Pa. It did so. That was a lesson, in the centrifugal motion, or that power by which a body thus whirled, continually endeavours to fly off from the centre round which it moves. This is owing to the force or impulse you give it at setting out, as if you were going to throw it away from you. The string by which you hold it, on the contrary, is the power which keeps the ball toward the centre, called the centripetal power. Thus you see there are two powers acting upon the ball at the same time, one to make it fly off, the other to hold it in; and the consequence is, that it moves directly according to neither, but between both; that is, round and round. This it continues to do while you swing it properly; but if the string breaks or slips off, away flies the ball; on the other hand, if you cease to give it the whirling force, it falls toward your hand.
Lu. I understand all this.
Pa. I will give you another instance of this double force acting at the same time. Do not you remember seeing some curious feats of horsemanship?
Lu. Yes.
Pa. One of them was, that a man standing with one leg upon the saddle, and riding full speed, threw up balls into the air, and catched them as they fell.
Lu. I remember it very well.
Pa. Perhaps you would have expected these balls to have fallen behind him, as he was going at such a rate?
Lu. So I did.
Pa. But you saw that they fell into his hand as directly as if he had been standing quite still. That was because at the instant he threw them up, they received the motion of the horse straight forward as well as the upright motion that he gave them, so that they made a slanting line through the air, and came down in the same place they would have reached if he had held them in his hand all the while.
Lu. That is very curious, indeed!