The Project Gutenberg EBook of Pioneers of Science, by Oliver Lodge This eBook is for the use of anyone anywhere at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this eBook or online at www.gutenberg.org Title: Pioneers of Science Author: Oliver Lodge Release Date: April 26, 2009 [EBook #28613] Language: English Character set encoding: ISO-8859-1 *** START OF THIS PROJECT GUTENBERG EBOOK PIONEERS OF SCIENCE *** Produced by Audrey Longhurst, Greg Bergquist and the Online Distributed Proofreading Team at http://www.pgdp.net (This file was produced from images generously made available by The Internet Archive/American Libraries.) Transcriber’s Note The punctuation and spelling from the original text have been faithfully preserved. Only obvious typographical errors have been corrected. This text contains a few phrases in Greek, with English transliterations given as mouse hover pop-ups: φενόμενα Your browser should be set to read the UTF-8 character set. PIONEERS OF SCIENCE NEWTON NEWTON From the picture by Kneller, 1689, now at Cambridge PIONEERS OF SCIENCE BY OLIVER LODGE, F.R.S. PROFESSOR OF PHYSICS IN VICTORIA UNIVERSITY COLLEGE, LIVERPOOL WITH PORTRAITS AND OTHER ILLUSTRATIONS London MACMILLAN AND CO. AND NEW YORK 1893 Richard Clay and Sons, Limited, LONDON AND BUNGAY. PREFACE This book takes its origin in a course of lectures on the history and progress of Astronomy arranged for me in the year 1887 by three of my colleagues (A.C.B., J.M., G.H.R.), one of whom gave the course its name. The lectures having been found interesting, it was natural to write them out in full and publish. If I may claim for them any merit, I should say it consists in their simple statement and explanation of scientific facts and laws. The biographical details are compiled from all readily available sources, there is no novelty or originality about them; though it is hoped that there may be some vividness. I have simply tried to present a living figure of each Pioneer in turn, and to trace his influence on the progress of thought. I am indebted to many biographers and writers, among others to Mr. E.J.C. Morton, whose excellent set of lives published by the S.P.C.K. saved me much trouble in the early part of the course. As we approach recent times the subject grows more complex, and the men more nearly contemporaries; hence the biographical aspect diminishes and the scientific treatment becomes fuller, but in no case has it been allowed to become technical and generally unreadable. To the friends (C.C.C., F.W.H.M., E.F.R.) who with great kindness have revised the proofs, and have indicated places where the facts could be made more readily intelligible by a clearer statement, I express my genuine gratitude. University College, Liverpool, November, 1892. CONTENTS PART I LECTURE I PAGE COPERNICUS AND THE MOTION OF THE EARTH 2 LECTURE II TYCHO BRAHÉ AND THE EARLIEST OBSERVATORY 32 LECTURE III KEPLER AND THE LAWS OF PLANETARY MOTION 56 LECTURE IV GALILEO AND THE INVENTION OF THE TELESCOPE 80 LECTURE V GALILEO AND THE INQUISITION 108 LECTURE VI DESCARTES AND HIS THEORY OF VORTICES 136 LECTURE VII SIR ISAAC NEWTON 159 LECTURE VIII NEWTON AND THE LAW OF GRAVITATION 180 LECTURE IX NEWTON'S "PRINCIPIA" 203 PART II LECTURE X ROEMER AND BRADLEY AND THE VELOCITY OF LIGHT 232 LECTURE XI LAGRANGE AND LAPLACE—THE STABILITY OF THE SOLAR SYSTEM, AND THE NEBULAR HYPOTHESIS 254 LECTURE XII HERSCHEL AND THE MOTION OF THE FIXED STARS 273 LECTURE XIII THE DISCOVERY OF THE ASTEROIDS 294 LECTURE XIV BESSEL—THE DISTANCES OF THE STARS, AND THE DISCOVERY OF STELLAR PLANETS 304 LECTURE XV THE DISCOVERY OF NEPTUNE 317 LECTURE XVI COMETS AND METEORS 331 LECTURE XVII THE TIDES 353 LECTURE XVIII THE TIDES, AND PLANETARY EVOLUTION 379 ILLUSTRATIONS FIG. PAGE 1. Archimedes 8 2. Leonardo da Vinci 10 3. Copernicus 12 4. Homeric Cosmogony 15 5. Egyptian Symbol of the Universe 16 6. Hindoo Earth 17 7. Order of ancient Planets corresponding to the Days of the Week 19 8. Ptolemaic System 20 9. Specimens of Apparent Paths of Venus and of Mars among the stars 21 10. Apparent Epicyclic Orbits of Jupiter and Saturn 22 11. Egyptian System 24 12. True Orbits of Earth and Jupiter 25 13. Orbits of Mercury and Earth 25 14. Copernican System as frequently represented 26 15. Slow Movement of the North Pole in a Circle among the Stars 29 16. Tychonic system, showing the Sun with all the Planets revolving round the Earth 38 17. Portrait of Tycho 41 18. Early out-door Quadrant of Tycho 43 19. Map of Denmark, showing the Island of Huen 45 20. Uraniburg 46 