American Fisheries Society. 145 
does not have energy. enough to force the warmer, lighter water 
down to the bottom of the lake: so that, as the water gets to the 
leeward side, it 1s pressed down to a certain distance, but to a 
certain distance only, a distance depending on the temperature 
of the upper water, the force of the wind, the area of the lake, 
and other conditions that I need not specify. 
The net result of this contest between the wind, seeking to 
mix the water and the sun, which tends to keep the hehter water 
on the top, is that during the warmer season the circulation of 
the water is confined to a small portion of the lake, a portion 
which differs in thickness in different lakes. The water moves 
not along the bottom of the lake but a certain distance below the 
surface—perhaps 10 or 12 feet down in a small lake, and per- 
haps 20, 30 or even 40 feet in an inland lake of larger size. 
This continued action results in the formation of a compara- 
tively warm layer of water on the top of the lake, within which 
circulation is going on, and the water of which is more or less 
continuously turned oyer and exposed to the action of the air, 
and beneath that there is a lower layer of water which is cooler, 
which does not circulate, and which, for a time varying from a 
month or two in certain lakes to four or five months in others, 
is shut off from all direct access to the external air by the layer 
of circulating water on the top of the lake. 
You will easily see that the oxygen conditions of the water 
are very different indeed in these two portions of the lake. The 
story can perhaps best be illustrated by some of these diagrams, 
which show in a very general way what goes on in Lake Mendota 
during the open season of the year. 
Lake Mendota, on whose south shore he the grounds of the 
University of Wisconsin, is a lake about 6 miles long, + miles in 
width and 84 feet in depth in the deepest portion ; several square 
miles of the lake are more than 7) feet in depth, and the water 
reaches a depth of 50 or 60 feet pretty close to the shore; the 
shore is fairly steep, reaching a maximum height of 150 feet 
above the lake. 
Figures 3 to 8 show the temperature and oxygen of the 
water from April to November, 1905. In each diagram the 
vertical column of figures represents the depth in meters from 
the surface down to 22 meters, which is as deep water as you 
