144 Thirty-fifth Annual Meeting 
we should find in our lakes in October, November, or December, 
according to the depth and area of the lake. Now, if the wind 

FiG. 1—Circulation caused by wind in lake of uniform temperature. 
blows from one direction, for any considerable length of time, 
it will start a current of water moving in the direction of the 
wind towards the leeward side of the lake; when the water driven 
by the wind reaches this side of the lake it has to return; a com- 
paratively small portion passes around the lake: more of it turns 
down on the shore, comes back at various depths along the bot- 
tom, or wherever it may find an opportunity. As a general re- 
sult, if the lake has a uniform temperature from top to bottom, 
the mass of water, even though it may be 159 or 200 feet in depth, 
is set into rotation by the wind with comparative ease, so that 
the bottom water is brought to the top and vice versa, and a very 
thorough mixture of the water is made to all depths of the lake. 
Under these circumstances all the water of the lake is brought 
into contact with the air and becomes saturated with oxygen. 
In the spring after the ice leaves the lakes, the conditions 
are substantially the same for a little while; the temperature is 
practically uniform, the water is set into rotation, kept in cir- 
culation and aerated by the action of the wind. But as the water 
warms, the conditions become different; and the warmer water 
Ne 

Fic. 2—Circulation in lake during summer. A.—Warm layer. B.—Cool water. 
of the surface becoming hghter than the cooler water of the bot- 
tom, tends to float upon it. As the days grow longer, the in- 
fluence of the sun becomes greater and as the mights become 
warmer, the cooling at night becomes less, with the result that 
the surface water gets very considerably heated. Under those 
circumstances when the wind blows the water across the lake, 1 
