386 Prof. Bischoff' on the Temperature of' 



depth, be thus reduced to 38o.7, the descending current will 

 cease. In this state it will continue whether the temperature 

 decrease towards 32°, or to whatever degree it rise. The de- 

 scending current will not again come into action until thiJ tem- 

 perature at the surface, having risen above 38°.7 shall again 

 begin to decrease. Whilst the temperature at the bottom of 

 the lake is 38°.7, or near 38o.7, and at the surface 32°, the 

 water having a temperature between 32° and 38°. 7 may be heat- 

 ed by the currents made to ascend by the warmth of the earth, 

 as high as 46°.79, without those currents reaching the surface; 

 for at 46o.79 water has the same specific gravity as at 32°. 



Observations on the temperature of lakes at different depths 

 during the time when the temperature at the surface remains 

 constant at 32° would give interesting results. For, accord- 

 ing to the above reflections, the temperature ought to increase 

 from the bottom up to a certain height, and then continue de- 

 creasing up to the surface. But nowhere could the temperature 

 rise above 46°.79j so long as the water at the surface remained 

 at 32°.* The only instance of an observation being made at a 

 great depth, at a time when the temperature at the surface was 

 near 32°, is that of Saussure of the 6lh of February 1777, 

 when he found the temperature at the surface 41°.74, whilst at 

 a depth of 950 feet it was 42o.09. 



Lakes receive a greater quantity of heat from the earth, the 

 hotter the climate is in which they are situated, and the deeper 

 they are. This quantity of heat, therefore, depends on the lati- 

 tude of the lakes, their elevation above the surface of the sea, 

 and their depth. The actual temperature of the bottom of a lake 

 cannot, it is true, be higher than that of the water immediate- 

 ly in contact with it, and consequently, not much above 38°. 75 ; 

 but more heat will l)e communicated from the interior of the 

 earth, the warmer the strata are which lie inunediately beneath 

 the bottom of the lake. Near the Lake of Geneva, a tempera- 

 ture of 62°. 8 was found in a sounding well at the depth of 680 



* I may perhaps have an opportunity of making such observations in the 

 Laacher Sec, where a depth of 200 feet has been found, witliout there beino 

 any reason for supposing that to be the deepest spot. 



