Rainfall and the Crops 53 



several crops would, in consequence of their different 

 variabilities, receive disproportionate weights. The 

 method of calculating the index which we have em- 

 ployed obviates this difficulty. 



Having now obtained an index of the fluctuation of 

 crops, we next consider the method of combining the rain- 

 fall of the critical periods of growth for the several crops. 

 The method will be clear if we bear in mind that the 

 critical period of growth of a crop is the combination of 

 months whose rainfall gives the highest correlation with 

 the yield. The mean effective monthly rainfall for the 

 critical period of a crop is the total rainfall of the critical 

 period of growth divided by the number of months mak- 

 ing up the critical period. In case of hay, for example, 

 the critical period of growth is March, April, May, 

 June. The mean effective rainfall for any given year 

 would be the total rainfall for the four months, March, 

 April, May, June, divided by the number of the months. 

 If the mean effective monthly rainfall for the several 

 crops is summed for each year and divided by the num- 

 ber of crops, a measure is obtained of the mean effective 

 monthly rainfall for the crops taken all together. In 

 Table IV of the Appendix to this chapter the mean 

 effective rainfall of the several crops, and of the crops 

 taken all together, is tabulated for each of the years 

 1870-1910. 



We have now an index of the fluctuation of crops and 

 an index of the mean effective rainfall of the critical 

 periods of the crops. The correlation between the 

 two series is r = .584. In Figure 14 are traced the graphs 



