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 11 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 timecovered in part at least into millions of years. 
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