344 Wisconsin Academy of Sciences , Arts , and Letters . 
Ideal cases are easily pictured: if the heat were delivered 
from the sun in small quantity and the wind were steady and 
violent, the surface current would have enough energy to 
overcome the thermal resistance to mixture, complete cir¬ 
culation of the water would result, and the heat delivered to 
the surface would be distributed to the entire mass of the 
lake. The temperature of the water would be approximately, 
but not exactly, the same at all depths. 
If there were no wind, or only light baffling airs, and the 
sun shone continually from a clear sky, the surface water 
would be highly heated and the deeper water would not be 
warmed at all. 
As matters actually go in a lake, the advantage in the 
contest between sun and wind turns now to one side and now 
to the other. But on the whole the wind has the worse until 
in late summer or autumn the sun’s energy declines and the 
water begins to cool. 
The work of the wind may be stated in another way. If 
the wind creates a horizontal current in the water the re¬ 
sistance which the water offers is that due to viscosity and all 
the work done is done against friction. If the water is ho- 
mothermous the same statement holds for the vertical 
component of the movement of the currents as guided by 
shores and bottom. But if the surface strata of the water 
are less dense than those below, any downward movement of 
these upper strata involves the doing of work against gravity; 
the lighter water must be forced down against the resistance 
of the denser. So far as the wind accomplishes this task it is 
the agent for distributing heat through the water of the 
lake, and the amount of this work done is the measure of the 
vertical component of the work of the wind. 
Obviously, it is impossible to ascertain the full amount of 
work thus done by the wind. The wind may blow all day, 
crowding the warm water to one side of the lake and depres¬ 
sing the cooler water. But at night the wind ceases and the 
water settles back, almost or quite reaching its original 
position. Work has been done against gravity but little or 
no record of it has been kept. It is like work done in com¬ 
pressing a spring which returns to its original position as soon 
as the pressure ceases. But if the water does not settle back 
to the position held before, if the isotherms at the junction 
