TUTOR. If one boat were three times the bulk of the other, how then? PUPIL. The lightest would move three times as far as the heaviest, or 15 yards whilst the heaviest moved only 5. TUTOR. Upon my word you reason philosophically. In both cases you are perfectly right. PUPIL. As the sun is so immense a body that his quantity of matter is so much greater than the planets, I am at a loss to know why they are not by the power of attraction drawn to him. TUTOR. And so they would if the attractive power were not counteracted by another of equal force. PUPIL. Did you not say, Sir, that the planets are kept in their orbits by attraction? TUTOR. I did. But you find that by attraction _only_ the sun would draw all the planets to himself. PUPIL. That is evident. But I wish to know what this counteracting power you speak of is? TUTOR. I will tell you presently.—You must remember that _simple_ motion is naturally rectilineal, that is, all bodies, if there were nothing to prevent them, would move in strait lines. PUPIL. Then as the planetary motion is circular, it cannot be simple? TUTOR. No. It is a _compound_ of the two forces I have been mentioning: the one is called the attractive or centripetal force; the other, the projectile or centrifugal force. PUPIL. The former I clearly comprehend, but not the latter. I can conceive, that if two bodies approach each other by attraction they must move in a right line. TUTOR. If you shoot a marble on a smooth piece of ice, in what direction will it run? PUPIL. Strait forward. TUTOR. This is a projectile force.—Could you, do you think, shoot it in any other direction? PUPIL. No, Sir. TUTOR. Then is not this motion also rectilineal? PUPIL. It is. TUTOR. When you strike a ball with your cricket-bat, or throw a stone with your hand, is it not projected or thrown forward by the force of the bat or hand? PUPIL. Certainly. TUTOR. And does it not move in a strait line? PUPIL. At first it appears to do so; but afterwards it inclines towards and falls to the earth. TUTOR. Cannot you account for this? PUPIL. I suppose it must be drawn to the earth by attraction. TUTOR. You are right. The attraction of the earth, and the resistance of the atmosphere or air through which it moves, retards its progress, or it would continue moving in a strait line, with a velocity equal to that which was at first impressed upon it. In like manner the beneficent Creator of the Universe impressed a force on all the planets which should be equal to that of the attractive power of the sun, that one might not overcome the other. PUPIL. This wants explaining. TUTOR. I would willingly gratify you, but as I have much more to say on the subject, I fear it will be too great a burthen on your memory; it will therefore be better to postpone it. PUPIL. As you please, Sir. ------------------------------------------------------------------------ DIALOGUE VII.