26 



IOWA STUDIES IN NATURAL HISTORY 



TABLE 16- 



-CONTRIBUTION OF THE SUN TO THE DISTRIBU- 

 TION OF HEAT 



Note: The calories which pass through each square centimeter are 

 stated in terms of the area at that depth, in the column headed Cal. 

 The dii-ect radiation which reaches the lake bottom, say between 100 

 cm. and 200 cm. depth, is not available for warming the deeper water. 

 The contribution of the sun must therefoi'e be measured as a fraction 

 of the energy passing through each sq. cm. of the depth in question 

 and not in terms of the surface of the lake. 



It appears, therefore, that the sun may contribute 9000 

 cal. of the 19,800 cal. which pass through each square centi- 

 meter of the one-meter level ; that it can contribute only 760 

 cal. at five meters of the 15,000 cal. which pass that level ; 

 and that at eight meters only one per cent of the heat comes 

 directly from the sun. 



This is the maximum possible contribution if the observa- 

 tions represent average conditions, as they doubtless do 

 approximately. The results assume that none of the heat 

 due to direct insolation is lost; that for instance all of the 

 9000 cal. delivered by the sun during the warming season 

 to the depth of one meter are part of the summer heat-in- 

 come, none being lost to the lake by cooling at night or dur- 

 ing cool periods. This assumption is manifestly far too 

 favorable to the sun, but at present we have no data on 

 which to correct it accurately. 



The data of table 16 may be applied to Fig. 3 and a curve 

 drawn through the points thus determined. Then the area, 

 ABDEH, represents the total work done in distributing the 

 heat to the depth of 25 m. ; the area, ACDB, represents the 

 contribution of the direct insolation to the work of distribut- 

 ing heat on the assumption that no sun-placed heat is lost ; 

 and the area, ACDEH, represents the contribution of the 



