From the elementary works of geometry, Galileo passed to the writings of Archimedes; and while he was studying the hydrostatical treatise[3] of the Syracusan philosopher, he wrote his essay on the hydrostatical balance[4], in which he describes the construction of the instrument, and the method by which Archimedes detected the fraud committed by the jeweller in the composition of Hiero's crown. This work gained for its author the esteem of Guido Ubaldi, who had distinguished himself by his mechanical and mathematical acquirements, and who engaged his young friend to investigate the subject of the centre of gravity in solid bodies. The treatise on this subject, which Galileo presented to his patron, was the source of his future success. Through the cardinal del Monte, the brother-in-law of Ubaldi, the reigning duke of Tuscany, Ferdinand de' Medici, was made acquainted with the merits of our young philosopher; and, in 1589, he was appointed lecturer on mathematics at Pisa. By the drudgery of private teaching he was obliged to add to the small salary of sixty crowns which was attached to the office. With this moderate Competency, Galileo commenced his philosophical career. At the early age of eighteen, when he had entered the university, he displayed his innate antipathy to the Aristotelian philosophy. This feeling was strengthened by his earliest inquiries; and upon his establishment at Pisa, he seems to have regarded the doctrines of Aristotle as the intellectual prey which, in his chace of glory, he was destined to pursue. Nizzoli, who flourished near the beginning of the sixteenth century, and Giordano Bruno, who was burned at Rome in 1600, led the way in this daring pursuit; but it was reserved for Galileo to track the Thracian boar through its native thickets, and, at the risk of his own life, to strangle it in its den.