Galileo Galilei, famous as an astronomer and as an experimental physicist, was born at Pisa, in Italy, Feb. 18, 1564. His talents were most multifarious and remarkable; but his mathematical and mechanical genius was dominant from the first. As a child he constructed mechanical toys, and as a young man he made one of his most important discoveries, which was that of the pendulum as an agent in the measurement of time, and invented the hydrostatic balance, by which the specific gravity of solid bodies might be ascertained. At the age of 24 a learned treatise on the centre of gravity of solids led to a lectureship at Pisa University. Driven from Pisa by the enmity of Aristotelians, he went to Padua University, where he invented a kind of thermometer, a proportional compass, a microscope, and a telescope. The last invention bore fruit in astronomical discoveries, and in 1610 he discovered four of the moons of Jupiter. His promulgation of the Copernican doctrine led to renewed attacks by the Aristotelians, and to censure by the Inquisition. (See Religion, vol. xiii.) Notwithstanding this censure, he published in 1632 his "Dialogues on the System of the World." The interlocutors in the "Dialogues," with the exception of Salviatus, who expounds the views of the author himself, represent two of Galileo's early friends. For the "Dialogues" he was sentenced by the Inquisition to incarceration at its pleasure, and enjoined to recite penitential psalms once a week for three years. His life thereafter was full of sorrow, and in 1637 blindness added to his woes; but the fire of his genius still burnt on till his death on January 8, 1642. _Does the Earth Move_ SALVIATUS: Now, let Simplicius propound those doubts which dissuade him from believing that the earth may move, as the other planets, round a fixed centre. SIMPLICIUS: The first and greatest difficulty is that it is impossible both to be in a centre and to be far from it. If the earth move in a circle it cannot remain in the centre of the zodiac; but Aristotle, Ptolemy and others have proved that it is in the centre of the zodiac. SALVIATUS: There is no question that the earth cannot be in the centre of a circle round whose circumference it moves. But tell me what centre do you mean? SIMPLICIUS: I mean the centre of the universe, of the whole world, of the starry sphere. SALVIATUS: No one has ever proved that the universe is finite and figurative; but granting that it is finite and spherical, and has therefore a centre, we have still to give reasons why we should believe that the earth is at its centre. SIMPLICIUS: Aristotle has proved in a hundred ways that the universe is finite and spherical.