Page:Popular Science Monthly Volume 1.djvu/29
THE RECENT ECLIPSE OF THE SUN.
flame. They are not, properly speaking, flames at all, but masses of gas glowing with intensity of heat.
Many of the most important discoveries recently made respecting the sun relate to these wonderful objects; but in this place I shall refrain from speaking more about them than seems necessary to illustrate the subject of the corona; for, as a matter of fact, the observers during the late eclipse turned scarcely a thought to the colored prominences, nor is it likely that any thing new respecting them will ever be learned during total eclipses of the sun.
Outside the sierra and the prominences, the true corona is seen. To ordinary vision, and probably also even under the scrutiny of powerful telescopes, it appears to be divided into two distinct portions. There is in the first place an inner and brighter region, extending apparently to a distance from the sun equal to about one fifth of his diameter. The outline of this inner corona is uneven but not radiated, and, though not sharply defined, appears yet to be very definitely indicated by the rapid falling off of luster beyond its limits. The inner corona has been described as of a white, pearly luster by some observers; but under the most favorable conditions it appears, when carefully observed, to have a somewhat ruddy hue.
Extending much farther from the sun, how far is not as yet known, is the radiated corona. It is much fainter than the inner corona, and its light grows fainter and fainter with distance from the sun, until lost to view on the dark but not black background of the sky. Through this faint and softly graduated corona extend radiations of somewhat greater brightness. It is between these radiations that those dark gaps or rifts appear, which have figured so much in the narratives of recent eclipse observations. The dark gaps are, indeed, more striking features than the radiations which form them; but it must be remembered, nevertheless, that the radiations are the only positive features in this case, the gaps being merely regions where there are no radiations.
We may typically represent the corona, as it had been revealed to us during former eclipses, by the accompanying sketch from a photograph taken by Mr. Brothers at Syracuse during the eclipse of December, 1870. Only, it must be remembered that the photograph may not represent the full extent of the corona, while many details of its structure are too delicate to be shown in a figure so small as is here given. It will be understood further that the inner part, marked R, is much brighter than the whole of the outer part, marked c, and that this outer part shades off gradually into the dark background of the sky.
- In a gas-flame there is (as our meters tell us) a continual supply of gas, which mixes with the oxygen of the air, and undergoes what is called combustion. But in the sun's colored prominences the hydrogen enters into no chemical combination, at least none such as we are familiar with. Simply by the intense heat to which it is exposed it glows, just as iron glows when it is heated sufficiently.