292 Birge — The Crustacea of the Plankton. 



torn. In six weeks the temperature of the bottom had risen 

 about 5° or 6°, while that of the surface had advanced about 15°. 



The relation of the wind to this warming of the lake is well 

 stated by Whipple ('95, p. 207). 



In both of the years of observation, and also in 1897, there 

 came in the middle or latter part of May a marked decline in tem- 

 perature accompanied with high northerly winds. The effect of 

 this was two-fold: first, the surface water was cooled; secondly, the 

 wind mingled pretty thoroughly the water of the lake, thus caus- 

 ing a sharp rise of temperature in the lower strata. On the 12th 

 of May, 1895, the difference in temperature between top (15.6°) 

 and bottom (7.7°) was 7.9°; on the 16th the difference was only 

 1.5°, and on the 18th only one degree (12.6°-11.6°). On May 

 11th, 1896, there was a difference of 8.3° between top (18°) and 

 bottom (9.7°), and a thermocline was evidently formed be- 

 tween 4 and 6 meters. On May 17th the difference between top 

 (15.6°) and bottom (13.4°) was only 2.2°. Thus in both 

 years there was a rapid rise of 3-4° in the temperature 

 of the bottom water. It is probable that if temperatures could 

 have been taken at the most favorable time the lake would have 

 been found nearly homothermous in late May, at a temperature 

 not far from 11° in 1895, and 13.5° in 1896. The effect 

 of the spring warming was therefore to warm a mass of water 

 18 to 24 meters deep from an average temperature between 2° 

 and 3° in March to an average of 11° to 14° at the latter part 

 of May; with the differences between the top and bottom not ex- 

 ceeding 1° to 2° at the beginning and end of the period. 



From these facts it appears that the bottom temperature of 

 the lake may vary greatly in different summers, and that the 

 bottom temperatures of lakes of the same depth, in the same 

 region and season may also vary greatly — much more than the 

 temperatures of the surface. Four factors are effective in de- 

 termining the bottom temperature; three constant, and one 

 variable: (1) the depth of the lake, (2) its area relatively to its 

 depth, (3) the shape of the lake and the nature of its surround- 

 ings as favoring or hindering the influence of the wind, and (4) 

 the amount of warmth and of wind during the spring and the times 

 of occurrence of gales and the succession of warm and cold 



