1,200 earths rolled into one to form a globe equal to the globe of Jupiter. Measured by weight, the disparity between the earth and Jupiter, though still enormous, is not quite so great; but this is a matter to be discussed more fully in a later chapter.

Even in this preliminary survey of the solar system we must not omit to refer to the vast host of planets which attract our attention, not by their bulk but by their multitude. In the ample zone, bounded on the inside by the orbit of Mars, and on the outside by the orbit of Jupiter, it was thought at one time that no planet revolved. Modern research has shown that this vast area of space is tenanted, not by one planet, but by hundreds of planets. The discovery of these planets is a charge undertaken by many diligent modern astronomers, while the discussion of their movements affords labour to many others. We shall find much to learn in the study of these tiny bodies, to which a chapter will be devoted further on in the course of this work.

But we do not propose to enter deeply into the mere statistics of the planetary system at present. Were such our intention, the tables at the end of this volume will show that ample materials are available. Astronomers have taken a more or less complete inventory of every one of the planets. They have measured their distances, the shapes of their orbits and the positions of those orbits, their times of revolution, and, in the case of all the larger planets, their sizes and their weights. Such results are of interest for many purposes in astronomy; but it is the more general features of the science which at present claim our attention.

Let us, in conclusion, note one or two important truths with reference to this beautiful planetary system. We have seen that the planets all revolve in nearly circular paths around the sun. We have now to supplement this by another statement of very great importance. Each of the planets pursues its path in the same direction. It may thus happen that one planet may overtake another, but it can never happen that two planets pass by each other as do the trains on adjacent lines of railway. We shall subsequently find that the whole welfare of our system, nay, its continuous existence, is dependent upon this remarkable uniformity.

Such is our solar system; a mighty organised group of planets circulating round a common sun, poised in space, and completely isolated from all external interference. No star, no constellation, has any appreciable influence on our solar system. We constitute a little island group, separated from the nearest stars by the most appalling distances. It may be that as the other stars are suns, so they too may have systems of planets circulating around them; but of this we know nothing. Of the stars we can only say that they are points of light, and if they had hosts of planets those planets must for ever remain invisible to us, even if they were many times as large as Jupiter.

At this limitation to our possible knowledge we need not repine, for just as we find in the solar system all that is necessary for our daily bodily wants, so shall we find ample occupation for whatever faculties we may possess, in endeavouring to understand those mysteries of the heavens which lie within our reach.

CHAPTER V.

THE LAW OF GRAVITATION.

Gravitation The Falling of a Stone to the Ground-All Bodies fall EquallySixteen Feet in a Second-Is this True at Great Heights?-Fall of a Body at a height of a Quarter of a Million Miles-How Newton obtained an Answer from the Moon-His great Discovery-Statement of the Law of Gravitation-Illustrations of the Law-How is it that all the Bodies in the Universe do not rush together?-The Effect of Motion-How a Circular Path can be produced by Attraction-General Account of the Moon's Motion-Is Gravitation a Force of great Intensity?-Two Weights of 50 lbs.-Two Iron Globes, 53 yards in diameter, and a mile apart, attract with a force of 1lb.-Characteristics of Gravitation-Orbits of the Planets not strictly Circles-The Discoveries of Kepler-Construction of an Ellipse-Kepler's First Law-Does a Planet move uniformly ?-Law of the Changes of Velocity-Kepler's Second Law-The Relation between the Distances and the Periodic Times-Kepler's Third LawKepler's Laws and the Law of Gravitation-Movement in a straight line—A Body unacted on by disturbing Forces would move in a straight line with constant Velocity-Application to the Earth and the Planets-The Law of Gravitation deduced from Kepler's Laws-Universal Gravitation.

