GALILEI
343
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-
