Temperatures — Winter. 
289 
tions of a week. Lines were then drawn connecting the posi¬ 
tions of the full degrees. In 1895 the diagram is carried to 18 
meters only, the depth at my regular station. In 1896 the 
temperatures were carried to 22 meters, observations being 
taken at that depth nearly every week. Two other temperature 
diagrams are given, showing the movement of the surface and 
bottom temperatures from April to December of the years 1895 
and 1896. 
B. Results. 
Winter Temperatures. 
Lake Mendota freezes at very different dates during the early 
winter in different years, and the time of opening also varies 
greatly. The lake is so large that continued high winds prevent 
its freezing even after long continued low temperatures, and as 
there is no large affluent, there are no spring floods to move the 
ice, which therefore remains until it is greatly weakened by the 
effect of the sun and is broken up by the wind. In 1894 the 
lake froze on December 28th, and opened April 8th, 1895, being 
closed for 100 days. In 1895-96 the lake froze December 6th and 
opened April 28th. The first and last observations through the 
ice were made on January 1st and March 23d, 1895; and De 
cember 9th, 1895, and March 28th, 1896. In the winter of 
1896-97 the lake froze December 29th, then broke up again and 
did not freeze the second time until January 7th, 1897. It op¬ 
ened on April 10th, 1897. The ice usually reaches a thickness of 
over 60 cm., and in 1895 became nearly 1 m. thick. 
During the winter the temperature of the surface of the water 
is, of course, zero. The water at the bottom when the lake 
freezes has a temperature which varies in different years. If 
the lake is prevented by wind from freezing during the first cold 
weather of December, it may remain open for days or even 
weeks, cooling very slowly. This was the case in 1894, and the 
temperature at the bottom on January 1st, 1895, was barely one 
degree, and at nine meters was about 0.5°. In 1895 
when the ice on December 9th permitted observations, the tem¬ 
perature was as follows: 0.5 m., 0.3°; 5 m., 1.2°; 18 m., 1.7° 
19 
