26 Trvciity-si.rth Annual Meeting 



one. During the spring, the period when the lake is warming 

 uj), the dechne of temperature from the surface to the bottom is 

 more or less uniform. But when the season has advanced, 

 from about the ist of July, in Lake Mendota, to the latter part 

 of September, we find a peculiar distribution of temperature. 

 The upper water of the lake, varying from about 20 feet in 

 thickness to some 45 feet, is very nearly uniform in tempera- 

 ture. One may say, speaking roughly, that in the early morn- 

 ing, before the sun has had any effect, the upper stratum of the 

 lake is practically uniform in temperature, falling, perhaps, in 

 this distance of 20 to 40 feet, i or 2 or perhaps 3 degrees Fah- 

 renheit. 



Immediately under this stratum there comes a thin layer in 

 which the temperature falls with great rapidity, sometimes falling 

 as nmch as 10 degrees Fahrenheit in a meter, at other times 

 falling less rapidly than that. But there is always a zone inn:ie- 

 diately below the warm water in which the temperature falls 

 very rapidly and below which the falling of the temperature is 

 quite uniform and slow imtil the bottom is reached. This little 

 chart which I have had drawn to go with the paper will illustrate 

 this. 



This diagram shows the condition of temperature on August 

 12, 1896. In the diagram the horizontal lines represent depth 

 in meters and the vertical lines temperature in degrees Fahren- 

 heit. You will see that at the surface the temperature is about 

 79 and at the bottom a little above 59 — a difference of 20 

 degrees between the top and bottom. But the line of tem- 

 perature slujws that the rajMdity of the fall in temperature 

 is very different at different depths. From the surface to 

 6 meters there is very little fall, somewhat more in the next 

 two meters, while there is a drop of nearly 10 degrees from 8 

 to 10 meters, and a fall of only about 6 degrees in the lower 

 8 meters. It is plain that nearly one-half of the difference in 

 temperature between the top and the bottom of the lake comes 

 in the two meters from 8 to 10. The effect of this is that the lake 

 becomes divided into two parts, horizontally. There is what 

 you may call a warm lake on the surface from 20 to 30 feet 

 thick, or of even greater thickness than that. This lake is sub- 

 jected to the action of the winds and the currents keep the water 

 stirred up, so that the water may be brought to the surface by 

 the action of the wind. IWlow lies another lake, say from 20 

 to 30 feet below the surface and extending to the bottom, which 

 is entirely undisturbed by the wind, in which the temperature 



