296 



Lakes 



remains constantly at the same temperature, all the year and 

 ar, and that in winter this stratum thickens so as often 

 to fill the lake, and gets thinner again in summer. By the 

 improvement of the instruments both of these suppositions 

 have been shown to be erroneous. In summer and in tem- 

 perate latitudes, however deep the lake may be, its temperature 

 falls as the depth increases, first rapidly and then very slowly, 

 and the bottom temperature observed in any summer depends 

 on the nature of the winter which preceded it, and may vary 

 from year to year by one or two degrees. It was also believed 

 that the deep water of a lake preserved constantly the mean 

 winter temperature or the mean temperature of the six coldest 

 months of the year in the locality. This was deduced from some 

 observations by Sir Robert Christison in Loch Lomond, who 

 found the bottom temperature at Tarbet to be 41-4 Fahr., 

 agreeing with the mean of the six winter months as observed 

 at Balloch Castle, which, however, is about 15 miles distant. 

 Although the theory may be accidentally true for Loch 

 Lomond, it has been proved not to hold for other lakes. Thus 

 Simony (Wien. Sitz. Ber. 1875, LXXI. p. 435) gives the following 

 table, comparing the temperature of the bottom water in the 

 G'miinder See with the winter (October to March) air tem- 

 perature 1 : 



It will be seen .that, with the exception of the end of 1872, 



1 These air temperatures are those of the observatory at Vienna 

 corrected for difference of level. 



