Do you remember my telling you how fond I was of swimming boats long ago? When my brother and I used to launch our paper boats--not on the river, but in that big tub in the yard--our great difficulty was to keep them from running each other down, and becoming dismal wrecks before they had completed their first voyage. We did not know why, but it seemed as if the vessels of our tiny fleet _would_ drift towards each other, in spite of all our efforts to keep them apart. Have you not found it so with your boats? It certainly was with ours, but we should have been surprised if anyone had told us that as they ran against each other, our paper boats were but obeying the "law of gravitation," each little vessel drawing the other to itself by a power which it had of attracting it. Knowing this rule makes many things plain. If you throw your ball high into the air, it is sure to come down again. Why? Because the earth, which is a much larger ball, attracts it to itself by the law of gravitation; by the same law, the drops of rain in a shower fall to the ground; by the same law, we and all the people upon the globe are able to stand firm on it; by the same law, the great earth itself, the moon, and all the planets are kept in their places. But what is the mighty magnet which has power to draw the earth to itself? It is that wonderful globe the sun, which is more than a million times as large as the earth; and though it is so far, far away--at a distance greater than we can have any idea of--yet by its mighty power of drawing them to itself, makes our earth, as well as the other planets, move round it in the most beautiful order, and keeps them all in their places. Although Newton felt sure that this unseen but resistless power, of which he afterwards spoke reverently as "the finger of God," kept the moon going round the earth and the earth round the sun, yet he was at first silent about his great discovery; he worked and waited for long years, until he had proved that it was not merely a happy guess, but that he had really discovered the rule which governs the motion of sun, moon and stars. Then he explained the reason why the moon is always moving _round_ the earth, and the earth and other planets _round_ the sun, instead of all moving on in a straight line; it is because everyone of the heavenly bodies attracts all the rest, and thus the smaller move round the larger, all in perfect order and harmony. [Illustration: SAILING THE BOAT] You must not think that this force set them all moving; it only governs their movements, the earth pulling the moon to itself, and the sun in like manner pulling all the planets with gentle but resistless power, and keeping them all moving round himself--their glorious centre.