must for ever remain invisible to us, even if
they were many times larger than Jupiter.

We need not repine at this limitation to our possible knowledge, for
just as we find in the solar system all that is necessary for our daily
bodily wants, so shall we find ample occupation for whatever faculties
we may possess in endeavouring to understand those mysteries of the
heavens which lie within our reach.

CHAPTER V.

THE LAW OF GRAVITATION.

Gravitation--The Falling of a Stone to the Ground--All Bodies fall
equally, Sixteen Feet in a Second--Is this true at Great
Heights?--Fall of a Body at a Height of a Quarter of a Million
Miles--How Newton obtained an Answer from the Moon--His Great
Discovery--Statement of the Law of Gravitation--Illustrations of
the Law--How is it that all the Bodies in the Universe do not rush
Together?--The Effect of Motion--How a Circular Path can be
produced by Attraction--General Account of the Moon's Motion--Is
Gravitation a Force of Great Intensity?--Two Weights of 50
lbs.--Two Iron Globes, 53 Yards in Diameter, and a Mile apart,
attract with a Force of 1 lb.--Characteristics of
Gravitation--Orbits of the Planets not strictly Circles--The
Discoveries of Kepler--Construction of an Ellipse--Kepler's First
Law--Does a Planet move Uniformly?--Law of the Changes of
Velocity--Kepler's Second Law--The Relation between the Distances
and the Periodic Times--Kepler's Third Law--Kepler's Laws and the
Law of Gravitation--Movement in a Straight Line--A Body unacted on
by Disturbing Forces would move in a Straight Line with Constant
Velocity--Application to the Earth and the Planets--The Law of
Gravitation deduced from Kepler's Laws--Universal Gravitation.

Our description of the heavenly bodies must undergo a slight
interruption, while we illustrate with appropriate detail an important
principle, known as the law of gravitation, which underlies the whole of
astronomy. By this law we can explain the movements of the moon around
the earth, and of the planets around the sun. It is accordingly
incumbent upon us to discuss this subject before we proceed to the more
particular account of the separate planets. We shall find, too, that the
law of gravitation sheds some much-needed light on the nature of the
stars situated at the remotest distances in space. It also enables us to
cast a glance through the