26 THE AMERICAN NATURALIST. (VoL. XXXII. 
where the water circulates to considerable depths, the at- 
mospheric temperature is usually higher than the water 
temperature. 
The temperature at the bottom of Lake Cochituate during 
the winter, when the surface is frozen, is not far from that of 
maximum density (39.2° F.). The heaviest water, therefore, is 
at the bottom and the lightest. at the top, the intermediate 
layers being arranged in the order of their density. Repeated 
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TEMPERATURE OF LAKME COCHITUATE 
30° | | | 30° 
JAN. | FEA | MAR | APA | MAY | JUNE | JULY | AUG | SEPT | OCT. | NOV | DEC. 
Fic. 1. 
observations have shown that the colder water occupies a com- 
paratively thin stratum under the ice, and that the temperature 
at a depth of ten feet is not much lower than at the bottom. 
With these conditions the water is in comparatively stable 
equilibrium. There is no tendency for it to circulate vertically. 
It is in a condition of “ inverse stratification,” as Forel calls it, 
when the colder water is above the warmer. It is the “ period 
of winter stagnation.” In the spring when the ice breaks up 
the cold surface water becomes mixed to a certain extent with 
the warmer water below it, and the bottom temperature drops 
