30 IOWA STUDIES IN NATURAL HISTORY 



Here is apparently a drop of nearly 2.0°C. in about three 

 meters followed by a rise of about 1.8°C. in 1.5 m. Such 

 changes at this depth are highly improbable and lead one 

 to query whether the thermometer functioned accurately. 

 If the 110-ft. reading is to be taken as correct, the bottom 

 temperature is 13.6° ; if we take the 115-ft. reading it is 

 15.5°. It may be added that the amount of the oxygen 

 found by us in the lower water forbids the idea of a pos- 

 sible rise of the bottom temperature due to fermentation. 

 (See p. 42). 



The observations of July 26, 1916, are open to the same 

 criticism. The readings on that date show the following : 



18 m. 17.6° 



19 m. 16.8^ 



20 m. 18.6° 



21 m. 17.1° 



22 m. 16.5° 



Such irregularities are unprecedented. It is not easy to 

 see how a temperature of 18.6° could be reached at 20 m. 

 with much lower temperatures at 19 m. and 21 m. 



Another though somewhat similar query arises regard- 

 ing the 1916 series. There is no objection to accepting 

 13.1° as a correct reading at 32 m. on July 18 and 13,3° at 

 27 m. on the same day. These readings were made with 

 the Leeds-Northrop instrument. But if these are correct 

 it is hard to see how we can accept the readings by the 

 minimum thermometer on July 26. This gives a tempera- 

 ture of 15.6° at 27 m. or a rise at that depth of 2.3° in eight 

 days. These days were bright hot days with little wind, 

 according to weather records at Madison. 



In this case the criticism is not based on the idea that 

 the temperature reported is impossibly high; on the con- 

 trary it seems quite within the possible limits. The diffi- 

 culty is to see how so great gains of heat could have been 

 made in the week available for them. 



The thermocline was well established on July 18 and 

 therefore conditions would be as unfavorable as possible 

 for the distribution of heat to the lower water. Still fur- 

 ther the record of the minimum thermometer on Julv 26 



