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]].
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.
Tutor. Well then, great as you may suppose the velocity of the earth on its axis to be, if the sun move round the earth his hourly motion will be nearly 25 millions of miles; and beyond conception would be that of the fixed stars. Which now do you think is most probable, that the sun and stars should move round the earth, or that they, by the simple motion of the earth, should appear to be in motion?
Pupil. The latter, to be sure, Sir.—I have one difficulty remaining, which is this; if a lark rise from a field near London and remain in the air a quarter of an hour, if the earth move at the rate of 560 miles an hour, it will go 140 miles whilst the lark is suspended, and yet it continues over the field,—how can this be?
Tutor. This objection to the motion of the earth has been made by those who were older and who thought themselves wiser too than yourself. They either did not know or did not consider, that the atmosphere which surrounds the earth is a part of itself, and gravitates towards it, and therefore partakes of the earth’s motion and carries the lark along with it. Besides, as the Sun, Venus, Mars, and Jupiter are known to revolve on their axes, we have reason to suppose that the other planets, together with the earth, must have the same motion[[14]].
Pupil. How is it known that they do revolve on their axes; and in what time do they perform their revolutions?