MAIN SUPPORT OF LEVERRIER'S DISCOVERY.

When Leverrier was working at his great discovery he did not strike out a new path in science; he was supported by a great law of nature, the base of all astronomical knowledge. It is the law of gravitation, discovered by Sir Isaac Newton.

Those of our readers who have fully understood what we said before (page 50) about light, will now easily comprehend, what we are going to say about the force of gravity.

Every heavenly body is endowed with the power of attraction; that is, it attracts every other body in the same manner that a magnet attracts iron. If the celestial bodies, or, to speak only of one class, if all the planets were at rest, that is, without motion, they would, on account of the great attractive power of the sun, rapidly approach it, and finally unite with it and form one body.

That this does not take place, may be ascribed solely to the fact that all planets have their own motion. This motion, combined with the attractive force of the sun, causes them to move in circles around it.

This may be illustrated by the following: Suppose a strong magnet to lie in the centre of a table. Now, suppose some one to place an iron ball on the table; then will this ball run straightway towards the magnet. But if some one were to roll the ball so that it should pass the magnet, it would at first run in a straight line, but the magnet attracting it at every moment of time, the ball would be compelled to deviate from its straight course and would begin to circulate round the magnet.

We see that this circular motion round the magnet springs from two forces: first, from the hand that starts the ball in a straight line; and secondly, from the attraction of the magnet, which at every moment draws the ball towards itself.

Newton, the greatest natural philosopher of all times, who lived in England two hundred years ago, proved, that all the orbits round the sun, as described by the planets, are caused by two such forces; by the motion of the planets peculiar to themselves, which, if not interfered with, would make them fly through space in a straight line; and by the attractive force of the sun, which is continually disturbing that straight course, thus forcing the planets to move in circles around him.

But Newton has discovered more than this. He succeeded in proving that, knowing the time of a planet's revolution around the sun, we can determine precisely with what force the attractive power of the sun affects it. For if the sun's attractive power is strong, the planet will revolve very quickly; if weak, it will move slowly.

Were the sun, for example, all of a sudden to lose a portion of his attractive force, the consequence would be that the earth must revolve around him more slowly. Our year, which now has three hundred and sixty-five days, would then have a much greater number of days.

Newton has also shown—and this is for us the main thing—that the attractive force of the sun is strong in his close proximity, but that it diminishes as the distance from him increases. In other words, the remoter planets are attracted by the sun with less force than those nearer to him. The attractive force decreases with the distance in the same proportion as light, which, we saw a little while ago, decreases in intensity as the square of the distance increases. This means, that a planet at a distance from the sun twice as great as that of the earth, is attracted with only one-fourth the force; one that is three times the distance, with one-ninth of the force, etc.

This great law pervades all nature. It is the basis of the science of astronomy, and was the main support of Leverrier's discovery.