A LIMNOLOGICAL RECONNAISSANCE 29 



of 5.6° and a maximum of 10.0°. The temperature of Oko- 

 boji was therefore nearly twice as high as the mean of lake 

 Geneva. The maximum gain of Geneva above 4° was 6°, or 

 about one-half that of Okoboji. 



There is no reason to doubt that Dr. Tilton's observations 

 show the general character of the lake's temperature. He 

 corrected his minimum thermometer for pressure, which at 

 135 ft. would have caused a rise of 0.24°C. This correction 

 is not made in the figures given above. The temperature of 

 the lower water of all lakes in this region was exceptionally 

 high in 1915, in most cases at a maximum. This was due 

 to the low temperature of the air during May, June, and 

 July, which favored the distribution of heat. 



'The air temperatures at Madison were as follows: 



Mean 1915 1916 



May 14.2° 11.1° 14.0° 



June 19.6° 16.5° 16.5° 



July 22.4° 20.0° 26.5' 



There is only one year on record in which the tempera- 

 ture of May was lower than in 1915. June was the coldest 

 on record in both 1915 and 1916, and July, 1915, was the 

 coldest since 1891. May, 1916, was practically at the mean, 

 while July was much above the mean. Under these con- 

 ditions Vv^e should expect to find unusually high bottom tem- 

 peratures in lakes during both years. This condition was 

 present in the Wisconsin lakes. Lake Geneva had a bottom 

 temperature of 9.8° in August, 1915. This is the highest 

 on record except in 1917 when it rose to 10.0°. In that year 

 the temperature of May and June was the same as in 1915, 

 and the first half of July was also cold. 



There is therefore no reason why the records from Oko- 

 boji lake in 1915 and 1916 should not be accepted as show- 

 ing the general situation. 



On the other hand it does not seem probable that they 

 are exactly accurate. The readings of 1915 near the bot- 

 tom are as follows: 



100 ft. 60.0°F. 



110 " 56.5°F. 



115 " 59.8°F. 



