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
place a bottle from which the air is exhausted in a balance and exactly
poise it with a counter-weight, and then open it and let in the air, it
will show at once that the air has weight or gravitates by immediately
descending. Thirdly, if we extend a piece of india-rubber over the end
of a vessel and begin to withdraw the air from it, we shall see the
india-rubber sink in, under the pressure of the air outside, to fill up
the space left vacant by the removal of the included air. The fact that
air gravitates we have already taken for granted in explaining the
ascent of a balloon; and the proofs now given are enough to show that
the cause assumed is a real one. The lighter gas rises and the heavier
sinks by law of gravitation.

_Gravitation and Cohesion._--Unlike the attraction of aggregation, or
cohesion, which acts only between particles separated from each other
by spaces that are imperceptible, gravitation takes effect at distances
which transcend conception, but it diminishes in force as the distance
increases. The law according to which it does so is expressed thus; its
intensity decreases with the square of the distance; that is to say, at
twice the original distance it is 1-4th; at thrice, 1-9th; at four
times, 1-16th, for 4, 9, 16 are the squares respectively of 2, 3, and 4.
To take an instance, a ball which weighs 144 lb. at the surface of the
earth will weigh 1-4th of that, or 36 lb., when it is twice as far from
the centre as it is at the surface; and 1-9th, or 16 lb. when it is
thrice as far; and 1-16th, or 9 lb. when it is four times as far. The
attraction of cohesion, on the other hand, as we say,