GIFT OF 
 MICHAEL REESE 
 
THE 
 
 STORY OF THE HEAVENS, 
 
 
 
 
FRONTISPIECE. 
 
 THE PLANET SATURN, 
 IN 1872. 
 
THE 
 
 STORY OF THE HEAVENS. 
 
 SIR ROBERT STAWELL BALL, LL.D., 
 
 FELLOW OF TUE ROYAL SOCIKTY OF LONDON, FELLOW OF THE ROYAL ASTRONOMICAL SOCIFTY, 
 
 HONORARY MEMBER OF THE CAMBRIDGE PHILOSOPHICAL SOCIETY, VICE-PRESIDENT 
 
 OF THE ROYAL IRISH ACADEMY, SCIENTIFIC ADVISER TO THE COMMISSIONERS OF 
 
 IRISH LIGHTS, ANDREWS PROFESSOR OF ASTRONOMY IN THE UNIVERSITY 
 
 OF DUBLIN, AND ROYAL ASTRONOMER OF IRELAND. 
 
 Coloured Jplatcc anU numerous ^lustrations, 
 
 SECOND EblTiQN? 
 
 CASSELL & COMPANY, LIMITED 
 
 LONDON, PARIS, NEW YORK & MELBOURNE. 
 
 1886. 
 [ALL RIGHTS RESERVED.] 
 
PREFACE. 
 
 I HAVE to acknowledge the kind aid which I have 
 received in the preparation of this book. 
 
 Mr. Nasmyth has permitted me to use some of the 
 beautiful drawings of the Moon, which have appeared 
 in the well-known work published by him in conjunc- 
 tion with Mr. Carpenter. To this source I am indebted 
 for Plates VII., VIII., IX., X., and Figs. 26, 27, 28. 
 
 Professor Pickering has allowed me to copy some of 
 the drawings made at Harvard College Observatory by 
 Mr. Trouvelot, and I have availed myself of his kind- 
 ness for Plates I., IV., XI., XIL, XV. 
 
 I am indebted to Professor Langley for Plate II., 
 to Mr. De la Eue for Plates III., and XIV., to Dr. 
 Huggins for Fig. 16, to Professor C. Piazzi Smyth for 
 Fig. 89, to Mr. Chambers for Fig. 6, which has been 
 borrowed from his " Handbook of Descriptive Astronomy," 
 to Dr. Stoney for Fig. 65, to Mr. Grubb for Fig: 4, and 
 to Dr. Copeland and Dr. Dreyer for Fig. 59. I have 
 to acknowledge the valuable assistance derived from Pro- 
 fessor Newcomb's " Popular Astronomy," and Professor 
 Young's " Sun." 
 
 I have also to thank Dr. Copeland and Mr. Steele 
 for their kindness in reading through the entire proofs ; 
 while I have also occasionally availed myself of the help 
 of Mr. Cathcart. ^^ g 
 
 OBSERVATORY, DUNSINK, Co. DUBLIN. 
 May, 1886. 
 
TABLE OF CONTENTS. 
 
 PAGE 
 
 INTRODUCTION . " '. 1 
 
 CHAPTER t. 
 THE ASTRONOMICAL OBSERVATORY. 
 
 Early Astronomical Observations The Observatory of Tycho TVahe The 
 
 Pupil of the Eye Vision of Faint Objects The Telescope The Object- 
 GlassAdvantages of Large Telescopes The Equatorial The Obser- 
 vatoryThe Power of a Telescope Reflecting Telescopes Lord Rosse's 
 Great Reflector at Parsonstown How the mighty Telescope is used 
 The Instruments of Precision The Meridian Circle The Spider Lines 
 Delicacy of pointing a Telescope The Precautions necessary in making 
 Observations The Ideal Instrument and the Practical one The Elimi- 
 nation of Error The ordinary Opera-Glass as an Astronomical Instru- 
 ment The Great Bear Counting the Stars in the Constellation How 
 to become an Observer . . . ." . * . . . 9 
 
 CHAPTER II. 
 
 THE SUN. 
 
 The vast Size of the Sun Hotter than Fusing Platinum Is the Sun the 
 Source of Heat for the Earth? The Sun is 92,700,000 miles distant- 
 How to realise the Magnitude of this Distance Day and Night 
 Luminous and Non-Luminous Bodies Revolution of the Earth The 
 Planets are Lighted by the Sun Contrast between the Sun and the 
 Stars The Sun a Star The Spots on the Sun Changes in the Form 
 of a Spot They are Depressions on the Surface The Rotation of the 
 Sun on its Axis The Size and "Weight of the Sun Is the Sun a Solid 
 Body? View of a Typical Sun-Spot Periodicity of the Sun-Spots 
 Connection between the Sun-Spots Terrestrial Magnetism The Faculae 
 The Granulated Appearance of the Sun The Prominences surround- 
 ing the Sun Total Eclipse of the Sun Size and Movement of the 
 Prominences Drawings of the Objects, coloured The Corona sur- 
 rounding the Sun The Heat of the Sun 2G 
 
 CHAPTER III. 
 THE MOON. 
 
