Temperatures—Autumn. 
299 
are not caused by the warming or cooling of the water but by 
the fluctuations in the level of the thermocline. These fluctua¬ 
tions go on to a certain extent without an assignable cause, but 
the larger movements* at the station where observations were 
taken, are plainly due to the wind. 4. The upper layers of the 
vcool water become mingled by the action of the wind with the 
lower part of the warm water above it and are taken into the 
'warm layer. Thus the thermocline moves constantly downward 
^during summer, while the water below it is little or not at all 
^changed in temperature. 5. The water below the thermocline 
is practically stagnant during the summer, and is cut off from 
direct exposure to sun and air. As a result, it may become unfit 
to support most forms of animal life, as is the case in lake Men- 
dota. 6. The larger changes in temperature below the thermo- 
'dine are due to currents caused by winds. 
Autumn temperatures. 
By the latter part of September the temperature of the sur¬ 
face water has fallen so that it exceeds that of the bottom by 
barely 5°. At this time also gales from the north are 
.apt to occur whose effect is to break the thermocline and 
render the lake homothermous. This result is reached at 
•different dates for different depths, but in both years the 
lake became homothermous in its deepest parts about two or 
three days after the time when a similar condition was reached 
•at 18 meters. In each year the homothermous condition was 
reached at a temperature not much exceeding 16°; and in 
.general the temperature for the 1st of October may be stated 
as about 16°. 
The breaking up of the thermocline is accompanied by a 
marked rise in the temperature of the bottom water. In 1895 this 
rise amounted to 2.8° from the 26th to the 28th of September; 
and in 1896, to about 1.5° in the same time. 
During October and November the temperature falls with 
singular uniformity, as indicated by the weekly averages, pass¬ 
ing the temperature of the maximum density of water late in 
November. The decline continues steadily until a temperature 
is reached between 2° and 3°, after which the cooling goes 
