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Rh Clavius at Rome, with whom he had become acquainted on his first visit there in 1587, Michael Coignet at Antwerp, Riccoboni, the Marquis Guidubaldo del Monte, etc., entered into correspondence with him. Intercourse with the latter, a distinguished mathematician, who took the warmest interest in Galileo's fate, became of the utmost importance to him. It was not merely that to his encouragement he owed the origin of his excellent treatise on the doctrine of centres of gravity, which materially contributed to establish his fame, and even gained for him from Del Monte the name of an "Archimedes of his time," but he first helped him to secure a settled and honourable position in life. By his opportune recommendation in 1589, the professorship of mathematics at the University of Pisa, just become vacant, was conferred on Galileo, with an income of sixty scudi. It is indicative of the standing of the sciences in those days that, while the professor of medicine had a salary of two thousand scudi, the professor of mathematics had not quite thirty kreuzers a day. Even for the sixteenth century it was very poor pay. Moreover, in accordance with the usage at the Italian Universities, he was only installed for three years; but in Galileo's needy circumstances, even this little help was very desirable, and his office enabled him to earn a considerable additional income by giving private lessons.

During the time of his professorship at Pisa he made his grand researches into the laws of gravitation, now known under the name of "Galileo's Laws," and wrote as the result of them his great treatise "De Motu Gravium." It then had but a limited circulation in copies, and did not appear in print until two hundred years after his death, in Albèri's "Opere complete di Galileo Galilei." Aristotle,