 The Moon and the Tides The Use of the Moon in Navigation The Changes 
 of the Moon The Moon and the Poets Whence the Light of the Moon ? 
 Sizes of the Earth and the Moon Weight of the Moon Changes in 
 
viii CONTENTS. 
 
 TAG* 
 
 Apparent Size Variations in its Distance Influence of the Earth on the 
 Moon The Path of the Moon Explanation of the Moon's Phases 
 Lunar Eclipses Eclipses of the Sun, how produced Visibility of the 
 Moon in a Total Eclipse How Eclipses are Predicted Uses of the 
 Moon in finding Longitude The Moon not connected with the Weather 
 Topography of the Moon Nasmyth's Drawing of Triesnecker 
 Volcanoes on the Moon Normal Lunar Crater Plato The Shadows of 
 Lunar Mountains The Micrometer Lunar Heights Former Activity 
 on the Moon Nasmyth's View of the Formation of Craters Gravitation 
 on the Moon^Varied Sizes of the Lunar Craters Other features of the 
 Moon Is there Life on the Moon ? Absence of Water and of Air 
 - Explanation of the Rugged Character of Lunar Scenery Possibility of 
 
 Life on Distant Bodies in Space 49 
 
 CHAPTER IV. 
 
 THE SOLAR SYSTEM. 
 
 Exceptional Importance of the Sun and Moon The Course to be pursued 
 The Order of Distance The Neighbouring Orbs How are they to be 
 discriminated ? The Planets Venus and Jupiter attract notice by their 
 brilliancy Sirius not a neighbour The Planets Saturn and Mercury 
 Telescopic Planets The Criterion as to whether a Body is to be ranked as 
 a neighbour Meaning of the word Planet Uranus and Neptune Comets 
 The Planets are Illuminated by the Sun The Stars are not The 
 Earth is really a Planet The four Inner Planets, Mercury, Venus, the 
 Earth, and Mars Velocity of the Earth The Outer Planets, Jupiter, 
 Saturn, Uranus, Neptune Light and Heat received by the Planets from 
 the Sun Comparative Sizes of the Planets The Minor Planets The 
 Planets all Revolve in the same direction The Solar System An Island 
 Group in Space 81 
 
 CHAPTER V. 
 THE LAW OF GRAVITATION. 
 
 Gravitation The Falling of a Stone to the Ground All Bodies fall Equally 
 Sixteen Feet in a Second Is this True at great Heights ? Fall of a 
 Body at a height of a Quarter of a Million Miles How Newton obtained 
 an Answer from the Moon His great Discovery Statement of the Law 
 of Gravitation Illustrations of the Law How is it that all the Bodies 
 in the Universe do not rush together ? The Effect of Motion How a 
 Circular Path can be produced by Attraction General Account of the 
 Moon's Motion Is Gravitation a Force of great intensity ? Two Weights 
 of 50 Ibs. Two Iron Globes, 53 yards in diameter, and a mile apart, 
 attract with a force of 1 Ib. Characteristics of Gravitation Orbits of 
 the Planets not strictly Circles The Discoveries of Kepler Construction 
 of an Ellipse Kepler's First Law Does a Planet move uniformly ? 
 
CONTENTS. ix 
 
 Law of the Changes of Velocity Kepler's Second Law The Relation 
 between the Distances and the Periodic Times Kepler's Third Law- 
 Kepler's Law and the Law of Gravitation Movement in a straight line 
 A Body unacted on by disturbing Forces would move in a straight line 
 with constant Velocity Application to the Earth and the Planets The 
 Law of Gravitation deduced from Kepler's Laws Universal Gravitation 96 
 
 CHAPTER VI. 
 
 THE PLANET OF ROMANCE. 
 
 Outline of the Subject Is Mercury the Planet nearest the Sun ? Transit of 
 an Interior Planet across the Sun Has a Transit of Vulcan ever been 
 seen ? Visibility of Planets during a Total Eclipse of the Sun Professor 
 Watson's Researches in!878 123 
 
 CHAPTER VLT. 
 
 MERCURY. 
 
