190 COMETS TAILS, THE CORONA, AND THE AURORA. 



The other variation of some 1») per cent each wa}" has a period of 

 25,03 (lays and affects both hemispheres alike. At first sig-ht it is 

 natural to refer this to the synodical time of revolution of the sun on 

 his axis as determined by observations of sun spots. But this is 27.3 

 days. Remembering- that the earth never departs more than 7'^ from 

 the sun's equator, we should rather take the time of revolution of the 

 equatorial belt for comparison. As estimated by the motion of the 

 facula\ this is 2<>.06 days, the equator moving faster than the sun-spot 

 l)elts, and prol)ably the time of revolution of the outermost layers, 

 from which the particles stream, is yet a little shorter. The agreement 

 with the period of the aurora (2.5.93 da3'S) would thus be within the 

 limits of error of the observations. 



ATMOSPHERIC ELECTRICITY. 



Let us now trace the effect of the auroras on the earth's atmosphere. 

 If they are really cathode rays on a grand scale, they must ionize the 

 air; the neg-ative "ions will form centers for condensation, and sinking 

 to the earth by gravitation, will charge it negatively, leaving the layers 

 at moderate heights positively charged. This agrees with the results 

 of recent observations made from balloons up to heights of 3,000 

 meters. 



Since condensation will depend on the number of ions available for 

 nuclei, we have at once an explanation of the curious fact that cloud 

 formation in the upper atmosphere is more copious in j^ears of frequent 

 aurora^ than when they occur rarely. In this connection another odd 

 coincidence ma}^ be mentioned. When sun spots are numerous Jupiter 

 shines with a white light; when the}^ are few his light has a reddish 

 tinge. Now, it is agreed that Jupiter is still at a high temperature. 

 If, therefore, sun spots cause auror;e on Jupiter, and consequent cloud 

 formation, we must see less of the heated interior in sun-spot years 

 than we do when his cloud layers are not so opaque. 



In isiil) Von Bezold showed that the daily variation of the compass 

 over the earth's surface could l)e simply represented as follows: 

 Imagine two points, one in latitude -lO'^ N., and one in 40^ S., to move 

 round with the sun. Then it is as if the north end of the compass 

 needle were attracted toward the northerly point and the south end 

 toward the southerly. Remembei'ing that the air immediately above 

 the earth has a positive charge, we see that this effect would follow by 

 Ampere's rule, if the sun's heat caused two air whirls, one in the 

 northern and one in the southern hemisphere, over the places of highest 

 temperature, the former rotating counter-clockwise, the latter clock- 

 wise. Such whirls would result from the sucking in of currents from 

 the slower-moving north latitudes and the faster-moving south lati- 

 tudes toward the mean latitude of 40^, in the northern hemisphere, 

 and similarly for the southern. If this ])e the true explanation, then 

 for a given f requenc}^ of sun spots the amplitude of the diurnal varia- 



