Introduction 



9 



If, now, we analyse Professor Henry's data ^ — which 

 run from 1881 to 1920 — with a view to discovering 

 whether there are important cycles in the rainfall of 

 the United States as a miit, we obtain these smprising 

 results : 



Figure 3 



The periodogram of annual rainfall departures in the United States as a unit. 



1881-1920. 



(1) Figure 3 shows that if there is an important real 

 cycle within the limits of three and twelve years 

 in the rainfall of the United States as a unit, its 

 period is about eight years; 



(2) Figure 1 shows that when the equation to the 

 eight-year rainfall cycle in the United States is 



1 The data are given in Table I of the Appendix to this chapter. 

 The figures in the column marked "Corrected Data" were used in the 

 computation. 