 The Ancient Astronomical Discoveries How Mercury was first found ? Not 
 easily seen Mercury was known in 265 B.C. Skill necessary in the 
 Discovery The Distinction of Mercury from a Star Mercury in the 
 East and in the West The Prediction How to Observe Mercury Its 
 Telescopic Appearance Difficulty of Observing its Appearance Orbit 
 of Mercury Velocity of the Planet Can there be Life on the Planet ? 
 Changes in its Temperature Transit of Mercury over the Sun 
 Gassendi's Observation Atmosphere around Mercury The Weight of 
 Mercury . . . . . ' . . . * .... .128 
 
 CHAPTER VIII. 
 
 VENTJS. 
 
 Interest attaching to this Planet The Unexpectedness of its Appearance 
 The Evening Star Visibility in Daylight Only Lighted by the Sun 
 The Phases of Venus Why the Crescent is not Visible to the unaided 
 Eye Variations in the Apparent Size of the Planet Resemblance of 
 Venus to the Earth The Transit of Venus Why of such especial Interest 
 
 The Scale of the Solar System Orbits of the Earth and Venus not in 
 
 the same Plane Recurrence of the Transits in Pairs Appearance of 
 Venus in Transit Transits of 1874 and 1882 The Early Transits of 
 1631 and 1639 The Observations of Horrocks and Crabtree The An- 
 nouncement of Halley How the Track of the Planet differs from different 
 
 places Illustrations of Parallax Voyage to Otaheite The result of 
 
 Encke Probable Value of the Sun's Distance Observations of the recent 
 Transit of Venus at Dunsink The Question of an Atmosphere to Venus 
 Dr. Copeland's Observations Utility of such Researches Other Deter- 
 minations of the Sun's Distance Statistics about Venus . . . .139 
 
* CONTENTS. 
 
 CHAPTER IX. 
 
 THE EAETH. 
 
 PAGE 
 
 The Earth is a great Globe How the Size of the Earth is Measured The 
 Base Line The Latitude found by the Elevation of the Pole A Degree 
 of the Meridian The Earth not a Sphere The Pendulum Experiment 
 Is the Motion of the Earth slow or fast ? Coincidence of the Axis of 
 Rotation and the Axis of Figure The Existence of Heat in the Earth 
 The Earth once in a Soft Condition Effects of Centrifugal Force Com- 
 parison with the Sun and Jupiter The Protuberance at the Equator 
 The Weighing of the Earth Comparison between the Weight of the Earth 
 and an equal Globe of Water Comparison of the Earth with a Leaden 
 Globe The Pendulum Use of the Pendulum in Measuring the Intensity 
 of Gravitation The Principle of Isochronism Shape of the Earth 
 Measured by the Pendulum 163 
 
 CHAPTER X. 
 
 MARS. 
 
 Our nearer Neighbours in the Heavens Surface of Mars can be Examined 
 in the Telescope Remarkable Orbit of Mars Resemblance of Mars to 
 a Star Meaning of Opposition The Eccentricity of the Orbit of Mars 
 Different Oppositions of Mars Apparent Movements of the Planet 
 Effect of the Earth's Movement Measurement of the Distance of Mars 
 Theoretical Investigation of the Sun's Distance Drawings of the 
 Planet Is there Snow on Mars ?- The Rotation of the Planet Gravi- 
 tation on Mars Has Mars any Satellites ? Mr. Asaph Hall's great 
 Discovery The Revolutions of the Satellites Deimos and Phobos 
 Gulliver's Travels 180 
 
 CHAPTER XI. 
 
 THE MINOR PLANETS. 
 
 The lesser Members of our System Bode's Law The Vacant Region in the 
 Planetary System The Research The Discovery of Piazzi Was the 
 small Body a Planet ? The Planet becomes Invisible Gauss undertakes 
 the Search by Mathematics The Planet Recovered Further Discoveries 
 Number of Minor Planets now known The Region to be Searched 
 The Construction of the Chart for the Search for Small Planets How a 
 Minor Planet is Discovered Physical Nature of the Minor Planets- 
 Small Gravitation on the Minor Planets The Bei-lin Computations 
 How the Minor Planets tell us the Distance of the Sun Accuracy of the 
 Observations How they may be Multiplied Victoria and Sappho The 
 most perfect Method . . '. ' V ' . . . ''-> . . 196 
 
 CHAPTER XII. 
 
 JUPITER. 
 
