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 Huygens, 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 Padua. He lectured here with unparalleled success. Scholars from the most distant regions of Europe crowded round him. He delivered his lectures in the Italian language instead of Latin, which was considered a daring innovation.

During eighteen years which he spent at Padua, he made many discoveries in natural philosophy, which he introduced into his lectures, without regard to their inconsistency with the doctrines previously taught. Among these may be mentioned his discovery of the rate of descent in falling bodies; certain improvements on the thermometer; some interesting observations on the magnet; and a number of experiments relative to the floating and sinking of solid bodies in water. In 1609, hearing that one Jansen, a Dutchman, had made an instrument by which distant objects were made to appear near, Galileo, whose mind was prepared for the discovery, instantly conceived on what principle it was constructed, and, without losing a day, he fashioned a similar instrument with many improvements: such was the origin of the telescope, the most interesting of all instruments connected with science.

Turning his optical tube towards the heavens, Galileo perceived the moon to be a body of uneven surface, the elevations of which he computed by their shadows; and the sun to be occasionally spotted; and from the regular advance from east to west of these spots, he inferred the rotation of the sun, and the inclination of its axis to the plane of the ecliptic. From a particular nebula, which his rude instrument enabled him to resolve into individual stars, he even conjectured, what Lord Fosse has but recently proved, that the whole Milky Way was but a vast assemblage of stars and systems. He discovered that the planet Venus waxed and waned like the moon, that Saturn had something like wings by its sides (afterwards found to be a ring), and that Jupiter was surrounded by four satellites. It is now altogether impossible to imagine the wonder and delight with which these discoveries must have filled the mind of a philosopher like Galileo, who had perhaps long surmised that all was not as it seemed in the heavens, but despaired of ever being able to penetrate the mystery. In the year 1611, while entering upon his investigations, he was induced, by the invitation of his prince, the Grand Duke of Tuscany, to return to Pisa, and resume the chair of mathematics there, with a large salary. It was consequently at that city that he first gave his discoveries to the world. That persecution which had only been suspended by accident in the case of Copernicus, now fell with full weight on the head of the Italian philosopher. Having openly declared, in a work which he published, that his discoveries proved the truth of the Copernican theory, he was denounced by the clergy as a heretic, and obliged, in 1615, to proceed to Rome, and appear before the court of Inquisition, who obliged him to promise that he would never more broach such dangerous doctrines. It has been stated, but is not quite certain, that he was on this occasion imprisoned by the Inquisition for five months, and that he would have suffered still more severely if the Grand Duke had not interceded for him.