Temperatures. 295 



The Thermocline. 



During the summer, then, the difference in temperature 

 between the surface and the bottom may amount to 10°, 12°, or 

 even 15°. The decline in temperature from surface to bottom 

 is, however, not uniform as the depth increases. If a series 

 of temperatures is taken about the first of August it will be 

 found that there is a layer of surface water from 8 to 12 

 meters in thickness whose temperature is nearly uniform, the 

 difference between that of the surface and that at 9 or 10 

 meters being usually only a fraction of a degree and frequently 

 nothing. Immediately below this mass of warm water lies a 

 stratum in which the decline of temperature is extremely rapid. 

 This stratum may be two or three meters in thickness with a de- 

 cline of as many degrees per meter. It may be only a meter or 

 even less in thickness, and a decline of as many as nine degrees 

 has been observed in a single meter. This layer in which the 

 temperature changes rapidly may be known as the thermo- 

 cline — the Sprungschicht of German authors. Below the ther- 

 mocline the temperature decreases toward the bottom at first 

 more rapidly and then more slowly as the depth of the water 

 increases, but never showing the sudden transitions which are 

 characteristic for the thermocline, the rate of decline rarely 

 exceeding one degree per meter of depth. The thermocline was 

 first noticed by Richter ('91) in a study of the Alpine lakes. 

 Its origin was attributed by him to the alternate action of the 

 sun warming the surface in the day, followed by a cooling at 

 night. The alternation of conditions resulted in the formation 

 of a layer of water of nearly uniform temperature above the 

 colder bottom water. I do not wish to argue against the cor- 

 rectness of this theory as applied to the lakes which have been 

 studied by Richter and others, but in lake Mendota the concur- 

 rence of gentle winds and hot weather are essential to the for- 

 mation of the thermocline. In other words, the warmth of the 

 surface water, received from the sun, is distributed by the wind 

 through a certain depth of the lake, a depth which is propor- 

 tional to the violence of the wind and the area of the lake. 

 (Cf. FitzGerald, '95; Whipple, '95.) It can readily be seen that 



