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. It is very evident.

Tutor. Again. In every direction, if a ship be seen at a distance, the first things observed are the top-mast and rigging, whilst the hull or body of the ship is hidden behind the convexity, that is roundness of the water, just as you would see a man coming over a hill, you would first see his head, he would be rising more and more to your view till he arrived at the top, where he would be full in sight.

Pupil. I am at a loss to account for the convexity of the water. How can its surface be round?

Tutor. Have you never observed the drops of water falling from the eaves of a house?

Pupil. Often, Sir.

Tutor. Of what shape were they?

Pupil. Globular.—But what is the cause of their being so?

Tutor. Attraction.—For as every particle of water which composes the drop tends to the same center, every part of the surface must be equidistant from the center, it must therefore be spherical. In like manner if you separate quicksilver, each portion will form itself into a globe.

Pupil. All this is very clear. And, for the same reason, the water in the ocean must be convex; for, I remember you told me that it gravitated towards the center of the earth.

Tutor. Once more.—I think you must have seen an eclipse of the moon.