148 Thirty-fifth Annual Meeting 
are substantially the same at all depths of the lake and all por- 
tions of the lake support an abundant life. 
If now we pass to Fig. 4, which represents the conditions on 
May 24, you will see that the lake has warmed a good deal—the 
surface temperature has increased to 13° while the bottom has 
increased to a little over 10°, but is beginning to lag behind the 
surface. That shows that the action of the wind is now failing 
to reach the bottom of the lake, and that as a result the surface 
is beginning to gain in temperature on the bottom. The amount 
of oxygen in the surface water has gone down to about 7 ce. per 
litre; in consequence of the rise of temperature it has declined a 
little at the bottom, and yet the oxygen line is comparatively 
straight. 
But as you pass into June and July, both as to temperature 
0 50 10 IS 2OG eh tee 



FiG, 5-Lake Mendota, July 29, 1905. 
and oxygen the conditions in late July are shown in Fig 5. 
The surface temperature has risen to 22.6° C, and down to the 
depth of 7 meters it is almost uniform. At this depth comes a 
sudden drop in the temperature, which is more marked at 10 
meters; the temperature line running down until at the bottom 