 The great Size of Jupiter Comparison of his Diameter with that of the Earth 
 Dimensions of the Planet and his Orbit His Rotation Comparison of 
 his Weight and Bulk with that of the Earth Relative Lightness of 
 
CONTENTS. xi 
 
 Jupiter How explained Jupiter still probably in a Heated Condition-r- 
 The Belts on Jupiter Spots on his Surface Time of Rotation of dif- 
 ferent Spots various Storms on Jupiter Jupiter not Incandescent 
 The Satellites Their Discovery Telescopic Appearance Their Orbits 
 The Eclipses and Occultations A Satellite in Transit The Velocity 
 of Light Discovered How is this Velocity to be Measured experi- 
 mentally ? Determination of the Sun's Distance by the Eclipses of 
 Jupiter's Satellites Jupiter's Satellites demonstrating the Copernican 
 System * . . .'..... . . . . . .211 
 
 CHAPTER XIII. 
 
 SATURN. 
 
 The Position of Saturn in the System Saturn one of Three most Interesting 
 Objects in the Heavens Compared with Jupiter Saturn to the unaided 
 Eye Statistics relating to the Planet Density of Saturn Lighter 
 than Water The Researches of Galileo What he found in Saturn A 
 Mysterious Object The Discovery made by Huyghens half a Century 
 later How the Existence of the Ring was Demonstrated Invisibility of 
 the Rings every Fifteen Ytars The Rotation of the Planet The Cele- 
 brated Cypher The Explanation Drawing of Saturn The Dark Line 
 W. Herschel's Researches Is the Division in the Ring really a Separa- 
 tion ? Possibility of Deciding the Question The Ring in a Critical 
 Position Are there other Divisions in the Ring ? The Third Ring 
 Has it appeared but recently? Physical 'Nature of Saturn's Rings Can 
 they be Solid ? Can they even be Slender Rings ? A Fluid Probable 
 Nature of the Rings A Multitude of Small Satellites Analogy of the 
 Rings of Saturn to the Group of Minor Planets Problems Suggested by 
 Saturn The Group of Satellites to Siitum The Discoveries of Additional 
 SateUites The Orbit of Saturn not the Frontier of our System . 232 
 
 CHAPTER XIV. 
 TJTtANUS. 
 
 Contrast between Uranus and the other great Planets William HerscheT 
 His Birth and Parentage Herschel's Arrival in England His love of 
 Learning Commencement of his Astronomical Studies The Construc- 
 tion of Telescopes Reflecting Telescopes Construction of Mirrors The 
 Professor of Music becomes an Astronomer The Methodical Research 
 The 13th March, 1781 The Discovery of Uranus Delicacy of Observa- 
 tion Was the Object a Comet? The Significance of this Discovery 
 The Fame of Herschel George III. and the Bath Musician The King's 
 Astronomer at Windsor Caroline Herschel The Planet Uranus Nu- 
 merical Data with reference thereto The Four Satellites of Uranus 
 Their Circular Orbits Early Observations of Uranus Flamsteed's 
 Observations Lemonnicr saw Uranus Utility of their Measurements 
 The Elliptic Path The great Problem thus Suggested . . . .257 
 
 CHAPTER XV. 
 
 NEPTUNE. 
 
 Discovery of Neptune A Mathematical Achievement The Sun's Attraction 
 All Bodies Attract Jupiter and Saturn The Planetary Perturbations 
 
CONTENTS. 
 
 Three Bodies Nature has Simplified the Problem Approximate Solution 
 The Sources of Success The Problem Stated for the Earth The Dis- 
 coveries of Lagrange The Eccentricity Necessity that all the Planets 
 Revolve in the same Direction Lagrange's Discoveries have not the 
 Dramatic Interest of the more Recent Achievements The Irregularities 
 of Uranus The unknown Planet must Revolve outside the Path of 
 Uranus The Data for the Problem Le Yerrier and Adams both Investi- 
 gate the Question Adams Indicates the Place of the Planet How the 
 
 Search was to be Conducted Le Verrier also Solves the Problem The 
 
 Telescopic Discovery of the Planet The Rival Claims Early Observa- 
 tion of Neptune Difficulty of the Telescopic Study of Neptune Numeri- 
 cal Details of the Orbit Is there any Outer Planet ? Contrast between 
 Mercury and Neptune ,275 
 
 CHAPTER XVI. 
 
 COMETS. 
 
 Comets Contrasted with Planets in Nature as well as in their Movements 
 Coggia's Comet Periodic Returns The Law of Gravitation Parabolic 
 and Elliptic Orbits Theory in Advance of Observations Most Cometary 
 Orbits are sensibly Parabolic The Labours of Halley The Comet of 
 1682 Halley's Memorable Prediction The Retardation produced by 
 Disturbance Successive Returns of Halley's Comet Encke's Comet 
 Effect of Perturbations Orbit of Encke's Comet Attraction of Mercury 
 and of Jupiter How the Identity of the Comet is secured How to 
 Weigh Mercury -Distance from the Earth to the Sun found by Encke's 
 Comet The Disturbing Medium The Comets of 1843 and 1858 Pas- 
 sage of a Comet between the Earth and the Stars Comets not composed 
 of Gas of appreciable Density Can the Comet be Weighed ? Evidence 
 of the Small Mass of the Comet derived from the Theory of Perturbation 
 The Tail of the Comet Its Changes Views as to its Nature Carbon 
 present in Comets 296 
 
 CHAPTER XVII. 
 
