16 IOWA STUDIES IN NATURAL HISTORY 



TABLE 11— DISTRIBUTION IN DEPTH OF SUMMER HEAT-IN- 

 COME STATED IN CALORIES PER SQUARE CENTI- 

 METER OF THE SURFACE OF THE LAKE 



It must be noted that this table and the account of the 

 heat exchanges of the lake in general take no account of the 

 heat which passes through the water and goes into the mud 

 of the lake. It is obvious, for example, that if the bottom 

 water at the deepest part of the lake stands at a tempera- 

 ture above 10° for months, no small quantity of heat must 

 pass through it into the mud ; and this heat must ultimately 

 be supplied from the surface. At less depths the passage 

 of heat into the bottom mud or sand must be even more 

 rapid since the temperature of the water is higher. The 

 number of 21,000 cal. does not represent the total heat 

 which passes through the water of the lake, but that which 

 remains in the water. Nothing is known about the mud 

 temperature of Okoboji lake, or indeed of any lake, except 

 Mendota; and information is far from complete regarding 

 that. Perhaps 10% is a fair estimate .of the addition which 

 should be made if the heat of the mud is to be accounted 

 for. This statement is not made with any idea of making 



