Birge—Heat Budgets of American and European Lakes. 209 
does not seem possible that they give correctly the mean tem¬ 
perature of the lake. 
Professor Halbfass has kindly informed me of certain correc¬ 
tions to be made in his computations for Mjosen. The volume 
of the lake should be 67.1 cu. km. not 69.1 eu. km. as given in 
Halbfass’ paper of 1910. This change reduces the mean depth 
to 186.7 m. instead of 192.9 m. I have computed again 
the temperatures of the lake on this basis and find them some¬ 
what different from those assigned by Halbfass. A source 
of difference besides the change in volume is in the method of 
computation. If in the 0-10 m. level, for instance, readings are 
given at 0 m., 5 m., 10 m. Halbfass takes that at 5 m. as repre¬ 
senting the mean of the stratum; while I use the mean of the 
three readings. 
The changes in computation make no essential difference ex¬ 
cept in the summer of 1901. There can be no doubt that the 
water south of the island of Helgoen was unusually warm dur¬ 
ing that season. The mean temperatures are as follows: Apl. 17, 
3.17° ; May 26, 3.62°; July 1, 5.27° ; Aug. 7, 6.41°. The follow¬ 
ing table shows the gains computed for a mean depth of 187 m. 
TABLE 5 
GAIN OP HEAT, MJOSEN 
Date 
Gain of 
temperature 
Calories 
Calories 
per day 
Apl. 17-May 26 . 
0.45® 
8,400 
215 
May 26-July 1 .. 
1.65° 
30,900 
858 
July 1-Aug. 7 .. 
1.14* 
21,300 
576 
60,600 
497 
A gain of nearly 61,000 cal. in 122 days is quite impossible. So 
great a gain as nearly 600 cal. per day during July seems be¬ 
yond the credible and a gain of over 850 cal. during June is very 
far above anything that can be accepted. The gains during 
April and May are normal. 
The observations were made south of the island Helgoen, which 
is about 4.0 km. long and lies some 50 km. from the north end 
14 — S. A. 
