138 Prof. Bischoff on the Temperature of 



lower station seldom corresponded with those in the upper, at 

 one time being altogether different, at another time, although 

 corresponding pretty exactly on the whole, yet each fluctuation 

 being greater than in the upper stations. 



In Chap. VIII. it has been remarked, how, during the ob- 

 servations in the Saxon Erzgebirge, even thermometers, the 

 bulbs of which were sunk 40 inches into the rock, and that at 

 depths to which heat cannot reasonably be supposed to be com- 

 municated through the earth's crust from without, still indi- 

 cated the influence of the external temperature. Indeed, at the 

 great depth of 1014 feet, at the lowest point of observation, 

 the effect of the air in the mine upon the thermometer, which 

 was increased by the bulb being only situated at a distance of 

 SO inches from the surface of the rock,* was very great. Here 

 the greatest difi^erence observed during twenty-eight months 

 amounted to 4°. 77. The result of the experiments was, that 

 out of 29 different points of observation, situated from 147 to 

 1014 feet below the surface, 18 were found to give evidence of 

 the more or less considerable influence of the seasons. In one 

 place, at the depth of 687 feet,-f during three years, the highest 

 temperature was observed in the middle of the winter, the 

 lowest in the middle or towards the close of summer. The in- 

 fluence of the air could be observed with the greater cer- 

 tainty, in these experiments, as other thermometers, exactly 

 corresponding with those sunk into the rock, were placed, as 

 we have before mentioned, close by them, for observing the 

 temperature of the air. 



From all this it follows, that it is extremely difficult, or, in 

 fact, literally speaking, impossible, perfectly to protect the bulbs 

 of the thermometers, buried in the rock, from the influence of 

 the seasons. This would be no inconvenience, if the effect con- 

 tinued, as far as the place of observation, to correspond pre- 

 cisely with the variations of the external temperature, provided 

 the observations were carried on for at least a year. However, 

 it may be supposed, with great probability, and the observa- 

 tions in the Erzgebirge seem to favour the supposition,J that 

 the effect is more frequently to depress than to raise the tem- 



* Rekh. p. 38. t Ibid, p, '31. J Ibid. p. 133. 



