Temperatures — Spring. 2 91 



deed, at the lower temperature the progress of the development 

 of eggs is almost suspended, while at a temperature of 2.5 to 3° 

 the development of eggs into nauplii and of oauplii into young 

 Cyclops goes on with considerable rapidity, and at 1.5-2° it 

 is present, though decidedly slower. The history of Cyclops in 

 the spring, therefore, depends to a considerable degree on this 

 warming of the water under the ice. If the winter is cold, so 

 that the warming does not take place, or the rise is only slight, 

 the number of Cyclops may remain almost unaltered during the 

 winter; while conditions like those of the winter of 1895-96 

 permit the development of large numbers of young Cyclops 

 ready to take advantage of the increased warmth and food in 

 early spring, and so to develop enormous numbers of this genus. 



The spring rise of temperature. 



A glance at Figs. 1 and 2 will show that the warming of 

 the lake in the springs of 1895 aod 1896 was singularly alike. 

 In each year the month of April was pretty steadily warm, and 

 the surface of the lake rose rapidly and uniformly in tempera- 

 ture for about six weeks following the breaking up of the ice. 

 Immediately after the disappearance of the ice the temperature 

 of the lake frequently falls, since the breaking up of the ice is 

 often caused by a north wind accompanied by a much lower 

 temperature than had preceded the breaking up of the ice. This 

 fall in the temperature of the water amounted to over one de- 

 gree in 1896. But this slight drop is quickly recovered, and if 

 the weekly averages are considered it will be seen that the sur- 

 face temperatures in both years rose rapidly and steadily. For 

 a time the rise in temperature at the bottom is as rapid as that 

 at the surface. The length of this time varies, of course, with 

 the amount of wind. A succession of warm days, accompanied 

 or followed by high wind, will mix the warmed surface water 

 with the body of the lake and thus secure uniformity in temper- 

 ature. In neither 1895 nor 1896 were these conditions long 

 realized; the temperature of the bottom began to lag behind that 

 of the surface, and by the middle of May there was a difference 

 of 7° to 8° between the surface temperature and that of the bot- 



