INVARIABLE TEMPERATURE. 39 



from the observations which were carried on for many 

 years by Arago in the garden of the Paris Observatory, that 

 very small differences of temperature were perceptible 30 

 feet below the surface. Bravais calculated one degree for 

 about every 50 feet on the high northern latitude of Bosse- 

 kop, in Finmark (69 58' JS. L.). The difference between 

 the highest and lowest annual temperature diminishes in 

 proportion with the depth, and according to Fourrier this 

 difference diminishes in a geometrical proportion as the 

 depth increases in an arithmetical ratio. 



The stratum of invariable temperature depends, in respect 

 to its depth, conjointly upon the latitude of the place, 

 the conductive power of the surrounding strata and the 

 amount of difference of temperature between the hottest 

 and the coldest seasons of the year. In the latitude 

 of Paris (48 50') the depth and temperature of the Caves 

 de rObservatoire (86 feet and 53.30 F.) are usually re- 

 garded as affording the amount of depth and temperature 

 of the invariable stratum. Since Cassini and Legentil in 

 1783 placed a very correct mercurial thermometer in 

 these subterranean caves, which are portions of old stone 

 quarries, the mercury in the tube has risen about 0.4. 39 

 Whether the cause of this rising is to be ascribed to an 

 accidental alteration in the thermometrical scale which, 

 however, was adjusted by Arago in 1817 with his usual 

 care, or whether it indicates an actual increase of heat is 

 still undecided. The mean temperature of the air at Paris 

 is 51. 478 F. Bravais is of opinion that the thermometer 

 in the Caves de V Observatoire stands below the limit cf 

 invariable temperature, although Cassini believes that he 

 has found a difference of T Vo tfts f a degree (Fahr.) between 

 the winter and summer temperature, the higher tempe- 



in the Trans, of the Royal Soc. of Edinburgh, vol. xvi, 1849, pt. ii, 

 p. 189. 



39 All numbers refeiTing to the temperature of the Caves de V Obser- 

 vatoire have been taken from the work of Poisson, Theorie Mathema- 

 tique de la Chaleur, pp. 415 and 462. The Annuaire Meteoroloyique de la 

 France, edited by Martins and Haeghens, 1849, p. 88, contains correc- 

 tions by Gay-Cussac for Lavoisier's subterranean thermometer. The 

 mean of three readings, from June till August, was 5 3. 9 5 F. for 

 this thermometer, at a time when Gay-Lussac found the temperature 

 to be 53.32, which was therefore a difference of 0.63. 



