One of the false doctrines which he first combated was that bodies of unequal weights would fall through the same altitude in unequal times : thus, if one body were ten times as heavy as another, it should fall through 100 yards while the lighter had only fallen through ten. But though the experiment was performed from the leaning tower at Pisa, and both bodies reached the ground at almost the same instant (the small difference, as Galilei rightly observed, being attri butable to the unequal resistances of the air), the witnesses of this experiment were not convinced, so inveterately were they prejudiced in favour of the doctrines in which they had been taught to place implicit belief.
Instead of making converts by his experiments, Galilei discovered that he had made many secret and some open enemies ; he therefore left Pisa and removed to the university of Padua (1592), where he was appointed to a professor's chair for the limited period of six years. Here he invented an imperfect species of thermometer, depending on the expansion of the air which remained after a portion was expelled by heat from a narrow glass tube, which was then inverted and immersed in water. His correspondence with Kepler commenced about the same period, and continued with the greatest mutual friendship and regard until his death. A treatise on the 'Sphere,' after the Ptolemaic system, which is attributed to Galilei, appeared about the same time. (Afterwards published at Rome, 1665.) On his reappointment to the professorship at Padua his salary was doubled, his fame iocreased, and his lectures were crowded; but these flattering events were overbalanced by a disagreeable intermittent disease to which he then first became subject, and which pursued him for the remainder of his life. A new star, almost as brilliant as that which directed Tycho BrahCa mind to the study of astrouomy, having appeared in 1604, in the constellation of Ophiuchus, he made it the subject of his lectures, which it may be presumed were less explana tory of its cause, than intended as an attack upon the Ptolemaic system. The conjecture now most generally adopted relative to these remarkable phwnomena is, that luminosity is not essential to the central body or sun of a planetary system, consequently the star may be quite opaque or partially luminous, and therefore would be either absolutely invisible or only seen when the luminous portion I was in the line joining the earth and star : this explanation is sufficient for those which appear and disappear with regularity ; in other cases this transitory phrenomenon may merely indicate an epoch of change in the cosmogony of the peculiar system of the star.
Astronomy did not however engross all tha attention of Galilei. He read and admired Gilbert's work, On the Nature of Bodies,' and adopted his views on the subject of terrestrial gravity, and con structed magnets after his example; about the same time he attacked with some bitterness one Capra, who ascribed to himself tho invention of a species of compass which Oalilei had made; and he wrote also on practical methods for the measurement of heights and distances. Shortly afterwards ho states in a letter, that "he intended hereafter to write three books on the system of the universe; three books on local motion; three books of mechanics; also on sound, speech, light, the tides, continuous quantity, animal motion, and castrameta tion; many of which, it is supposed, were destroyed by his relatives after his death, at the instance of the family confessor.
The year 1609 was signalised by the construction of the Galilean telescope, which consisted of a plano.convex object-glass, and a piano concave eye-glass, and thus he laid the foundation of the brilliant discoveries iu the solar system, which have rendered that science the most perfect of which the objects are the most remote. It is true that Jansen, a Ditch optician, and some others previous to him, had constructed microscopes, and perhaps imperfect telescopes, but they cannot claim the invention of the astronomical telescope, their articles having been more intended for toys and puerile amusement than any valuable practical purpose; and as they had no notion of applying them to the heavenly bodies, it is obvious that their random constructions would be totally inapplicable to such a purpose. How ever the long-mooted question of the invention of this noble instru ment of science may be decided, its application by Galilei to astronomy, for the first time, is indisputable. His first telescope was presented to the Doge of Venice, by whom the professorship at Padua was confirmed to him for life, with the greatest salary which had ever been there given to the mathematical professor, namely about 1000 florins.