340 Prof. Bischoff' oti the Temperature q/' 



tions, tlieimometers, placed in holes of 8 to 12 inches deep, pro- 

 tected from the rays of the siin by a roofing, shewed variations 

 ^f at most but a few tenths of a degree. De Saussure* found in 

 a medium latitude that at the depth of 29.5 feet there was still a 

 variation of 2°.25 Fahr. It must, however, be remarked, that this 

 observation was made in a well, where the influence of the tem- 

 perature of the atmosphere was not precluded. Arago found at 

 Paris that a thermometer did not remain constant at a depth of 

 25 feet below the surface. Indeed, at a depth of 86 feet, a year- 

 ly variation of ^1^, or 0.0703 of Fahr. was observed. However, 

 the observations made at Paris on the 20th July 1825, by sinking 

 thermometers to different depths into the earth, shew that the 

 direct influence of the external temperature, that is to say, inde- 

 pendently of the temperature of the air, does not extend much 

 below 25 feet. It was found, namely, that at a depth of 



14 feet the temperature was 82°.06 



3 72M9 



6 62°.5 



10 .... 57°.9 



20 .... 52°.76 



26 .... 52°.49 



86 (in a cellar) 53M4 



•whilst a thermometer in the shade shewed 91.4, and another im- 

 mersed in sand shewed, in the sun, 127.4.+ D'Aubuisson esti- 

 mates the depth in question at between 46 and 61 feet, and 

 Kupff*er"]: at 77 feet below the surface. 



The determination of this depth is extremely difficult, as in 

 wells, shafts, and caverns, it is scarcely possible thoroughly to 

 prevent the influence of the external temperature; whilst the 

 sinking of thermometers to considerable depths into the solid 

 rock, or into the earth, and the observing of their temperature, 

 is hardly practicable. 



" Voyages III. in theBibliot. Britann. viii. 341. 



t Annal. de Chini. et de Pbys. vol. xxx. p. 398. 



J In a late memoir in PoggendorflF's Annal. vol. xxxii. p. 270, KupfFer ex- 

 presses his opinion that all points of the interior of the earth, of which the 

 greatest variation of temperature is 0.36, are situated at the same depth, what- 

 ever be the variations of temperature at the surface. He calculates this depth 

 for points near the equator at 25,2 feet, and for the polar regions at 35,5 feet. 

 This is contradictory to theory, as well as to the actual observations made by 

 Boussingault between the tropics. 



