o where it
started in about four weeks. It would then commence another revolution,
four weeks more would find it again at the starting point, and this
motion would go on for ages.

Do not suppose that we are entirely romancing. We cannot indeed show the
cannon, but we can point to a great projectile. We see it every month;
it is the beautiful moon herself. No one asserts that the moon was ever
shot from such a cannon; but it must be admitted that she moves as if
she had been. In a later chapter we shall enquire into the history of
the moon, and show how she came to revolve in this wonderful manner.

As with the moon around the earth, so with the earth around the sun. The
illustration shows that a circular or nearly circular motion harmonises
with the conception of the law of universal gravitation.

We are accustomed to regard gravitation as a force of stupendous
magnitude. Does not gravitation control the moon in its revolution
around the earth? Is not even the mighty earth itself retained in its
path around the sun by the surpassing power of the sun's attraction? No
doubt the actual force which keeps the earth in its path, as well as
that which retains the moon in our neighbourhood, is of vast intensity,
but that is because gravitation is in such cases associated with bodies
of enormous mass. No one can deny that all bodies accessible to our
observation appear to attract each other in accordance with the law of
gravitation; but it must be confessed that, unless one or both of the
attracting bodies is of gigantic dimensions, the intensity is almost
immeasurably small.

Let us attempt to illustrate how feeble is the gravitation between
masses of easily manageable dimensions. Take, for instance, two iron
weights, each weighing about 50lb., and separated by a distance of one
foot from centre to centre. There is a certain attraction of gravitation
between these weights. The two weights are drawn together, yet they do
not move. The attraction between them, though it certainly exists, is an
extremely minute force, not at all comparable as to intensity with
magnetic attraction. Everyone knows that a magnet will draw a piece of
iron with considerable vigour, but the intensity of gravitation is very
much less on masses of equal amount. The attraction between these two
50lb. weights is less than the ten-millionth part of a single pound.
Such a force is utterly infinitesimal in comparison with the friction
between the weights