.--Attraction, which may be illustrated by the effect a
magnet has on a piece of iron, may be viewed generally as an influence
which two bodies, say, exert on each other, under which, though at a
distance, they tend to move towards each other till they come into
contact. The force by which a body has weight, and, when free, falls to
the ground, is of this nature; and it is called, from _gravis_, "heavy,"
the gravitating force of the earth, because it causes weight, and
because, though emanating in a small degree from the falling body, it is
mainly exerted by the earth itself. It is under the action of gravity
that a pendulum oscillates: it is by that unseen influence it begins to
sway alternately downward and upward as soon as it is moved to a side;
and it is only because it is withheld by the rod that the ball or bob
keeps traversing the arc of a circle and does not fall straight to the
earth.

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