FINGER LAKES OP NEW YORK. 



215 



Table ,2. — Distribution op Summbr Hbat Income to Thermai, Regions op Lakes — Continued. 



SENriCA I,AKE. 



SUMMER HEAT INCOME. 



The summer heat income represents the gains in heat of the water of the lake 

 above the temperature of 4°- This notion was put forward in our paper on the New 

 York Lakes (Birge and Juday, 1914, p. 562) under the name of "wind-distributed 

 heat." In the following year (1915, p. 167) I proposed the name "summer heat in- 

 come" for the same gains of heat, preferring a term which does not imply any theory 

 as to the method of distributing such heat. For reasons discussed in the same paper 

 (19 15, p. 186) the summer heat income of lakes can be used as an index of their heat 

 exchanges in much the same way as the annual heat budget. The heat income, like 

 the heat budget, is stated in gram calories per square centimeter of lake surface. For 

 the sake of brevity it is ordinarily stated as so many calories without adding in every 

 case the qualifsdng terms. The whole question of heat budgets and the methods of 

 computing them is discussed in the paper aheady referred to (Birge, 1915). 



Table 2 shows the summer heat income of the three lakes concerned. Canandaigua 

 Lake shows a mean income of nearly 27,000 cal./cm.^, ranging from 23,000 to more 

 than 29,000 cal. Cayuga Lake has an income of about 29,500 cal., ranging from less 

 than 27,000 to nearly 31,000 cal. The income of Seneca Lake is about 34,000 cal., 

 ranging from less than 30,000 to nearly 38,000 cal. The smaller income of Canandaigua 

 Lake is mainly due to the thinner epilinmion, which, in turn, is due to the smaller size 

 of the lake and to the protection from wind afforded by its high shores. So far as 

 area goes, Cayuga Lake is as well off as Seneca and its depth is ample to secure as large 

 an income. But while the epilimnion in both lakes is 15 m. thick, its reduced thickness 

 is nearly 12 per cent less in Cayuga Lake than in Seneca. This fact is due to the large 

 extent of shallow water at the north end of the lake, which causes a corresponding reduc- 



