accounted for just as completely by a rotation of the earth as by a
rotation of the heavens. He alludes to the fact that, to those on
board a vessel which is moving through smooth water, the vessel
itself appears to be at rest, while the objects on shore seem to be
moving past. If, therefore, the earth were rotating uniformly, we
dwellers upon the earth, oblivious of our own movement, would wrongly
attribute to the stars the displacement which was actually the
consequence of our own motion.

Copernicus saw the futility of the arguments by which Ptolemy had
endeavoured to demonstrate that a revolution of the earth was
impossible. It was plain to him that there was nothing whatever to
warrant refusal to believe in the rotation of the earth. In his
clear-sightedness on this matter we have specially to admire the
sagacity of Copernicus as a natural philosopher. It had been urged
that, if the earth moved round, its motion would not be imparted to
the air, and that therefore the earth would be uninhabitable by the
terrific winds which would be the result of our being carried through
the air. Copernicus convinced himself that this deduction was
preposterous. He proved that the air must accompany the earth, just
as his coat remains round him, notwithstanding the fact that he is
walking down the street. In this way he was able to show that all a
priori objections to the earth's movements were absurd, and therefore
he was able to compare together the plausibilities of the two rival
schemes for explaining the diurnal movement.

[PLATE: FRAUENBURG, FROM AN OLD PRINT.]

Once the issue had been placed in this form, the result could not be
long in doubt. Here is the question: Which is it more likely--that