21. Astrolabe 47 22. Tycho's large Sextant 48 23. The Quadrant in Uraniburg 49 24. Tycho's Form of Transit Circle 50 25. A Modern Transit Circle 51 26. Orbits of some of the Planets drawn to scale 60 27. Many-sided Polygon or Approximate Circle enveloped by Straight Lines 61 28. Kepler's Idea of the Regular Solids 62 29. Diagram of Equant 67 30. Excentric Circle supposed to be divided into equal Areas 68 31. Mode of drawing an Ellipse 70 32. Kepler's Diagram proving Equable Description of Areas for an Ellipse 71 33. Diagram of a Planet's Velocity in Different Parts of its Orbit 72 34. Portrait of Kepler 76 35. Curve described by a Projectile 82 36. Two Forms of Pulsilogy 87 37. Tower of Pisa 91 38. View of the Half-Moon in small Telescope 97 39. Portion of the Lunar Surface more highly magnified 98 40. Another Portion of the Lunar Surface 99 41. Lunar Landscape showing Earth 100 42. Galileo's Method of estimating the Height of Lunar Mountain 101 43. Some Clusters and Nebulæ 102 44. Stages of the Discovery of Jupiter's Satellites 103 45. Eclipses of Jupiter's Satellites 105 46. Old Drawings of Saturn by Different Observers, with the imperfect Instruments of that day 111 47. Phases of Venus 112 48. Sunspots as seen with Low Power 113 49. A Portion of the Sun's Disk as seen in a powerful modern Telescope 114 50. Saturn and his Rings 115 51. Map of Italy 118 52. Portrait of Galileo 126 53. Portrait of Descartes 148 54. Descartes's Eye Diagram 151 55. Descartes's Diagram of Vortices from his "Principia" 152 56. Manor-house of Woolsthorpe 162 57. Projectile Diagram 170 58. } { 171 59. Diagrams illustrative of those near the Beginning of Newton's "Principia" 174 60. 175 61- 2. 175 63. Prismatic Dispersion 182 64. A single Constituent of White Light is capable of no more Dispersion 183 65. Parallel Beam passing through a Lens 184 66. Newton's Telescope 186 67. The Sextant, as now made 187 68. Newton when young 196 69. Sir Isaac Newton 200 70. Another "Principia" Diagram 207 71. Well-known Model exhibiting the Oblate Spheroidal Form as a Consequence of spinning about a Central Axis 219 72. Jupiter 221 73. Diagram of Eye looking at a Light reflected in a Distant Mirror through the Teeth of a revolving Wheel 238 74. Fizeau's Wheel, showing the appearance of distant Image seen through its Teeth 239 75. Eclipses of one of Jupiter's Satellites 241 76. A Transit instrument for the British Astronomical Expedition, 1874 243 77. Diagram of equatorially mounted Telescope 245 78. Aberration Diagram 250 79. Showing the three Conjunction Places in the Orbits of Jupiter and Saturn 259 80. Lord Rosse's Drawing of the Spiral Nebula in Canes Venatici 269 81. Saturn 271 82. Principle of Newtonian Reflector 278 83. Herschel's 40-foot telescope 283 84. William Herschel 285 85. Caroline Herschel 287 86. Double Stars 288 87. Old Drawing of the Cluster in Hercules 290 88. Old Drawing of the Andromeda Nebula 291 89. The Great Nebula in Orion 292 90. Planetary Orbits to scale 297 91. Diagram illustrating Parallax 307 92. The Königsberg Heliometer 312 93. Perturbations of Uranus 320 94. Uranus' and Neptune's Relative Positions 325 95. Meteorite 333 96. Meteor Stream crossing Field of Telescope 334 97. Diagram of Direction of Earth's Orbital Motion 335 98. Parabolic and Elliptic Orbits 340 99. Orbit of Halley's Comet 341 100. Various Appearances of Halley's Comet when last seen 342 101. Head of Donati's Comet of 1858 343 102. Comet 344 103. Encke's Comet 345 104. Biela's Comet as last seen in two Portions 346 105. Radiant Point Perspective 348 106. Present Orbit of November Meteors 349 107. Orbit of November Meteors before and after Encounter with Uranus 351 108. The Mersey 355 109. Co-tidal Lines, showing the way the Tidal Wave reaches the British Isles from the Atlantic 359 110. Whirling Earth Model 364 111. Earth and Moon Model 365 112. Earth and Moon (Earth's Rotation Neglected) 366 113. Maps showing how comparatively Free from Land Obstruction the Ocean in the Southern Hemisphere Is 369 114. Spring and Neap Tides 370 115. Tidal Clock 371 116. Sir William Thomson (Lord Kelvin) 373 117. Tide-gauge for recording Local Tides 375 118. Harmonic Analyzer 375 119. Tide-predicter 376 120. Weekly Sheet of Curves 377 PIONEERS OF SCIENCE PART I FROM DUSK TO DAYLIGHT DATES AND SUMMARY OF FACTS FOR LECTURE I Physical Science of the Ancients. Thales 640 b.c., Anaximander 610 b.c., Pythagoras 600 b.c., Anaxagoras 500 b.c., Eudoxus 400 b.c., Aristotle 384 b.c., Aristarchus 300 b.c., Archimedes 287 b.c., Eratosthenes 276 b.c., Hipparchus 