TEMPERATURE OF SCOTTISH LAKES 
109 
Before going on to deal more in detail with the temperature in 
fresh-water lakes reference may be made in passing to the difference 
of the conditions in salt Jakes. The differences in density which may 
occur in the same lake owing to varying degrees of salinity produce 
complications. As an example of the temperature distribution which 
is possible, reference may be made to the lakes of Austria-Hungary,^ 
some of which contain as much as 25 per cent, common salt. In 
Lake Medve, which has an average depth of 32 feet, the surface 
temperature during summer varies from 68° to 86° Fahr. Below the 
surface the temperature I'ises gradually, and at a depth of about 
4 feet reaches a maximum of about 133° Fahr., after which it again 
falls to about 86° Fahr. at a depth of 18 feet. Other lakes show simi- 
larly a median zone of water of high temperature, and the point of 
maximum temperature is also found to be the point of maximum 
salinity. 
Surface Temperatures 
As it is through the surface that a lake gains and loses its 
greatest quantity of heat, the investigation of surface temperatures is 
always of importance, although it is not so interesting as the investi- 
gation of abysmal temperatures. 
The observation of the temperature of the surface is, of course, 
easier than the observation of the temperature of deep water, as 
an ordinary mercury thermometer may be used. As a rule, when 
observing surface temperatures the reversing thermometer was used 
and immersed in the water at the surface. In this way it is not 
actually the temperature of the uppermost layer of water which is 
measured, but a sort of average of the temperature of the first 9 or 
12 inches of water at the surface. The observation of the tempera- 
ture of a thin film at the surface would be a matter of considerable 
difficulty, even on perfectly calm days, and I am not aware that any- 
thing has been done in this direction. 
In calm weather a diurnal variation in the temperature at the 
surface can be distinctly traced; but when there is a wind blowing, 
this diurnal variation is masked by the changes produced by the wind. 
Forel had at his command a great number of observations made 
in the Lake of Geneva, and by taking the means of morning and 
evening observations he found that the average diurnal variation was 
aboivfc 2°'5 Fahr., the maximum variation being about 8° Fahr. He 
1 See Professor Kaleczinsky, Scott. Geogr. Mag., vol. xviii. p. 317, 1902, and 
vol. XX. p. 216, 1904. Professor Kaleczinsky suggests that deposits of rock-salt 
will take place more rapidly in winter than in summer because of the reduced 
temperature of the water, and that deposits during summer will chiefly consist of 
anhydrite. The stratification of the deposits thus indicates their age. 