 SHOOTING STARS. 
 
 Small Bodies of our System Their Numbers How they are Observed The 
 Shooting Star The Theory of Heat A great Shooting Star The 
 November Meteors Their Ancient History The Route followed by the 
 Shoal Diagram of the Shoal of Meteors How the Shoal becomes Spread 
 out along its Path Absorption of Meteors by the Earth The Discovery 
 of the Relation between Meteors and Comets The remarkable Investiga- 
 tions concerning the November Meteors Two Showers in Successive 
 Years No Particles have ever been Identified from the great Shooting 
 Star Showers Meteoric Stones Chladni's Researches Early Cases of 
 Stonefalls The Meteorite at Ensisheim Collections of Meteorites The 
 Rowton Siderite Relative frequency of Iron and Stony Meteorites 
 Fragmentary Character of Meteorites No Reason to connect Meteorites 
 with Comets Tschermak's Theory Effects of Gravitation on a Missile 
 ejected from a Volcano Can they have come from the Moon ? The 
 
CONTENTS. xiii 
 
 Claims of the Minor Planets to the Parentage of Meteorites Possible 
 
 Terrestrial Origin The Ovifak Iron ' . ..-. ,330 
 
 CHAPTER XVIII. 
 
 THE STARRY HEAVENS. 
 
 Whence the Importance of the Solar System ? Home View in Space Other 
 Stellar Systems The Sun a Star Stars are Self-Luminous We see the 
 Points of Light, but nothing else The Constellations The Great Bear 
 and the Pointers The Pole Star Cassiopeia Andromeda, Pegasus, and 
 Perseus The Pleiades Auriga, Capella, Aldebaran, Taurus, Orion, 
 Sirius ; Castor and Pollux The Lion Bootes, Corona, and Hercules- 
 Virgo and Spica Vega and Lyra The Swan , '. - * . . ,369 
 
 CHAPTER XIX. 
 THE DISTANT SUNS. 
 
 Comparison between the Sun and the Stars Sirius Contrasted with the 
 Sun Stars can be Weighed, but not Measured The Companion of 
 Sirius Determination of the Weights of Sirius and his Companion 
 Dark Stars Variable Stars Enormous Numbers of Stars . . . 388 
 
 CHAPTER XX. 
 DOUBLE STARS. 
 
 Interesting Stellar Objects What is .a Double Star? Stars Optically Double 
 The great Discovery of the Binary Stars made by Herschel The 
 Binary Stars describe Elliptic Paths Why is this so important ? The 
 Law of Gravitation Special Double Stars Castor Mizar The Pole 
 Star The Coloured Double Stars Cygni, 7 Andromedse . . . 399 
 
 CHAPTER XXI. 
 
 THE DISTANCES OF THE STARS, 
 
 Sounding-line for Space The Labours of W. Herschel His Reasonings 
 Illustrated by Vega Suppose this Star to recede 10 times, 100 times, 
 or 1.000 times Some Stars 1,000 times as far as others Herschel' s 
 Method incomplete The Labours of Bessel Meaning of Annual Paral- 
 laxMinuteness of the Parallactic Ellipse Illustrated The case of 61 
 Cygni Different Comparison Stars used Difficulty owing to Refrac- 
 tion How to be avoided The Proper Motion of the Star Bessel's 
 Preparations - The Heliometer Struve's Investigations Can they be 
 Reconciled ? Researches at Dunsink Conclusion obtained Accuracy 
 which such Observations admit Examined How the Results are Dis- 
 cussedThe Proper Motion of 61 Cygni The Permanence of the Side- 
 real Heavens Changes in the Constellation of the Great Bear since the 
 time of Ptolemy The Star Groombridge 1,830 Large Proper Motion 
 Its Parallax Velocity of 200 Miles a Second The New Star in Cygnus 
 Its History No Appreciable Parallax A Mighty Outburst of Light 
 The Movement of the Solar System through Space Herschel' s Dis- 
 coveryJourney towards Hercules Probabilities Conclusion . . 407 
 
*iv CONTENTS. 
 
 CHAPTER XXTI. 
 
