106 THE FKESH-WATER LOCHS OF SCOTLAND 
temperature may be 39° Fahr. or higher, and the layers nearer the 
surface are all of an equal or higher temperature. 
A lake may in winter belong apparently to two classes. A series 
of temperatures taken in a shallow bay will reveal a lake apparently 
of the temperate or polar type, the colder water being at the sur- 
H 
eating 
Cooling Heating 
Cooling 
Spring 
Summer 
Aurumn Winter Spring Summer 
AuPumn Winter 
Fig. 39.— Polar Lakes. 
face, or the surface may even be covered with ice ; while a series of 
temperatures taken in the deepest portions, of the lake will indicate 
that it belongs to the tropical class — the temperature being all over 
89° Fahr. Further reference to this state of affairs will be made 
when discussing the formation of ice, but an illustration is given in 
Fig. 40. » 
fig. 40, which represents the section of part of a lake-basin, and shows 
temperature distribution where one part of the lake belongs to the 
temperate class and another part to the tropical class. This diagram 
is also reproduced from Le Leman with ForeFs kind permission. 
It is the tropical class which is of the greatest interest to the 
limnologist, as it includes the grandest and largest of our inland 
