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Fig. 11.

1,000, then that of the penumbra would be 469, and that of the nucleus 7. There may be much light and heat radiated by a spot, which seems totally black as compared with the sun : we remember that when we look through even a Drumrnond light at the sun, it appears as a black spot on the disk of that luminary.

Jo Facuke, willow-leaf, and mottled appearance.—Besides the variety of spots already described, there are other curious appearances worthy of note. Bright ridges or streaks appear, which constitute the most brilliant portions of the sun.— These are called faculxz. They vary from barely discernrACDI"B- ible, softly-gleaming

tracts 1,000 miles long, to lofty, piled-up, mountainous regions 40,000 miles long and 4,000 broad. Outside of the spots, the entire disk of the sun is covered with minute shady dots, giving it a mottled appearance not unlike that of the skin of an orange, though less coarse. Under a large telescope the surface seems to be entirely made up of luminous masses, imperfectly separated by dark dots called pores. These masses are said by Mr. Nasmyth to have a "willow-leaf" shape; many observers apply other descriptive terms, such as "rice grains," "untidy circular masses," "things twice as long as broad," "granules," etc. The accompanying cut represents the willow-leafed structure of the luminous surface, and also the "bridges"

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spanning the solar spot. Indeed, it is said that the spots themselves always have their origin in a "pore," which appears to slowly increase and assume the blackness of an umbra, after which the penumbra begins to appear. Physical Constitution Of The Sun.—Of the constitution of the sun, and consequent cause of the solar spots, very little is definitely known. We shall notice the various theories now adopted by different astronomers.

"f\ Wilson's Theort.—This theory supposes that the sun is composed of a solid, dark globe, surrounded by three atmospheres. The first, nearest the black body of the sun, is a dense, cloudy covering, possessing high reflecting power. The second is called the photosphere. It consists of an incandescent gas, and is the seat of the fight and heat of the sun. The third, or outer one, is transparent, very like our atmosphere. According to this theory, the spots are to be explained in the following manner. They are simply openings in these atmospheres made by powerful upward currents. At the bottom of these chasms we see the dark sun as a nucleus at the centre, and around this the cloudy atmosphere—the penumbra. This explains a black spot with its penumbra. Sometimes the opening in the photosphere may be smaller than that in the inner or cloudy atmosphere; in that case there will be a black spot without a penumbra. It will be natural to suppose that when the heated gas of the photosphere or second atmosphere is thus violently rent asunder by an eruption or current from below, luminous ridges will be formed on every side of the opening by the heaped-up gas. This will account for the faculce surrounding the sun-spots. It will be natural, also, to suppose that sometimes the cloudy atmosphere below will close up first over the dark surface of the sun, leaving only an opening through the photosphere, disclosing at the bottom a grayish surface of penumbra. We can readily

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see, also, how, as the sun revolving on its axis brings a spot nearer and nearer to the centre, thus giving us a more direct view of the opening, we can see more and more of the dark body. Then as it passes by the centre the nucleus will disappear, until finally we can see only the side of the fissure, the penumbra, which, in its turn, will pass from our sight. The existence of an outer atmosphere will account for the fact that the sun's margin is not so bright as its centre.


Kirchhoff's Theory.—This view differs essentially

from that of Wilson. It considers the sun as an . intensely white-hot solid or fluid body surrounded by a dense atmosphere of flame, filled with substances volatilized by the vivid heat. Changes of temperature take place, which give rise to tornadoes and violent tempests. Descending currents produce openings filled with clouds, which appear as black spots on the sun's disk. A cloud once formed becomes a screen to shield the upper regions from the direct heat of the body of the sun. Thus a lighter cloud is produced, which gives the appearance of a penumbra around the spots.

Spectrum analysis.—The hypothesis just given of the constitution of the sun rests upon the discoveries of the spectroscope. This subject will be treated hereafter under the head of Celestial Chemistry. Wilson's theory is time-honored, but complicated; Kirchhoff s is modern, and partakes of the simplicity of true science.

"A The Heat Of The Sun.—This subject is not understood. Many theories have been advanced, but none has been generally adopted. Some have supposed the heat is produced by condensation, whereby the size of the sun is being constantly decreased. The dynamic theory accounts for the heat

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