htiest of all, is the continuous
manner in which the stars overhead give place to others as one travels
about the surface of the earth. When in northern regions the Pole Star
and its neighbours--the stars composing the Plough, for instance--are
over our heads. As one journeys south these gradually sink towards the
northern horizon, while other stars take their place, and yet others are
uncovered to view from the south. The regularity with which these
changes occur shows that every point on the earth's surface faces a
different direction of the sky, and such an arrangement would only be
possible if the earth were a sphere. The celebrated Greek philosopher,
Aristotle, is known to have believed in the globular shape of the earth,
and it was by this very argument that he had convinced himself that it
was so.

The idea of the sphericity of the earth does not appear, however, to
have been generally accepted until the voyages of the great navigators
showed that it could be sailed round.

The next point we have to consider is the rotation of the earth about
its axis. From the earliest times men noticed that the sky and
everything in it appeared to revolve around the earth in one fixed
direction, namely, towards what is called the West, and that it made one
complete revolution in the period of time which we know as twenty-four
hours. The stars were seen to come up, one after another, from below the
eastern horizon, to mount the sky, and then to sink in turn below the
western horizon. The sun was seen to perform exactly the same journey,
and the moon, too, whenever she was visible. One or two of the ancient
Greek philosophers perceived that this might be explained, either by a
movement of the entire heavens around the earth, or by a turning motion
on the part of the earth itself. Of these diverse explanations, that
which supposed an actual movement of the heavens appealed to them the
most, for they could hardly conceive that the earth should continually
rotate and men not be aware of its movement. The question may be
compared to what we experience when borne along in a railway train. We
see the telegraph posts and the trees and buildings near the line fly
past us one after another in the contrary direction. Either these must
be moving, or we must be moving; and as we happen to _know_ that it is,
indeed, we who are moving, there can be no question therefore about the
matter. But it would not be at all so easy to be sure of this m