175 
cline is a gradual one; in others, as 1595 and 1595, it is subject 
to some irregularities. With December the winter minimum 
returns, but with less persistence than in the months which 
follow, flood waters at this season bringing their higher tem- 
peratures. 
The annual temperature cycle thus falls into four periods: 
one of minimum and quite constant temperatures, including 
December, January, and February, and a varying portion of 
March ; one of maximum and more fluctuating temperature, 
approaching 80° and extending, with some interruptions, from 
the early part of June till about the middle of September; and, 
separating these, the two shorter intervals of change. The 
period of increase in temperature, which is also one of rapid 
change and increase of the plankton, includes the latter part 
of March and the months of Apriland May. The period of de- 
cline, which is sometimes more abrupt than the spring rise, as 
in 1895, 1897, and 1898, extends from the latter part of Septem- 
ber until the end of November. This is also a period of change 
and of frequent but not universal diminution in the plankton. 
The average temperature for the years, as expressed approxi- 
mately in the table, is 57.08°. This point is passed about the 
middle of April and again about the middle of October with 
considerable regularity. Since, however, these dates both he 
in periods of rapid change, the average temperatures are of 
much less duration than the more extreme ones. The existence 
of these well-defined periods of maximum, minimum, increase. 
and decline of temperatures affords the basis for corresponding 
seasonal changes in the minute life of the water as fundamental 
and extensive as those which affect the plant and animal life of 
terrestrial and aerial environment. This subject of the rela- 
tion between temperature and organisms of the plankton will 
be fully discussed in connection with the statistical study of 
their seasonal distribution. 
A comparison of the thermographs (Pl, VITI.—XIII.) of the 
different years and an inspection of the table on page 171, 
reveal but few significant annual differences. The spring rise 
