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 GALILEI

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GALILEI

science may be said to owe its existence to him. Be- fore he was twenty, observation of the oscillations of a swinging lamp in the cathedral of Pisa led him to the discovery of the isochronism of the pendulum, which theory he utilized fifty years later in the construction of an astronomical clock. In 15S8, a treatise on the centre of gravity in solids obtained for him the title of the Archimedes of his time, and secured him a lecture- ship in the University of Pisa. During the years imme- diately following, taking advantage of the celebrated leaning tower, he laid the foundation experimentally of the theory of falling bodies and demonstrated the falsity of the peripatetic maxim, hitherto accepted without question, that their rate of descent is propor- tional to their weight. This at once raised a storm on the part of the .\ristotoleans, who would not accept even facts in contradiction of their master's dicta. Galileo, in conseiiuence of this and other troubles, found it prudent to quit Pisa and betake himself to Florence, the original home of his family. By the in- fluence of friends with the Venetian Senate he was nominated in 1592 to the chair of mathematics in the University of Padua, which he occupied for eighteen years, with ever-increasing renown. He afterwards be- took himself to Florence, being appointed philosopher and mathematician extraor- dinary to the Grand Duke of Tuscany. During the whole of this period, and to the close of his life, his inves- tigation of Nature, in all her fields, was unwearied. Fol- lowing up his experiments at Pisa with others upon inclined planes, Galileo established the laws of falling bodies as they are still formulated. He likewise demonstrated the laws of projectiles, and largely anticipated the laws of motion as finally estab- lished by Newton. He studied the properties of the cycloid and attpni]ited the problem of its ((uadrature: while in the "infinitesimals", which he was one of the first to intro- duce into geometrical demon- strations, was contained the germ of the calculus. In statics, he gave the first direct and entirely satisfactory demonstration of the laws of equilibrium and the principle of virtvial velocities. In hydrostatics, he set forth the true principle of flota- tion. He invented a thermometer, though a defective one, but he did not, as is sometimes claimed for him, invent the microscope.

Though, as has been said, it is by his astronomical discoveries that he is most widely remembered, it is not these that constitute his most substantial title to fame. In this connexion, his greatest achievement was undoubtedly his virtual invention of the telescope. Hearing early in 1609 that a Dutch optician, named Lippershey, had proflucctl an instrument by which the apparent size of remote objects was magnified, Galileo at once realized the principle by which such a result could alone be attained, and, after a single night de- voted to consideration of the laws of refraction, he succeeiied in constructing a telescope which magnified three times, its magnifying power being soon increased to thirty-two. This instrument being provided and turned towanls the heavens, the discoveries, which have made Galileo famous, were bound at once to fol- low, though undoubtedly he was quick to grasp their

full significance. The moon was shown not to be, as the old astronomy taught, a smooth and perfect sphere, of different nature to the earth, but to possess hills and valleys and other features resembling those of our own globe. The planet Jupiter was found to have satellites, thus displaying a solar system in min- iature, and supporting the doctrine of Copernicus. It had been argued against the said system that, if it were true, the inferior planets, Venus and Mercury, between the earth and the sun, should in the course of their revolution exhibit phases like those of the moon, and, these being invisible to the naked eye, Copernicus had to advance the quite erroneous explanation that these planets were transparent and the sun's rays passed through them. But with his telescope Galileo found that Venus did actually exhibit the desired phases, and the objection was thus turned into an argument for Copernicanism. Finally, the spots on the sun, which Galileo soon perceived, served to prove the rotation of that luminary, and that it was not incorruptible as had teen assumed.

Prior to these discoveries, Galileo had already aban- doned the old Ptolemaic astronomy for the Coperni- can, but, as he confessed in a letter to Kepler in 1597, lie had refrained from mak- ing himself its advocate, lost like Copernicus himself he should be overwhelmed with ridicule. His telescopic discoveries, the significance of which he immediately per- ceived, induced him at once to lay aside all reserve and come forward as the avowed and strenuous champion of Copernicanism, and, appeal- ing as these discoveries did to the evidence of sensible phe- nomena, they not only did more than anything else to recommend the new- system to general acceptance, liut in- vested Galileo himself with the credit of being the great- est astronomer of his age, if not the greatest who ever lived. They w-ere also the cause of his lamentable con- troversy with ecclesiastical authority, which raises ques- tions of graver import than any others connected with his name. It is necessary, therefore, to understand clearly his exact position in this regard.

The direct services which Galileo rendered to astron- omy are virtually simimed up in his telescopic dis- coveries, which, brilliant and important as they were, contributed little or nothing to the theoretical perfec- tion of the science, and were sure to be made by any careful observer provided with a telescope. Again, he wholly neglected discoveries far more fundamental than his own, made by his great contemporary Kepler, the value of which he either did not perceive or en- tirely ignored. Since the first and second of his famous laws were already published by Kepler in 1609 and the third, ten years later, it is truly inconceivable, as Delambre says, that Galileo should not once have made any mention of these discoveries, far more difli- cult than his own, which finally led Newton to deter- mine the general principle which forms the very soul of the celestial mechanism thus established. It is, more- over, undeniable, that the proofs which Cialileo ad- duced in support of the heliocentric .system of Coperni- cus, as against the geocentric of Ptolemy and the ancients, were far from conclusive, and failed to con-