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 which were numerous, and written in Latin, are still extant. The Emperor Rodolphus purchased his expensive astronomical and other instruments; but they were mostly destroyed after the battle of the Weisseberg, near Prague, in 1620. A large sextant alone remains in Prague. The famous brass celestial globe, which was six feet in diameter, and cost about a thousand pounds, returned to Copenhagen after various adventures, but perished in the great fire of 1728. The castle of Uranienborg, where he nightly watched and pondered, has long been in ruins, leaving scarcely a trace of its structure and character. All, however, has not perished, nor been fruitless. 'It was the friendship of Tycho,' says an eminent authority, 'which formed Kepler, and directed him in the career of astronomy. Without this friendship, and without the numerous observations of Tycho, of which Kepler found himself the depositary after the death of his master, he would never have been able to discover those great laws of the system of the world which have been called 'Kepler's Laws,' and which, combined with the theory of central forces, discovered by Hüygens, conducted Newton to the grandest discovery which has ever been made in the sciences—that of universal gravitation.'

GALILEO.

The Copernican theory, which Tycho had labored in vain to supersede, was next received and supported by an Italian philosopher, whose name and history are inseparably interwoven with the progress of astronomy. That illustrious individual, Galileo Galilei, usually known by his Christian name, was born at Pisa in 1564. His father, a Tuscan nobleman of small fortune, caused him to be educated for the profession of medicine at the university of his native city. While studying there, he became deeply sensible of the absurdities of the philosophy of Aristotle, as it had then come to be taught, and he became its declared enemy. That spirit of observation for which he was so distinguished was early developed. When only nineteen years old, the swinging of a lamp suspended from the ceiling of the cathedral in Pisa, led him to investigate the laws of the oscillation of the pendulum, which he was the first to employ as a measure of time. He left it incomplete, however, and it was brought to perfection by his son, Vincenzo, and particularly by Hüygens, the latter of whom must be regarded as the true inventor of the pendulum. About this period Galileo devoted himself exclusively to mathematics and natural science, and in 1586 was led to the invention of the hydrostatic balance. In 1589, his distinction in the exact sciences gained for him the chair of mathematics in his native university, where, immediately on his installation, he began to assert the laws of nature against a perverted philosophy. In the presence of numerous spectators, he performed a series of experiments in the tower of the cathedral, to show that weight has no influence on the velocity of falling bodies. By this means he excited the opposition of the adherents of Aristotle to such a degree, that, after two years, he was forced to resign his professorship. Driven from Pisa, he retired into private life; but his genius being appreciated in another part of Italy, he was, in 1592, appointed professor of mathematics in Pudua. He lectured here with unparalleled success. Scholars from the most distant regions of Europe crowded round