186 coMpyrs' tails, the corona, and thk aurora. 



those which have less than the critical diameter will carr}- a nei>ative 

 charge off into space. On both counts the corona will be left with a 

 surplus of positive charge. The same arguments hold for the vapors 

 emitted by the nucleus of a comet. Thus comets' tails should consist 

 of negatively charged particles. 



Let us follow the career of the particles launched into space. They 

 proceed radiall}^ from the sun alcove the regions of sun spots with 

 rapidl}' increasing speed, which, however, ma}^ })e shown to approach 

 a finite limit at a distance of about ten radii from the sun. If thej'' 

 encounter another bodv, such as the earth, they charge its outer atmos- 

 phere negatively, and when this charge reaches a certain value, it will 

 begin to repel them. The oncoming rush will l)e deflected, and stream 

 past the earth on each side in hyperbolic orbits. Far out in space they 

 must sooner or later meet particles from other bodies, and, if by col- 

 lision or agijregation they increase beyond the critical diameter, they 

 will first lose speed and then drift back with ever increasing velocity 

 past the earth, clirectl}- toward the sun. The space immediately ])ehind 

 the earth would be screened b}" her, and so be void of particles. Could 

 we take our stand on the moon, we should thus see the earth attended 

 by a faint double tail with a dark dividing line (so conspicuous a fea- 

 ture in comets), immediatel}^ behind it, pointing from the sun; and a 

 similar, though perhaps fainter, tail, pointing toward him. Not only 

 so, but the earth helps to form her own tail. For when the negative 

 charge in the upper atmosphere is high enough, discharges are })rought 

 about by the powerful ultra-violet radiation from the sim, and parti- 

 cles are driven off' radiall}' from the earth on the side turned toward the 

 sun, only to l)e drifted back with the other streams into the tail. The 

 effect will be as if a sheaf of light projected from her toward the sun. 



Compare with this the description of the zodiacal light (Newcomb, 

 Popidar Astronomy, p. -iH)): 



"This object consists of a very soft faint column of light, which may 

 be seen rising from the western horizon after twilight on any clear 

 winter or spring evening; it may also be s(^en rising from the eastern 

 liorizon just before daybreak in the summer or autumn. It really 

 extends out on each side of the sun, and lies nearly in the plane of the 

 ecliptic. * ''" " Near the equator, where the ecliptic always rises 

 high above tiie horizon, the light can be seen about equally well all 

 the year round. * "" '^^ It is due to a lens-shaped appendage of 

 some sort surrounding the sun, and extending out a little beyond the 

 earth's orl)it. 



"The nature of the su])stance from which this light emanates is 

 entirely unknown. * * '^^ Professor Wright, of Yale College, finds 

 its sp(H'trum to be continuous. Accepting this, we should be led to 

 the conclusion that the phenomenon in question is due to reflected sun- 

 light, probabl}' from an inuuense cloud of meteorites, filling up the 

 space between the earth and the sun.'' 



