184 comets' tails, the corona, and the aurora. 



from near th(^ sun's surface to the earth's orl)it, 3'et whirled round 

 unl)roken, in the latter case throuj^h an angle of 180-* in little more 

 than two hours. It seems utterly incredible that in such a case it is 

 one and the same material object which is thus brandished. If there 

 could be conceived such a thin^ as a neg-ative shadow, a momentary 

 impression made upon the luminiferous ether behind the comet, this 

 would represent in some degree the conception such a phenomenon 

 irresistibly calls up. But this is not all. Even such an extraordinary 

 excitement of the ether, conceive it as we will, will alt'ord no account 

 of the projection of lateral streamers; of the effusion of light from the 

 nucleus of a comet toward the sun, and of its subsetpient rejection; 

 of the irregular and capricious mode in which that effusion has been 

 seen to take place." 



These passages give a vivid picture of the utter puzzledom of 

 astronomers over difficulties which arise from precisely those phe- 

 nomena which fit most naturally into the theory of Arrhenius. 



THE PROMINENCES AND THE CORONA. 



At the moment when the sun's disc is obscured in a total eclipse 

 enormous red flames, sometimes curving over toward the sun and 

 sometimes floating like clouds at heights up to •i(),000 miles above his 

 surface, are seen projecting over the reg-ion of sunspots, where the 

 sun's eruptive activity is greatest; and silvery streamers with a radial 

 structure form a lens-shaped envelope about the same region, often 

 extending to a distance of several times the sun's radius. These are 

 known as the prominences and the corona. 



The sun must itself project vapors into space. When these con- 

 dense, the drops will, if larger than the critical size, fall back to the 

 sun, g'iving rise to the curved prominences; and if smaller, they will 

 be driven off' into space, and l)e seen as the streamers of the corona. 

 Since the eruptions will not always be perpendicular to the sun's sur- 

 face, the prominences will often exhibit parabolic curves, and the 

 streamers may not alwa3-s be strictly radial, though the greater part 

 of this effect is to be attributed to the foreshortening under which 

 some of them are viewed from the earth. 



Those particles which have approximately the critical diameter will 

 float as clouds, sustained by the pressure of light. This point is 

 specially interesting, since it has been difiicult to account for the main- 

 tenance of the cloudlike prominences without assuming the existence 

 of a considerable atmosphere about the sun. Yet the comet of 1843 

 described 300,000 miles within a distance of less than one-third of the 

 sun's radius from his surface with a velocity of 350 miles per second, 

 and came out without having suft'eredany visible damage or retardation. 



The corona has ))een as great a stumbling-block to astronomers as 

 the comet's tail. Thus Newcomb (Popular Astronomy, p. 263) says: 



"•The corona is not a mass of foggy or milky light. ])ut has a hairy 

 structure like long tufts of flax. * * "" (3f this appendage we may 



