room; in which case, Mercury would answer to a good large marble; Venus, to a small penny ball; the Earth, to another ball, much the same size; Mars, to a very large marble; Jupiter, to a big foot-ball; and Saturn, to a little one; Uranus, to a cricket-ball; and Neptune, to another cricket-ball, a shade larger. Their distances from the sun are most easily understood by calculating the time it would take a fast train to go to them from the sun. Starting from the sun, and going 50 miles an hour, without stopping, a train would take 76 years to get to Mercury; 144, to Venus; 200-as has been said before— to the Earth; 300, to Mars; 1,000, to Jupiter; 1,900, to Saturn; 3,800, to Uranus; and 6,000 years to reach Neptune.

FIXED STARS.

After all that has been said in the other chapters, you will now have learned about seven only out of all the stars in the sky; or rather about nine, counting the sun and the earth as stars, which we should do. But if you have quite understood the motions of these nine stars, your work is more than half done, for the remaining stars, so far as is known, obey the same laws.

We are careful to say so far as is known, for one of the great facts to know about them is, that we know very little indeed, on account of their immense distance. They are commonly called the Fixed Stars, as distinguished from the Planets, because they appear fixed in the sky, and keep the same apparent distance apart. Not that they are really fixed, for there is every reason to believe that they

all move, but on account of their distance the motion can hardly be observed.

Till of late years their distance was thought to be immeasurable, and even now astronomers have found out only five or six of the nearest, and that only in an imperfect kind of way. But it seems pretty clear that the nearest must be more than 19,000,000,000,000 (nineteen billion) miles away.

This number is easy enough to write or to read, but not to realise; and, indeed, we can hardly help you to understand how very great it is. Our instance of the railway train would be of no use, for such a train as has been mentioned would want more than 40,000,000 (forty million) years to reach the nearest fixed star. And it is not much easier to realise forty millions than nineteen billions. You may, however, get a notion of the latter number in this way. Take ten small stones. Put one down on the ground, and call it the sun, put another a yard off for the earth. Now, if the earth's distance from the sun be shown by one yard-and remember that it is really so far off that a train would take 200 years to do the journey-the other planets must be as follows, to be at the proper scale :-Mercury's stone a foot from the middle one; Venus's two feet; Mars's a yard and a half; Jupiter's five yards; Saturn's nine; Uranus's nineteen; Neptune's thirty. You have now put down nine out of your ten stones, for the sun and the eight planets. The tenth is for the nearest* fixed star. Where is that to be? If you would keep the same scale, you must put that down 120 miles off. But this is the nearest. The next

* Alph. Centauri.

is twice as far; the next six times as far; and after that our power of measuring fails us altogether.

Far beyond this system of fixed stars, lie others, which can only be seen through a telescope, as small white clouds. These also are systems of stars, for anything we know, as large as the immense body already described.* Whether the stars extend for ever and ever into space, or whether there is an end of space where nothing is, we cannot say. Each state is inconceivable; yet one of them must exist.

* We have not meddled with the nebular hypothesis. Its truth is very doubtful, and the question raised here is independent of it.

APPENDIX

TO THE SECTION ON ASTRONOMY.

THE planet nearest the sun is Mercury, a small star about 3,000 miles in thickness, that is to say, less than half as thick as the earth. Mercury is about 35,000,000 miles from the sun, less than half our distance. Mercury spins round once in twenty-four hours, and moves round the sun in eighty-eight days. Its day, therefore, is of the same length as ours, and its year about a quarter as long.

Mercury can seldom be seen without a telescope, and not easily with one, for it is so near the sun as generally to be lost in its light.

Venus is very nearly, but not quite, as large as the earth, being about 7,700 miles thick. It is about threequarters of our distance from the sun, that is to say, about 66,000,000 miles. Its day is a little over twentythree hours, and its year about seven months long. Venus may often be seen as a large bright star early in the morning and late in the evening.

The next of the planets is the Earth, on which we live, and which has been already described.

After the earth comes Mars, which is something like 4,000 miles thick, half as thick as the earth, and 140,000,000 miles from the sun. Its day is about halfan-hour longer than our own, and its year about equal to

a year and three-quarters of our year. be seen as a rather large reddish star.

Mars is often to

Jupiter, which is the largest planet, comes next. It is about 87,000 miles thick, and 476,000,000 miles from the sun, that is to say, more than ten times the earth's thickness, and about five times the distance. Jupiter has four moons, which move round it, just as our moon moves round us. Its day is about ten hours long, and its year as long as eleven and a half of ours. Jupiter can often be seen as a very large bright star, with a clear steady light.

Outside Jupiter is Saturn, which is almost as large, being nine times the thickness of the earth, 74,000 miles, and nine times its distance from the sun, 870,000,000. It has no less than eight moons attending it, as well as another contrivance for lighting it up, which shall be described directly. Its day is about the length of ten hours and a half, and its year is twenty-nine years and a half long, as we measure years on the earth. Saturn looks to the eye like a middlesized star of a yellowish colour, and you would at first see nothing remarkable in its appearance.

If, however, you were to look at it through a small telescope one that magnifies twenty or twenty-five times would do you would see that, instead of being round as the other stars are, it sticks out all round in the middle, and looks something like a teetotum. On looking at it through a larger telescope-one to magnify one hundred or one hundred and fifty times-you would see that there is a large ring that goes all round the planet without touching it, and which reflects the sun's light