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CARNEGIE INSTITUTION OF WASHINGTON. 



strengths of sun-spots, have been made as in previous years by Messrs. 

 EUerman, Nicholson, Pettit, E. R.Hoge, and Benioff. Photographs of 

 spot spectra have also been taken, and special attention has been 

 given by Mr. Nicholson to a study of the direction of the lines of force in 

 different portions of the spots. The following table, prepared by Mr. 

 Nicholson, gives the polarities of all spots observed in the northern and 

 southern hemispheres of the sun during the calendar year 1920: 



GREAT SUN-SPOT OF MAY 1921. 



The sun-spot which appeared at the east limb on May 8, while not 

 of the very largest size, was a very exceptional group, especially during 

 the time of reduced solar activity. The sun's equator passed longi- 

 tudinally through the spot, and intermittent eruptions near its prin- 

 cipa;l members were photographed throughout the period of observa- 

 tion. A violent terrestrial magnetic storm, accompanied by brilliant 

 auroras, began just when the preceding edge of the calcium flocculi 

 Burrdunding the group reached the central meridian and continued 

 with fluctuating intensity for several days. The very active eruptive 

 region following the largest umbra was on the meridian when the third 

 or most intense phase of the magnetic storm began. On May 16, at 

 4^ lO"* G. M. T., there was a gi-eat outburst in this region, coinciding 

 very closely with a sudden break in the terrestrial magnetic record, 

 which rose to a maximum 30 minutes later, when &n Ha spectro- 

 heliogram showed the eruption to cover an enormous area. 



In the Director's first paper (1908) on the existence of magnetic 

 fields in sun-spots, he pointed out the improbability of any direct 

 influence of these fields on the earth. Subsequently, Schuster, assum- 

 ing an extreme spotted area, the same polarity for all spots, and a 

 length of vortex (separation of poles) equal to an entire solar diameter, 

 calculated that their collective magnetic effect at the earth could not 

 exceed 0.1 mm. displacement on the Greenwich scale. When actual 

 areas, mixed polarities, and short vortices are taken into account, 

 the probability of any appreciable influence is still further reduced. 

 In the present case the mixed polarities of the two principal components 

 of the group, which superficially resembled a typical bipolar spot^ 

 would be an important factor. 



