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. S. 

 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 37.85 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 collec- 

 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 



