MOUNT WILSON SOLAR OBSERVATORY.* 



George E. Hale, Director. 

 INTRODUCTORY SKETCH OF THE YEAR'S WORK. 



The far-reaching influences of the war, so disastrous in their effect 

 upon the progress of science in Eiu-ope, are clearly reflected in the 

 Observatory's history for the past year. The resulting diminution in 

 research activity, however, has not been serious. The Du-ector has 

 been called upon to devote nearly all of his time to the organization 

 and work of the National Research Council, which since February has 

 served as the Department of Science and Research of the Council of 

 National Defense. Fortunately he was able to leave the Observatory's 

 work in the hands of the Assistant Dii'ector, who deserves full credit 

 for its successful conduct during the greater part of the year. 



The prolonged activity of the sun, hardly yet at its maximum, has 

 afforded excellent opportunity for a variety of work. Sun-spots are 

 so numerous that an average of nearly 40 (including the members of 

 groups) have been observed daily for magnetic phenomena, while other 

 evidences of solar unrest have been on a corresponding scale. The 

 successful continuation of laboratory researches on the Stark effect 

 has provided means for another test of the presence of free electricity 

 in sun-spots, but the outcome, as in previous years, has been negative. 

 Thus, while we can hardly doubt that the intense magnetic fields in 

 sun-spots are due to electrically charged particles whirling in vortices, 

 the full explanation of the effect is yet to be developed. The same 

 may be said of the natm-e of the spot vortex. The association in pairs 

 of sun-spots of opposite magnetic polarity remains a phenomenon of 

 almost unbroken occurrence and undoubted significance. But the 

 simple hypothesis that the two members (single or multiple) of the 

 group represent the extremities of a half-ring vortex, while fruitful as 

 a guide to research, can hardly be taken at its face value, in view of 

 the complex polarity phenomena presented by large and active spot- 

 groups; so long as this hypothesis remains useful in suggesting new 

 lines of work, however, it will not be wholly discarded. 



The sudden reversal of magnetic polarity exhibited by sun-spots at 

 the last minimum of solar activity (1912) has given no sign of repetition 

 since that date. If the maximum is to be accompanied by a reversion 

 to the former state, the effect should soon appear. Meanwhile we 

 may speculate sparingly on the meaning of this unexpected change of 

 polarity. If, as we suppose, the magnetic fields in spots are due to 

 the whirl of charged particles in vortices, the direction of the rotary 



♦Situated on Mount Wilson, California. Address, Pasadena, California. 



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