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 vistas of time past, and to trace with plausibility, if not with certainty, certain early phases in the history of our system. The sun and the moon, the planets and the comets, the stars and the nebulae, all alike are subject to this universal law, which is now to engage our attention. What is more familiar than the fact that when a stone is dropped it will fall to the ground? No one at first thinks the matter even worthy of remark. People are often surprised at seeing a piece of iron drawn to a magnet. Yet the fall of a stone to the ground is the manifestation of a force quite as interesting as the force of magnetism. It is the earth which draws the stone, just as the magnet draws the iron. In each case the force is one of attraction; but while the magnetic attraction is confined to a few substances, and is of comparatively limited importance, the attraction of gravitation is significant throughout the universe.