Lakes 295 



An exceedingly important and valuable series of observa- 

 tions was made by Fischer and Brunner 1 in the Lake of Thun 

 throughout the course of a whole year (March 1848 to February 

 1849). They used, after Saussure's method, thermometers 

 protected by non-conducting envelopes, which were pulled up 

 as quickly as possible. The depth of the water where they 

 observed was 540 feet, and they made a series of observations 

 of the temperature at that depth, at the surface, and at eleven 

 intermediate depths, and repeated the series of observations 

 at eight different dates over the year. From these series, which 

 afford the first information of the yearly march of temperature 

 at different depths, we learn that the lake as a whole gains 

 heat till the end of September, then loses it until the month 

 of February, when it begins to warm again, though slowly. 

 The maximum temperature occurs in October at depths from 

 the surface to 70 feet, in November at depths from 70 to 120 

 feet, in December from 120 to 200 feet, and in February at 

 500 feet. As the whole yearly variation of the temperature at 

 200 feet is less than a degree, the epoch at which the greater 

 depths attain their maximum and minimum temperatures 

 cannot be certainly deduced from one year's observations. 

 The minimum temperature of depths from the surface to 80 

 feet is attained in the month of February, at greater depths in 

 the month of March. During the course of the whole year 

 the temperature at the bottom varied between 40-7 and 

 40-9 Fahr., and in the month of February the whole of the 

 water from the surface to the bottom was between 40-7 and 

 41 Fahr, 



:id other observations showed that, from depths of 

 400 feet, the variation of t -mp T atun- with increasing depth 

 is quitr insignificant. BO that even though the lake might be 

 1000 feet deep the temperature at 400 feet is only one or two 

 truths of a derive different from that of the bottom; further. 

 MII many <>f tin- thermometers recmtlv UM-d. it is impossijr 

 distinguish with certaintv temperature ditlerini; by less than 

 half a degree, consequent I v it was not difficult t<> believe that 

 in all di considerable stratum of water which 



1 Mtm. Soc. Pkys. Gentve, xu. p. 255. 



