All the first time announced. Some twelve years later, in his work, entitled 'Harmonies' he announced the third law of planetary motion, fully establishing his right to the title, by which he has since been distinguished 'The Legislator of the Heavens.'
These laws of Kepler are: First, that the orbits of the planets are elliptical, the sun being at one of the foci; second, that the radius vector, that is, a line drawn from a planet to the sun, passes over equal spaces in equal times; third, that the squares of the times of revolution of the different planets, are to each other as the cubes of the mean distances from the sun.
Together with these laws of planetary motion, two of the three axioms of the science of mechanics, known as the Laws of Motion, were about this time discovered, or rather, were now for the first time distinctly apprehended and enunciated. The first of these was given by Kepler—the law of inertia, namely, that a body will persevere in the state in which it is, whether of rest or motion, until it is acted on by some force; or more precisely, a body at rest will continue at rest until acted on by some force, and when acted on by any single force, if free to move, its motion will be rectilinear, uniform and continuous until the body is acted on by some other force. The second law of motion was announced by Galileo, and is known as the law of the coexistence of motions, or independence of forces. It may be expressed as follows: If a body be acted on by several forces simultaneously, it will obey the impulse of each force, just as it would if the others were not acting. [The simplest illustration of this law is what is known as the parallelogram of forces. If the direction and intensity of two forces acting simultaneously on a body be represented by the sides of a parallelogram, the body will describe the diagonal of the parallelogram; that is, at the end of a unit of time the body will be just where it would have been if the forces had, each for a unit of time, acted consecutively.]
The true system of the universe, the laws of planetary motion and the fundamental principles of mechanics having become known, for the first time in the history of the race any intelligent inquiry as to the physical causes of the motions of the heavenly bodies became possible. With earnestness and assiduity proportioned to the interest and grandeur of the problem, men of science at once applied themselves to its solution, and yet half a century of gradual progress elapsed before the desired result was reached. From the facts which we shall have occasion to mention it will appear how much, or rather how little, foundation there is for the common belief that the idea of the law of gravitation was wholly original with Newton—suggested to him for the first time by observing the fall of an apple, and then suddenly coming forth from his brain like Minerva from the head of Jove, un-
