Birge—Heat Budgets of American and European Lakes. 201 
the observations were made may cause minor variations in the 
record. This period may have been cold, warm, or average, and 
the temperature of the water will vary accordingly. In Wet- 
tern, the mean temperature for July 11, 1900, was 7.39° (Halb- 
fass, ’10, p. 62) ; Aug. 12, 6.48°; Sept. 2, 8.09°. There is thus 
indicated a loss of 3,000 cal. between the first and the second 
observation and a gam of 6,300 cal. between the second and third. 
The middle of August normally gives a reading close to the 
maximum, but on this occasion the series taken on August 12 
would have given an annual budget nearly* 20% smaller than 
it really was. 
Thirdly, the mean temperature derived from the observations 
may not fairly represent the mean temperature of the lake. This 
general question has been discussed elsewhere in this paper. 
There are but few cases which seem to be of this character. The 
most conspicuous is that of Mjosen in 1901, which is discussed 
later in these notes. 
Nos. 1-7.—The mean depths of the American lakes are given 
to tenths of a meter, not to indicate greater accuracy of the sur¬ 
vey, but that the number of calories may agree with those stated 
in the paper of Birge and Juday (T4) to which reference has 
been made. This paper should be consulted for a more com¬ 
plete account of the heat budgets of these lakes. 
In the case of Green lake, the summer temperatures are given 
to the first decimal place only. This has been my usual custom 
with the temperatures of Wisconsin lakes, and it did not seem 
worth while to recompute the temperatures of Green lake, sincer 
in the case of any lake the value of the figure in the second 
decimal place is wholly uncertain. In all summers when the 
lake was visited more than once, the highest reading has been 
taken. 
The uniformity of the heat budget is noteworthy, especially 
in comparison with the irregularities of most European lakes. 
The maximum departures of the annual heat budget from the 
mean are about 7% in either direction; those of the summer 
heat-income are less. This last fact means that there is more 
difference between the loss of heat below 4° than in the gain 
of heat above this point. That part of the heat budget which 
lies below 4° ranged from 6200 cal. in 1911 to 10,500 cal. in 
1901. It should be noted, however, that the higher temperature- 
was taken in March and would be above the minimum temper- 
