natural that he should inquire whether the doctrine of a moving earth
might not remove the difficulties presented in other celestial
phenomena. It had been universally admitted that the earth lay
unsupported in space. Copernicus had further shown that it possessed
a movement of rotation. Its want of stability being thus recognised,
it seemed reasonable to suppose that the earth might also have some
other kinds of movements as well. In this, Copernicus essayed to
solve a problem far more difficult than that which had hitherto
occupied his attention. It was a comparatively easy task to show how
the diurnal rising and setting could be accounted for by the rotation
of the earth. It was a much more difficult undertaking to
demonstrate that the planetary movements, which Ptolemy had
represented with so much success, could be completely explained by
the supposition that each of those planets revolved uniformly round
the sun, and that the earth was also a planet, accomplishing a
complete circuit of the sun once in the course of a year.

[PLATE: EXPLANATION OF PLANETARY MOVEMENTS.]

It would be impossible in a sketch like the present to enter into any
detail as to the geometrical propositions on which this beautiful
investigation of Copernicus depended. We can only mention a few of
the leading principles. It may be laid down in general that, if an
observer is in movement, he will, if unconscious of the fact,
attribute to the fixed objects around him a movement equal and
opposite to that which he actually possesses. A passenger on a
canal-boat sees the objects on the banks apparently moving backward
with a speed equal to that by which he is himself advancing
forwards. By an application of this principle, we can account for
all the phenomena of the movements of the planets, which Ptolemy had