al event to be much easier in the former case than in the latter。 For the ruler of the universe； however； whose might is infinite； it is no less easy to move the universe than the earth or a straw balm。 But if his power is infinite； why should not a greater； rather than a very small； part of it be revealed to me？ 〃Salviati。 If I had said that the universe does not move on account of the impotence of its ruler； I should have been wrong and your rebuke would have been in order。 I admit that it is just as easy for an infinite power to move a hundred thousand as to move one。 What I said； however； does not refer to him who causes the motion； but to that which is moved。 In answer to your remark that it is more fitting for an infinite power to reveal a large part of itself rather than a little； I answer that； in relation to the infinite； one part is not greater than another； if both are finite。 Hence it is unallowable to say that a hundred thousand is a larger part of an infinite number than two； although the former is fifty thousand times greater than the latter。 If； therefore； we consider the moving bodies； we must unquestionably regard the motion of the earth as a much simpler process than that of the universe； if； furthermore； we direct our attention to so many other simplifications which may be reached only by this theory； the daily movement of the earth must appear much more probable than the motion of the universe without the earth； for； according to Aristotle's just axiom； 'Frustra fit per plura； quod potest fieri per p auciora' （It is vain to expend many means where a few are sufficient）。〃＇2＇
The work was widely circulated； and it was received with an interest which bespeaks a wide…spread undercurrent of belief in the Copernican doctrine。 Naturally enough； it attracted immediate attention from the church authorities。 Galileo was summoned to appear at Rome to defend his conduct。 The philosopher； who was now in his seventieth year； pleaded age and infirmity。 He had no desire for personal experience of the tribunal of the Inquisition； but the mandate was repeated； and Galileo went to Rome。 There； as every one knows； he disavowed any intention to oppose the teachings of Scripture； and formally renounced the heretical doctrine of the earth's motion。 According to a tale which so long passed current that every historian must still repeat it though no one now believes it authentic； Galileo qualified his renunciation by muttering to himself； 〃E pur si muove〃 （It does move； none the less）； as he rose to his feet and retired from the presence of his persecutors。 The tale is one of those fictions which the dramatic sense of humanity is wont to impose upon history； but； like most such fictions； it expresses the spirit if not the letter of truth； for just as no one believes that Galileo's lips uttered the phrase； so no one doubts that the rebellious words were in his mind。 After his formal renunciation； Galileo was allowed to depart； but with the injunction that he abstain in future from heretical teaching。 The remaining ten years of his life were devoted chiefly to mechanics； where his experiments fortunately opposed the Aristotelian rather than the Hebrew teachings。 Galileo's death occurred in 1642； a hundred years after the death of Copernicus。 Kepler had died thirteen years before； and there remained no astronomer in the field who is conspicuous in the history of science as a champion of the Copernican doctrine。 But in truth it might be said that the theory no longer needed a champion。 The researches of Kepler and Galileo had produced a mass of evidence for the Copernican theory which amounted to demonstration。 A generation or two might be required for this evidence to make itself everywhere known among men of science； and of course the ecclesiastical authorities must be expected to stand by their guns for a somewhat longer period。 In point of fact； the ecclesiastical ban was not technically removed by the striking of the Copernican books from the list of the Index Expurgatorius until the year 1822； almost two hundred years after the date of Galileo's dialogue。 But this； of course； is in no sense a guide to the state of general opinion regarding the theory。 We shall gain a true gauge as to this if we assume that the greater number of important thinkers had accepted the heliocentric doctrine before the middle of the seventeenth century； and that before the close of that century the old Ptolemaic idea had been quite abandoned。 A wonderful revolution in man's estimate of the universe had thus been effected within about two centuries after the birth of Copernicus。
V。 GALILEO AND THE NEW PHYSICS After Galileo had felt the strong hand of the Inquisition； in 1632； he was careful to confine his researches； or at least his publications； to topics that seemed free from theological implications。 In doing so he reverted to the field of his earliest studies namely； the field of mechanics； and the Dialoghi delle Nuove Scienze； which he finished in 1636； and which was printed two years later； attained a celebrity no less than that of the heretical dialogue that had preceded it。 The later work was free from all apparent heresies； yet perhaps it did more towards the establishment of the Copernican doctrine； through the teaching of correct mechanical principles； than the other work had accomplished by a more direct method。 Galileo's astronomical discoveries were； as we have seen； in a sense accidental； at least； they received their inception through the inventive genius of another。 His mechanical discoveries； on the other hand； were the natural output of his own creative genius。 At the very beginning of his career； while yet a very young man； though a professor of mathematics at Pisa； he had begun that onslaught upon the old Aristotelian ideas which he was to continue throughout his life。 At the famous leaning tower in Pisa； the young iconoclast performed； in the year 1590； one of the most theatrical demonstrations in the history of science。 Assembling a multitude of champions of the old ideas； he proposed to demonstrate the falsity of the Aristotelian doctrine that the velocity of falling bodies is proportionate to their weight。 There is perhaps no fact more strongly illustrative of the temper of the Middle Ages than the fact that this doctrine； as taught by the Aristotelian philosopher； should so long have gone unchallenged。 Now； however； it was put to the test； Galileo released a half…pound weight and a hundred…pound cannon…ball from near the top of the tower； and； needless to say； they reached the ground together。 Of course； the spectators were but little pleased with what they saw。 They could not doubt the evidence of their own senses as to the particular experiment in question； they could suggest； however； that the experiment involved a violation of the laws of nature through the practice of magic。 To controvert so firmly established an idea savored of heresy。 The young man guilty of such iconoclasm was naturally looked at askance by the scholarship of his time。 Instead of being applauded； he was hissed； and he found it expedient presently to retire from Pisa。 Fortunately； however； the new spirit of progress had made itself felt more effectively in some other portions of Italy； and so Galileo found a refuge and a following in Padua； and afterwards in Florence； and while； as we have seen； he was obliged to curb his enthusiasm regarding the subject that was perhaps nearest his heartthe promulgation of the Copernican theoryyet he was permitted in the main to carry on his experimental observations unrestrained。 These experiments gave him a place of unquestioned authority among his contemporaries； and they have transmitted his name to posterity as that of one of the greatest of experimenters and the virtual founder of modern mechanical science。 The experiments in question range over a wide field； but for the most part they have to do with moving bodies and with questions of force； or； as we should now say； of energy。 The experiment at the leaning tower showed that the velocity of falling bodies is independent of the weight of the bodies； provided the weight is sufficient to overcome the resistance of the atmosphere。 Later experiments with falling bodies led to the discovery of laws regarding the accelerated velocity of fall。 Such velocities were found to bear a simple relation to the period of time from the beginning of the fall。 Other experiments； in which balls were allowed to roll down inclined planes； corroborated the observation that the pull of gravitation gave a velocity proportionate to the length of fall； whether such fall were direct or in a slanting direction。 These studies were associated with observations on projectiles； regarding which Galileo was the first to entertain correct notions。 According to the current idea； a projectile fired； for example； from a cannon； moved in a straight horizontal line until the propulsive force was exhausted； and then fell to the ground in a perpendicular line。 Galileo taught that the projectile begins to fall at once on leaving the mouth of the cannon and traverses a parabolic course。 According to his idea； which is now familiar to every one； a cannon…ball dropped from the level of the cannon's muzzle will stri