2 74 POPULAR SCIENCE MONTHLY. 



in motion is (pace M. Cremieux) equivalent to a negative current, the 

 passage of electrified particles through the upper atmosphere should 

 affect magnetic instruments on the earth. Sunspots, aurorae, magnetic 

 storms should therefore vary together. 



It has long been known empirically that they do agree in a gen- 

 eral way. Arrhenius' discussion of the mass of statistics of observed 

 auroraj forms so striking an example of the 'Method of Concomitant 

 Variations' that at the risk of wearying the reader we shall give it in 

 some detail. 



1. Slow secular periods. 



(a) Both sunspots and aurorae show marked maxima at the middle 

 of the eighteenth and the end of the nineteenth centuries. 



(b) Sunspots, aurorae, and magnetic storms go through a simul- 

 taneous increase and decrease in the well-known period of 11.1 years. 



The source of these slow variations must be looked for in the little 



understood variations of the sun's activity. 



2. Annual period. 



The number of aurorae is greatest in March and September, and 

 least in June and December; and the mean frequency for both hemi- 

 spheres is somewhat less in June than in December. 



Now the sun's activity, as indicated by the number of sunspots, is a 

 minimum at his equator, the spots occurring principally in belts about 

 15° north and south of his equator. Since the streams of particles 

 issue radially from the sun, the earth will be most exposed to them, 

 when she is most nearly opposite the active belts. But the earth stands 

 opposite the sun's equator on June 4 and December 6, and is at her 

 farthest north and south of it (7°), t. e., most nearly opposite the sun- 

 spot belts, on March 5 and on September 3. Moreover, she is somewhat 

 nearer to the sun in December than in June. 



As between the two hemispheres, the same conditions apply as those 

 which regulate the seasons, viz., altitude of the sun above the horizon, 

 and length of time during which he remains above it daily. Aurora? 

 should therefore be more frequent in summer than in winter, a result 

 which is verified by the records. And just as the highest daily tem- 

 perature occurs from two to three hours after mid-day, so we oiight to 

 lind a daily maximum of aurorae about 3 p. m. It is not possible to 

 verify tliis directly, since aurora? are not visible in daylight. But 

 Arrhenius remarks (1) that the majority of them occur before mid- 

 night and not after it, which is so far in general agreement with the 

 theory; (2) that Carlheim-Gyllenskiold, discussing the observations 

 made at Cape Thordsen in Spitzbergen during the winter of 1882-1883, 

 with a view to correcting the numbers recorded for the effect of day- 

 light in concealing them, deduces a probable maximum for the num- 

 ber actually occurring at 2.40 p. m. 



