however； that Da Vinci understood the principle of this mechanism； for he describes how such a camera can be made by cutting a small； round hole through the shutter of a darkened room； the reversed image of objects outside being shown on the opposite wall。 Like other philosophers in all ages； he had observed a great number of facts which he was unable to explain correctly。 But such accumulations of scientific observations are always interesting； as showing how many centuries of observation frequently precede correct explanation。 He observed many facts about sounds； among others that blows struck upon a bell produced sympathetic sounds in a bell of the same kind； and that striking the string of a lute produced vibration in corresponding strings of lutes strung to the same pitch。 He knew； also； that sounds could be heard at a distance at sea by listening at one end of a tube； the other end of which was placed in the water； and that the same expedient worked successfully on land； the end of the tube being placed against the ground。 The knowledge of this great number of unexplained facts is often interpreted by the admirers of Da Vinci； as showing an almost occult insight into science many centuries in advance of his time。 Such interpretations； however； are illusive。 The observation； for example； that a tube placed against the ground enables one to hear movements on the earth at a distance； is not in itself evidence of anything more than acute scientific observation； as a similar method is in use among almost every race of savages； notably the American Indians。 On the other hand； one is inclined to give credence to almost any story of the breadth of knowledge of the man who came so near anticipating Hutton； Lyell； and Darwin in his interpretation of the geological records as he found them written on the rocks。 It is in this field of geology that Leonardo is entitled to the greatest admiration by modern scientists。 He had observed the deposit of fossil shells in various strata of rocks； even on the tops of mountains； and he rejected once for all the theory that they had been deposited there by the Deluge。 He rightly interpreted their presence as evidence that they had once been deposited at the bottom of the sea。 This process he assumed bad taken hundreds and thousands of centuries； thus tacitly rejecting the biblical tradition as to the date of the creation。 Notwithstanding the obvious interest that attaches to the investigations of Leonardo； it must be admitted that his work in science remained almost as infertile as that of his great precursor； Bacon。 The really stimulative work of this generation was done by a man of affairs； who knew little of theoretical science except in one line； but who pursued that one practical line until he achieved a wonderful result。 This man was Christopher Columbus。 It is not necessary here to tell the trite story of his accomplishment。 Suffice it that his practical demonstration of the rotundity of the earth is regarded by most modern writers as marking an epoch in history。 With the year of his voyage the epoch of the Middle Ages is usually regarded as coming to an end。 It must not be supposed that any very sudden change came over the aspect of scholarship of the time； but the preliminaries of great things had been achieved； and when Columbus made his famous voyage in 1492； the man was already alive who was to bring forward the first great vitalizing thought in the field of pure science that the Western world had originated for more than a thousand years。 This man bore the name of Kopernik； or in its familiar Anglicized form； Copernicus。 His life work and that of his disciples will claim our attention in the succeeding chapter。
IV。 THE NEW COSMOLOGYCOPERNICUS TO KEPLER AND GALILEO We have seen that the Ptolemaic astronomy； which was the accepted doctrine throughout the Middle Ages； taught that the earth is round。 Doubtless there was a popular opinion current which regarded the earth as flat； but it must be understood that this opinion had no champions among men of science during the Middle Ages。 When； in the year 1492； Columbus sailed out to the west on his memorable voyage； his expectation of reaching India had full scientific warrant； however much it may have been scouted by certain ecclesiastics and by the average man of the period。 Nevertheless； we may well suppose that the successful voyage of Columbus； and the still more demonstrative one made about thirty years later by Magellan； gave the theory of the earth's rotundity a certainty it could never previously have had。 Alexandrian geographers had measured the size of the earth； and had not hesitated to assert that by sailing westward one might reach India。 But there is a wide gap between theory and practice； and it required the voyages of Columbus and his successors to bridge that gap。 After the companions of Magellan completed the circumnavigation of the globe； the general shape of our earth would； obviously； never again be called in question。 But demonstration of the sphericity of the earth had； of course； no direct bearing upon the question of the earth's position in the universe。 Therefore the voyage of Magellan served to fortify； rather than to dispute； the Ptolemaic theory。 According to that theory； as we have seen； the earth was supposed to lie immovable at the centre of the universe； the various heavenly bodies； including the sun； revolving about it in eccentric circles。 We have seen that several of the ancient Greeks； notably Aristarchus； disputed this conception； declaring for the central position of the sun in the universe； and the motion of the earth and other planets about that body。 But this revolutionary theory seemed so opposed to the ordinary observation that； having been discountenanced by Hipparchus and Ptolemy； it did not find a single important champion for more than a thousand years after the time of the last great Alexandrian astronomer。 The first man； seemingly； to hark back to the Aristarchian conception in the new scientific era that was now dawning was the noted cardinal； Nikolaus of Cusa； who lived in the first half of the fifteenth century； and was distinguished as a philosophical writer and mathematician。 His De Docta Ignorantia expressly propounds the doctrine of the earth's motion。 No one； however； paid the slightest attention to his suggestion； which； therefore； merely serves to furnish us with another interesting illustration of the futility of propounding even a correct hypothesis before the time is ripe to receive itparticularly if the hypothesis is not fully fortified by reasoning based on experiment or observation。 The man who was destined to put forward the theory of the earth's motion in a way to command attention was born in 1473； at the village of Thorn； in eastern Prussia。 His name was Nicholas Copernicus。 There is no more famous name in the entire annals of science than this； yet posterity has never been able fully to establish the lineage of the famous expositor of the true doctrine of the solar system。 The city of Thorn lies in a province of that border territory which was then under control of Poland； but which subsequently became a part of Prussia。 It is claimed that the aspects of the city were essentially German； and it is admitted that the mother of Copernicus belonged to that race。 The nationality of the father is more in doubt； but it is urged that Copernicus used German as his mother…tongue。 His great work was； of course； written in Latin； according to the custom of the time； but it is said that； when not employing that language； he always wrote in German。 The disputed nationality of Copernicus strongly suggests that he came of a mixed racial lineage； and we are reminded again of the influences of those ethnical minglings to which we have previously more than once referred。 The acknowledged centres of civilization towards the close of the fifteenth century were Italy and Spain。 Therefore； the birthplace of Copernicus lay almost at the confines of civilization； reminding us of that earlier period when Greece was the centre of culture； but when the great Greek thinkers were born in Asia Minor and in Italy。 As a young man； Copernicus made his way to Vienna to study medicine； and subsequently he journeyed into Italy and remained there many years； About the year 1500 he held the chair of mathematics in a college at Rome。 Subsequently he returned to his native land and passed his remaining years there； dying at Domkerr； in Frauenburg； East Prussia； in the year 1543。 It would appear that Copernicus conceived the idea of the heliocentric system of the universe while he was a comparatively young man； since in the introduction to his great work； which he addressed to Pope Paul III。； he states that he has pondered his system not merely nine years； in accordance with the maxim of Horace； but well into the fourth period of nine years。 Throughout a considerable portion of this period the great work of Copernicus was in manuscript； but it was not published until the year of his death。 The reasons for the delay are not very fully established。 Copernicus undoubtedly taught his system throughout the later decades of his life。 He himself tells us that he had even questioned whether it were not better for him to confin