14 



IOWA STUDIES IN NATURAL HISTORY 



Cal. 0. *00 ■COO eOO MO 1000 iOOO 



ST o 16 10 JO ^o JO 60 70 aq so /co 



Fig. 2. Temperature, summer heat-income, and reduced thickness of Okoboji lake. 

 The curve RT gives at any point the reduced thickness in centimeters (p. 8) of a 

 stratum one meter thick whose center is at that point. The curve cal. gives the 

 summer heat-income per meter of depth corresponding to the temperature curve. It 

 is given in calories per square centimeter of the surface. It is derived from the two 

 other curves by the formula (T-4) ; RT. See p. 12. 



is multiplied by the sum of the five RT's. The reason is 

 obvious : The temperatures in the upper part of the stratum 

 are above the mean and are multiplied by the larger num- 

 bers representing the reduced thickness. The temperatures 

 in the lower part of the stratum, which are as much below 

 the mean as the others are above it, are multiplied by the 

 smaller RT. The result is to make the sum of the calories 

 greater in the 10 m.-15 m. stratum by as much as 75; in 

 the others the difference is small since the change of tem- 

 perature is not so great. The larger amount is the more 

 accurate, since it gives proper weight to smaller units of 

 thickness, but in general the difference is not worth noting. 

 In each case the summer heat-income of Okoboji lake would 

 be stated as about 21,000 cal. in 1919. The more accurate 

 total is convenient to use in discussing the distribution of 

 heat. Employing single meters to 20 m. and 5 m. intervals 

 below that depth, it is 20,933 cal. 



