INVARIABLE TEMPERATURE. 41 



terior of our earth, experience at very different times the 

 maximum and minimum of atmospheric temperature, which 

 is modified by the sun's place and by the seasons of the year. 

 According to the very accurate observations of Quetelet, 

 daily variations of temperature are not perceptible at depths 

 of 3*ths feet below the surface ;* and at Brussels the high- 

 est temperature was not indicated until the 10th of Decem- 

 ber, in a thermometer which had been sunk to a depth of 

 more than 25 feet, while the lowest temperature was ob- 

 served on the loth of June. In like manner, in the admira- 

 ble experiments made by Professor Forbes, in the neighbor- 

 hood of Edinburgh, on the conductive power of different 

 rocks, the maximum of heat was not observed until the 8th 

 of January in the basaltic trap of Calton Hill, at a depth of 

 24 feet below the surface.f It would appear, from the ob- 

 servations which were carried on for many years by Arago 

 in the garden of the Paris Observatory, that very small dif- 

 ferences of temperature were perceptible 30 feet below the 

 surface. Bravais calculated one degree for about every 50 

 feet on the high northern latitude of Bossekop, in Finmark 

 (69° 58' N. lat.). The difference between the highest and 

 lowest annual temperature diminishes in proportion with 

 the depth, and according to Fourrier this difference dimin- 

 ishes 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 con- 

 ductive 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 V Observatoire (86 feet 

 and 53 o, 30 F.) are usually regarded as affording the amount 

 of depth and temperature of the invariable stratum. Since 

 Cassini andLegentil, 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°-44 Whether the cause of this rising is to be ascribed to 



* Quetelet, in the Bultetin de VAcad. de Bruxelles, 1836, p. 75. 



f Forbes, Exper. on the Temperature of the Earth at different Depths, 

 in the Trans, of the Royal Soc. of Edinburgh, vol. xvi., 1819, pt. ii., 

 p. 189. 



J All numbers referring to the temperature of the Caves de P Ob- 

 servatoire have been taken from the work of Poisson, Theorie Mathe- 

 matique de la Chaleur, p. 415 and 4G2. The Annuaire Mctcorologique 

 de la France, edited by Martins and Haeghens, 1849, p. 88, contains 



