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tution 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.
Wilson's Theory.—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 light 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
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 oui 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; Kirchhoffs is modern, and partakes of the simplicity of true science.
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 and the solar spots by assuming that there are vast numbers of meteors revolving around the sun, and that these constantly rain down upon the surface of that luminary. Their motion being stopped and changed to heat, feeds this great central fire. Were Mercury to strike the sun in this way, it would generate sufficient heat to compensate the loss by radiation for seven years. Many suppose that the heat of the sun is gradually diminishing. Of this we may be assured, there is enough to support life on our globe for millions of years yet to come.
We shall describe these in regular order, passing outward from the sun. In this journey we shall examine each planet in turn, noticing its distance, size, length of its year, duration of day and night, temperature of the climate, the number of its moons, and many other interesting facts, showing how much we can laiow of its world-life in spite of its wonderful distance. We shall encounter the earth in our imaginary wanderings through space, and shall explain many celestial phenomena already partially familiar to us. In all these worlds we shall find traces of the same Divine hand, moulding and directing in conformity to one universal plan. The laws of light and heat will be invariable. The law of gravitation, which causes a stone to fall to the ground, will be found to apply equally to the most distant planets. Even the very elements of which they are composed will be familiar to us, so that a book of natural science published here would, in all its general features, answer for use in a school on Mars or Jupiter.
Characteristics Common To The Planets. (Hind.) —1. They move in the same invariable direction around the sun; their course, as viewed from the north side of the ecliptic, being contrary to the motion of the hands of a watch.
2. They describe oval or elliptical paths round the sun—not, however, ^differing greatly from circles.
3. Their orbits are more or less inclined to the ecliptic, and intersect it in two points, which are the nodes—one half of the orbit lying north and the other south of the earth's path.
4. They are opaque bodies like the earth, and shine by reflecting the light they receive from the sun.
5. They revolve upon their axes in the same way as the earth. This we know by telescopic observation to be the case with many planets, and by analogy the rule may be extended to all. Hence they will have the alternation of day and night like the inhabitants of the earth; but their days are of different lengths from our own.
6. Agreeably to the principles of gravitation, their velocity is greatest at those parts of their orbit