296 Birge — The Crustacea of the Plankton. 



in a lake of the size of Mendota the water would be of uniform 

 temperature from top to bottom if the lake were always agitated 

 by violent winds. On the other hand, if the weather were per- 

 fectly calm, the lake would be warmed only to the depth which 

 the rays of the sun could directly penetrate. As a matter of 

 fact, the formation of the thermocline is due to the concurrence 

 of gentle winds and a temperature high enough to warm the sur- 

 face water rapidly. 



The temperature observations on lake Mendota have been 

 made chiefly at a station about one-half of a mile from the south 

 shore. On bright days in May, with a gentle north (on shore) 

 breeze, it not infrequently happens that a thermocline is 

 formed, there being a mass of water four or five meters in 

 thickness of uniform temperature, below - which there is a rapid 

 descent in temperature to the cooler water below. When, how- 

 ever, the direction of the wind changes and blows off shore, 

 this warm water is carried to the other side of the lake, and 

 the temperature shows a fairly uniform rate of descent from the sur- 

 face to the bottom. If, however, this condition of warm weather 

 and gentle wind continues, there is produced a mass of warm 

 water on the surface, so thick that however the wind may blow 

 there is always a warm stratum floating on the colder water; 

 and when this condition has been established, a permanent 

 thermocline has been formed. 



A study of Figs. 3 and 4 will show the formation and movements 

 of the thermocline as disclosed by the weekly averages. It will be 

 seen that in the early part of May the gain of heat is rapidly dis- 

 tributed through the whole mass of water. The bottom lags behind 

 the surface, of course, but the difference in temperature between 

 them rarely exceeds 5° and the temperature of the surface water 

 reaches the bottom in 10 days or 2 weeks. During the rapid 

 warming of the early summer this condition ceases. The sur- 

 face warms rapidly, the winds are not constant or strong 

 enough to distribute the heat throughout the water, and the 

 ownward movement of the isotherms no longer extends to the 

 bottom, but they penetrate for an increasingly shorter distance 

 into the water. In 1895, for example, the surface reached an 

 average temperature of 15° during the last week in May, 



