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 iLpTii+ 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° 



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