160 b.c., Ptolemy 100 a.d. Science of the Middle Ages. Cultivated only among the Arabs; largely in the forms of astrology, alchemy, and algebra. Return of Science to Europe. Roger Bacon 1240, Leonardo da Vinci 1480, (Printing 1455), Columbus 1492, Copernicus 1543. A sketch of Copernik's life and work. Born 1473 at Thorn in Poland. Studied mathematics at Bologna. Became an ecclesiastic. Lived at Frauenburg near mouth of Vistula. Substituted for the apparent motion of the heavens the real motion of the earth. Published tables of planetary motions. Motion still supposed to be in epicycles. Worked out his ideas for 36 years, and finally dedicated his work to the Pope. Died just as his book was printed, aged 72, a century before the birth of Newton. A colossal statue by Thorwaldsen erected at Warsaw in 1830. [Pg 3] [Pg 4] PIONEERS OF SCIENCE LECTURE I COPERNICUS AND THE MOTION OF THE EARTH The ordinary run of men live among phenomena of which they know nothing and care less. They see bodies fall to the earth, they hear sounds, they kindle fires, they see the heavens roll above them, but of the causes and inner working of the whole they are ignorant, and with their ignorance they are content. "Understand the structure of a soap-bubble?" said a cultivated literary man whom I know; "I wouldn't cross the street to know it!" And if this is a prevalent attitude now, what must have been the attitude in ancient times, when mankind was emerging from savagery, and when history seems composed of harassments by wars abroad and revolutions at home? In the most violently disturbed times indeed, those with which ordinary history is mainly occupied, science is quite impossible. It needs as its condition, in order to flourish, a fairly quiet, untroubled state, or else a cloister or university removed from the din and bustle of the political and commercial world. In such places it has taken its rise, and in such peaceful places and quiet times true science will continue to be cultivated. The great bulk of mankind must always remain, I suppose, more or less careless of scientific research and scientific result, except in so far as it affects their modes of locomotion, their health and pleasure, or their purse. But among a people hurried and busy and preoccupied, some in the pursuit of riches, some in the pursuit of pleasure, and some, the majority, in the struggle for existence, there arise in every generation, here and there, one or two great souls—men who seem of another age and country, who look upon the bustle and feverish activity and are not infected by it, who watch others achieving prizes of riches and pleasure and are not disturbed, who look on the world and the universe they are born in with quite other eyes. To them it appears not as a bazaar to buy and to sell in; not as a ladder to scramble up (or down) helter-skelter without knowing whither or why; but as a fact—a great and mysterious fact—to be pondered over, studied, and perchance in some small measure understood. By the multitude these men were sneered at as eccentric or feared as supernatural. Their calm, clear, contemplative attitude seemed either insane or diabolic; and accordingly they have been pitied as enthusiasts or killed as blasphemers. One of these great souls may have been a prophet or preacher, and have called to his generation to bethink them of why and what they were, to struggle less and meditate more, to search for things of true value and not for dross. Another has been a poet or musician, and has uttered in words or in song thoughts dimly possible to many men, but by them unutterable and left inarticulate. Another has been influenced still more directly by the universe around him, has felt at times overpowered by the mystery and solemnity of it all, and has been impelled by a force stronger than himself to study it, patiently, slowly, diligently; content if he could gather a few crumbs of the great harvest of knowledge, happy if he could grasp some great generalization or wide-embracing law, and so in some small measure enter into the mind and thought of the Designer of all this wondrous frame of things. These last have been the men of science, the great and heaven-born men of science; and they are few. In our own day, amid the throng of inventions, there are a multitude of small men using the name of science but working for their own ends, jostling and scrambling just as they would jostle and scramble in any other trade or profession. These may be workers, they may and do advance knowledge, but they are never pioneers. Not