S84! Prof. BischofF on the Temperature of' 



From the above observations on the temperature of the lakes 

 in Sxvitzerland, which were made at various seasons of the year, 

 we see that the temperature on the bottom ajjproaches very near to 

 the degree at which water assumes its greatest density ; but ne- 

 ver quite reaches it. It is always 1°.35 to 5°. 74 higher than 

 38°. 7. Suppose a lake in the autumn to have everywhere in its 

 depth a temperature of 54<°.5 ; and that the water neither gives 

 nor receives heat from the earth on which it rests. Now, if 

 cold set in, and the surface of the lake be depressed below 5^°.5, 

 the water at the surface becoming more dense, will sink to the 

 bottom, and continue to do so until it be everywhere reduced to 

 38°.7. If the cooling of the surface continue still further, it 

 will have no more influence on the temperature of the water be- 

 low ; for now the water at the bottom of the lake can only be- 

 come colder by imparting its heat to the colder water above ; 

 which, however, cannot take place, water being so bad a conduc- 

 tor of heat, and the lake having so great a depth. The lake 

 will therefore have a temperature of S8°.7 from the bottom, 

 nearly to its surface. If the temperature at the surface then 

 begin to rise again, the only increase which will be caused in the 

 temperature of the water below, will be by the communication 

 of its heat. But for the reason just mentioned, this cannot ex- 

 tend far ; and, in fact, the observations of De la Beche gave equal 

 temperatures at depths of 40 to 70 fathoms ; and also at depths 

 of 80 to 174 fathoms. They shew farther, a decrease of 22°.50, 

 from the surface to 40 fathoms ; but from that depth to 164 fa- 

 thoms, which is a depth four times as deep as the former, only 

 0.450. We may, therefore, imagine a stratum of water below 

 40 fathoms, acting as an absolute non-conductor of heat from 

 above. From De la Beche's observations, this stratum may 

 have a thickness of 30 to 84 fathoms. It is consequently diffi- 

 cult to conceive how the temperature at the bottom of the lake 

 should ever exceed 38°.7, especially since at least twice in each 

 year the surface of the lake falls to 38°.7, at which seasons the 

 stratum of water bearing that temperature increases in thickness. 

 If, notwithstanding, we find the temperature at the bottom of 

 the Swiss Lakes, at all seasons, 1°.35 to 5°. 74 higlier than 

 38°.7, the water must have obtained that excess of temperature 

 from another source, and there is no other source of heat left 

 but the earth. 



