CLIMATIC CYCLES. 



337 



as next in importance to the 11-year period (l. c, 553), and he refers to it 

 frequently (1914 : 4, 89, 140, 242 ; 1914" : 553, 563) . 



Major sun-spot cycles.-^The evidence of the occurrence of major sun-spot 

 cycles is fairly convincing, though in any particular case it is suggestive rather 

 than conclusive. Himtington (1914^:555) states that all attempts to find a 

 definite period have broken down because of the great irregularity of sxm-spots, 

 and that, while there may be a distinct periodicity for a few cycles, it soon 

 changes. This doubtless serves to explain the number of major cycles based 

 upon the work of one or two investigators. Moreover, since accurate data 

 upon sun-spot numbers have only been available since 1749, it becomes clear 

 why evidence of the larger cycles is scanty or lacking. It is indeed only as a 

 consequence of the correlation of sun-spots and tree-growth during this period 

 that we have a method of tracing sun-spot cycles weU back into historic times. 



The major cycles that have been suggested are 50, 72, 100, 150, 300-400, 

 1,000 ±, and 10,000* years. Fritz's cycle of 50 years and Hansky's of 72 

 years need not detain us, except to point out that they may be related to the 

 shorter cycles. The cycle of a century more or less rests upon Huntington's 

 curves of major and minor sun-spot cycles since 1749 (1914^:554) (fig. 31). 



1750 neO 1770 1780 1790 1800 1810 1820 1830 1840 1850 I860 1870 1880 1890 1900 1910 



Fig. 31. — Major and minor sun-spot cycles. Asterisks indicate two absolute minima of 

 sun-spots in 1810 and 1913, and middle years (1780 and 1854) of two periods when 

 the sun-spot maxima never fell below 95. After Huntington. 



For this one period the intervals between minima as well as between maxima are 

 very suggestive. It is naturally impossible to confirm this cycle from the sun- 

 spot record at present, and its value is much affected by the fact that Douglass 

 finds it not at all in the growth-curve of Ariaona trees, and that, while it is 

 suggested in a few places in the Sequoia curve, Huntington does not call 

 attention to it in this connection. Douglass (1914^:117) finds much agree- 

 ment between the curve of tree-growth in Arizona since 1400 and a curve 

 representing a 150-year cycle. The coincidence of the 4 crests and troughs is 

 not only fairly convincing (figs. 32, 33), but the curves for each cycle corre- 



■ /-\/\/-' /-v.-'\ ,'--..- 



Fig. 32. 



Fig. 33. 



Fig. 32. — 150-year cycle, shown by placing above one another the three successive cycles 



of curve in figure 33. After Douglass. 

 Fig. 33. — 500-year curve of tree-growth: 20-year means. The major cycle of 150 years 



is indicated by dotted line, the 21-year cycle by broken line. After Douglass. 



