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. 8. 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 
