AND ITS DISTRIBUTION. 39 



Observatory at Paris, only very slight differences of tempe- 

 rature are discernible at 28 Paris, or 30 English, feet below 

 the surface. Bravais still found a difference of 1 Cent. 

 (l-8 Pahr.) at a depth of 26i Prench feet in the high North, 

 at Bossekop in Pinmarken, lat. 69 58'. The difference be- 

 tween the maximum and minimum temperatures of the year 

 becomes rapidly less as we descend; diminishing, according 

 to Pourier, in geometrical, as the depth increases in arith- 

 metical, progression. 



In reality, however, the depth of the " invariable stra- 

 tum" below the surface, or the depth at which no sensible 

 change of temperature takes place, is not everywhere the 

 same. It depends on the latitude of the place, on the heat- 

 conducting power of the rock, and on the amount of 

 difference between the temperatures of the hottest and coldest 

 seasons. In the latitude of Paris (48 50'), the observa- 

 tions in the " Caves de 1'Observatoire " give 86 Paris feet, 

 or 92 English feet as the depth, and 11'834 Cent., or 

 53'3 Pahr., as the temperature of the "invariable stratum." 

 Since Cassini and Legentil in 1783 fixed a very exact 

 mercurial thermometer in those subterranean spaces which 

 are parts of ancient stone- quarries, the mercury has risen 

 0'22 (Cent.) in the tube.( 39 ) It is still doubtful whether 

 this rise is caused by an accidental alteration in the thermo- 

 meter scale, (which, however, was very carefully verified by 

 Arago in 1817), or whether it is to be ascribed to a real 

 increase of temperature. The mean temperature of the air 

 at Paris is 10'822 Cent., or 51*4<8 Pahr. Bravais believes 

 the thermometer in the Caves de 1'Observatoire to be below 

 the invariable stratum ; although Cassini considered he had 

 found a difference of two-hundredths of a centesimal degree 



