The word =gravity=, when it was first used, had exactly the same meaning
as weight; and a body which has weight is said to =gravitate= towards
the center of the earth. But gravity has now acquired a much wider sense
than weight. For an immense number of careful observations and
experiments have established the general rule, or law of nature, that
every material substance, tends to approach every other material
substance, just in the same way as a drop of rain falls towards the
earth; and, in fact, that any two portions of matter, whatever the
nature of that matter may be, will move towards one another if there is
nothing to prevent them from doing so.
To make this clear, let us suppose that the only material bodies in the
universe were two spherical drops of water, each a tenth of an inch in
diameter. Each of these drops would have the same bulk as the other, and
would be a quantity of matter exactly equivalent to the other. Then,
however great the distance which separated these two drops, they would
begin to approach one another; and, each moving with gradually
increasing swiftness, they would at length meet in a point exactly
half-way between the positions which they at first occupied. But if the
bulk of one drop were greater than that of the other drop, then the
larger would move more slowly, and the point of meeting would be by so
much nearer the larger drop. It follows that, if the one body of water
were as big as the earth and the other remained of its original size, no
bigger than a rain-drop—the motion of the large mass towards the small
one would be an inconceivably minute fraction of the total distance
travelled over. It would appear as if the large body were perfectly
still and drew the small body to itself.
This is just what happens when a single drop of water falls from a
cloud, say through a distance of a mile, to the earth. The earth really
moves towards it, just as it moves towards the earth, on the straight
line which joins the centres of the two. But the length of this line
which each travels over is =inversely proportional= to the quantity of
matter in each, that is to say is the less the bigger the quantity. So
that we have a rule-of-three sum. As the quantity of matter in the earth
is to that in a rain-drop, so is a mile to the distance travelled over
by the earth. And if any one worked out this sum, he would find that the
fourth term of the proportion would be an inconceivably minute fraction
of an inch. For all practical purposes, therefore, we may consider the
earth to be at rest in relation to all falling bodies, inasmuch as the
quantity of matter in any falling body is insignificant, in comparison
with that contained in the earth.
What is true of water is true, so far as we know, of all kinds of
matter, and we therefore say that it is a law of nature that all kinds
of matter possess gravity; that is to say, that of any two, each tends
to move towards the other, at a speed which is the slower the greater
the quantity of matter it contains in proportion to that which the other
contains; and this speed gradually becomes quicker as the two bodies
approach.
What is usually called the =law of gravitation= is a statement of the
same observed facts in another and more complete fashion. (See _Physics
Primer_.)