128 
The average rainfall for the State of Illinois for a period 
of forty-nine years, from 1851 to 1899 inclusive, is 37.859 inches. 
Leverett (96) gives a tabulation of the records of the U. 5. 
Weather Bureau from stations within the state and on its bor- 
ders from 1851 to 1895 inclusive. The average for this period 
of forty-five years, was 37.55 inches per year. The averages 
for subsequent years, kindly furnished by Mr. M. E. Blystone, 
of the U. S. Weather Bureau, increase this amount by 0.008 
inches. 
The above diagram (Fig. A) shows the variations in the 
rainfall in the period above mentioned. The irregularity in 
the earlier years of observation may in part be due to the small 
number of stations from which records are available. In 1895 
records were made at ninety-seven points within or adjacent 
to the state ; in 1885, at twenty-seven ; in 1875, at twenty ; in 
1865, at sixteen; and in 1851, at but five. The range of the 
annual averages is 24.8 inches, ranging from 54.1 inches in 
1851 to 29.3 inches in 1894—a year of extreme drouth. Of the 
forty-nine records twenty-four lie above and twenty-five below 
the mean. 
An examination of this diagram of the rainfall of Illi- 
nois shows that the period covered by our plankton collee- 
tions, 1894 to 1899 inclusive, was predominantly one of mini- 
mum rainfall. The average for the six years is 35.5 inches, or 
2.3 inches below the general average. It includes one year, 
1894, when the rainfall was only 29.3 inches, the lowest on 
record, while the remaining years with the single exception of 
1898 are all more or less below the average. Omitting 1898, a 
year of excessive rainfall, 46.6 inches, the average for the re- 
maining years is only 33.2 inches, 4.4 inches below the general 
average. The reduction of overflow stages of the river conse- 
quent upon this lowered rainfall doubtless affected the plankton 
by the restriction of the breeding areas, the concentration of 
sewage, and the prolongation of the low-water period with its 
slackened current. Our collections thus, as a whole, are repre- 
sentative of a period of minimum rainfall and its attendant 
