seventeenth century, recognizing the truth of the Copernican system, determined the three laws of planetary motion: 1. That the orbit of each planet is an ellipse, the sun being in one focus. 2. That as each planet moves around the sun, the line which joins it to the sun passes over equal areas in equal times. 3. That the square of the time of a planet's revolution around the sun is in proportion to the cube of its mean distance from the sun. These laws were discovered by Kepler as deductions made upon mathematical principles from observations which had to be carried on without the aid of the telescope, and without that knowledge of the general laws of motion which came later. Kepler's laws, although in the main correct, were subsequently found to be subject to certain deviations in the planetary motions. It was when Galileo, the contemporary of Kepler, who, if he was not the first inventor of the telescope, was the first to use it in astronomical observations, was able by means of it to discover the general laws of motion, that the substantial accuracy of Kepler's three laws could be proved, while at the same time the deviations from them were accounted for. Still, there was wanting the grand discovery, which would disclose the cause of these motions of the planets in elliptical orbits, and the relations between their distances and their times of revolution, and thus reduce the whole of the phenomena to a general law. Descartes, who flourished 1596-1650, first attempted to do this by his theory of Vortices. He supposed the sun to be immersed in a vast fluid, which, by the sun's rotation, was made to rotate in a whirlpool, that carried the planets around with it, the outer ones revolving more slowly because the parts of the ethereal fluid in which they were immersed moved more slowly. This was a reversion back to some of the ancient speculations. It was reserved for Newton to discover the law of universal gravitation, by which, in the place of any physical connection between the bodies of the solar system by any intervening medium, the force of attraction exerted by a larger body upon a smaller would draw the smaller body out of the straight line that it would pursue when under a projectile force, and would thus convert its motion into a circular revolution around the attracting body, and make the orbit of this revolution elliptical by the degree in which the attracting force varied in intensity according to the varying distance between the two bodies. When Newton's laws of motion were discovered and found to be true, the phenomena of the solar system were explained.