warmer than the lodes, and the Icdes 

 than the cross-veins. (Table III.) 



It is well known to miners that the largest streams of water 

 flow through the cross-veins ;\\ smaller ones through the lodes ; 



* My own Papers, Thomsons Eecoids of General Science, iv. (1836) 

 p. 198 ; Reports of the British Association, \-i. (1837), p- 36. 



t Mr E. W. Fox, Annals of Philosophy, iv. (1822), p. 447 ; Phil. Mag. 

 and Annals, is. (1831), p. 98. 



X " From observations ciirried on in various mining establishments in the 

 Prussian dominions, M. Von Declion finds that the increase of temperature 

 is, in general, much more rapid in coal than in metalliferous mines."' Pro- 

 fessor Bischof, Edin. Nei'j Phil. Jovrn., xxiv. (1838), p. 141. 



§ Mr R. W. Fox, Com. Geo. Trans., ii. p. 21 ; Report of the Royal Corn. 

 Polytech. Soc. (1836), p. J07; Dr Forbes, Corn. Geo. Trans., ii. p. 217; my 

 own Summary of Experiments made in Cornwall (by other observers), Ediu. 

 Jour, of Science, x. o. s. (1829), p. 244. 



II As a general fact, the crofs-vchis traverse the lodes ; and thus, by falling 

 in contact with and separating all of them, become the main subterranean 

 aqueducts or channels for the circulation of water. 



