from a nice balance between gravitation and speed
of motion.

The mass of the earth is ascertained to be about eighty times that of
the moon. Our knowledge of the mass of a planet is learned from
comparing the revolutions of its satellite or satellites around it, with
those of the moon around the earth. We are thus enabled to deduce what
the mass of such a planet would be compared to the earth's mass; that is
to say, a study, for instance, of Jupiter's satellite system shows that
Jupiter must have a mass nearly three hundred and eighteen times that of
our earth. In the same manner we can argue out the mass of the sun from
the movements of the planets and other bodies of the system around it.
With regard, however, to Venus and Mercury, the problem is by no means
such an easy one, as these bodies have no satellites. For information in
this latter case we have to rely upon such uncertain evidence as, for
instance, the slight disturbances caused in the motion of the earth by
the attraction of these planets when they pass closest to us, or their
observed effect upon the motions of such comets as may happen to pass
near to them.

Mass and weight, though often spoken of as one and the same thing, are
by no means so. Mass, as we have seen, merely means the amount of matter
which a body contains. The weight of a body, on the other hand, depends
entirely upon the gravitational pull which it receives. The force of
gravity at the surface of the earth is, for instance, about six times as
great as that at the surface of the moon. All bodies, therefore, weigh
about six times as much on the earth as they would upon the moon; or,
rather, a body transferred to the moon's surface would weigh only about
one-sixth of what it did on the terrestrial surface. It will therefore
be seen that if a body of given _mass_ were to be placed upon planet
after planet in turn, its _weight_ would regularly alter according to
the force of gravity at each planet's surface.

Gravitation is indeed one of the greatest mysteries of nature. What it
is, the means by which it acts, or why such a force should exist at all,
are questions to which so far we have not had even the merest hint of an
answer. Its action across space appears to be instantaneous.

The intensity of gravitation is said in mathematical parlance "to vary
inversely with the square of the distance." This means that at _twice_
the distance the pull will become only _one-quarter_ as strong, and