.liiniiion I'islicrics Society. 101 



lake; or. in other words, if you coni])are lakes in the same rep;ion 

 and of approximately the same depth, you will find that the tem- 

 jierature at anv oivon dei)th will he less as the area of the lake is 

 smaller. In order to illustrate this point. I have hrought in a dia- 

 fjrani on which I have platted the temperature curves of four lakes. 

 The larpi-est of these is Lake Mendota. 6 miles lonj:::. l\v 3i miles 

 wide, and about 85 feet deep. The second is Okauchee Lake, 

 about 2 miles by i\, and 95 feet deej). The third is Mouse Lake, 

 about 1 mile bv 1-3 of a mile, and 60 feet deej). And the fourth, 

 (larvin Lake, is about 1-4 of a mile lono^ and half as wide, and 

 about 40 feet deep. 



The temperature of these four lakes was taken on the same 

 dav. on the afternoon of the 12th of July, 1898. Tf you look at 

 the curves vou will see in the first place that the lakes have sub- 

 stantially the same surface tem]ierature. They are all within about 

 one degree of each other at the surface. 



Secretarv W'hitaker: All taken at the same hour? 



Prof. Ilirge: No; because one has to go from one lake to the 

 other. 



In the accompanying diagram each vertical space represents 

 10 feet in depth of water, and each horizontal space represents 

 5 degrees Fahrenheit of temperature. The temperature of the 

 water in each lake was taken at every meter of depth, the result 

 platted in the diagram in its appropriate place, and the points so 

 marked connected for each lake by a line. Several things appear 

 plainly from the diagram. In the first place, the layer of warm 

 water at the top of the lake is thinner in the case of the smaller 

 lake. In Garvin Lake this layer is about 13 feet thick, while in 

 Mendota. the largest lake, it is nearly 30 feet in thickness, and in 

 the two lakes of intermediate size it is of. an intermediate thick- 

 ness. This shows, of course, the depth to which the wind has 

 thoroughly distributed the warmer surface water of the summer. 



A second fact which is very plain is that at equal depths these 

 lakes have a very different temperature. At 30 feet, for example, 

 Garvin Lake has a temperature but little above 4=; degrees, while 

 Mouse and Okauchee Lakes have temperatures about 10 degrees 

 higher, and Mendota has been warmed at this depth to a tempera- 

 ture of more than 6y degrees. Similar relations appear at all 

 dei)ths below 10 feet; the larger lake in every case having a higher 

 temperature at any given depth than the smaller lake. A third 

 fact appears with equal clearness, namely, that the temperature 

 at the bottom of these four lakes is very unecjual. In Garvin 



