538 THE POPULAR SCIENCE MONTHLY, 



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 oscillat- 

 ing 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 fol- 

 lows is chiefly confined to a statement of fact, since a review of oppos- 

 ing 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 appear- 

 ance changes. We are approaching what is really the outer rim of an 

 enormous shallow funnel (that shown is 20,000 miles across) ; shal- 

 low, 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 ofi" into the photosphere, but be- 

 gins, as has just been said, abruptly; and this sudden transition is a 

 thing to be noted. We also observe that the edge is extremely irreg- 

 ular — full of indentations and subdivisions, patches of the photosphere 

 pusliing 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 surround- 

 ing the pupil. A closer look shows that these threads are not ar- 



