156 



elevation*, have, in the middle of winter, at 

 nine hundred, at six hundred, and sometimes 

 even at one hundred and fifty feet of depth, a 

 uniform temperature from 4*3 to 6 cent, degrees; 

 but these experiments have not yet been re- 

 peated in lakes situate under the torrid zone. 

 The strata of cold water in Switzerland are of 

 an enormous thickness. They have been found 

 so near the surface in the lakes of Geneva and 

 Bienne, that the decrement of heat in the 

 water was one centesimal degree for ten or 

 fifteen feet: that is to say, eight times more 

 rapid than in the ocean, and forty-eight times 

 more rapid than in the atmosphere f . Under the 

 temperate zone, where the heat of the atmos- 

 phere sinks to the freezing point, and far lower, 

 the bottom of a lake, even were it not sur- 

 rounded by glaciers and mountains covered 

 with eternal snow, must contain particles of 

 water, which, having during winter acquired at 

 the surface the maximum of their density, be- 

 tween 3*4° and 4*4°, have consequently fallen to 

 the greatest depth. Other particles, the tem- 

 perature of which is + 0*5°, far from placing 

 themselves below the stratum at 4°, can only 



* This is the difference between the absolute elevations of 

 the lake of Geneva and that of Thun. 



t See vol. ii, p. 56 ; and Arago, in the Annates de Physique, 

 vol* % p. 403. 



