344 



Prof. Bischoff on the Temperature of 



circumstances which may influence the temperature of the rock, 

 we cannot be astonished that, in the foregoing table, the differ- 

 ences between maximum and minimum do not decrease in direct 

 proportion to the depths, so that, notwithstanding the care with 

 which the ob.servations in the Saxon Erzgehirge were conduct- 

 ed, they still leave us in uncertainty respecting the depth to 

 which the influence of the external temperature penetrates. 

 Thus much, however, may be deduced from them, that, in cer- 

 tain circumstances, there is a yearly difference of temperature of 

 scarcely 2° (4°.5 Fahr.) at a depth of 17 feet ; whilst imder other 

 circumstances, at twice that depth, a variation of S° (6°.7 Fahr.) 

 may be produced. It is therefore hardly to be expected, that 

 the depth to which the external temperature penetrates should 

 be discovered, for any places in high latitudes, by observations 

 in mines, wells, or caverns. 



Finally, let it be observed, that the maximum in no case took 

 place before August, and most commonly in September, Octo- 

 ber, and November ; and the minimum never before February, 

 generally in Marcli, and in two cases as late as June. Herren- 



