According to our ordinary experience,
the very idea of an object poised without support in space, appears
preposterous. Would it not fall? we are immediately asked. Yes,
doubtless it could not remain poised in any way in which we try the
experiment. We must, however, observe that there are no such ideas
as upwards or downwards in relation to open space. To say that a
body falls downwards, merely means that it tries to fall as nearly as
possible towards the centre of the earth. There is no one direction
along which a body will tend to move in space, in preference to any
other. This may be illustrated by the fact that a stone let fall at
New Zealand will, in its approach towards the earth's centre, be
actually moving upwards as far as any locality in our hemisphere is
concerned. Why, then, argued Ptolemy, may not the earth remain
poised in space, for as all directions are equally upward or equally
downward, there seems no reason why the earth should require any
support? By this reasoning he arrives at the fundamental conclusion
that the earth is a globular body freely lying in space, and
surrounded above, below, and on all sides by the glittering stars of
heaven.
The perception of this sublime truth marks a notable epoch in the
history of the gradual development of the human intellect.
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