Twenty-eight years had elapsed from the day when the Canon of Thorn expired while holding in his trembling hands the first copy of the work which was to glorify the name of Poland, when Würtemberg witnessed the birth of a man who was destined to achieve a revolution in science not less fertile in consequences, and still more difficult to accomplish. This man was Kepler. Endowed with two qualities which seem incompatible,--a volcanic imagination, and a dogged pertinacity which the most tedious calculations could not tire,--Kepler conjectured that celestial movements must be connected with each other by simple laws; or, to use his own expression, by harmonic laws. These laws he undertook to discover. A thousand fruitless attempts--the errors of calculation inseparable from a colossal undertaking--did not hinder his resolute advance toward the goal his imagination descried. Twenty-two years he devoted to it, and still he was not weary. What are twenty-two years of labor to him who is about to become the lawgiver of worlds; whose name is to be ineffaceably inscribed on the frontispiece of an immortal code; who can exclaim in dithyrambic language, "The die is cast: I have written my book; it will be read either in the present age or by posterity, it matters not which; it may well await a reader since God has waited six thousand years for an interpreter of his works"? These celebrated laws, known in astronomy as Kepler's laws, are three in number. The first law is, that the planets describe ellipses around the sun, which is placed in their common focus; the second, that a line joining a planet and the sun sweeps over equal areas in equal times; the third, that the squares of the times of revolution of the planets about the sun are proportional to the cubes of their mean distances from that body. The first two laws were discovered by Kepler in the course of a laborious examination of the theory of the planet Mars. A full account of this inquiry is contained in his famous work, 'De Stella Martis' [Of the Planet Mars], published in 1609. The discovery of the third law was announced to the world in his treatise on Harmonics (1628).