MEMOIR OF GALILEO. 85 



drostatics, he wrote an essay on the hydrostatical balance, 

 explaining its construction, and the mode by which the phi- 

 losopher of Syracuse detected the fraud committed by the 

 jewellers making Hiero's crown. This work introduced 

 Galileo to Guido Ubaldi, an eminent mathematician, who 

 engaged him to investigate the subject of the centre of 

 gravity in solid bodies ; and the treatise which he produced 

 upon this subject was the foundation of his future celebrity 

 Through his connexion with Ubaldi, Galileo was appoin- 

 ted lecturer on mathematics at Pisa in 1589, with a yearly 

 salary of sixty crowns, which he increased by devoting some 

 time to private teaching. At the early age of eighteen, 

 Galileo doubted the philosophy of Aristotle; and on his 

 establishment at Pisa, commenced to overthrow the doc- 

 trines of this philosopher. His first inquiries were into 

 the mechanical doctrines of Aristotle, which he soon dis- 

 covered to be untenable. The errors which he found exist- 

 ing, he exposed to his pupils, and a rancorous controversy 

 commenced between the followers of Aristotle on the one 

 side, and Galileo and his pupils on the other. Argument, 

 and even experiment, failed in convincing Galileo's oppo- 

 nents. The doctrine of Aristotle, that the heavier of two 

 falling bodies would fall quicker, was proved by the experi- 

 ment of dropping bodies of different weights from the lean- 

 ing tower at Pisa; but although these bodies struck the 

 ground nearly at the same instant, the followers of Aristo- 

 tle remained unconvinced, or at least unconverted. Con- 

 scious of his superiority, and the truth of his doctrines, 

 Galileo turned not only the powers of argument, but the 

 shafts of ridicule and sarcasm against his opponents ; thus 

 raising up a personal enmity, which afterwards developed 

 itself in bitter persecution. Other circumstances increased 

 the rancor of his enemies, and at last made his position so 

 uncomfortable, that he gave up his situation at Pisa, and 

 accepted the professorship of mathematics at the university 



