No. 373-] CLASSIFICATION OF LAKES. 31 
In lakes of the polar type there is but one opportunity for 
vertical circulation (except in the third order), namely, in the 
summer season, when the water approaches the temperature of 
maximum density. In a lake of the first order, that is, in one 
where the bottom temperature remains constantly at 39.2°, the 
circulation period would be very short indeed, if not lacking 
altogether. In a lake of the second order circulation might and 
POLAR TYPE TEMPERATE TYPE TROP/CAL TYPE 
FIRST ORDER FIRST ORDEA FIAST ORDER 
POLAR TYPE TEMPERATE TYPE TROPICAL TYPE 
4a 
SECOND ORDER SECOND ORDER 
POLAR TYPE TEMPERATE TYPE TROPICAL TYPE 
pen 39.2" j JN 3q2° 
320° 320° 
THIRD ORDER THIRD ORDER THIRO ORDER 
CLASSIFICA TION OF LAKES. 
Fic. 3. 
probably would continue for a longer period. In a lake of the 
third order the water would be in circulation nearly all the time 
except when frozen. The minimum temperature limit indicated 
for this order, z.e., 32° at all depths, would be possible only in 
very shallow bodies of water, and would simply indicate that 
all the water was frozen; the temperature of the ice would 
probably be below 32° at the surface. It is probable that very 
few polar lakes exist. 
In lakes of the tropical type there is likewise but one period 
of circulation each year (except in the third order). This would 
