294 
Birge—The Crustacea of the Plankton. 
the 1st of July in 1898, when the maximum was about 2.5° 
higher. The maximum surface temperature recorded was 25.2° 
Aug. 1, 1895, and 27.8° July 28, 1896, both at 5 p. m. After 
the maximum has been reached there follows a period in 
which the temperature of the surface is nearly stationary, and 
in which the weekly averages do not vary more than two 
degrees. This period was exceptionally long in 1895, lasting 
from the middle of June to the third week of September, about 
three and one-half months, in which time the weekly averages 
were between 22° and 24°. In 1896 it lasted only about six 
weeks, from the first week of July to the middle of August, 
at a temperature of 24° to 26°. At the close of this period the 
surface temperature falls and the decline once started goes on 
pretty uniformly as shown by the weekly averages, until the 
lake nears the freezing point. In 1895 the temperature fell 3° 
in as many days at the last of September. In 1896 there was 
a fall of 4.4° during the last ten days of August. 
At the opening of the summer period the temperature of the 
bottom rises somewhat rapidly in the latter part of May, gain¬ 
ing perhaps 1.5-2° in two weeks. After this the bottom 
temperature is stationary or rises very slowly, not gaining 
a degree in three months. The bottom temperature at 18 me¬ 
ters lay between 13° and 14° in 1895; close to 15° in 1896, 
and near 12° in 1897. At the depth of 22-23 meters the 
temperature was from 0.4° to 0.6° lower in each year. Late in 
September the water of the lake becomes mingled from top to 
bottom and the temperature becomes uniform. At this time the 
bottom temperature rises rapidly by the mixture of the bottom 
water with the warmer water above. 
During the early parts of the period when the bottom tem¬ 
perature is nearly stationary, that of the surface rises until the 
difference between bottom and surface amounts to 10° and even 
15° in late July or early August. As the surface tempera¬ 
ture declines, the difference between top and bottom becomes 
less and usually amounts to between 4° and 5° in late Septem¬ 
ber, just before the time when the lake is rendered homother- 
mous by the fall gales. 
