Page:Popular Science Monthly Volume 5.djvu/556

538 above that at which the materials chiefly composing the earth's crust would become fluid. It is, then, at the outset, an unlikely supposition that the surface of the sun should be solid; but, independently of such considerations, the behavior of this, or any other spot, is decisive as against this alternative. It was formed and grew to its present size in a comparatively short time, and, according to past experience, it will shortly break up and disappear.

Besides its rotation with the sun, the spot has an absolute motion on it, advancing, as a whole, at a greater angular velocity round the solar axis than spots nearer its poles, besides having a slight oscillating movement, which carries it alternately nearer to and farther from the sun's equator; all this going on simultaneously with changes in its form and size. The spot then moves about on the sun as a ship on the ocean, or, to employ a less inaccurate simile, like a rent in the clouds of our sky, which, while turning with the earth, both shifts its place and alters its appearance from hour to hour, the spot not being something above the sun's surface, but a gap in and below it. We seem irresistibly led to the conclusion, then, that the surface of the sun is not a solid, and, considering this freedom of motion, we are led to question if it can even be a liquid, and whether we must not look upon it as wholly vapor-like.

But we may approach the spot and look within it for answers to these questions, though, as we do so, the reader should distinguish between the facts stated and the inferences drawn from them. As to the former, observers may be said to agree with little exception; as to the latter, astronomers are, in some cases, at variance, and what follows is chiefly confined to a statement of fact, since a review of opposing hypotheses would not be at present in place.

The approach to the spot is scarcely marked in the engraving by any variation of the surface; though there is, in reality, a very slight blurring of the luminous masses (rice-grains) which makes these look less distinct as we draw near the edge. Here, all at once, the appearance changes. We are approaching what is really the outer rim of an enormous shallow funnel (that shown is 20,000 miles across); shallow, that is, in the outer portion only which is saucer-shaped, while the spout of the funnel is indefinitely deep. The first or gentle slope is the penumbra. It does not shade off into the photosphere, but begins, as has just been said, abruptly; and this sudden transition is a thing to be noted. We also observe that the edge is extremely irregular—full of indentations and subdivisions, patches of the photosphere pushing out here and there over it, and at times apparently hanging suspended above the abyss.

The sides of the slope are filled with what seem at first sight like long white threads, radially disposed, so that a circular penumbra, looked at casually, has somewhat the appearance of the iris surrounding the pupil. A closer look shows that these threads are not