until finally we can see only the side of the fissure, the penumbra, which, in its turn, will vanish.

THE PRESENT THEORY* is deduced from the results of Spectrum Analysis, of which we shall hereafter speak. It is constantly being modified by new discoveries. But we may, in general, believe the sun to be a vast, fiery body, surrounded by an atmosphere of substances volatilized by the intense heat. Among these, we recognize familiar elements, as iron, copper, &c.

The different portions of the sun are thought to be arranged thus: (1). The nucleus, probably gaseous; † (2). The photosphere, an envelope several thousand miles thick, which constitutes the visible part of the sun; (3). The chromosphere, composed of luminous gas, mostly hydrogen, and the seat of enormous protuberances, tongues of fire, which dart forth, sometimes at the rate of 150 miles per second, and to a distance of over 100,000 miles; (4). The corona, an outer appendage of faint, pearly light, consisting of streamers reaching out often several hundred thousand miles. Of these solar constituents, the eye and the telescope ordinarily reveal only the photosphere; the rest are seen during a total eclipse or by means of the spectroscope.

The outer portion of the sun radiates its heat and

* As Kirchhoff, by his discoveries in Spectrum Analysis, laid the foundation of this theory, it is often called after him.

The interior of the sun, if gaseous, must be powerfully condensed, because of the tremendous pressure of the atmosphere. The high temperature, however, prevents the gas from liquefying. The rain-storms on the sun, if such ever occur, consist of drops of molten iron, copper, zinc, &c., vaporized by the enormous heat; and often a tempest would drive before it this white-hot, metallic blast, with a speed of 100 miles per second.

This is so called because, during a total eclipse, it forms around the moon a corona, or glory, that is the most wonderful feature of this rare event. (See p. 141.)

light, and, becoming cooler, sinks; the hotter matter in the interior then rises to take its place, and thus convection currents are established (Physics, p. 193). The cooler, descending currents are darker, and the hotter, ascending ones are lighter; this gives rise to the mottled look of the sun. At times, this occurs on a grand scale, and the heated, up-rushing masses form the faculæ, and the cooler, down-rushing ones produce the solar spots.

The Heat of the Sun is generally considered to be produced by condensation, whereby the size of the sun is constantly decreasing, and its potential energy thus converted into kinetic. 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, thus stopped, is changed to heat, and 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.

Doubtless, the solar heat is gradually diminishing, and will ultimately be exhausted. In time, the sun will cease to shine, as the earth did long since. Newcomb says that in 5,000,000 years, at the present rate, the sun will have shrunk to half its present size, and that it cannot sustain life on the earth more than 10,000,000 years longer. Of this we may be assured, there is enough to support life on our globe for millions of years yet to come.

* The heat of the sun could be maintained by an annual contraction of 220 feet in its diameter, a decrease so insignificant as to be imperceptible with the best instruments; or by the annual impact of meteors equal in amount to the mass of Mercury.

II. THE PLANETS.

INTRODUCTION.

The Planets will be described in regular order, passing outward from the sun. In this journey, we shall examine each planet in turn, noticing its distance, size, length of year, duration of day and night, temperature, climate, number of moons, and other interesting facts, showing how much we can know 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, molding and directing in conformity to one universal plan. We shall discover that the laws of light and heat are invariable, and that the force of gravity, which causes a stone to fall to the ground, acts similarly upon the most distant planet. Even the elements of which the planets are composed will be familiar to us, so that a book of natural science published here might, in its general features, answer for use in a school on Mars or Jupiter.

Common Characteristics (Hind).-1. The planets move in the same 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 elliptical paths around the sun,not differing much from circles.

3. Their orbits are more or less inclined to the ecliptic, and intersect it in two points-the nodes,one-half of the orbit lying north, and the other south of the earth's path.

4. They are opaque bodies, and shine by reflecting the light they receive from the sun.

5. They rotate 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 have the alternation of day and night.

6. Agreeably to the principles of gravitation, their velocity is greatest at that part of their orbit nearest the sun, and least at that part most distant from it; in other words, they move quickest in perihelion, and slowest in aphelion.

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 and Mars, are not attended by any satellite, while all the exterior planets have satellites.

2. The average density of the first group consider

ably exceeds that of the second, the approximate ratio being 5:1.

3. The mean duration of the axial rotations, or the 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.

Properties of the Ellipse.-In Fig. 20, S and S' are the foci of the ellipse; A C is the major axis; B D, the minor or conjugate axis; O, the center: or, astronomically, O A is the semi-axis-major or mean

distance, O B the semi-axis-minor: the ratio of OS to O A is the eccentricity; the least distance, S A, is the perihelion distance; the greatest distance, SC, the aphelion distance.

Characteristics of a Planetary Orbit.-It will not be difficult to follow in the mind the additional characteristics of a planet's orbit. Take two hoops, and bind them into an oval shape. Incline one