to them is it given to open out great tracts of unexplored territory, or to view the promised land as from a mountain-top. Of them we shall not speak; we will concern ourselves only with the greatest, the epoch-making men, to whose life and work we and all who come after them owe so much. Such a man was Thales. Such was Archimedes, Hipparchus, Copernicus. Such pre-eminently was Newton. Now I am not going to attempt a history of science. Such a work in ten lectures would be absurd. I intend to pick out a few salient names here and there, and to study these in some detail, rather than by attempting to deal with too many to lose individuality and distinctness. We know so little of the great names of antiquity, that they are for this purpose scarcely suitable. In some departments the science of the Greeks was remarkable, though it is completely overshadowed by their philosophy; yet it was largely based on what has proved to be a wrong method of procedure, viz the introspective and conjectural, rather than the inductive and experimental methods. They investigated Nature by studying their own minds, by considering the meanings of words, rather than by studying things and recording phenomena. This wrong (though by no means, on the face of it, absurd) method was not pursued exclusively, else would their science have been valueless, but the influence it had was such as materially to detract from the value of their speculations and discoveries. For when truth and falsehood are inextricably woven into a statement, the truth is as hopelessly hidden as if it had never been stated, for [Pg 5] [Pg 6] [Pg 7] [Pg 8] we have no criterion to distinguish the false from the true. Fig. 1. Fig. 1.—Archimedes. Besides this, however, many of their discoveries were ultimately lost to the world, some, as at Alexandria, by fire— the bigoted work of a Mohammedan conqueror—some by irruption of barbarians; and all were buried so long and so completely by the night of the dark ages, that they had to be rediscovered almost as absolutely and completely as though they had never been. Some of the names of antiquity we shall have occasion to refer to; so I have arranged some of them in chronological order on page 4, and as a representative one I may specially emphasize Archimedes, one of the greatest men of science there has ever been, and the father of physics. The only effective link between the old and the new science is afforded by the Arabs. The dark ages come as an utter gap in the scientific history of Europe, and for more than a thousand years there was not a scientific man of note except in Arabia; and with the Arabs knowledge was so mixed up with magic and enchantment that one cannot contemplate it with any degree of satisfaction, and little real progress was made. In some of the Waverley Novels you can realize the state of matters in these times; and you know how the only approach to science is through some Arab sorcerer or astrologer, maintained usually by a monarch, and consulted upon all great occasions, as the oracles were of old. In the thirteenth century, however, a really great scientific man appeared, who may be said to herald the dawn of modern science in Europe. This man was Roger Bacon. He cannot be said to do more than herald it, however, for we must wait two hundred years for the next name of great magnitude; moreover he was isolated, and so far in advance of his time that he left no followers. His own work suffered from the prevailing ignorance, for he was persecuted and imprisoned, not for the commonplace and natural reason that he frightened the Church, but merely because he was eccentric in his habits and knew too much. The man I spoke of as coming two hundred years later is Leonardo da Vinci. True he is best known as an artist, but if you read his works you will come to the conclusion that he was the most scientific artist who ever lived. He teaches the laws of perspective (then new), of light and shade, of colour, of the equilibrium of bodies, and of a multitude of other matters where science touches on art—not always quite correctly according to modern ideas, but in beautiful and precise language. For clear and conscious power, for wide-embracing knowledge and skill, Leonardo is one of the most remarkable men that ever lived. About this time the tremendous invention of printing was achieved, and Columbus unwittingly discovered the New World. The middle of the next century must be taken as the real dawn of modern science; for the year 1543 marks the publication of the life-work of Copernicus. [Pg 9] [Pg 10] Fig. 2. Fig. 2.