24 IOWA STUDIES IN NATURAL HISTORY 



one-meter stratum is transmitted to the surface of the next 

 one-meter stratum. Stated in terms of absorption, about 

 46%-47% of the energy of the solar radiation present at any 

 depth below one meter is absorbed in passing through a 

 stratum of that water one meter thick. Thus it appears 

 that only a small fraction of the solar radiation penetrates 

 the lake to the depth of one meter and that this small 

 amount is very rapidly absorbed. 



If we assume that the conditions present at the time of 

 observation are average ones, we may compute the general 

 role of the sun in distributing heat. For this purpose we 

 estimate as before that the sun delivers to the lake 60,000 

 cal. cm.^ during the warming season. We take from the 

 pyrlimnimeter reading that 22% of this reaches a depth of 

 50 cm. ; that 15% penetrates to the depth of 100 cm. ; and 

 that the transmission below 100 cm. is 54% per meter. The 

 result is then as follows : 



TABLE 15— HEAT DELIVERED TO THE SURFACE OF THE 

 LAKE DURING THE WARMING SEASON AND THE 

 AMOUNT DELIVERED TO VARIOUS DEPTHS BY 

 DIRECT INSOLATION. STATED IN CALORIES PER 

 SQUARE CENTIMETER OF HORIZONTAL SURFACE 



Depth Cal. 



60,000 



13,200 



9,000 



4,860 



2,620 



1,440 



760 



410 



220 



120 



65 



35 



Table 15 shows in a very striking way how the direct in- 

 fluence of the sun is confined to the surface strata. At the 

 depth of 5 m., for instance, there would be directly delivered 

 only 760 cal. during the entire warming season of four 

 months, or only about 6 cal. per day. The total is less than 

 may be delivered to the surface in a day. At the same rate 

 of transmission only 35 cal. would reach the depth of 10 m. 



