1904 - 5 .] Study of the Lakes of Scotland and Denmark. 407 
sides, the more will the temperature of the water differ from that 
of the air ; it will take a longer time to freeze over, but will 
remain ice-bound much longer than a shallow lake, and the 
temperature of the water will rise more slowly, never attaining the 
high temperature of the shallower lake. Only in one Danish lake 
(Haldso) does the temperature of the water appear to differ 
essentially from that of the other lakes. Thus the mean tempera- 
ture of the air in July 1901 was extremely high, — 19’9° C. 
(67*8° F.), and the surface temperature of all our lakes except 
Haldso was 21° to 23° C. (70° to 73° F.), while in Haldso the 
temperature never exceeded 18° C. (64° F.); in the winter 
of 1901-2 the other lakes were ice-bound from 39 to 65 days, 
whereas Haldso was only ice-bound for 35 days. It may be 
added that Haldso is one of our deepest lakes (about 120 feet), 
and has more precipitous shores than any of the others. 
The transparency of the water in our lakes is small, and varies 
regularly with the season of the year, being always greatest in 
spring, diminishing during the last days of April, and least in 
August. During the ice-bound period the water becomes much 
clearer, all the detritus and huge masses of phytoplankton being 
precipitated to the bottom.. 
The colour of the water in the Danish lakes in April, after the 
ice has broken up, is nearly always a bright blue, but this colour 
only continues till the beginning of May, when most of the lakes 
become of a yellowish-green colour, which continues to be the 
predominant colour till the frost sets in. In hot summers the 
surface is generally covered by a coating of “ wasserbliithe,” and 
then the colour changes to blue-green or green ; in cold summers 
no “ wasserbliithe ” appears on the deeper and colder lakes. 
As regards the chemical composition of the water, very few 
observations have as yet been made, but I hope this will soon 
be remedied. 
B. The Scottish Lakes. 
Comparing the natural conditions of the Danish lakes, as 
indicated in the foregoing pages, with those of the Scottish lakes, 
we shall find the greatest differences in nearly every detail. It 
must be borne in mind that geologically Scotland is a very old 
