GALILEO GALILEI (Galileo, by which he is commonly known, being his Christian name), an Italian philosopher and mathematician, born in Pisa, Feb. 15, 1564, died in Arcetri, Jan. 8, 1642. He came of a noble Florentine family, whose original name was Bonajuti, which they exchanged for that of Galilei about the middle of the 14th century. Vincenzo, the father of the philosopher, was a man of learning and the author of a number of treatises on music. He was unable to give his sons a thorough education, but Galileo acquired, amid various discouragements, a fair knowledge of the classics and the common branches of learning, and also of music, drawing, and painting. The last named art he seems to have resolved upon cultivating as a profession, but his father sent him to Pisa to study medicine, where he was matriculated at the university as a scholar in arts, Nov. 5, 1581, and became a pupil of the celebrated botanist Cæsalpinus. He still employed his leisure in his favorite branches of the fine arts, and his love of drawing led him to study geometry. After many fruitless remonstrances his father left him to the natural bent of his genius. His first discovery was about 1583, when he was led to infer the isochronism of the vibration of the pendulum by noticing the regular swinging of a lamp in the cathedral of Pisa. Though it was 50 years before the philosopher applied his discovery to clockwork, he at once perceived its importance, and caused it to be employed by physicians in counting the pulses of their patients. Some time afterward, having read the treatise of Archimedes on floating bodies, he invented a hydrostatic balance, and wrote a description of it, which introduced him to the friendship of Guido Ubaldi, the mechanist and mathematician. A paper on the centre of gravity was indirectly the means of securing for him at the age of 25 a professorship of mathematics in the university of Pisa. The salary was but 60 crowns, and he had to look for his support partly to private pupils. His sarcastic attacks upon the notions of the Aristotelians, although his arguments were fortified with careful experiments, raised up a host of enemies, whose animosity pursued him for the rest of his life. He demonstrated the error of supposing that the velocity of falling bodies is proportional to their weight, by letting fall unequal weights at the same time from the top of the leaning tower of Pisa, explaining that the trifling difference of time noticed in their respective descents was owing solely to the resistance of the air. The death of his father in 1591 imposed upon him the duty of supporting the family. Soon after this the interest of Ubaldi procured him the appointment of professor of mathematics for six years in the university of Padua. This new position, upon which he entered in September, 1592, gave him a salary of 180 florins, and enabled him to remove from a city where the hostility of the Aristotelians embittered his existence. He constructed several useful machines for the state, and composed treatises on gnomonics, astronomy, mechanics, architecture, and even fortification, which he delivered in the form of lectures. In 1597 he made a kind of thermometer in which both air and water were employed. During this period he began a friendly correspondence with Kepler, which continued until the death of the latter; and about the same time appeared a treatise on the sphere after the Ptolemaic system, which has been attributed to Galileo on rather insufficient grounds. It was published from a MS. in the library of Somaschi at Venice (Rome, 1656).