Copernicus was born in the Polish village of Thorn, in 1473, and died in 1543, at Warmia, of which he was canon, and where he built an observatory. The voyages of his youth, his labours, adversities, and old age at last broke him down, and in the winter of 1542 he took to his bed, and was incapable of further work. His work, which was just finished printing at Nuremberg, was brought to him by his friends before he died. He soon after completely failed in strength, and passed away tranquilly on the 23rd of May, 1543. [Illustration: PLATE VIII.--DEATH OF COPERNICUS.] The Copernican system required, however, establishing in the minds of astronomers generally before it took the place it now holds, and this work was done by Galileo--a name as celebrated as that of Copernicus himself, if not more so. This perhaps is due not only to his demonstration of the motion of the earth, but to his introduction of experimental philosophy, and his observational method in astronomy. The next advance was made by Kepler, who overthrew at one blow all the excentrics and epicycles of the ancients, when by his laborious calculations he proved the ellipticity of the orbit of Mars. The Grecian hypotheses were the logical consequences of two propositions which were universally admitted as axioms in the early and middle ages. First, that the motions of the heavenly bodies were uniform; second, that their orbits were perfect circles. Nothing appeared more natural than this belief, though false. So then when Kepler, in 1609, recognised the fact, by incontestable geometrical measurements, that Mars described an oval orbit round the sun, in which its velocity varied periodically, he could not believe either his observation or his calculation, and he puzzled his brain to discover what secret principle it was that forced the planet to approach and depart from the sun by turns. Fortunately for him, in this inquietude he came across a treatise by Gilbert, _De Magnate_, which had been published in London nine years before. In this remarkable work Gilbert proved by experiment that the earth acts on magnetized needles and on bars of iron placed near its surface just as a magnet does--and by a conjectural extension of this fact, which was a vague presentiment of the truth, he supposed that the earth itself might be retained in its constant orbit round the sun by a magnetic attraction. This idea was a ray of light to Kepler. It led him to see the secret cause of the alternating motions that had troubled him so much, and in the joy of that discovery he said, "If we find it impossible to attribute the vibration to a magnetic power residing in the sun, acting on the planet without any material medium between, we must conclude that the planet is itself endowed with a kind of intelligent perception which gives it power to know at each instant the proper angles and distances for its motion." In the result Kepler was led to enunciate to the world his three celebrated laws:--