Birge—Heat Budgets of American and European Lakes. 185 
at the point indicating the number of calories necessary to raise 
its water from 4° to the winter temperature. It represents in 
some sense the permanent stock of heat in the lake above 4°. 
The diagram shows, for instance, that Cayuga lake is a tem¬ 
perate lake; that its annual heat budget is about 37,000 gram 
calories per square centimeter of the surface of the lake; that 
of these calories more than 9,000 lie below 4° ; that about 28,000 
cal. constitute the summer heat-income. It shows that lago di 
Bolsena is a tropical lake; that at its winter temperature the 
water still contains wind-distributed heat to the amount of about 
29,000 gram calories per square centimeter of its surface; and 
that its annual heat budget is about 32,000 calories. 
The temperature of 4° is taken as the starting point for four 
reasons: 
1. The lakes all approach within a measurable distance of 4° 
in the autumn or winter of each year. 
2. If 0° C. were taken as the starting point the distances 
from zero line to beginning of lake line would vary greatly. In 
Owasco lake in 1911 only about 2,400 cal. would remain in the 
lake; in Mjosen some 60,000 cal.; in Seneca nearly 64,000 cal.; 
in Como 126,000 cal. Yet Mjosen had lost below 4° some 15,000 
cal. per sq. cm. and Owasco only about 6,200, and it is well worth 
while to bring out this fact. 
3. Losses of heat below 4° and gains of heat up that point are 
subject to very different laws from those at a higher temperature 
and should be treated separately.' 
4. The principal reason, however, for placing the zero line 
at the temperature of 4° is the fact that in this way the place of 
the lake in Forel’s classification can be shown in the diagram, as 
well as its position in the class to which it belongs. 
The diagram shows the main facts of Table A in a way which 
conspicuously strikes the eye. Among th.em I should name: 
1. The small heat budget of a series of mid-European lakes 
from Thuner See to Traun-See, and their general'uniformity in 
spite of differences of size, depth, situation, etc. The differences 
(with two possible exceptions—the largest of Zuger See and 
Traun-See) all seem to be within the possible range of annual 
variation of any one of the lakes. 
