84 CLIMATIC CYCLES AND TREE-GROWTH. 



slower-moving latitudes on each side would present much mechanical 

 disturbance and favor the formation of local vortices. Such a process 

 as this would be accompanied by the increased radiation in sunspot 

 maximum which has been observed. If this hypothesis has a basis of 

 fact, it is probable that the increased radiation at that time would come 

 from the sun's equator, where there are no spots. Increased rotational 

 movement of the equatorial zone at the sunspot maximum should be 

 susceptible of observation by spectroscopic means. The meaning of the 

 slow movement of this spot-forming zone toward the equator, as sun- 

 spot maximum changes to minimum, is not clear under this hypothesis ; 

 nor does one see why the secondary spot described by Hale (1919) 

 should have its definite location following the principal spot, nor why 

 the magnetic polarity of spots changed near the last sunspot minimum. 

 These phenomena, recently observed by Hale and his collaborators, 

 point toward causes within the sun. 



Length of the sunspot period. For many years Newcomb's figure 

 of 11.13 years has been commonly quoted. However, recently some of 

 the best authorities say frankly that it may be anywhere from 1 1 years 

 to nearly 12 years. Schuster (1898-1906) discussed analytically the 

 best known sunspot numbers, those since 1750. This has been followed 

 by the work of Kimura (1913), and especially Turner (1913) and 

 Michelson (1913). In general, the analyses by Schuster and Kimura, 

 and by Turner in his earlier papers, produce a large number of possible 

 periods of small amplitude. Michelson, however, goes to the other 

 extreme. "Indeed," he says, "it would seem that with the exception 

 of the 11-year period and possibly a very long period (of the order of 

 100 years) the many periods found by previous investigators are 

 illusory." Turner in his hypothesis referred to above reduces the 

 number to a few, which supply a basis for his reasoning. Michelson had 

 favored a period of about 11.4 years and Turner says that only this 

 11.4-year period is sensible at the present time. 



Tree-growth and solar activity. The correlation shown in this chap- 

 ter suggests a possible use of the annual rings of trees in the study of 

 solar activity. There are two lines which such a study might take. 

 An intensive line already mentioned includes the search for wet-climate 

 trees showing the solar rhythm in their growth and the determination 

 of the conditions under which they produce this curve. An extensive 

 line of study is obviously possible also in reconstructing, as far as 

 possible, a history of the sunspot cycle from very old trees. The 

 yellow pines of Arizona give evidence that 500 years ago the cycle was 

 operating very much as now. The sequoias, if correctly interpreted, 

 already carry the history back over 3,000 years, and beyond that fossil 

 trees may stretch the time covered in part at least into millions of years. 