 THE SPECTROSCOPE. 
 
 A New Department of Science The Materials of the Heavenly Bodies 
 
 Meaning of Elementary Bodies Chemical Analysis and Spectroscopic 
 
 Analysis The Composite Nature of Light Whence Colours? The 
 
 Rainbow The Prism Passage of Light through a Prism Identification 
 of Metals by the Rays they emit when Incandescent The great Dis- 
 covejy of the Identity of the n-Lines with Sodium The Dark Lines in 
 the Solar Spectrum Interpreted Metals present in the Sun Examination 
 of Light from the Moon or the Planets The Prominences Surrounding 
 the Sun Photographs of Spectra Measurement of the Motion of the 
 Stars along the Line of Sight . 440 
 
 CHAPTER XXIII. 
 STAR CLUSTERS AND NEBULAE. 
 
 Interesting Sidereal Objects Stars not Scattered uniformly Star Clusters 
 
 Their Varieties The Cluster in Perseus--The Globular Cluster in Her- 
 cules The Milky Way A Cluster of Minute Stars Nebula? distinct 
 from Clouds Number of known Nebula} The Constellation of Orion 
 The Position of the Great Nebula The Wonderful Star 6 Orionis 
 The Drawing of the Great Nebula in Lord Rosse's Telescope Photo- 
 graphs of this wonderful object The Importance of Accurate Drawings 
 The Great Survey Photographic Charts Magnitude of the Nebula 
 The Question as to the Nature of a Nebula Is it composed of Stars 
 or of Gas ? How Gas can be made to Glow Spectroscopic Examination 
 of the Nebula The Great Nebula in Andromeda Its Examination by 
 the Spectroscope The Annular Nebula in Lyra Resemblance to a Vortex 
 Ring Planetary Nebula Drawings of several Remarkable Nebulae 
 Distance of Nebulas Conclusion , 452 
 
 CHAPTER XXIV. 
 THE PRECESSION AND NUTATION OF THE EARTH'S AXIS. 
 
 The Pole is not a Fixed Point Its Effect on the Apparent Places of the 
 Stars The Illustration of the Peg-top The Disturbing Force which 
 acts on the Earth Attraction of the Sun on a Globe The Protuberance 
 at the Equator The Attraction of the Protuberance by the Sun and by 
 the Moon produces Precession The Efficiency of the Professional Agent 
 varies inversely as the Cube of the Distance The Relative Efficiency of 
 the Sun and the Moon How the Pole of the Earth's Axis Revolves 
 Round the Pole of the Ecliptic . . . . ...... _. . .472 
 
 CHAPTER XXV. 
 
 THE ABERRATION OF LIGHT. 
 
 The Real and Apparent Movements of the Stars How they can be Discrimi- 
 nated Aberration produces Effects dependent on the Position of the 
 Stars The Pole of the Ecliptic Aberration make Stars seems to Move 
 
CONTENTS. 
 
 xv 
 
 in a Circle An Ellipse or a Straight Line according to Position All the 
 Ellipses have Equal Major Axes How is this Movement to be Explained ? 
 How to be Distinguished from Annual Parallax ? The Apex of the 
 Earth's Way How this is to be Explained by the Velocity of Light 
 How the Scale of the Solar System, can be Measured by the Aberra- 
 tion of Light ....... ..... 482 
 
 CHAPTER XXVL 
 
 THE ASTRONOMICAL SIGNIFICANCE OP HEAT. 
 
 Heat and Astronomy Distribution of Heat The presence of Heat in the 
 Earth Heat in other Celestial Bodies Varieties of Temperature The 
 Law of Cooling The Heat of the Sun Can its Temperature be 
 Measured ? Radiation connected with the Sun's Bulk Can the Sun 
 be Exhausting his Resources ? No marked Change has Occurred 
 Geological Evidence as to the Changes of the Sun's Heat Doubtful The 
 Cooling of the Sun The Sun cannot be merely an Incandescent Solid 
 Cooling Combustion will not Explain the matter Some Heat is 
 obtained from Meteoric Matter, but this is not Adequate to the Main- 
 tenance of the Sun's Heat The Contraction of a Heated Globe of Gas 
 An Apparent Paradox The Doctrine of Energy The Nebular 
 Theory Evidence in Support of this Theory Sidereal Evidence of the 
 Nebular Theory Herschel's View of Sidereal Aggregation The NebivUe 
 do not Exhibit the Changes within the Limits of our Observation . . 492 
 
 CHAPTER XXVII. 
 
 THE TIDES. 
 
