Rainfall and the Crops 55 



of the compound cycles that describe the two series, 

 each graph consisting of two cycles and their semi- 

 harmonics, a thirty-three years cycle describing the 

 ground-swell and the smaller cycle of eight years sum- 

 marizing the minor cyclical movements. The measure 

 of the degree of fit to the observations is, in case of the 

 yield curve, K = 2.46, and in case of the rainfall curve, 

 K = l.QS. The yield curve reproduces the general 

 characteristic features of the rainfall curve. 



Our findings with reference to the crops taken to- 

 gether are similar to what we discovered in case of the 

 single crops: The yield per acre and the rainfall of the 

 critical season are highly correlated; the rhythmical 

 movements of the yield and of the effective rainfall 

 may be accurately described by a compound cycle of 

 thirty-three years and eight years with their semi- 

 harmonics; and the yield curve reproduces the general 

 characteristics of the curve of effective rainfall. 



Passing now to a summary of the contents of this 

 chapter, we may collect our results in a series of prop- 

 ositions. 



(1) The yield per acre of the four representative 



crops, corn, hay, oats, and potatoes is associ- 

 ated with the amount of the rainfall of their 

 respective critical periods of growth. In 

 three out of the four cases the degree of cor- 

 relation lies between r = .589 and r = .666. 



(2) The rhythmical changes in the yield per acre of 



the crops and in the rainfall of the respective 



