not
one-half as otherwise might be expected. At _four_ times the distance,
therefore, it will be _one-sixteenth_ as strong. At the earth's surface
a body is pulled by the earth's gravitation, or "falls," as we
ordinarily term it, through 16 feet in one _second_ of time; whereas at
the distance of the moon the attraction of the earth is so very much
weakened that a body would take as long as one _minute_ to fall through
the same space.

Newton's investigations showed that if a body were to be placed _at
rest_ in space entirely away from the attraction of any other body it
would remain always in a motionless condition, because there would
plainly be no reason why it should move in any one direction rather than
in another. And, similarly, if a body were to be projected in a certain
direction and at a certain speed, it would move always in the same
direction and at the same speed so long as it did not come within the
gravitational attraction of any other body.

The possibility of an interaction between the celestial orbs had
occurred to astronomers before the time of Newton; for instance, in the
ninth century to the Arabian Musa-ben-Shakir, to Camillus Agrippa in
1553, and to Kepler, who suspected its existence from observation of the
tides. Horrox also, writing in 1635, spoke of the moon as moved by an
_emanation_ from the earth. But no one prior to Newton attempted to
examine the question from a mathematical standpoint.

Notwithstanding the acknowledged truth and far-reaching scope of the law
of gravitation--for we find its effects exemplified in every portion of
the universe--there are yet some minor movements which it does not
account for. For instance, there are small irregularities in the
movement of Mercury which cannot be explained by the influence of
possible intra-Mercurial planets, and similarly there are slight
unaccountable deviations in the motions of our neighbour the Moon.

CHAPTER V

CELESTIAL DISTANCES

Up to this we have merely taken a general view of the solar system--a
bird's-eye view, so to speak, from space.

In the course of our inquiry we noted in a rough way the _relative_
distances at which the various planets move around the sun. But we have
not yet stated what these distances _actually_ are, and it were
therefore well now to turn our attention to this important matter.

Each of us has a fair idea of what a mile is. It is a quarter of an
hour's sharp walk, for instance; or yond