300 Birge — The Crustacea of the Plankton. 



on very slowly. The difference of temperature between the 

 surface and bottom of the lake during this time is very small. 

 In the morning the lake is entirely homothermous. On bright, 

 calm days, the temperature of the surface rises, and may become 

 as much as 2° warmer than the bottom. This condition 

 of things, however, is uncommon, and ordinarily it is difficult to 

 find differences between the surface and bottom exceeding 0.1° or 

 0.2°. It is a feature of especial interest in lake Mendota 

 that the fall homothermous period begins so early and at so 

 high a temperature. The autumnal multiplication of many of 

 the species of Crustacea goes on after this period has been fully 

 established, and their vertical distribution at this time is there- 

 fore independent of temperature. In the deeper lakes, or 

 in smaller lakes of the same depth the homothermous condition 

 is reached much later. In Green lake, as reported by Professor 

 Marsh (Marsh, '97, p. 187), it occurs in November at a bottom 

 temperature of 4.7°, and at a depth of about 45 meters. 

 The rise at the bottom was 1.4°. In Cochituate lake, near 

 Boston, at a depth of 18 meters, the homothermous condition 

 is reached at about the same time, and at the same temperature. 

 (FitzGerald, '95, p. 74.) This lake has an area of less than one and 

 a half square miles. 



During the last of November and the early part of December 

 cooling goes on very slowly. The surface temperature fre- 

 quently falls to zero, as the result of a calm night, and the lake 

 may skim with ice, which is broken up again by the wind. 



