2l8 



BULLETIN OF THE BUREAU OF FISHERIES. 



DISTRIBUTION OF HEAT. 



The radiation from the sun which enters a lake is rapidly absorbed by the strata of 

 water near the surface. Even in very clear lakes only 20 per cent of the total radiation 

 passes the i-meter level, and only about 10 per cent passes the 3-meter level. (See p. 

 228.) All of the wanning of the deeper water and most of the warming of that which 

 lies below the surface meter is due not to insolation but to mixture of wanner water 

 carried down from near the surface. This mixture of warm surface water with the 

 cooler water below is effected by the wind for all temperatures above 4°. It involves 

 work against gravity, since the warmer water is lighter than the cooler. This work may 

 be measured and may be conveniently stated in gram-centimeters per square centimeter 

 of lake area. These facts were stated in our paper on the New York Lakes (Birge and 

 Juday, 1914, p. 562). The principles underlying these ideas were later published as a 

 special paper (Birge, 1916, pp. 341-391) and were applied to Lake Mendota. It was 

 there shown that about 1,210 gram-centimeters of work per square centimeter of area 

 are needed to distribute 18,400 gram-calories of heat per square centimeter of area 

 through the waters of a lake 24 meters in maximum depth and 12.1 meters in mean depth. 

 It is understood that in this statement the term "work" is not used in an exact sense; 

 since in it are included both the action of the wind in distributing heat, which is properly 

 work; and also the direct effect of insolation, which does not involve work. (See Birge 

 and Juday, 1914, p. 574; Birge, 1916, p. 360.) This division of the distribution of heat 

 between sun and wind is discussed later in the paper. For the present, however, the 

 matter is discussed as though the entire distribution of heat were due to wind. 



Table 4. — Temperature (T) in Degrees Centigrade and Gram Centimeters (G. Cm.) of Work 

 Necessary to Distribute the Summer Heat Income. 



[Note, — This table shows the " direct curve of work." i. e.. the work necessary to carry the warmed water from the surface to 

 the stratum in question. It is stated in gram centimeters per square centimeter of surface of the lake.l 



canandaigua lake. 



Depth in meters. 



5-10 

 10-15 

 15-20 

 20-25 

 25-30 

 30-40 

 40-50 

 50-60 

 60-70 

 jo-84 



T. G. 



18.0 

 10. 6 



8.2 



70 

 6.2 

 5.8 

 5-6 

 5-5 

 5-4 



236- 2 

 582.0 

 630. 7 

 199- I 

 100. 4 

 60.8 

 75- 2 

 57- 2 

 42. o 

 22.3 

 6.7 



T. G, 



20. o 

 19. 6 

 17- S 

 10. 6 

 6.2 

 5-4 

 4-9 

 4.6 

 4-5 

 4.4 

 4.5 



2, 022. 6 I. 583. 5 



224. 



505. 



589. 



199. 

 25 

 13 

 13. 

 6 



T. G. cm. 



21.5 

 20. 7 

 16. 9 

 10. 2 



7-4 



6.9 



6.3 



5-4 

 4.8 

 4.6 

 4- 5 



1916 



T. G. 



1.782.9 2,235.8 



240. 8 

 613.0 

 612. 4 

 288.3 

 149.8 

 106. I 

 104.9 

 63.8 

 36. o 

 17-4 

 3-3 



1918 



T. G. cm. 



20. 6 

 17.0 

 12. s 

 9.8 

 8.0 

 6.8 

 5-9 

 5-6 

 5,4 

 5-3 



324. 



568. 

 548. 

 309. 

 19J 

 106. 

 122. 



63. 



42. 



19 

 5. 



Mean. 



20. 6 

 17-5 



7. I 

 6.1 



S-S 



S-2 



S-I 



4.9 



G. cm. 



224.0 

 568.1 

 589.5 

 235-5 

 lOO. 4 

 64. 2 

 69- 3 

 39-6 

 24. o 

 12.4 

 2.9 



