Birge—Work of Wind in Warming a Lake. 365 
Thus the temperature gradient in the thermocline is so 
steep that very little mixture takes place there except when 
the wind is exceptionally violent; and when the wind is 
strong, so that most of the epilimnion is pushed to one side of 
the lake, this process compresses the thermocline and greatly 
increases the temperature gradient at the very place where 
the currents set up by the wind are most vigorous. The same 
statement may be made of oscillations, like the temperature 
seiche. These are robbed of much of their possible effect in 
warming the lake by the fact that each of their movements 
which tends to mix the water at the junction of epilimnion 
and thermocline increases also the thermal resistance to 
mixture in that stratum. 
Thus much of the energy of the wind is lost so far as con¬ 
cerns its effect in distributing heat. The temperature gradient 
in the thermocline acts somewhat like a spring resisting the 
action of the wind. It is compressed when the wind pushes 
harder; it releases when the wind dies down; but in either 
case its resistance is proportioned to the vigor of the pressure 
on it, and it is almost equally effective at all times in prevent¬ 
ing mixture. Hence there are found only small results, or even 
none that are measurable, from violent winds in summer, un¬ 
less these are accompanied by a decided fall of temperature. 
The distribution of heat below the thermocline depends on 
currents indirectly occasioned by the wind, and therefore 
relatively feeble under ordinary conditions. They operate, 
however, on water which has a decreasing thermal gradient 
and therefore are in position to effect some mixture. The 
discussion of this matter, however, does not belong in this 
paper. 
16. THE “STABILITY 55 OF A LAKE. 
In 1915 Professor Wilhelm Schmidt of Vienna published a 
very thoughtful and suggestive paper entitled “Uber den 
Energiegehalt der Seen” (Schmidt T5). In this he discussed 
a subject nearly related to that of the present paper, under 
the head of the stability of a lake. The meaning of this 
may be stated as follows: A lake in a homothermous condi¬ 
tion is in a condition of indifferent equilibrium; if its upper 
strata are warmed the equilibrium becomes proportionally 
stable. The amount of this stability may be measured by the 