 Mathematical Astronomy Recapitulation of the Facts of the Earlier Re- 
 searches Another great Step has been taken Lagrange's Theories, how 
 far they are really True The Solar System is not Made of Rigid Bodies 
 Kepler's Laws True to Observation, but not Absolutely True when the 
 Bodies are not Rigid The Errors of Observation Growth of certain 
 Small Quantities Periodical Phenomena Some Astronomical Phenomena 
 are not Periodic The Tides How the Tides were Observed Discovery 
 of the Connection between the Tides and the Moon Solar and Lunar 
 Tides Work done by the Tides Whence do the Tides obtain the Power 
 to do the Work Tides are Increasing the Length of the Day Limit to 
 the Shortness of the Day Early History of the Earth-Moon System- 
 Unstable Equilibrium Ratio of the Month to the Day The Future 
 Course of the System Equality of the Month and the Day The Future 
 Critical Epoch The Constant Face of the Moon accounted for The 
 other Side of the Moon The Satellites of Mars Their Remarkable 
 Motions Have the Tides Possessed Influence in Moulding the Solar 
 System generally ? Moment of Momentum Tides have had little or no 
 appreciable Effect on the Orbit of Jupiter Conclusion . . . .510 
 
 APPENDIX :- Astronomical Quantities 539 
 
LIST OF ILLUSTRATIONS. 
 
 PLATES. 
 
 PLATE 
 
 I. Saturn Frontispiece 
 
 II. A Typical Sun-spot ... ... ... ... ... ... To face page 9 
 
 III. Spots and Faculae on the Sun ... ... 41 
 
 IV. Solar Prominences or Flames ... ... . .;... ... ,, 45 
 
 V. The Solar Corona 48 
 
 VI. Chart of the Moon's Surface *.; 60 
 
 VII. The Lunar Crater Triesnecker ..-, ... ,. 68 
 
 \ III. A Normal Lunar Crater ., ... ... 73 
 
 IX. The Lunar Crater Plato , 80 
 
 X. The Lunar Crater Tycho ... ... ....'. 88 
 
 XI. The Planet Jupiter ... ' 218 
 
 XII. Coggia's Comet .. ... ,, 297 
 
 XIII. Spectra of the Sun and of three Stars 451 
 
 XIV. The Great Nehula in Orion '-...' 461 
 
 XV. The Great Nebula in Andromeda ... ... 467 
 
 XVI. Nebulae observed with Lord Rosse's Telescope 470 
 
 ENGRAVINGS. 
 
 FIG. PAGE 
 
 1. Principle of the Refracting Telescope ... ... ... 11 
 
 2. Dome of the South Equatorial at Dunsink Observatory, Co. Dublin ... 12 
 
 3. Section of the Dome of Dunsink Observatory ... ... 13 
 
 4. The Great Vienna Telescope ... ... ... 15 
 
 5. Principle of Herschel's Reflecting Telescope ... ... '... 16 
 
 6. Lord Rosse's Telescope ... ... ... ... ... ... 17 
 
 7. Meridian Circle 19 
 
 8. The Great Bear 24 
 
 9. Comparative Sizes of the Earth and the Sun 27 
 
 10. The Sun, photographed Sept. 22, 1870 30 
 
 11. An ordinary Sun-spot ... ... 31 
 
 12. Successive Appearances of a Sun-spot ... 32 
 
 13. Schemer's Observations on Sun-spots 33 
 
 14. Zones on the Sun's surf ace in which spots appear ... ... 36 
 
 15. Texture of the Sun and a small spot ... 38 
 
 16. Dr. Huggins' Drawing of a remarkable arrangement of Solar Granules ... 39 
 
xviii LIST OF ILLUSTRATIONS. 
 
