Temperatures — Spring. 
291 
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 nauplii 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 and 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- 
