224 Wisconsin Academy of Sciences, Arts, and Letters. 
Na 
K 
197.7 
34.5 
253.2 
301.5 
Cl 
It will be noted that this water contains about two and a half 
times as much matter in solution as that of lake Amatitlan. 
Especially significant is the large amount of sodium and chlor¬ 
ine. 
Temperatures. 
All of these lakes belong to the tropical class whose waters are 
disturbed throughout their entire depths but once each year. 
That is, the so-called process of overturning and circulation is 
confined to the late autumn and winter months. With the ad¬ 
vent of cooler weather in the autumn, the temperature of the up¬ 
per water begins to fall. The cooling takes place at the surface 
and it results in the formation of convection currents which aid 
in the mixture of the upper strata of water. These currents af¬ 
fect the water to greater and greater depths as the cooling pro¬ 
gresses. The wind, however, is a more active as well as a more 
important agent in causing the mixture and circulation of the 
upper water. As the cooling progresses and the temperature of 
the upper water approaches that of the lower, the thickness of 
the stratum that is disturbed by the wind gradually increases 
Finally the thermal resistance to the mixture of these two strata 
becomes so small that the wind is able to produce a general mix¬ 
ture of the water from surface to bottom. This is known as the 
autumnal overturning and it probably takes place between thk 
middle of November and the middle of December. Owing to 
differences in altitude as well as in latitude the time is different 
for the different lakes. There are, doubtless, variations in the 
same lake from year to year owing to annual differences in the 
weather. 
This overturning is followed by a period during which the 
water is subject to disturbance by the wind throughout its en¬ 
tire depth. This is the socalled period of circulation. During 
its existence the water has a uniform temperature from surface 
to bottom; that is, the lake is homothermous. The exact time at 
which this circulation ceases is not known, but in 1910 it took 