—Leonardo da Vinci. Nicolas Copernik was his proper name. Copernicus is merely the Latinized form of it, according to the then prevailing fashion. He was born at Thorn, in Polish Prussia, in 1473. His father is believed to have been a German. He graduated at Cracow as doctor in arts and medicine, and was destined for the ecclesiastical profession. The details of his life are few; it seems to have been quiet and uneventful, and we know very little about it. He was instructed in astronomy at Cracow, and learnt mathematics at Bologna. Thence he went to Rome, where he was made Professor of Mathematics; and soon afterwards he went into orders. On his return home, he took charge of the principal church in his native place, and became a canon. At Frauenburg, near the mouth of the Vistula, he lived the remainder of his life. We find him reporting on coinage for the Government, but otherwise he does not appear as having entered into the life of the times. He was a quiet, scholarly monk of studious habits, and with a reputation which drew to him several earnest students, who received vivâ voce instruction from him; so, in study and meditation, his life passed. He compiled tables of the planetary motions which were far more correct than any which had hitherto appeared, and which remained serviceable for long afterwards. The Ptolemaic system of the heavens, which had been the orthodox system all through the Christian era, he endeavoured to improve and simplify by the hypothesis that the sun was the centre of the system instead of the earth; and the first consequences of this change he worked out for many years, producing in the end a great book: his one life-work. This famous work, "De Revolutionibus Orbium Cœlestium," embodied all his painstaking calculations, applied his new system to each of the bodies in the solar system in succession, and treated besides of much other recondite matter. Towards the close of his life it was put into type. He can scarcely be said to have lived to see it appear, for he was stricken with paralysis before its completion; but a printed copy was brought to his bedside and put into his hands, so that he might just feel it before he died. [Pg 11] [Pg 12] Fig. 3. Fig. 3.—Copernicus. That Copernicus was a giant in intellect or power—such as had lived in the past, and were destined to live in the near future—I see no reason whatever to believe. He was just a quiet, earnest, patient, and God-fearing man, a deep student, an unbiassed thinker, although with no specially brilliant or striking gifts; yet to him it was given to effect such a revolution in the whole course of man's thoughts as is difficult to parallel. You know what the outcome of his work was. It proved—he did not merely speculate, he proved—that the earth is a planet like the others, and that it revolves round the sun. Yes, it can be summed up in a sentence, but what a revelation it contains. If you have never made an effort to grasp the full significance of this discovery you will not appreciate it. The doctrine is very familiar to us now, we have heard it, I suppose, since we were four years old, but can you realize it? I know it was a long time before I could. Think of the solid earth, with trees and houses, cities and countries, mountains and seas—think of the vast tracts of land in Asia, Africa, and America—and then picture the whole mass spinning like a top, and rushing along its annual course round the sun at the rate of nineteen miles every second. Were we not accustomed to it, the idea would be staggering. No wonder it was received with incredulity. But the difficulties of the conception are not only physical, they are still more felt from the speculative and theological points of view. With this last, indeed, the reconcilement cannot be considered complete even yet. Theologians do not, indeed, now deny the fact of the earth's subordination in the scheme of the universe, but many of them ignore it and pass it by. So soon as the Church awoke to a perception of the tremendous and revolutionary import of the new doctrines, it was bound to resist them or be false to its traditions. For the whole tenor of men's thought must have been changed had they accepted it. If the earth were not the central and all-important body in the universe, if the sun and planets and stars were not attendant and subsidiary lights, but were other worlds larger and perhaps superior to ours, where was man's place in the universe? and where were the doctrines they had maintained as irrefragable? I