and the table on which they stand, and hence there
is no response to the attraction by even the slightest movement. Yet, if
we can conceive each of these weights mounted on wheels absolutely
devoid of friction, and running on absolutely perfect horizontal rails,
then there is no doubt that the bodies would slowly commence to draw
together, and in the course of time would arrive in actual contact.

If we desire to conceive gravitation as a force of measurable intensity,
we must employ masses immensely more ponderous than those 50lb. weights.
Imagine a pair of globes, each composed of 417,000 tons of cast iron,
and each, if solid, being about 53 yards in diameter. Imagine these
globes placed at a distance of one mile apart. Each globe attracts the
other by the force of gravitation. It does not matter that buildings and
obstacles of every description intervene; gravitation will pass through
such impediments as easily as light passes through glass. No screen can
be devised dense enough to intercept the passage of this force. Each of
these iron globes will therefore under all circumstances attract the
other; but, notwithstanding their ample proportions, the intensity of
that attraction is still very small, though appreciable. The attraction
between these two globes is a force no greater than the pressure exerted
by a single pound weight. A child could hold back one of these massive
globes from its attraction by the other. Suppose that all was clear, and
that friction could be so neutralised as to permit the globes to follow
the impulse of their mutual attractions. The two globes will then
commence to approach, but the masses are so large, while the attraction
is so small, that the speed will be accelerated very slowly. A
microscope would be necessary to show when the motion has actually
commenced. An hour and a half must elapse before the distance is
diminished by a single foot; and although the pace improves
subsequently, yet three or four days must elapse before the two globes
will come together.

The most remarkable characteristic of the force of gravitation must be
here specially alluded to. The intensity appears to depend only on the
quantity of matter in the bodies, and not at all on the nature of the
substances of which these bodies are composed. We have described the two
globes as made of cast iron, but if either or both were composed of lead
or copper, of wood or stone, of air or water, the attractive power would
s