AN UNREGARDED FACTOR IN LAKE TEMPER¬ 
ATURES. 
EDWARD A. BIRGE. 
[Notes from the Laboratory of the Wisconsin Geological and Natural History 
Survey. V.] 
In this paper I wish to call attention to one of the factors 
regulating the distribution of heat in lakes, which seems to 
have been overlooked hitherto. It is well understood that the 
heat of the sun is delivered almost wholly to the surface strata 
of a lake; most of it to the upper meter. This heat is distrib¬ 
uted from the surface to the lower strata by various agencies. 
Chief of these is the wind, which mixes the warmer surface 
water with the cooler water below. The efficiency of the wind 
as a distributing agent is opposed and limited by the thermal 
resistance to mixture offered by the decreased density of the 
warmed surface water. I wish to point out that the effective¬ 
ness of this thermal resistance increases as the temperature of 
the water, which the wind is trying to mix, departs from the 
temperature of maximum density and decreases as the tempera¬ 
ture approaches 4°. A given temperature difference causes 
a thermal resistance which varies according to the position of 
that difference on the scale of the thermometer. This vari¬ 
ability aids to explain many of the phenomena associated with 
the distribution of heat. 
It is a well known fact that the density of water is at a 
maximum at 4°, and that it decreases as the water is cooled be¬ 
low or warmed above that temperature. This fact lies at the 
