PUPIL. I do not clearly comprehend what you mean by the axis of the earth; for, as it moves in open space and has no support, it can have nothing to resemble the spit on which it turns. TUTOR. Certainly not. By the earth’s axis is meant an imaginary line passing through its center, on which it is supposed to turn; as your ball if rolled on the ground would revolve on an axis whilst it was moving forward. PUPIL. I can now answer your question in the affirmative: and, as our year consists of 365 days, I imagine the earth must make as many revolutions on its axis whilst it is going once round the sun. TUTOR. Undoubtedly: and as only one half of a spherical body can at any time be enlightened by a luminous body, that part of the earth only which is turned to the sun, can receive the benefit of his enlivening rays, when it will be day; whilst the opposite part will be involved in darkness, and it will be night. PUPIL. I perceive it must be so. But, if the earth move in the manner you describe, I cannot conceive how it is that we are not sensible of its motion. TUTOR. If the motion of the earth were irregular it would be perceptible; but as it meets with no obstruction the motion must be so uniform as not to be perceived. PUPIL. Had I recollected this, I need not have given you this trouble.—But I am continually meeting with fresh difficulties. TUTOR. You have only to mention what they are, and I shall take a pleasure in removing them. PUPIL. I thank you, Sir; and shall be obliged to you to inform me, how the motion of the earth can cause the sun to appear to move? TUTOR. When in a carriage which went smoothly on the road, or in a boat whose motion was scarcely perceptible on the water, did you never fix your attention on the objects you passed? PUPIL. Yes, often, Sir. TUTOR. And had you not known that you really moved, and that the trees, &c. were immoveable in the ground, what then would have been your opinion? PUPIL. That the trees, &c. moved in a direction contrary to that in which I was moving. TUTOR. Is not this sufficient to convince you that the apparent motion of the sun may be occasioned by the revolution of the earth on its axis? PUPIL. It is:—But if so large a body as the earth make a revolution on its axis in 24 hours, it must move with great velocity. TUTOR. It does so; and the inhabitants of London by this motion are carried at the rate of 560 miles an hour[13]. [Footnote 13: The hourly motion under the equator is 900 miles.] PUPIL. What an astonishing rapidity! TUTOR. Now, the sun with the rest of the heavenly bodies must move round the earth, or the earth must revolve on its axis in 24 hours, to cause that appearance. PUPIL. That is plain.