Temperatures — Summer. 293 



waves. The same factors are also the chief powers in deter- 

 mining the position of the thermocline and its rate of down- 

 ward movement. 



Very few of the inland lakes of Wisconsin are more than 25-30 

 meters in depth, and their bottom temperatures vary more with 

 relation to their area than to any other one factor. In the 

 Oconomowoc lakes, which are in the same region as lake Men- 

 dota, and are of the same depth approximately, but are much 

 smaller in area, the temperature of the bottom water does 

 not rise much above 7° during the summer. The same is 

 true of Cochituate lake, Massachusetts, having a depth of 60 

 feet and an area of less than one and one-half square miles. 

 (FitzG-erald, '95. ) Green lake and lake Geneva, Wisconsin, 

 both of them not greatly differing in area from lake Mendota, 

 but having a depth of 150 to 200 feet, have bottom temperatures 

 of about 6°. 



In a lake of large area, like lake Mendota, and about 24 me- 

 ters in greatest depth, the temperature at the bottom may dif- 

 fer widely in different summers. In 1896 the bottom tempera- 

 ture at 18 meters at the first of June was nearly 15°; 

 in 1895 about 12°, and in 1897 about 11.4°. At 22 meters it 

 was about 0.5° lower in each year. Had it not been for the 

 gales in the latter part of May the bottom temperatures would 

 have been much lower; possibly from 7° to 9°. The extreme pos- 

 sible range of bottom temperature in summer for lake Men- 

 dota in different years may perhaps be stated as from 8° as a 

 minimum to 18°, as a maximum, and the probable range as 

 from 10° to 15°. 



Summer temperatures. 



The temperature of the surface rose rapidly and evenly after the 

 fall in the temperature and mixture of water in the latter part of 

 May. In 1895 the weekly average rose from about 13.6° to 22.5° 

 in three weeks, a rate of nearly three degrees per week. In 1896 

 the surface rose from 15.4° to 25.1° in six weeks, rising some 

 what less regularly and at a much lower average rate. The period 

 of the summer maximum was reached about the middle of June 

 in 1895, when the average temperature was 23.5°, and about 