by no means assert that the new doctrines were really utterly irreconcilable with the more essential parts of the old dogmas, if only theologians had had patience and genius enough to consider the matter calmly. I suppose that in that case they might have reached the amount of reconciliation at present attained, and not only have left scientific truth in peace to spread as it could, but might perhaps themselves have joined the band of earnest students and workers, as so many of the higher Catholic clergy do at the present day. But this was too much to expect. Such a revelation was not to be accepted in a day or in a century—the easiest plan was to treat it as a heresy, and try to crush it out. Not in Copernik's life, however, did they perceive the dangerous tendency of the doctrine—partly because it was buried in a ponderous and learned treatise not likely to be easily understood; partly, perhaps, because its propounder was himself an ecclesiastic; mainly because he was a patient and judicious man, not given to loud or intolerant assertion, but content to state his views in quiet conversation, and to let them gently spread for thirty years before he published them. And, when he did publish them, he used the happy device of dedicating his great book to the Pope, and a cardinal bore the expense of printing it. Thus did the Roman Church stand sponsor to a system of truth against which it was destined in the next century to hurl its anathemas, and to inflict on its conspicuous adherents torture, imprisonment, and death. [Pg 13] [Pg 14] To realize the change of thought, the utterly new view of the universe, which the Copernican theory introduced, we must go back to preceding ages, and try to recall the views which had been held as probable concerning the form of the earth and the motion of the heavenly bodies. Fig. 4. Fig. 4.—Homeric Cosmogony. The earliest recorded notion of the earth is the very natural one that it is a flat area floating in an illimitable ocean. The sun was a god who drove his chariot across the heavens once a day; and Anaxagoras was threatened with death and punished with banishment for teaching that the sun was only a ball of fire, and that it might perhaps be as big as the country of Greece. The obvious difficulty as to how the sun got back to the east again every morning was got over—not by the conjecture that he went back in the dark, nor by the idea that there was a fresh sun every day; though, indeed, it was once believed that the moon was created once a month, and periodically cut up into stars—but by the doctrine that in the northern part of the earth was a high range of mountains, and that the sun travelled round on the surface of the sea behind these. Sometimes, indeed, you find a representation of the sun being rowed round in a boat. Later on it was perceived to be necessary that the sun should be able to travel beneath the earth, and so the earth was supposed to be supported on pillars or on roots, or to be a dome-shaped body floating in air—much like Dean Swift's island of Laputa. The elephant and tortoise of the Hindu earth are, no doubt, emblematic or typical, not literal. Fig. 5. Fig. 5.—Egyptian Symbol of the Universe. The earth a figure with leaves, the heaven a figure with stars, the principle of equilibrium and support, the boats of the rising and setting sun. Aristotle, however, taught that the earth must be a sphere, and used all the orthodox arguments of the present children's geography-books about the way you see ships at sea, and about lunar eclipses. To imagine a possible antipodes must, however, have been a tremendous difficulty in the way of this conception of a sphere, and I scarcely suppose that any one can at that time have contemplated the possibility of such upside-down regions being inhabited. I find that intelligent children invariably feel the greatest difficulty in realizing the existence of inhabitants on the opposite side of the earth. Stupid children, like stupid persons in general, will of course believe anything they are told, and much good may the belief do them; but the kind of difficulties felt by intelligent and thoughtful children are most instructive, since it is quite certain that the early philosophers must have encountered and overcome those very same difficulties by their own genius. [Pg 15] [Pg 16] [Pg 17] Fig. 6. Fig. 6.