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, nearly two thousand years before, had raised the statement to the rank of a proposition, that the rate at which a body falls depends on its weight. Up to Galileo’s time this doctrine had been generally accepted as true, on the mere word of the old hero of science, although individual physicists, like Varchi in 1544, and Benedetti in 1563, had disputed it, maintaining that bodies of similar density and different weight fall from the same height in an equal space of time. They sought to prove the correctness of this statement by the most acute reasoning, but the idea of experiment did not occur to any one. Galileo, well aware that the touchstone of experiment would discover the vulnerable spot in Aristotelian infallibility, climbed the leaning tower of Pisa, in order thence to prove by experiment, to the discomfiture of the Peripatetic school, the truth of the axiom that the velocity with which a body falls does not depend on its weight but on its density.[22] It might have been thought that his opponents would strike sail after this decisive argument. Aristotle, the master, would certainly have yielded to it—but his disciples had attained no such humility. They followed the bold experiments of the young professor with eyes askance and miserable sophistries, and, being unable to meet him with his own weapons of scientific research, they eagerly sought an opportunity of showing the impious and dangerous innovator the door of the _aula_. An unforeseen circumstance came all at once to their aid in these designs. An illegitimate son of the half-brother of the reigning Grand Duke,—the relationship was somewhat farfetched, but none the less ominous for Galileo—John de’ Medici, took an innocent pleasure in inventing machines, and considered himself a very skilful artificer. This ingenious semi-prince had constructed a monster machine for cleaning the harbour of Leghorn, and proposed that it should be brought into use. But Galileo, who had been commissioned to examine the marvel, declared it to be useless, and, unfortunately, experiment fully confirmed the verdict. Ominous head-shakings were seen among the suite of the deeply mortified inventor. They entered into alliance with the Peripatetic philosophers against their common enemy. There were cabals at court. Galileo, perceiving that his position at Pisa was untenable, voluntarily resigned his professorship before the three years had expired, and migrated for the second time home to Florence.[23]