Tutor. The vicissitudes of the seasons, the cause of day and night, &c. shall be the subject of future lessons: we shall find sufficient to employ us at present.
Pupil. I think you told me just now that the earth is nearest the sun in December; that is our winter; this seems a little mysterious.
Tutor. It may appear so to you now, by-and-by you will be of a different opinion. I shall explain this matter to you with that of the seasons, &c.
Pupil. I fear I have interrupted you.—As you said you had sufficient employment for us, I shall be glad to know what it is.
Tutor. Hitherto I have spoken of the sun’s being fixed, and that the planets revolve about him as a center. Instead of which the sun and planets move round one common center, called the center of gravity.
Pupil. What is this center of gravity?
Tutor. Have you never seen a person raise a heavy weight by means of a long pole or leaver, which it was not in his power to lift without it?
Pupil. Yes, Sir, and it excited my astonishment.
Tutor. Now, suppose the weight to see raised to be 10 Cwt. and the prop on which the leaver rested 1 foot from the body to be raised; and the person at the other end of the leaver 10 feet from the prop; with what weight must he press to raise the 10 Cwt.?
Pupil. I think that very easy; for, as he is ten times as far from the prop as the weight is, a pressure of 1 Cwt. which is one-tenth of the weight to be raised will do it.