METHOD OF ESTIMATING RAINFALL BY GROWTH OF TREES. 



119 



very regular curve, but with three subordinate minima dividing the whole into three 

 equal parts; this secondary cycle seems hkely to be due to traces of the 11-year period 

 now to be mentioned. 



The last cycle to be considered is that of 11 years. In the 60 years during which the 

 1 1-year sun-spot and magnetic cycle has been recognized, this period has been of the greatest 

 interest, for it deals with a connection between the sun and the earth other than gravity 

 which holds the earth in place, and it indicates that the energy given out by our great 

 central luminary is not constant. Since 1873 many writers have found variations in 

 the ordinary meteorological elements, rainfall, temperature, and pressure corresponding 

 to this period. Hence it is of peculiar interest to see whether the trees which carry the 

 rainfall record back so far with a comparativelj^ high degree of accuracy show the same 

 cycle. In nearly all parts of the long, 500-year curve, there are suggestions of an 11- 

 year variation. By tracing this throughout the record, the period is found to have a 

 length of very nearly 11.4 years, which is sufficiently close to the length of the sun-spot 

 cycle to be considered identical with it. The average total variation is 16 per cent of the 

 mean. The average conditions of growth during eight different intervals of approximately 

 60 years each are shown in figure 23. From this it appears that the 11-year cycle is not 

 uniform throughout the whole period of 492 years covered by the curve. In general the 

 cycle shows two maxima and two minima. From 1400 to about 1670 the second minimum 

 is generally the deeper. Then from about 1670 to about 1790 the cycle flattens out, and 

 has no marked rhythmic character. From about 1790 to the present time there are again 

 two minima, but here the fu-st is, on the whole, more conspicuous. 



The average of all the 11-year periods from 1492 to the present time is shown in the 

 upper curve of figure 24. Below this is the rainfall for 50 years on the California Coast, 



Arizona tree growth 

 492 years 



California coast 



rainfall-50 years 



1863-1912 



California coast 

 temperature— 50 years 

 1863-1912 



Inverted sun spot 

 numbers 125 years 



Fig. 24.— Comparison of 11.4-year Cycles in Tree Growth, Rainfall, Temperature, 

 and Inverted Sun-spot Numbers. 



