no. 9 



FORECAST OF U. S. PRECIPITATION — ABBOT 



17 



= 180 + (8 X 4) = 32 + (4 X 2) = 8, making 220 separate tabulations 

 in all. 



In tables of periods 1/18 to 1/63 of 273 months, there are many 

 cases when the number of columns for a 1} b lf c 1} a 2 , b 2 , c 2 , and a\, b\, 



3 5 7 9 13 5 



Fig. 8. — Sixfold grouping of periods to form generalized means. 



c\, o! 2 , b\, c' 2 are too few to give a trustworthy mean. Accordingly, 

 as I stated above, I have made the assumption that in form and 

 amplitude groups of SS>20 will be fairly similar, though of different 

 phases, and in form and amplitude groups for SS<20 will also be 



