118 



THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



have been placed, one above another, instead of consecutively. A cycle of this length is 

 interesting, not only in itself, but because it is one-half of the period which Clough thinks 

 that he has discovered.* The lower wavy Une in figure 20, representing a cycle of 33.8 years, 

 agrees with a cycle of similar length in the curve of growth for the last 180 years, but in 

 the years preceding that time no such cycle is apparent. On the whole, then, the growth 

 of these trees seems to indicate that a cycle of this length is not here a permanent feature. 

 This is important because of the large amount of discussion in regard to the 35-year cycle 

 of Bruckner. 



In his "Discussion of Australian Meteorology" (South Kensington Solar Physics 

 Observatory, 1909), Dr. W. J. S. Lockyer finds a pronounced 19-year cycle in barometric 



Date in 1 1 -year cyclp 1 



Mm. 

 1.10 



1850] 

 to I 



191«il,00 



-11.4 years - 



Fig. 23.— Variations of the ll-}'ear Cycle. 



pressures exhibited in Australia and South America. A year or two before Lockyer's 

 publication, I had worked out a distinct period in the northern Arizona trees, which at 

 first seemed to be 19 years, but on close analysis proved to be 21 years. With all the 

 improvements of method now made, this variation is evident for more than 400 out of the 

 500 years and its length is 21.0 years. For a great majority of the time the crests and 

 troughs follow each other with great regularity. On the average the total variation is 20 

 per cent of the mean (see figures 19 and 22). When this variation is plotted it shows a 



*H. W. Clough: Synchronous Variations in Solar and Terrestrial Phenomena. Astroph. Jour., pp. 22-42, 1905. 



