124 



low-water conditions. They are, however, fairly representative 

 of the whole range of rainfall and river conditions, including, as 

 they do, two years (1896 and 1897) which approximate the 

 average rainfall, and a year of minimum (1894) and one of 

 maximum (1898) rainfall. 



The periodicity of the rainfall of Illinois and adjoining 

 states was noted by Leverett ('96). There are alternating wet 

 and dry periods of eleven years which correspond somewhat 

 closely in duration to the sun-spot cycle as shown in Plate VI. 

 There is, however, in the sun-spot cycle no recorded alternation 

 of elevation and depression similar to that of the rainfall, the 

 maximum of sun-spot occurrences appearing at intervals of 

 eleven years while those of the rainfall appear in a twenty- 

 two-year cycle. In the diagram this alternation is brought 

 out by the horizontal lines, which represent the average 

 rainfall for periods of the eleven years included. The first 

 wet period within the time covered by the records of the 

 Weather Bureau lies about 1853-1863, the average being 38.5 

 inches — only 0.59 inch above the general average, though this 

 amount will be considerably increased if the period is shifted 

 back to include one or two preceding years. These earlier 

 records are, however, less reliable owing to the few places of 

 observation. A dry period from 1864 to 1874 falls 2.96 inches 

 below the general average, while the wet period of 1875 to 1885 

 rises 2.9 inches above it. The following dry period, 1886 to 

 1896, falls 3.05 inches below the general average, while the few 

 years of the current wet period already yield an average above 

 the general one. The average difference in rainfall between 

 the wet and dry periods is about 6 inches. 



Our plankton collections are about equally divided between 

 the closing years of the last dry period and the opening years 

 of the current wet one. 



A second cycle or rhythm in the rainfall has been discov- 

 ered by Sir Norman Lockyer and Dr. W. J. S. Lockyer ('00 and 

 '01 ) as a result of their study of the temperature changes in the 

 sun, with their accompanying sun-spot phenomena, and the 



