222 



BULLETIN OF THE BUREAU OP FISHERIES. 



needed for distribution in the upper water as the temperature rises equally makes clear 

 the reason why the lower water soon ceases to gain heat as the season advances. 



In Seneca Lake work amounting to only 0.4 g. cm. is needed to distribute 512 cal. 

 to depths below 70 m. , while no appreciable amount of work is needed to distribute 347 

 cal. through the water below 80 m. The last statement is obviously not strictly accurate, 

 but it is not worth while to compute the work in those cases where the decrease in den- 

 sity due to increase of temperature is less than one part per million. 



Table 7. — Subtraction Curve Means: Amount and Per Cent of Heat in Summer Heat Income 

 AND DP Work Necessary to Distribute this Heat Found at the Surface and at Different 

 Depths of the Several Lakes. 



HEAT AND WORK AS MEASURED AT DEPTH. 



In Table 7 the data are given in terms of the surface of the lake — so many calories, 

 or gram centimeters, per square centimeter of surface. If the datum plane is taken 

 as the area of the lake at the depth in question, the number of calories and gram 

 centimeters at each level will be increased proportionally to the decrease of area as 

 compared with that of the surface; but the ratio between the amounts of work and of 

 heat would remain unaltered. This relation is shown in Table 8. Perhaps the most 

 interesting fact shown by it is the very close agreement between Cayuga and Seneca 

 Lakes in both heat and work after the surface level is passed. Approximately equal 

 quantities of heat pass through the 5 to 40 m. levels of both lakes. Cayuga Lake shows 

 at the surface considerably less heat per unit of area than Seneca has, but this is largely 

 due to the great area of shallow water at the north end of Cayuga Lake. No such area 

 is found anywhere in Seneca Lake. The area of Cayuga Lake at 5 m. is about 79 per 

 cent of the surface area, while that of Seneca Lake at the same depth is 87 per cent of 

 the surface. The area of the 10 m. level in C.ayuga is about 92 per cent of that at 5 ra., 

 and in Seneca about 93 per cent of the 5 m. level, a very close correspondence, which 

 shows itself in the heat and work. The large area of shallow water in Cayuga Lake 

 adds nothing to its effective area in absorbing heat, nor does it seem to diminish the 

 efficiency of the lake. Canandaigua Lake, however, is plainly less efficient than either 

 of the others. Its smaller area and higher banks cause this condition, since both factors 

 lessen the efficiency of the wind. (See Birge and Juday, 1914, Pis. CXIII, CXIV, CXVI.) 



