Pupil. Had you not told me that bodies attract one another with forces which are proportioned to the quantities of matter they contain, I should say the boat to which the rope is fastened would come to that in which the man stands: but as I imagine you mean to apply this to attraction, by the above rule, they will meet at a point which is half way between them.
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.