comets' tails, the corona, and the attrora. 1<S9 



15 north and south of hiy (Hiuator. Since the streams of particles 

 issue radiall}' from the sun, tliQ earth will be most exposed to them 

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

 opposite the sun's ecjuator on June 4 and December (>, and is at her 

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

 spot l)elts, on March 5 and on Septem]>er o. Moreover, she is some- 

 what 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 leng'th of time durin^- which h(^ remains al)ove it daily. Aurorte 

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

 which is >eritied by the records. And just as the hig'hest daily tem- 

 perature occurs from two to three hours after midday, so we ou^ht to 

 tind a daily maximum of aurorte about 3 p. m. It is not possible to 

 verify this directly, since aurorse are not visible in daylii^'ht. Hut 

 Arrhenius remarks (1) that the majoritv of them occur ))efore mid- 

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

 theory; {'2) that Carlheim-(Tyllenskir)ld. discussing the observations 

 made at Cape Thordsen in Spitzbergcn during the winter of iSSii-SS, 

 with a view to correcting the mimbers recorded for the eti'ect of day- 

 light in concealing thcin, deduces a probable maximiunfor the nund)er 

 jictually occurring at 2.4<) p. m. 



But though we can not observe aurorte in daylight we are not with- 

 out resource, for even when invisible they give notice of their presence 

 by disturbing the ordinary course of the records photograj)hic;dly taken 

 in our magnetic observatories. In lSi>l> Van Benunelen discussed the 

 records of such magnetic storms taken in Batavia. He found thtit they 

 show maxima in March and September, minima in January and flune, 

 and a daily maximum at 'S p. m., and mininuun at i a. m. 



•>'. Aloiit/ih/ rarliit/oiis. -H is only recently (ISi)S) that the collection 

 of statistics of aurone published by pA-kholm and Arrhenius has 

 l)rought to light two curious monthly variations in their number. 



One of these, with ii variation of 20 per cent on each side of tin; 

 mean, depends on the revolution of the moon in her orbit, showing in 

 the northern hemisphere a maximum when the moon is fai'thest south 

 of the equator, a minimum when she is farthest north; and vice versa 

 for the southern hemisphere. 



The explanation appears highly ingenious. It is as follows: The 

 moon, being unprotected by an atmosphere, is charged by the sti'cams 

 of particles that reach her much as the outer layers of our own atmos- 

 phere are charged, and therefore, as we have good reason to belie\i', to 

 a far higher negative potential than is o))served at the surface of the 

 earth. If so, she will seriously affect th(> munbei- of aurovie at any 

 place over which she stands, by lowering the potential gradient, and 

 thus reducing thcnumberof negative discharges in the highest regions 

 of our atmosphere. 



