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by which a greater truth was to be reached, and the law of the universe recognised. He might have spoken of himself, had he known what was to come, as the Moses of the astronomy of the future, who saw the promised land afar off, but entered not therein. But he chose rather to use the words of the ancient mystics: I will rejoice!' he exclaimed; I will triumph in my sacred fury; for I have found the golden vases of the Egyptians !'1

But it was not till Newton came that the true meaning of these laws was ascertained, and very wonderful is the history of the process by which he solved the noble problem which Nature had presented to mankind for investigation. Everyone has heard the story of the apple, whose fall is said to have suggested to Newton the great discovery for which his name will be deservedly celebrated for all time. The story may be true in a sense, though not in the sense usually given to it. Newton certainly did not ask why the apple fell, since it was well understood in his day, and had been known for many centuries, that bodies fall to the earth by virtue of her attractive influence. But it is quite possible that Newton, who had long been engaged in profound meditation on the laws of planetary motion, should have suddenly seen revealed to him the possibility that a far wider law of attraction exists. His mind was full of the thoughts sug

1 Referring to the belief of the Pythagoreans that certain sacred secrets were preserved in golden vases shown to Pythagoras by Egyptian priests.

gested by the mysterious energies which appear to sway the motions of the planets; and here, suddenly, his attention was called to the mysterious energy by which the earth draws bodies to her surface. What if one and the same form of force is exerted in all such cases? What if the sun draws the planets towards him, as the earth draws unsupported bodies towards her? What if the law exemplified in the fall of the apple is a universal law, indicating a property of matter itself, not limited to this or that kind of matter, but common to all matter and exerted on all matter, operating as certainly on every particle of the thin air we breathe as on the heaviest metals?

Newton at once saw that it was to the moon we should look for an answer to these questions. And, by the way, I might add to the advantages we derive from having a moon the fact that but for her we should assuredly not be now acquainted with the law of the universe. The moon supplied Newton with an intermediate stepping-stone enabling him to pass over the wide gap separating terrestrial gravity from the sun's action as ruler of the planetary system. The earth has an orb circling round her as the planets of the sun; and the orb thus obeying her attrac

1

This is not inconsistent with what I formerly said as to the moon circling in reality around the sun. The fact is, that if we consider the moon's motion solely with reference to the earth, taking no account of their common motion around the sun, then the moon may be regarded as circling round the earth. It is only as viewed from some standpoint far away from the solar system that the moon must be regarded as an orb circling round the sun. Both views are just.

tion was shown by Newton to be subject to a degree of force corresponding precisely to the force which the earth exerts on bodies which fall to her surface, on the supposition that the force diminishes with distance from the earth's centre, according to a certain easily explained law. At twice the distance the law is reduced to one-fourth, at thrice the distance to one-ninth part, and so on. Now, the importance of this fact resides in the circumstance that, granting this to be the law of diminution in the sun's attracting force also, then-all Kepler's laws are explained. The planets ought to travel in paths such as they actually follow; they ought to move at rates varying as their rates of motion actually vary; and, lastly, the third law, which Kepler called the harmony of the system, is, like the others, a necessary consequence of the law according to which the solar action diminishes with distance.

Only one kind of evidence was required to make the demonstration of the law complete. The general motions of the moon, the planets, and the planets' families had been fully accounted for. But if the law of gravitation is true, then these different bodies must disturb each other. The planet Jupiter must disturb our earth, for example, as she circuits round the sun, and must disturb the moon as she circuits round the earth. Of all such instances of disturbance the most marked and the one we could recognise best should be the disturbarce of the moon by the sun.

Instead of following the course round the earth

which she would have if the earth were the sole centre of her motion, the moon would be now swayed on one side and now on another side of her course, now hastened and now checked, by the sun's disturbing influence.

It was in dealing with these disturbances that Newton showed with what wonderful mental powers he had been endowed. He tracked the moon through all her movements, and measured the sun's action on her in all positions; he showed where she would be hastened, where retarded, where drawn away from the earth, where drawn closer, where her path would be more tilted, where less, where its eccentricity would be increased, where diminished. All the peculiarities of motion thus calculated from the law of gravitation were found to accord in the most convincing manner with those peculiarities actually observed in the moon's motions which had long perplexed astronomers. The demonstration of the law of gravitation was so complete, as it thus first came from Newton's hands, that within a very short time men of science were thoroughly convinced, and the law of gravitation has not been seriously questioned from that day to this.

Such is a brief history of the greatest scientific discovery ever made by man-the recognition, in fact, of the law of the universe-a law affecting every particle of matter, operating at all distances, ruling the tiniest sandgrains, and swaying the mightiest orbs--the universal law of gravitation.

THE DISCOVERY OF TWO GIANT PLANETS.

THE history of astronomy presents two remarkable instances of the discovery of planets. In the present day the discovery of objects which by courtesy are called planets is not an uncommon event. Not a year passes without the recognition of two or three and sometimes ten or twelve of those bodies termed asteroids, which travel between the orbits of Mars and Jupiter. These bodies, so far as their motions are concerned, resemble the planets. But they are very minute compared with even the smallest of that family of minor planets to which our earth belongs. Our moon and the satellites or secondary planets which attend on Jupiter, Saturn, and Uranus are gigantic bodies compared with the largest of the asteroids, or, as they are sometimes called, the planetoids. And then the zone of asteroids is so crowded that no very great interest or importance can be attached to the detection of new members of this family. Already their number is approaching the middle of the second hundred, and the cry is " Still they

come.

Luther, the German astronomer, has discovered nineteen, and until lately stood easily first among asteroidhunters; but Peters, of America, has been gaining steadily

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