of the Mines of Cornwall and Devon. 247 



tation with others who have also been engaged in this inquiry, 

 it has been thought best to confine the observations, as much 

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

 diately issuing from the unbroken portions of the rocks and 

 veins. 



The reasons for this preference are : — That the temperature 

 of the air in mines is affected, not only by the presence of the 

 workmen, the combustion of candles, and the explosion of gun- 

 powder, but also by the warm or cold air which is brought to 

 the same spot, by the varying directions of the currents under- 

 ground, which are more or less influenced by the changes of 

 wind at the surface ; — that the rocks, forming the sides of the 

 shafts and levels, must, to a certain extent, partake of the 

 temperature of the air circulating through them, and, of 

 course, be affected by its changes ;* — and, that the water 

 flowing through, or standing in pools, in the levels, is exposed 

 to the same modifying causes, and probably, also, warmed by 

 the workmen who frequently stand in it. 



As all these causes operate irregularly, it is difficult, if not 

 altogether impossible, to select any distant periods at which 

 their influence should be the same. The present observations 

 were therefore made on the water as it issued from the un- 

 broken rock, before the streams could be aftected by the tem- 

 perature of the levels ; and they were, for the most part, in- 

 stituted where frequent excavations scarcely permit even the 

 apertures to partake of that influence, whilst the more deeply 

 seated portions of their channels are perfectly free from it. 



The following table presents a comparison of the tempera- 

 tures prevailing at nearly similar depths, in the different dis- 



* " During his observations on the hourly variations of the magnet, Reich 

 bad an opportunity of observing the rapidity with which the air operates 

 upon the rock even at a distance of 40 inches. The air in the mine shewed 

 48°.6 with but slight variations, and a thermometer sunk into the rock 48°.64 ; 

 but when, after 44 hours' observation of the magnet, the temperature of the 

 air had been raised by the presence of the observers, and their two candles, 

 to 49°.7, the thermometer in the rock, which was subject to no change of 

 air whatsoever, was found to have risen to 48°.71, 48°.73. This destroyed 

 all hope of obtaining the temperature of the rock, quite free from the influ- 

 ence of the air, by sinking thermometers even 40 inches deep into the rock." 



