Birge—Heat Budgets of American and European Lakes. 183 
difference between Como and Geneva, separated by only 0° 21' 
of latitude, than between Geneva and Mjosen, which lies over 14° 
further north. In view of the fact that Wettern and Ladoga 
show rather small annual heat budgets and that lochs Katrine, 
Lochy, and Morar, near neighbors of loch Ness, have budgets of 
no extraordinary size, it may well be questioned whether the 
large average budgets of Ness and Mjosen are not due to other 
causes than their northern situation. 
So far as the data now before us can warrant a conclusion, 
it would be that the variations in the annual heat budgets of 
these lakes lie within the range due to the variations of the 
weather of the season. Thus lakes, whose latitude ranges from 
that of Mjosen (lat. 60°) to Geneva (lat. 46°) or Seneca (lat. 
42°) may have identical budgets. We can not say that the mean 
budgets of the several lakes will not differ; indeed we may be 
sure that they will not be identical. But there is no reason to 
think that those of the northern lakes will have any great su¬ 
periority or that any difference found will be due to the latitude. 
In a word, there is no evidence that the annual heat budget in¬ 
creases with latitude, within the limits of the zone between 40° 
and 60° north. Still further, the data from these lakes do not 
show that a temperate lake has a larger heat budget than a trop¬ 
ical lake of comparable area and depth. 
The lakes of central Europe show on the whole smaller heat 
budgets than would be expected from American experience. The 
high winter temperature of these lakes which regularly freeze 
is noticeable, and this reduces the amount of that part of the 
annual heat budget which lies below 4°. Something of the same 
sort appears in the upper part of the budget also. While in the 
American lakes of the first class the wind-distributed heat will 
ordinarily exceed 25,000 cal., a decidedly lower figure would nat¬ 
urally be selected for those of central Europe. 
An instructive comparison may be made between lake Cajmga 
and Wurm-See which have closely similar mean and maximum 
depth. The comparison of their heat volumes shows that the 
main difference between these lakes lies in the upper 10 m. or 
20 m. (Figs. 2-3). This region in Cayuga lake, down to 15 m., 
has a temperature substantially uniform and high, while in 
