150 Thirty-fifth Annual Meeting 
(0.8 cc. per liter) and there is even less oxygen at greater 
depths. 
As we pass to the next diagram, (Fig. 6) which shows the 
facts for the last of August, we find substantially the same ar- 
rangement, so far as the temperatures are concerned. The tem- 
perature line for the last of August shows pretty nearly 23° C. 
down to a depth of 8 meters; then comes a rapid fall of tempera- 
ture, followed by a slower one, until at the bottom a temperature 
0 5 0 10 [Siro a0 

FIG. 7—Lake Mendota, Sept. 26. 1905. 
of about 12° is reached. The oxygen of the lower water has been 
practically used up and ends near the of the cool water. 
As the season passes on and the lake cools the upper warmed 
layer increases in thickness as it declines in temperature. In the 
latter part of September, as Fig. 7 shows, the circulating part of 
the lake has reached a thickness of about 13 meters, with a tem- 
perature of about 18°. The oxygen has followed on down with 
