Page:Popular Science Monthly Volume 60.djvu/278

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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 condense, the drops will, if larger than the critical size, fall back to the sun, giving rise to the curved prominences; and if smaller, they will be driven off into space, and be seen as the streamers of the corona. Since the eruptions will not always be perpendicular to the sun's surface, the prominences will often exhibit parabolic curves, and the streamers may not always 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 difficult to account for the maintenance 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 suffered any visible damage or retardation.

The corona has been 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, but has a hairy structure like long tufts of flax. . . Of this appendage we may say with confidence that it cannot be an atmosphere, that is, a continuous mass of elastic gas held up by its own elasticity. . . What then is the corona? Probably detached particles partially or wholly vaporized by the intense heat to which they are exposed. . . The difficult question which we meet is, How are these particles held up? To this question only conjectural replies can be given.

Three conjectures are then mentioned, of which we may note the first.

That the matter of the corona is in what we may call a state of projection, being constantly thrown up by the sun, while each particle thus projected falls down again according to the law of gravitation. The difficulty we encounter here is that we must suppose velocities of projection rising as high as 200 miles per second constantly maintained in every region of the solar globe. The prominences are of two classes—the cloud-like and the eruptive. The first class presents the appearance of clouds floating in an atmosphere; but as no atmosphere dense enough to sustain anything can possibly exist there, we find the same difficulty in accounting for them, that we do in accounting for the suspension of the matter of the corona.

Professor Young is frankly despairing.

I do not know what to make of the corona. . . By what forces the peculiar radiated structure of the corona is determined I have no definite idea. The analogies of comets' tails and auroral streamers both appear suggestive; but on the other hand, the spectra of the corona, the aurora borealis, the comets and the nebulæ are all different, no two in the least alike. . . Nor