166 Wisconsin Academy of Sciences, Arts, and Letters. 
THE HEAT BUDGETS OF AMERICAN AND EUROPEAN 
LAKES. 
Edward A. Birge. 
Notes from the Laboratory of the Wisconsin Geological and 
Natural History Survey. VII. 
This paper was suggested during the preparation of a report 
on the Finger lakes of New York, which was recently completed 
by Mr. C. Juday and myself, under the direction of the United 
States Bureau of Fisheries. (Birge and Juday, ’14.) That 
part of the report which deals with the temperatures of these 
lakes was written by myself and its preparation led me to com¬ 
pare the results of our observations with those made on similar 
lakes in Europe. 
In this paper an inland lake of the first class is defined as one 
whose size and depth are such as to permit the lake to acquire 
the maximum amount of heat possible under the weather condi¬ 
tions of the season. The lower limits for such lakes in the east¬ 
ern and central United States seem to be about 10 km. of length 
with, at least, 2 km. of breadth; and 30 m. of mean depth, which 
means 50 m. or more of maximum depth. Such lakes must also 
lie under ordinary conditions of topography and altitude. Lakes 
whose conditions of climate or location are exceptional, such as 
those of alpine lakes at considerable elevations, cannot be com¬ 
pared directly with those in lower and more normal situations. 
(See Birge and Juday, ’14, p. 561.) 
The European lakes selected for comparison are, with one or 
two exceptions, all of the first class as regards size and depth. 
They lie at elevations rarely exceeding 500 m., so that the eleva¬ 
tion can not play any considerable part in determining their 
heat cycle. These lakes are chiefly taken from lists used by 
