394 THE FEESH-WATER LOCHS OF SCOTLAND 
what similar. This is never appreciably high, and the great variability 
which might be expected in lakes of which some are of the con- 
spicuously tropical type, others, owing to the exceedingly long 
duration of the ice-covering, almost of the arctic type, is not met with. 
Neither Vierwaldstiittersee (Amberg) nor the Lake of Zurich 
(Pfenninger) generally exceed 22° C. even in the warmest summer 
time, Lac Leman at most 23° C. Lakes under even almost arctic 
conditions may, on the other hand, have temperatures which, though 
of brief duration, are high (12-15° C.) (see Zschokke, Pitard, Monti). 
Even in lakes frozen during about 300 days in the year, the summer 
temperature still attains 6 and probably more. The reasons for the 
comparatively slight difference in summer temperature are principally 
that the affluents are to a pretty considerable extent derived from 
glaciers and thus everywhere carry very cold water, that the littoral 
zone, which in the Baltic lakes plays the great heating part, is of 
small extent in the alpine lakes, that the organic processes in these do 
not work with such an intensity as to raise the temperature, and 
finally, that many of the less highly situated lakes and those from 
which we have most information are very large lakes with great depths, 
while the high alpine lakes are small and comparatively shallow lakes. 
Zschokke maintains, as the result of the records of the temperature in 
lakes over 1500 m. above sea-level, that the summer temperature does 
not exceed the deep-lake temperature in the less highly situated, 
somewhat large lakes ; still, I think that it might also be emphasised 
as a biological factor, that these high alpine lakes so often attain 
relatively high temperatures. Whether the latter temperature extends 
over short or long periods is of slight importance : the main point, 
according to my opinion, is that the temperature is reached. One 
thing is at any rate certain : all differ from the Baltic lakes in having 
a lower summer temperature (even in the larger of the latter lakes 
this is often about 24° C.) ; further, many of them have a much smaller 
range of temperature variation. In Lac Leman it is, for instance, 
only 4-22° C, in the high alpine lakes rarely over 0-12°, in the Baltic 
lakes 0-25°. Finally, it may be emphasised that the warming up and 
cooling in the various lakes, according as these approach the tropical 
or the arctic type, differ greatly, and that the length of the summer 
and the winter periods of stagnation respectively, as well as of the 
periods pf circulation, must likewise vary much more in the lakes of 
this than of the other zones. 
The water is on the whole remarkable for its great transparency ; 
in the Lake of Geneva the white disc has been visible even at a depth 
of 21 m. in February, in July at a depth of only 4 m. (Delebecque, 
1898a, p. 179). The mean for transparency in the Lake of Geneva 
is 10'2 m., in Vierwaldstattersee 9*4 m., in the Lake of Zurich 6"5 m., and 
