Origin of Eight-Year Generating Cycle 77 



and apt to be torrential in character, with heavy run- 

 off." The San Francisco Peaks, about ten miles north 

 of Flagstaff, ^'illustrate how meteorological data may 

 vary in rugged localities. The west slopes of these 

 mountains are exposed to the westerly stonns and have 

 an immense snowfall. Springs abound, and all favor- 

 able localities are taken up as ranches. East of the 

 mountain, however, the land is dry and barren, and 

 long distances intervene between watering places. 

 . . . Ina very rugged country like that about Prescott 

 similar differences between east and west mountain 

 slopes must constantly occur." 



The longest meteorological record in the Arizona 

 pine forest was begun at ^Miipple Barracks, near Pres- 

 cott, in 1867, and then continued at Prescott, which has 

 an elevation of about 5200 feet. With the variability of 

 rainfall from station to station which we have just 

 noted, and with the change of the meteorological obser- 

 vation from WTiipple Barracks to Prescott, one would 

 scarcely expect any normality to be revealed in this 

 particular rainfall record. An examination of Figure 21 

 is, nevertheless, surprisingly suggestive. The top graph^ 

 shows the relation between the annual rainfall and the 

 growth of pines near Prescott. The growth seems to 

 show a secular trend downwards and for that reason, 

 in seeking the degree of relation between the annual 

 rainfall and the annual growth, I have computed the 

 correlation between their first differences. The measure 

 of this relation is r = .56, which shows that there is an 

 unmistakable relation between the rainfall and the 

 growth of pines in this region. 



^ The top graph is taken from Professor Douglass' work, p. 29. 



