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 , having thus acquired some economic security, threw himself with increased ardour into the investigations that he loved, and set to work systematically to test by experiment everything in Aristotle's mechanics. The stirring in men's minds which produced the Reformation had already caused a few isolated attempts to replace dogma by proof, but no one previously had made such determined assaults on the "peripatetic" stronghold. No sooner did Galileo find any rebutting experiment than he promptly pilloried the discredited dogma, exposing it in his lectures. Once more, therefore, he drew upon himself the enmity of those professors who had objected to his sceptical attitude as a student. The best-known instance of the success of his campaign is that associated with the celebrated Leaning Tower of Pisa. According to Aristotle the velocity of falling bodies is proportional to their weight, so that a weight of a hundred pounds would fall a hundred times as fast as a weight of one pound. Galileo asserted that but for the resistance of the air, which varies with the size and shape of the body, both would fall at the same rate. The parapet of the Leaning Tower offered a very convenient opportunity of testing the point. The experiment was tried by Galileo, and in the event the heavy weight beat the light weight by about two inches. Curiously enough, many of his opponents claimed the victory for Aristotle, as the heavy weight fell more quickly than the light one, and Galileo caustically