reference to the sun, and the solar system with reference to the
stellar. As for the moon, it maintains its orbit and revolves round the
earth under the action of two forces, the one akin to that by which a
ball is projected from the mouth of a cannon, and the other the
attraction of the earth, which, by its constant and equal operation,
bends its otherwise rectilineal track into a circular one, as we might
show if we could only project a ball with such a force as exactly to
balance the power of gravity, so that it would at no point in its course
be drawn nearer the earth than at starting.

That the force we are considering pervades the solar system is
demonstrable, for it is on the supposition of it and the laws it is
known to obey that all the calculations of astronomy--and they never
miscarry--are grounded; and it is by noticing disturbances in the
otherwise regular movements of certain planets that astronomers have
been led more than once to infer and discover the presence of some
hitherto unknown body in the neighbourhood. It was actually thus the
planet Neptune was discovered in 1846. Certain irregularities had been
observed in the movements of Uranus, which could not be accounted for by
the influence of any other bodies known to be near it; and these
irregularities, being carefully watched and studied, gradually led more
than one astronomer first to the whereabouts, and then to the vision of
the disturbing planet.

Notwithstanding what we said about the universality of this force, and
how it affects all forms of matter, it may still appear as if the air
were an exception. But it is not so; the air also gravitates. The fact
that it gravitates is proved in various ways. First, if it did not, it
would not accompany the earth in its movements round the sun; the earth
would sweep along into space, and leave it behind it. Secondly, if we