—Hindoo Earth. However, somehow or other the conception of a spherical earth was gradually grasped, and the heavenly bodies were perceived all to revolve round it: some moving regularly, as the stars, all fixed together into one spherical shell or firmament; some moving irregularly and apparently anomalously—these irregular bodies were therefore called planets [or wanderers]. Seven of them were known, viz. Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn, and there is little doubt that this number seven, so suggested, is the origin of the seven days of the week. The above order of the ancient planets is that of their supposed distance from the earth. Not always, however, are they thus quoted by the ancients: sometimes the sun is supposed nearer than Mercury or Venus. It has always been known that the moon was the nearest of the heavenly bodies; and some rough notion of its distance was current. Mars, Jupiter, and Saturn were placed in that order because that is the order of their apparent motions, and it was natural to suppose that the slowest moving bodies were the furthest off. The order of the days of the week shows what astrologers considered to be the order of the planets; on their system of each successive hour of the day being ruled over by the successive planets taken in order. The diagram (fig. 7) shows that if the Sun rule the first hour of a certain day (thereby giving its name to the day) Venus will rule the second hour, Mercury the third, and so on; the Sun will thus be found to rule the eighth, fifteenth, and twenty-second hour of that day, Venus the twenty-third, and Mercury the twenty- fourth hour; so the Moon will rule the first hour of the next day, which will therefore be Monday. On the same principle (numbering round the hours successively, with the arrows) the first hour of the next day will be found to be ruled by Mars, or by the Saxon deity corresponding thereto; the first hour of the day after, by Mercury (Mercredi), and so on (following the straight lines of the pattern). The order of the planets round the circle counter-clockwise, i.e. the direction of their proper motions, is that quoted above in the text. To explain the motion of the planets and reduce them to any sort of law was a work of tremendous difficulty. The greatest astronomer of ancient times was Hipparchus, and to him the system known as the Ptolemaic system is no doubt largely due. But it was delivered to the world mainly by Ptolemy, and goes by his name. This was a fine piece of work, and a great advance on anything that had gone before; for although it is of course saturated with error, still it is based on a large substratum of truth. Its superiority to all the previously mentioned systems is obvious. And it really did in its more developed form describe the observed motions of the planets. Each planet was, in the early stages of this system, as taught, say, by Eudoxus, supposed to be set in a crystal sphere, which revolved so as to carry the planet with it. The sphere had to be of crystal to account for the visibility of other planets and the stars through it. Outside the seven planetary spheres, arranged one inside the other, was a still larger one in which were set the stars. This was believed to turn all the others, and was called the primum mobile. The whole system was supposed to produce, in its revolution, for the few privileged to hear the music of the spheres, a sound as of some magnificent harmony. [Pg 18] [Pg 19] Fig. 7. Fig. 7.—Order of ancient planets corresponding to the days of the week. The enthusiastic disciples of Pythagoras believed that their master was privileged to hear this noble chant; and far be it from us to doubt that the rapt and absorbing pleasure of contemplating the harmony of nature, to a man so eminently great as Pythagoras, must be truly and adequately represented by some such poetic conception. Fig. 8. Fig. 8.—Ptolemaic system. The precise kind of motion supposed to be communicated from the primum mobile to the other spheres so as to produce the observed motions of the planets was modified and improved by various philosophers until it developed into the epicyclic train of Hipparchus and of Ptolemy. It is very instructive to observe a planet (say Mars or Jupiter) night after night and plot down its place with reference to the fixed stars on a celestial globe or star-map. Or, instead of direct observation by alignment with known stars, it is easier to look out its right ascension and declination in Whitaker's Almanac, and plot those down. If this be done for a year or two, it will be found that the motion of the planet is by no means regular, but that though on the whole it advances it sometimes is stationary and sometimes goes back.[1] [Pg 20] [Pg 21]