8 IOWA STUDIES IN NATURAL HISTORY 



most equal per cent of the surface. Half of the volume of 

 Okoboji lake is found above the depth of about 7.5 m., while 

 the same volume lies at the depth of 12.5 m. in lake Geneva. 

 Seventy-five per cent of the volume of lake Okoboji lies 

 above the level of 13.8 m., while in lake Geneva the same 

 level lies about 7 m. deeper, at 21 m. 



The small volume of the deeper water in Okoboji lake has 

 an important bearing on the temperature of the lower 

 water, and so on the mean temperature and on the quantity 

 of heat absorbed by the lake during the open season. 



In computing the quantity of heat taken in by the lake, 

 it is necessary to know one more physical constant, viz., the 

 reduced thickness of the strata, or the thickness of any 

 given stratum of water if its area is made equal to that of 

 the surface. The mean depth is the reduced thickness of 

 the entire lake, L e., it is equal to the depth of a vertical 

 sided tank necessary to hold all the water of the lake, if its 

 area is equal to that of the lake's surface. It is convenient 

 for various purposes of computation to select strata as thin 

 as one meter, especially near the surface and in the thermo- 

 cline. The volume of such thin strata cannot be computed 

 accurately from the soundings, nor is such accuracy need- 

 ful. The percentile areas at m., 5 m., etc., are platted on 

 coordinate paper and connected by a smooth curve. Then 

 the percentages indicated at the middle of each meter of 

 depth will give the reduced thickness of that stratum in 

 centimeters. From the same curve the reduced thickness 

 of any other stratum can also be derived. 



