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. CHAPTER III. THE GREAT DISCOVERY. Perhaps the question presents itself to the thinking reader: If it be true that the heavenly bodies attract each other, why do not the planets attract one another in such a manner that they will run round and about each other?