Birge—An Unregarded Factor in Lake Temperatures. 997 
or more. The thermal resistance to mixture is, therefore, very 
great and it is increased by the processes which tend to cause 
mixture. When the wind sets up currents in the epilimnion 
and blows it to the leeward side of the lake, the accumulating 
mass of warm water presses the cooler hypolimnion downward 
and outward. The first effect of this process is to condense the 
isotherms at the very point where the influences tending to 
cause mixture are greatest. A decline of temperature amount¬ 
ing to 8° or 10° in a meter is thus often produced. In this 
way is developed a resistance to mixture several hundred times 
as great as any that is possible in April, or early May, and 
this lies exactly at the place where it is most effective in pre¬ 
venting mixture. Thus we may explain the fact that the ther- 
mocline is but little affected in summer, even by violent and 
long continued winds. 
During early and midsummer the temperature of the epilim¬ 
nion is not uniform but the surface is always somewhat warmer 
than the stratum immediately above the thermocline. Since 
the temperature of the surface at this time is high—from 22° 
to 25°, or even more—a small temperature difference between 
the surface and the strata below presents a very great resist¬ 
ance to mixture. This is an important factor among those 
which keep the thermocline at a practically constant average 
position during several weeks in summer. 
It is obvious that no fair comparison can be made between 
the ability of the wind to mix the water in summer and that 
which it may have in late autumn or winter after the surface 
has fallen below 4°. Many writers have found it hard to be¬ 
lieve that the wind is able to mix the water of a lake from 
top to bottom during the process of cooling below 4°. Richter, 
for example, ( ? 97, p. 49) finds it necessary to reject the wind 
as the agent in effecting this cooling, because the wind is 
not able to disturb the thermocline in summer. He is forced 
to set up a rather complex theory to account for the fall of 
temperature in deep water below 4°, and one which is not 
satisfactory to himself. In a similar way Ule ( ? 01, p. 124) 
neglects the wind in discussing this process while Groll (’05, 
p> 54) rejects it for lakes of considerable depth. Yet the re- 
