Birge—Work of Wind in Warming a Lake. 343 
The surface water would be stripped by the wind from the 
windward side of the lake and transported to the leeward 
side; it would return along the side or bottom to the wind¬ 
ward side. Thus the entire mass of the water of the lake may 
be brought into a sort of rotatory movement and a complete 
circulation may be set up. 
This programme is interfered with and modified by two 
factors: (1) the cessation or change of the wind; (2) the 
warming of the surface water by heat from sun and sky. 
We have nothing to do at present with the first of these 
factors, but are concerned with the second. 
The effect of the heat received by the surface of the water 
is to warm the upper stratum and thereby decrease its den¬ 
sity. This decrease of density tends to keep the water at the 
surface as it moves and to prevent the influence of the wind 
from reaching the deeper strata. This warmer and lighter 
water pushed along by the wind is stopped as it approaches 
the leeward shore. It accumulates there, piling up on top 
of the cooler water whose surface it depresses. 
If the wind soon dies out the movement of the water 
ceases; the cooler water settles back into place and the warm 
water spreads out above it as a surface stratum. In this 
process there will be some mixture of the warmer and the 
cooler water at the junction plane. 
If the wind is strong enough and continues long enough 
and if the surface water is not too much warmed, part of the 
warmer water will be pushed down into the cooler water and 
mingled with it. But in general most of it will return to the 
windward side of the lake in horizontal currents on top of the 
cooler water. As it flows along the cooler water there will 
be more or less mixture with consequent warming of the 
colder and deeper lying strata and cooling of the surface 
strata. 
There arises in this way a continual contest between wind 
and sun. The wind creates surface currents in the water 
of the lake which, guided by shores and bottom, would 
establish and maintain a complete circulation. The sun 
warms the surface stratum and so tends to confine the wind 
currents to the surface. From the interaction of these two 
forces result the phenomena of the actual warming of the 
lake. 
