Therm ocliiie 37 



These figures indicate the resistance to mixing that 

 gravity imposes, and show that the wind is not able to 

 overcome it below rather slight depths. 



Vernal and autumnal periods of circulation ha^e a 

 very great influence upon the distribution of both 

 organisms and their food materials in a lake; to the 

 consideration of this we will have occasion to return 

 later. 



The thermocline — In the study of lake temperatures 

 at all depths, a curious and interesting peculiarity of 

 temperature interval has been commonly found per- 

 taining to the period of direct stratification (mid- 

 summer). The descent in temperature is not regular 

 from surface to bottom, but undergoes a sudden acceler- 

 ation during a space of a very few meters some distance 

 below the surface. The stratum of water in which 

 this sudden drop of temperature occurs is known as the 

 thermocline (German, Sprimgschicht). It appears to 

 represent the lower limit of the intermittent summer 

 circulation due to winds. Above it the waters are more 

 or less constantly stirred, below it they lie still. This 

 interval is indicated by the shading on the right side of 

 figure 4. Birge has designated the area above the 

 thermocline as the epilimnion; the one below it as 

 hypolimnion. 



Further study of the thermocline has shown that it is 

 not constant in position. It rises nearer to the surface 

 at the height of the midsummer season and descends a 

 few meters with the progress of the cooling of the 

 autumnal atmosphere. This may be seen in figure 7, 

 which is Birge and Juday's chart of temperatures of 

 Lake Mendota as followed by them through the season 

 of direct stratification and into the autumnal circula- 

 tion period in 1906. This chart shows most graphically 

 the growing divergence of surface and bottom tempera- 

 tures up to August, and their later approximation and 



