A UMNOLOGICAL STUDY OF THE FINGER LAKES. 
559 
Table vii shows that the mean temperature of the deeper lakes is lower in summer 
and higher in winter than that of the shallower, and that the difference between sum- 
mer and winter temperatures (or the annual range of temperature) is smaller in the 
case of the deeper lake. This must obviously be true if lakes are similar in other respects 
but differ in depth.® The relation in these lakes between area, depth, and mean tem- 
perature is much more interesting than this simple statement indicates and will be 
described on a subsequent page. 
ANNUAL HEAT BUDGET. 
Forel was the first limnologist — first in this as in so many other matters — to deter- 
mine the amount of heat absorbed by a lake. He computed the number of calories 
necessary to raise a column of water of unit base in the deepest part of the lake to the 
temperature found in summer and he compared on this basis the amount of heat gained 
by different lakes. This method obviously permits accurate comparison only between 
lakes of similar area and depth. Halbfass in 1905 improved the method in that he 
determined the mean temperature of the whole mass of the water of the lake. Klnowing 
the volume of the water he was able to compute the total number of calories contained 
in the lake and also those gained or lost by the lake as it warmed or cooled. In an 
elaborate paper he gave the result of this method as applied to many European lakes.^ 
If this method is employed it is still necessary to select for comparison lakes of similar 
area and volume. 
In our judgment, if the heat budgets of lakes are to be compared at all, it is best 
to employ units of measurement of such a character that all lakes may be compared 
with each other, and such that this comparison ma^, if possible, reveal the relation of 
area and depth to the amount of the heat budget as well as the relation of geographical 
position and climate. Since all heat is taken in and given out by the surface of the 
water it seems best to us to express the amount of heat in the water and its variations 
in terms of calories per unit of that surface; and on the whole we have decided to 
employ the same units as those used by the meteorologist for measuring the energy 
received by the earth from the sun — the gram-calorie and the square centimeter. 
If the mean temperature of the w’ater of a lake is known, it is easy to compute the 
amount of heat which was received by the lake in order to produce this temperature. 
If the mean temperature of the water is multiplied by the mean depth in centimeters, 
the result will be the number of gram-calories which the lake must receive on each square 
centimeter of its surface in order to raise the temperature of the w^ater from 0° C. to the 
temperature observed. The following table gives this result for the six major New York 
lakes and also for Green Lake, Wisconsin. 
flFor a clear statement of this see Wedderbum, E. M., Temperatures of Scottish lochs, in Bathymetrical survey of the 
fresh-water lochs of Scotland, vol. i, p. 97, 1910. 
ft Halbfass, W.: Ergebnisse neuerer simultaner Temperaturemessungen in einigen tieferen Seen Europas. Petermanns 
Mitteilungen, 1910, bd. n, p. 59, 
