calcium, magnesium, iron and manganese. calcium, sodium, and aluminium.

Silicate of

(B) Carbonaceous ... Carbon in combination with hydrogen and oxygen. Sulphates of magnesium, calcium, sodium and potassium.

Gases, too, especially hydrogen and carbonic-oxide, are usually occluded in these bodies, and escape from them on the application of heat.

The first substances volatilised out of the meteorites constituting celestial bodies (See Art. 65) by the heat due to collisions, are magnesium, manganese, iron and sodium, that is, the constituents which are volatilised at the lowest temperatures.

320. Thinking that, unlike terrestrial rocks, meteorites are probably portions of cosmical matter which has not been acted on by water or volcanic heat, Mr. Sorby was led to study their microscopical structure. He has thus been able to ascertain that the material was at one time certainly in a state of fusion; and that the most remote condition of which we have positive evidence was that of small, detached, melted globules, the formation of which cannot be explained in a satisfactory manner, except by supposing that their constituents were originally in the state of vapour, as they now exist in the atmosphere of the Sun; and, on the temperature becoming lower, condensed into these "ultimate cosmical particles." These afterwards collected together into larger masses, which have been variously changed by subsequent metamorphic action, and broken up by repeated mutual impact, and often again collected together and solidified. The meteoric irons are probably those portions of the metallic constituents which were separated from the rest by fusion when the metamorphism was carried to that extreme point.

Stop

CHAPTER IV.

APPARENT MOVEMENTS OF THE HEAVENLY

BODIES.

LESSON XXVI.-The Earth a moving Observatory. The Celestial Sphere. Effects of the Earth's Rotation upon the apparent Movements of the Stars. Definitions.

321. In the previous chapters we have studied in turn the whole universe, of which we form a part; the nebulæ and stars of which it is composed; the nearest star to us -the Sun; and lastly, the system of bodies which centre in this star, our own Earth being among them.

We should be now, therefore, in a position to see exactly what "the Earth's place in Nature"-what its relative importance—really is. We find it, in fact, to be a small planet travelling round a small star, and that the whole solar system is but a mere speck in the universe-an atom of sand on the shore, a drop in the infinite ocean of space.

322. But, however small or unimportant the Earth may be compared to the universe generally, or even to the Sun, it is all in all to us inhabitants of it, and especially so from an astronomical point of view; for although in what has been gone through before, we have in imagination looked at the various celestial bodies from all points of view, our bodily eyes are chained to the Earth-the Earth is, in fact, our Observatory, the very centre of the visible creation; and this is why, until men

knew better, it was thought to be the very centre of the actual one.

323. More than this, the Earth is not a fixed observatory; it is a moveable one, and, as we know, has a double movement, turning round its own axis while it travels round the Sun. Hence, although the stars and the Sun are at rest, they appear to us, as every child knows, to have a rapid movement, and rise and set every twenty-four hours. Although the planets go round the Sun, their circular movements are not visible to us as such, for our own annual movement is mixed up with them.

Having described the heavens then as they are, we must describe them as they seem. The real movements must now give way to the apparent ones; we must, in short, take the motion of our observatory, the Earth, into account.

324. To make this matter quite clear, before we proceed, let the Earth be supposed to be at rest neither turning round on its own axis, nor travelling round the Sun. What would happen is clearly this-that the side of it turned towards the Sun would have a perpetual day, the other side of it perpetual night. On one side the Sun would appear at rest-there would be no rising and setting; on the other side the stars would be seen at rest in the same manner the whole heavens would be, as it were, dead.

325. Again, let us suppose the Earth to go round the Sun as the Moon goes round the Earth, turning once on its axis each revolution, which would result in the same side of the Earth always being turned towards the Sun. The inhabitants of the lit-up hemisphere would, as before, see the Sun motionless in the heavens; but in this case, those on the other side, although they would never see the Sun, would still see the stars rise and set once a year.

These examples should give you an idea of the way in which the various apparent movements of the heavenly bodies are moulded by the Earth's real movements, and

we shall find that the former are mainly of two kindsdaily apparent movements and yearly apparent movements, which are due, the first to the Earth's daily rotation, or turning on its axis, and the second to the Earth's yearly revolution or journey round the Sun. In each case the apparent movement is, as it were, a reflection of the real one, and in the opposite direction to the real one; exactly what we observe when we travel smoothly in a train or balloon. When we travel in an express train, the objects appear to fly past us in the opposite direction to that in which we are going; and in a balloon, in which not the least sensation of motion is felt, to the occupant of the car it is always the Earth which appears to fall down from it, and rush up to meet it, while the balloon itself rises or descends.

326. We will first study the effects of the Earth's rotation on the apparent movements of the stars. The daily motion of the Earth is very different in different parts at the equator and at a pole, for instance. An observer at a pole is simply turned round without changing his place, while one at the equator is swung round a distance of 24,000 miles every day. We ought, therefore, to expect to see corresponding differences in the apparent motions of the heavens, if they are really due to the actual motions of our planet. Now this is exactly what is observed, not only is the apparent motion of the heavens from east to west-the real motion of the Earth being from west to east-but those parts of the heavens which are over the poles appear at rest, while those over the equator appear in most rapid motion. In short, the apparent motion of the celestial sphere--the name given to the apparent vault of the sky-to which the stars appear to be fixed, and to which, in fact, their positions are always referred, is exactly similar to the real motion of the terrestrial one, our Earth; but, as we said before, in an opposite direction.

327. Before we proceed further, however, we must

say something more about this celestial sphere, and explain the terms employed to point out the different parts of it.

328. In the first place, as the stars are so far off, we may imagine the centre of the sphere to lie either at the centre of the Earth or in our eye, and we may imagine it as large or as small as we please. The points where the terrestrial poles would pierce this sphere, if they were long enough, we shall call the celestial poles; the great circle lying in the same plane as the terrestrial equator we shall call the celestial equator, or equinoctial; the point overhead the zenith; the point beneath our feet the nadir.

Now, as the whole Earth is belted by parallels of latitude and meridians of longitude, so are the heavens belted to the astronomer with parallels of declination and meridians of right ascension. If we suppose the plane in which our equator lies extended to the stars, it will pass through all the points which have no declination (0°). Above and below we have north and south declination, as we have north and south latitude, till we reach the pole of the equator (90°). As we start from the meridian of Greenwich in the measure of longitude, so do we start from a certain point in the celestial equator occupied by the Sun at the vernal equinox, called the first point of Aries, in the measure of right ascension. As we say such a place is so many degrees east of Greenwich, so we say such a star is so many hours, minutes, or seconds east of the first point of Aries.

329. In short, as we define the position of a place on the Earth by saying that its latitude and longitude (in degrees) are so-and-so, so do we define the position of a heavenly body by saying that, referred to the celestial sphere, its declination (in degrees) and right ascension (in time reckoned from Aries) are so-and-so.

Sometimes the distance from the north celestial pole is given instead of that from the celestial equator. This is called north-polar distance.