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penumbra, which, in its turn, will pass from out 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. 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.

THE PLANETS.

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 'mow 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

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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 which are nearest the sun, and least at the parts which are most distant from it; in other words, they move quickest in perihelion, and slowest in aphelion. JX

Comparison Of The Two Groups Of The Major Planets. {Chambers.)—Separating the major planets into two groups, if we take Mercury, Venus, the Earth, and Mars as belonging to the interior, and Jupiter, Saturn, Uranus, and Neptune to the exterior group, we shall find that they differ in the following respects:

1. The interior planets, with the exception of the earth, are not, so far as we know, attended by any satellite, while the exterior planets all have satellites. We can but consider this as one of the many instances to be met with, in the universe, of the beneficence of the Creator, and that the satellites of these remote planets are designed to compensate for the small amount of light their primaries receive from the sun, owing,to their great distance from that luminary.

2. The average density of the first group considerably exceeds that of the second, the approximate ratio being 5 :1.

3. The mean duration of the axial rotations, or mean length of the day of the interior planets, is much longer than that of the exterior; the average in the former case being about twenty-four hours, but in the latter only about ten hours.

The Properties Of The Ellipse.—In the figure, S

X

and S' are the foci of the ellipse; AC is the major axis; BD, the minor or conjugate axis; O, the centre: or, astronomically, OA is the semi-axis-major or mean distance, OB the semi-axis-minor: the ratio of OS to OA is the eccentricity; the least distance, SA, is the perihelion distance; the greatest distance, SC, the aphelion distance.

Fig. 14.

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Characteristics Of Planetary Orbit.—It will not be difficult to follow in the mind the additional

Fig. 15.

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characteristics of a planet's orbit. The orbit or ellipse just giyen is laid on a plane surface. Now,

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