42 Economic Cycles: Their Law and Cause 



referring to the yield per acre of the several crops after 

 the secular trends have been eliminated. These two 

 Tables furnish the statistical material for ascertaining 

 the critical periods of the respective crops. The facts 

 as to the times of planting and harvesting may be 

 obtained from an article in the Yearbook of the United 

 States Department of Agriculture, 1910, pp. 488-494, 

 on "Seedtime and Harvest: Average Dates of Planting 

 and Harvesting in the United States." The method of 

 detecting the period of critical relation between yield 

 and rainfall consists in ascertaining, for each crop, the 

 month or combination of months, within the interval 

 between planting and harvesting, 1 whose rainfall gives 

 the highest correlation with the ultimate yield per 

 acre of the crop. The time for planting corn in Illinois, 

 according to the official publication cited above, begins 

 about April 30, it is general about May 13, and it ends 

 about June 2. The average time for harvesting, accord- 

 ing to the same publication, begins about September 26, 

 is general by October 29, and ends about December 10. 

 The correlation between the yield of corn per acre 

 (secular trend eliminated), and the rainfall for June is, 

 r = .069; for July, r = .496; for August, r = .293; for 

 September, r = .087 ; for July and August combined, 

 r = .589. The critical period of growth for corn has, 

 therefore, been assumed to be the interval of two 

 months — July and August. 2 



1 For some purposes it would be desirable to test the correlation 

 beyond these limits. 



2 Of course all possible combinations of months have not been 



