Tutor. This line you must remember is what mathematicians call the tangent of a circle, as A a, B b, &c. (Plate II. fig. 5.) for all bodies moving in a circle have a natural tendency to fly off in that direction. Thus a body at A will tend towards a; at B towards b, and so on; but the central force acting against it preserves its circular motion.
Pupil. By the central force here you mean the action of the hand, do you not?
Tutor. Yes. For, as soon as the stone is released and that power is lost, it assumes its natural, that is, its rectilineal motion.—Again. If you are left at liberty, cannot you run strait forward?
Pupil. Yes, Sir.
Tutor. Now, suppose one of your companions were to fasten a rope round your body, and at the extent of it were to stand still and hold it tight, with a force equal to that with which you run, could you, do you think, move in a strait line, that is, in a tangent of a circle?
Pupil. No, Sir. I must run in a circle.
Tutor. Why?
Pupil. Because, whilst the rope is extended I am prevented running in any other direction.
Tutor. Just so it is with the planets: the attractive or centripetal force of the sun being equal to that of the projectile or centrifugal force of the planets, they are by attraction prevented moving on in a strait line, and, as it were, drawn towards the sun; and by the projectile force from being overcome by attraction. They must therefore revolve in circular orbits.
Pupil. What I have so long wished is now accomplished. I understand it perfectly.