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.

For several years he observed the silence enjoined upon him, but continued to pursue the study of the true theory of the heavens. Panting to make known to the world a complete account of the system of Copernicus, yet dreading the prejudices of his enemies, he fell upon the expedient of writing a work, in which, without giving his own opinion, he introduces three persons in a dialogue, of whom the first defends the Copernican system , the second the Ptolemaean (or that of Aristotle), and the third weighs the reasons of both in such a way, that the subject seems problematical, though it is impossible to mistake the preponderance of arguments in favor of Copernicus. With this great work, which is still held in reverence, Galileo went to Rome in 1630, in the sixty-sixth year of his age, and, by an extraordinary stretch of favor, received permission to print it. Scarcely had it appeared at Rome and Florence, when it was attacked by the disciples of Aristotle, and most violently of all by the teacher of philosophy at Pisa. A congregation of cardinals, monks, and mathematicians, was appointed to examine his work, which they unhesitatingly condemned as highly dangerous, and summoned him before the tribunal of the Inquisition. This blow fell heavily on the head of Galileo, now an old man, and left defenseless by the death of his friend and patron, Cosmo II. He was compelled to go to Rome in the winter of 1633, and was immediately immured in a cell in one of the prisons of the Inquisition. There he remained for several months, when, being brought before an assembly of his judges, he was condemned to renounce, kneeling before them, with his hand upon the gospels, what were called the sinful and detestable errors and heresies' which he had maintained. The firmness of Galileo gave way at this critical moment of his life: he pronounced the recantation. But at the moment he rose, indignant at having sworn in violation of his conviction, he exclaimed, stamping his foot, E pur si muove! (It still moves!) Upon this dreadful relapse into heresy, he was sentenced to imprisonment in the Inquisition for life, and every week for three years was to repeat the seven penitential psalms; his Dialogues' were also prohibited, and his system utterly condemned. Although Galileo was in this manner sentenced to confinement, it appeared to those who judged him that he would not be able, from his age, to endure such a severe punishment, and they mercifully banished him to a particular spot near Florence.

Here Galileo lived for several years, employing his time in the study of mechanics and other branches of natural philosophy. The results are found in two important works on the laws of motion, the foundation of the present' system of physics and astronomy. At the same time he tried to make use of Jupiter's satellites for the calculation of longitudes; and though he brought nothing to perfection in this branch, he was the first who reflected systematically on such a method of fixing geographical longitudes. He was at this time afflicted with a disease in his eyes, one of which was wholly blind, and the other almost useless, when, in 1637, he discovered the libration of the moon. Blindness, deafness, want of sleep, and pain in his limbs, united to embitter his declining years; still his mind was active. In my darkness,' he writes in the year 1638, muse now upon this object of nature, and now upon that, and find it impossible to soothe my restless head, however much I wish it. This perpetual action of mind deprives me almost wholly of sleep.' In this condition, and affected by a slowly-consuming fever, he expired in January, 1642, in the seventy-eighth year of his age. His relics were deposited in the church of Santa Croce, at Florence, where posterity did justice to his memory by erecting a splendid monument in 1737.

Galileo is represented by his biographers as of diminutive stature, but strong and healthy, of agreeable countenance, and lively conversation and manner. He preferred living in the country, where his relaxations consisted in the cultivation of his garden, and in the company and conversation of his friends. He loved music, drawing, and poetry; and is said to have been so fond of Ariosto, that he knew the whole of the 'Orlando' by heart. He had few books; the best book,' he said, is nature.' A complete edition of his works, in thirteen volumes, appeared at Milan in 1803, the style of which is natural and fluent, so elegant and pure, that it has been held up by competent judges as a model of classical Italian. Altogether,' says Professor Playfair, 'Galileo is one of those to whom human knowledge is under the greatest obligation. His discoveries in the theory of motion, in the laws of the descent of heavy bodies, and in the motion of projectiles, laid the foundation of all the great improvements which have since been made by the application of mathematics to natural philosophy. If to these we add the invention of the telescope, the discoveries made by that instrument, the confirmation of the Copernican system which these discoveries afforded, and lastly, the wit and argument with which he combated and exposed the prejudice and presumption of the schools, we must admit that the history of human knowledge contains few greater names than that of Galileo.