Circulation 39 



cooled some degrees they would descend, displacing the 

 layers underneath and setting up shallow currents 

 which would tend to equalize the temperature of all the 

 strata involved therein. And if the gradation of tem- 

 peratures downward were regular before this mixing, 

 the result of it would be a sudden descent at its lower 

 limit, after the mixing was done. This would account 

 for the upper boundary of the thermocline, but not for 

 its lower one. Perhaps an occasional deeper mixing, 

 extending to its lower boundary, and due possibly to 

 high winds, might bring together successional lower 

 levels of temperature of considerable intervals. Perhaps 

 the thermocline is but an accumulation of such sort of 

 thermal disturbance-records, ranged across the vertical 

 section of the lake, somewhat as wave-drift is ranged in 

 a shifting zone along the middle of a sloping beach. 

 At any rate, it appears certain that the thermocline 

 marks the lower limit of the chief disturbing influences 

 that act upon the surface of the lake. That it should 

 rise with the progress of summer is probably due to the 

 increasing stability of the lower waters, as differences 

 in temperature (and therefore in density) between upper 

 and lower strata are increased. Resistance to mixing 

 increases until the maximum temperature is reached, 

 and thereafter declines, as the influence of cooling and 

 of winds*' penetrates deeper and deeper. 



In running water the mixing is more largely mechani- 

 cal, and vertical circulation due to varying densities is 

 less apparent. Yet the deeper parts of quiet streams 

 approximate closely to conditions found in shallow 

 lakes. Such thermal stratification as the current 

 permits is direct in summer and inverse in winter, and 

 there are the same intervening periods of thermal over- 

 turn when the common temperature approaches 4° C. 

 In summer and in winter there is less ''stagnation" of 

 bottom waters owing to the current of the stream. 



