30*0 jM- BischofF on the Temperature of' 



librium with the temperature at the bottom. If in this case the 

 lake be cooled down to 38°.75 up to the surface, ascending cur- 

 rents will again take place, the temperature of which will be be- 

 low 38°. 75, and the upper beds will be cooled down by degrees. 

 But if the surface of the lake be warmed to as many degrees 

 above 38°.75 as the bottom is cooled below 38°.75, no current 

 whatsoever will take place. 



Such relations must exist in the elevated alpine lakes, for ex- 

 ample, on the Gemmif the Grimsel, St Gothard, St Bernhards 

 and so forth, where the mean temperature at the bottom of the 

 lakes is near 32°. It would therefore be very interesting if ob- 

 servations wei'e to be made on the temperature in the depths of 

 such lakes. For this purpose the small lake near the monastery 

 on the Great St Bernhard, which has a depth of 33 feet, is pecu- 



direction in which the above observations were made, this would be another 

 argument in favour of our hypothesis. The increase of temperature ob- 

 served in receding from the shore, seems to be connected with this pheno- 

 menon. For as the surface of the earth on the shore was cooled to a much 

 greater degree than the ice, the temperature of the air near the shore must 

 be found lower than at a greater distance from it. 



It is very much to be wished that accurate observers may on future occa- 

 sions keep these points in view, in order that they may, by means of ob- 

 servations directed immediately to this subject, place that in a clear light 

 which at present can only be given as conjecture. 



Von Humboldt (Reise, &c III. 131) mentions, that the temperature at 



the surface of the Lake of Valencia, during his stay in the valleys oi Aragua, 

 in the month of February, remained constantly between 73°.4 and 74°.75. 

 It was from 1°.03 to 2°.34 below the mean temperature of the air, whether in 

 consequence, as he remarks, of the evaporation, by which the water and the 

 air are deprived of a portion of their heat, or because the changes of tem- 

 perature in a large body of water do not keep equal pace with those of the 

 air, and because small streams run into the lake, proceeding from several 

 cold springs on the neighbouring mountains. But as this philosopher found 

 in Cumana that the temperature of water, after having been exposed to the 

 sun in vessels during seven or eight hours, was always 1°.8 to 3°.24 lower 

 than the temperature of the air in the shade ; then, supposing the depres- 

 sion of temperature, caused in the lake by evaporation, to be equal to this, 

 we should have for the real temperature of the lake, independently of this 

 depression, a de;;ree 0°.720 to 0°.9 higher than that of the air ; without 

 taking the other causes of a depression of temperature into consideration. 

 We are therefore justified in assuming that rising warm currehts of water 

 also exist in the Lake of Valencia. 



