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 
own ward 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, 
