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Page 24
All material substances, however small, and however light, buoyant, and
ethereal they may seem, are subject to this force: the tiniest speck in
a sunbeam and the most volatile vapour, equally with the heaviest metal
and the hugest block, the particles of bodies as well as the bodies
themselves. The rising of a balloon in the air may seem an exception to
this law; but it is not so; for the balloon rises, not because the
particles of the gas with which it is inflated are not acted upon by the
earth's attraction, but because the air outside being bulk for bulk
heavier than the air inside, its particles press in below the balloon
and buoy it up, until it reaches a stratum of the atmosphere where, the
pressure being less, the air outside is no heavier than the air
within--a fact which rather proves than disproves the universal action
of gravitation; because the greater weight of the air in the lower
strata of the atmosphere is due to the pressure of the air in those
above, and the balloon ceases to ascend because it has reached a point
where the air outside is the same weight as the air within, and the
weight in both cases is caused by the attraction of the earth.
And not only is the force of attraction universal, it is the same for
every particle; for though this may seem to be contradicted by the fact
that some bodies fall faster to the ground than others, that fact is
fully accounted for by the greater resistance which the air offers to
the falling of lighter bodies than to the falling of heavier. A
particles of bodies, and all bodies, tend to fall with the same
velocity, and, in fact, all do; for though, for the reason just stated,
a feather will take longer to reach the ground than an ounce of lead, an
ounce of lead will fall as fast as a hundredweight. And that it is the
resistance of the air, and not any diminution in the power of
attraction, which causes the feather to lag behind, may be proved by
experiment; for if you let a feather and a coin drop together from the
top of the exhausted receiver of an air-pump, they will both be seen to
descend at the same rate, and reach the bottom at the same instant; a
fact which may be demonstrated more simply by placing the coin and
feather free of each other in a paper cone, and letting the cone fall
with its apex downwards, so as to break the air's resistance; or by
suspending a piece of gold-leaf in a bottle, and letting the bottle
drop--of course short of the ground--in which case the included leaf
will be seen to have gone as fast and as far as the bottle.
It is to be especially noticed that attraction is no lopsided affair;
that it is mutual; that, while the larger body attracts the less, the
less also attracts and moves the larger in proportion; and that, indeed,
every body and every particle attracts every other, far as well as near,
to the utmost verge of the universe of matter. Under it the moon
maintains its place with reference to the earth, the planets with
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
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.
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