OUR narrative of the heavenly bodies must undergo a slight and temporary interruption, while we now enunciate and describe with appropriate detail the extremely important principle known as the law of gravitation, which underlies the whole of astronomy. To the law of gravitation must be ascribed the movements of the moon around the earth, and of the planets around the sun. Those movements can be completely accounted for when once the law of gravitation has been admitted. It is accordingly incumbent upon us to explain that law, before we proceed to the more detailed account of the separate planets. We shall find, too, that the law of gravitation opens up vast chapters in the history of the stars situated at the most stupendous distances in space, while it also affords the key by which we are enabled to cast a retrospective glance into the vistas of time past, and trace with plausibility, if not with certainty, early phases in the history of our system.

The sun and the moon, the planets and the comets, the stars and the nebulæ, all alike are subject to this universal law, which is now to engage our attention.

What is more common than the fact that when a stone is dropped it will fall to the ground? so common is it that no one at first thinks it worthy of remark. People are often surprised at seeing a piece of iron drawn to a magnet. Yet the fall of a stone to the ground is the manifestation of a force quite as interesting as the force of magnetism. It is the earth which draws the stone, just as the magnet draws the iron. In each case the force is one of attraction; but while the magnetic attraction is confined to a few substances, and is of comparatively limited importance, the attraction of gravitation extends far and wide throughout the whole universe.

Let us commence with a few very simple experiments upon the force of gravitation. Take in the hand a small piece of lead, and allow it to fall upon a cushion. The lead requires a certain time to move from the fingers to the cushion, but that time is always the same when the height is the same. Take now another and larger piece of lead, and hold one in each hand at the same height. If both are released at the same moment, they will both reach the cushion at the same moment. It might have been thought that the heavy body would fall more quickly than the light body; but when the experiment is tried it is seen that this is not the case. Repeat the experiment with various other substances. Try an ordinary marble. It also will fall in the same time as the piece of lead. With a piece of cork we again try the experiment, and again obtain the same result. At first it seems to fail when we compare a feather with the piece of lead; but that is solely on account of the air, which resists the feather more than it resists the lead. If, however, the feather be placed upon the top of a penny, and the penny be horizontal when dropped, it will clear the air out of the way of the feather in its descent, and then the feather will fall as quickly as the penny, as quickly as the marble, or as quickly as the piece of lead.

If the observer were in a gallery when trying these experi

H

ments, and if the cushion were sixteen feet below his hands, then the time the marble would take to fall through the sixteen feet would be one second. The time occupied by the cork or by the lead would be the same, and even the feather itself would fall through sixteen feet in one second, if it could be screened from the interference of the air. Try this experiment where we like, in London, or in any other city, in any island or continent, on board a ship at sea, at the North Pole, or the South Pole, or the equator, it will always be found that any body of any size, or any material, will fall about sixteen feet in one second of time.

Lest any erroneous impression should arise, we may just mention that the distance traversed in one second does vary slightly at different parts of the earth, but from causes which need not at this moment detain us. We shall for the present regard sixteen feet as the distance through which any body, free from interference, would fall in one second at any part of the earth's surface. But now let us extend our view above the earth's surface, and inquire how far this law of sixteen feet in a second may find obedience elsewhere. Let us, for instance, ascend to the top of a mountain and try the experiment there. It would be found that at the top of the mountain a marble would take a little longer to fall through sixteen feet than the same marble would if let fall at its base. The difference would be very small; but yet it would be measurable, and sufficient to show that the power of the earth to pull the marble down to the ground was somewhat weakened when we ascend high above the earth's surface. Yet no matter how high we ascend, either to the top of a high mountain, or to the still greater heights that have been reached in balloon ascents, we shall find that the tendency of bodies to fall to the ground remains, though no doubt the higher we go the more is that tendency weakened. It would be of the greatest interest to find how far this power of the earth to draw bodies towards it extends. We cannot get more than about five or six miles above the earth's surface in a balloon; yet we want to know what would happen if we could go 500 miles, or 5,000 miles, or still further, into the depths of space.

Conceive that a traveller were endowed with some means of soar