Birge—Heat Budgets of American and European Lakes. 177 
perature at all depths has been followed from day to day for a 
year, still less for a series of years. The Wisconsin Survey has 
done this for lake Mendota, but the results are still unpublished. 
The same is also to be said of the observations made by the sta¬ 
tion at Lunz. There is no lake the mean temperature of whose 
water is known in the same way as the mean temperature of the 
air is known for thousands of places all over the world. 
In the paper on the Finger lakes, reasons are presented for 
believing that a single series of observations taken near the 
middle of a lake gives a fair account of the mean temperature of 
its water at the time of observation. (Birge and Juday T4, 
p. 556.) I shall not repeat these arguments at this place, but 
will add somewhat to them, mainly from the facts contained in 
Table A. 
The mean temperatures of Green lake in nine seasons on dates 
varying from August 14 to September 8 show a total range of 
less than 1°, and a maximum range of about 12% in the wind- 
distributed heat. In those years in which observations were 
made on different dates, that one was selected which shows the 
highest result. I can not believe that so close an agreement 
would be present if the figures did not fairly show the mean 
temperature of the water. Certainly if the temperature of the 
water in different parts of the lake varies greatly, such a dif¬ 
ference ought to cause a more considerable variation in results. 
If such observations are “futile”, the futility ought to show 
itself by large variations in the temperatures indicated. 
In the paper on the New York lakes, attention was directed 
to the close agreement between the mean temperature of lakes 
as deduced from observations made at the center and that de¬ 
rived from the mean temperature of several series of observa¬ 
tions made along the axis of the lake. A maximum variation in 
the New York and Wisconsin lakes was found not exceeding 5%. 
The same result is reached if similar observations on European 
lakes are compared. Wedderbum (’07) gives several instances 
where temperatures in loch Ness were taken at Inverfarigaig at 
the center of the lake and at the same time near Ft. Augustus 
and Dores at its ends. The mean temperature of the lake, de¬ 
rived from the middle series and that derived from the three 
series, do not often differ by more than 0.1° C. This difference 
of temperature would correspond to a difference of about 1300 
cal. in a budget of about 40,000 cal. Such a difference of about 
12—S. A. 
