16 MEA.N TEMPE3ATUBE. 



ture of the atmosphere, and the lake recems streams whic/i 

 rise from several cold springs in the neighbouring mountains. 

 i have to regret that, notwithstanding its small depth, I could 

 not determine the temperature of the water at thirty or 

 forty fathoms. I was not provided with the thermometrical 

 sounding apparatus which I had used in the Alpine lakes 

 of Salzburg, and in the Caribbean Sea. The experiments 

 of Saussure prove that, on both sides of the Alps, the lakes 

 which are from one hundred and ninety to two hundred and 

 seventy-four toises of absolute 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 degrees : but these experiments 

 have not yet been repeated in lakes situated 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. In the temperate zone, where the heat of 

 the atmosphere sinks to the freezing point, and far lower, 

 the bottom of a lake, even were it not surrounded 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, between 3'4 and 

 4*4, have consequently fallen to the greatest depth. Other 

 particles, the temperature of which is + 0'5, far from 

 placing themselves below the stratum at 4, can only find 

 their hydrostatic equilibrium above that stratum. They 

 will descend lower only when their temperature is aug- 

 mented 3 or 4 by the contact of strata less cold. If 

 water in cooling continued to condense uniformly to the 

 freezing point, there would be found, in very deep lakes 

 and basins having no communication with each other (what- 

 ever the latitude of the place), a stratum of water, the 

 temperature of which would be nearly equal to the maxi- 

 mum of refrigeration above the freezing point, which the 

 lower regions of the ambient atmosphere annually attain. 



* This is the difference between the absolute elevations of the lakeg ot 

 Geneva and Thun. 



