No. 373-] CLASSIFICATION OF LAKES. 29 
The horizontal divisions represent temperatures ‘in Fahrenheit 
degrees increasing towards the right, and the vertical divisions 
represent depth. The three types of lakes are designated as 
polar, temperate, and tropical. In lakes of the polar type the 
surface temperature is never above that of maximum density; 
in lakes of the tropical type it is never below that point; in 
lakes of the temperate type it is sometimes below and some- 
times above it. This division into types corresponds somewhat 
closely with geographical location. 
The three orders of lakes may be defined as follows: lakes 
of the first order have bottom temperatures which are prac- 
TAOPICAL TYPE 
POLAR TYPE TEMPERATE TYPE SUAPACE 
° . . : . . 
sE 8 Ys $ 
FIAST ORDER FIAST OARDER FIAST ORDEA 
POLAR TYPE TEMPERATE TYPE TROPICAL TYP, sunrae 
$ 
FCONO ORDER 
POLAR TYPE 
SECONO ORDEA 
CLASSIFICATION OF LAKES 
Fic. 2. 
tically constant at or very near the point of maximum density ; 
lakes of the second order have bottom temperatures which 
undergo annual fluctuations, but which are never very far 
from the point of maximum density; lakes of the third order 
have bottom temperatures which are seldom very far from the 
surface temperatures. The division into orders corresponds in 
a general way to the characters of lakes; 7.e., size, contour, 
depth, surrounding topography, etc. 
This classification is essentially the same as that recently 
proposed by Forel. He divides lakes into three types, polar, 
