32 IOWA STUDIES IN NATURAL HISTORY 



scale. This lake had been sounded by this Survey but no 

 temperature readings had ever been made in it. It was 

 visited in August, 1918 in consequence of the work of Dr. 

 Tilton and its bottom temperature was found to be 20.0% 

 while that of Mendota (a much larger lake) at the same 

 time and depth was 13. 0\ No reading approaching 20° had 

 ever been made before at such a depth in any Wisconsin 

 lake. It is plain that in this case also the form of the lake 

 basin profoundly influences temperature. It may be added 

 that in 1919 the bottom temperature, like that of Okoboji, 

 was exceptionally low — 13.1°. 



IV. COMPARISONS OF TEMPERATURES IN LAKES 

 OKOBOJI AND GENEVA 



While the bottom temperature of lake Geneva in 1919 was 

 not as low as it has been on other years, the mean temper- 

 ature is the lowest recorded — 14.2°, as compared with a 

 16-year mean of 16.5°. The next lowest temperature was 

 15.4° in 1897, so that in 1919 the water was 1.2° colder than 

 in any other August on record. The lov/ record of 1919 

 was due to the temperature of the water below 10 m. since 

 the epilimnion was by no means exceptionally cold on Au- 

 gust 30, and earlier in the month it must have been quite 

 up to the average. The most conspicuous difference was 

 in the 10 m.-15 m. stratum which was 5.2° below the mean 

 of 18 years. The thermocline showed a drop of 10.0° in 

 three meters, from 21.0° at 9 m. to 11.0° at 12 m. This is 

 a remarkable decline considering the direction and force of 

 the wind which was from the northwest on August 30. 

 This would tend to spread rather than to condense the iso- 

 therms at the place of observation. The thin epilimnion 

 is no doubt an inheritance from the hot weather in June 

 ai>d to this is due the low temperature of the 10 m.-15 m. 

 stratum. 



The summer heat-income of Geneva as measured by the 

 observations on August 30 is as follows: 



