The Study of Astronomy, adapted to the capacities of youth / In twelve familiar dialogues, between a tutor and his pupil: explaining the general phænomena of the heavenly bodies, the theory of the tides, &c.

by teacher of astronomy John Stedman

Pupil. You say, Sir, that the distance is twice as great at B as at A?

Tutor. I do.

Pupil. Then as the square of the distance 2 is 4, the decrease of attraction at B, the planet at A will be attracted with four times the force it would be at B.—Am I right, Sir?

Tutor. Perfectly so. And if the distance at B were three times as great as at A, it would be attracted with a force nine times as great.

Pupil. I perceive it must be so.

Tutor. I shall now give you the 2d law, namely, That bodies attract one another with forces proportionable to the quantities of matter they contain.

Pupil. Do all bodies of the same magnitude contain equal quantities of matter?

Tutor. No, certainly: For a ball of cork may be as large as one of lead, and yet not contain the same quantity of matter, because it is more porous, and not so compact or dense a body as the lead; neither will a ball of lead of the same magnitude as one of gold contain an equal quantity of matter.—So the sun, though a million of times as big as the earth, contains a quantity of matter only 200,000 as great, therefore attracts the earth with a force 200,000 as great as the earth attracts him.

Pupil. I think this is clear.

Tutor. We will now suppose that in the river are two boats of equal bulk, at the distance of twenty yards from each other, and that a man in one boat pulls a rope which is fastened to the other, what effect will be produced, or where do you think the boats will meet?