340 POPULAR SCIENCE MONTHLY 



warmed water is, of course, lighter than the cooler water below and 

 tends to float upon it. The difference in density thus caused makes it 

 increasing^ difficult for the wind to create and maintain a complete 

 circulation of the water. For a time the action of the wind may con- 

 tinue to mix each successive stratum of water with that below it, the 

 mixture extending to the bottom of the lake. But this action is a very 

 different thing from a complete overturning of the water; and while 

 it results in raising the temperature of the lower water, it does not carry 

 freely oxygen to the bottom. Thus, when the surface becomes decidedly 

 warmer than the water below it, the bottom water, though it con- 

 tinues to warm, is withdrawn from direct contact with the air and is 

 therefore at a disadvantage in the matter of gaining a new supply of 

 oxygen. 



As the season advances this stratification of water dependent on 

 temperature becomes accentuated, and the lake becomes separated into 

 two parts: an upper warm stratum of nearly uniform temperature, 

 beneath which lies the cold water consisting of a transition layer — the 

 thermocline — in which the temperature is rapidly falling, and below 

 this the mass of the cold water whose temperature ordinarily falls 

 rather slowly with the depth until the bottom of the lake is reached. 

 The thickness of the upper layer varies with the size of the lake, from 

 ten to twelve feet to thirty or forty feet. It is present as a definite and 

 permanent layer at a date varying with the area of the lake from late 

 April to the middle of July. It increases in thickness after the cooling 

 of the lake begins, but does not change much before that process com- 

 mences. 



This upper layer is subject to the direct action of the wind, is kept 

 in circulation, and may be saturated with oxygen, or nearly so, but the 

 only new supply of oxygen which the lower water can gain must come 

 to it indirectly from the upper stratum. This condition of permanent 

 stratification of the water comes on at the time when the life of the 

 lake and its consequent need of oxygen are rising to the maximum, with 

 the increasing warmth of summer and the development of life. The 

 consumption of oxygen for the purposes of decomposition is also at a 

 maximum. The separation of the lower water from the atmosphere in 

 summer by a thick layer of warm water is therefore a much more serious 

 thing than the separation of the water from the air in winter by ice. 

 In winter the demand for oxygen is at a minimum and the stock con- 

 tained in the water is at a maximum. In summer both of these condi- 

 tions are exactly reversed. It is therefore necessary for us to inquire 

 as to the means which the lake has for absorbing oxygen from the air 

 and its means of transporting the gas from the surface to the place 

 where it is to be used, and to note the efficiency of these processes as 

 compared with the call for oxygen in the summer life of the lake. 



