Pa. In the same manner you may have observed, in riding in a carriage, that if you throw anything out of the window, it falls directly opposite, just as if the carriage was standing still, and is not left behind you.
Lu. I will try that the next time I ride in one.
Pa. You are then to imagine the sun to be a mighty mass of matter, many thousand times bigger than our earth, placed in the centre, quiet and unmoved. You are to conceive our earth, as soon as created, launched with vast force in a straight line, as if it were a bowl on a green. It would have flown off in this line for ever, through the boundless regions of space, had it not at the same instant received a pull from the sun by its attraction. By the wonderful skill of the Creator, these two forces were made exactly to counterbalance each other; so that just as much as the earth, from the original motion given to it, tends to fly forward, just so much the sun draws it to the centre; and the consequence is, that it takes a course between the two, which is a circle round and round the sun.
Lu. But if the earth was set a rolling like a bowl upon a green, I should think it would stop of itself, as the bowl does.
Pa. The bowl stops because it is continually rubbing against the ground, which checks its motion, but the ball of the earth moves in empty space, where there is nothing to stop it.
Lu. But if I throw a ball through the air, it will not go on for ever, but it will come down to the ground.
Pa. That is because the force with which you can throw it is much less than the force by which it is drawn to the earth. But there is another reason, too, which is the resistance of the air. This space all round us and over us is not empty space; it is quite full of a thin transparent fluid called air.
Lu. Is it?
Pa. Yes. If you move your hand quickly through it, you will find something resisting you, though in a slight degree. And the wind, you well know, is capable of pressing against anything with almost irresistible force; and yet wind is nothing but a quantity of air put into violent motion. Everything, then, that moves through the air is continually obliged to push some of this fluid out of the way, by which means it is constantly losing part of its motion.
Lu. Then the earth would do the same?