 FIG. PAGE 
 
 17. Willow-leaf Texture of the Sun's Surface 40 
 
 18. Prominences seen in Total Eclipse ... .. ... ... ... ... 41 
 
 19. View of the Corona in a Total Eclipse ... ... ... 45 
 
 20. The Zodiacal Light in 1874 47 
 
 21. Comparative Sizes of the Earth and the Moon ... ... ... ... 52 
 
 22. The Moon's Path around the Earth ... 55 
 
 23. The Phases of the Moon 55 
 
 24. The Earth's Shadow and Penumbra ... ... ... .'.. ... ... 57 
 
 25. Key to Chart of the Moon (Plate VI.) 60 
 
 26. Lunar Volcano in Activity : Nasmyth's Theory ... ... ... ... 73 
 
 27. ,, Subsequent Feeble Activity ... ... ... ... 73 
 
 28. ,, Formation of the Level Floor by Lava ... ... ... 74 
 
 29. Orbits of the Four Interior Planets 88 
 
 30. The Earth's Movement 90 
 
 31. Orbits of the Four Giant Planets 91 
 
 32. Apparent Size of the Sun from various Planets ... .. ... ... 92 
 
 33. Comparative Sizes of the Planets ... ... ... ... ... ... 93 
 
 34. Illustration of the Moon's Motion ... ... ... 104 
 
 35. Drawing an Ellipse ... ... ... 110 
 
 36. Varying Velocity of Elliptic Motion 113 
 
 37. Equal Areas in Equal Times ... ... ,. ... ... 114 
 
 38. Transit of the Planet of Romance ... ... ... ... ... ... 125 
 
 39. Variations in Phase and Size of Mercury ... ... ... 132 
 
 40. Mercury as a Crescent ... ... ... ... ... ... ... ... 133 
 
 41. Different Aspects of Venus in the Telescope ... 142 
 
 42. Venus on the Sun at the Transit of 1874 147 
 
 43. Paths of Venus across Sun in the Transits of 1874 and 1882 149 
 
 44. A Transit of Venus, as seen from Two Localities 153 
 
 45. Orbits of the Earth and of Mars ..... 182 
 
 46. Apparent Movements of Mars in 1877 ... ... ... ... ... 184 
 
 47. Relative Sizes of Mars and the Earth ... - . ,.:^ 188 
 
 48. Views of Mars ... 189 
 
 49. The Zone of Minor Planets between Mars and Jupiter ... ... ... 200 
 
 50. Relative Dimensions of Jupiter and the Earth ... ... ... ... 212 
 
 51. Jupiter and his Four Satellites 222 
 
 52. Disappearances of Jupiter's Satellites ... ... ... ... ... ... 223 
 
 53. Mode of Measuring the Velocity of Light ... ... ... ... ... 228 
 
 54. Relative Sizes of Saturn and the Earth .. ... 236 
 
 55. Parabolic Path of a Comet .... 299 
 
 56. Orbit of Encke's Comet *.' 306 
 
 57. Resisting Medium around the Sun ... ... ... ... 312 
 
 58. Tail of a Comet directed from the Sun 321 
 
 59. Bredichin's Theory of Comets' Tails ... 323 
 
 60. Tails of the Comet of 1858 ... 324 
 
 61. Cheseaux's Comet of 1744 ... 325 
 
 62. Path of the Fire-ball of November 6, 1869 ... 333 
 
 63. The Orbit of a Shoal of Meteors 337 
 
 64. Radiant Point of Shooting Stars .. ... 339 
 
 65. The History of the Leonids "-";;- 345 
 
 66. Section of the Chaco Meteorite ... ... 356 
 
 67. The Great Bear and Pole Star ... ... ... ... ... ... ... 373 
 
LIST OF ILLUSTRATIONS. xix 
 
 FIG. PAGE 
 
 68. The Great Bear and Cassiopeia 374 
 
 69. The Great Square of Pegasus 376 
 
 70. Perseus and its Neighbouring Stars 378 
 
 71. The Pleiades 379 
 
 72. Orion, Sirius, and Neighbouring Stars 381 
 
 73. Castor and Pollux 382 
 
 74. The Great Bear and the Lion 383 
 
 75. Bootes and the Crown 384 
 
 76. Virgo and Neighbouring Constellations ... ... ... 385 
 
 77. The Constellation of Lyra 386 
 
 78. Vega, the Swan, and the Eagle 387 
 
 79. The Parallactic Ellipse 413 
 
 80. 61 Cygni and the Comparison Stars ... ... 416 
 
 81. Parallax in Declination of 61 Cygni ... ... 423 
 
 82. The Prism .... 444 
 
 83. Dispersion of Light by the Prism 445 
 
 84. Globular Star-cluster in Hercules 454 
 
 85. Position of the Great Nebula in Orion 458 
 
 86. The Multiple Star 6 Orionis ' 460 
 
 87. Lyra, with the Annular Nebula 468 
 
 88. Two Views of the Annular Nebula in Lyra ... ... .,. 468 
 
 89. Star-Map, showing Precessional Movement ... ... 473 
 
 90. Illustration of the Motion of Precession 475 
 

THE 
 
 STORY OF THE HEAVENS. 
 
 " THE Story of the Heavens" is the title of our book. We have 
 indeed a wondrous story to narrate ; and could we tell it ade- 
 quately, it would prove of boundless interest and of exquisite 
 beauty. It leads to the contemplation of the mightiest efforts of 
 nature and the greatest achievements of human genius. 
 
 Let us enumerate a few of the questions which will be naturally 
 asked by one who seeks to learn something of those glorious bodies 
 which adorn our skies : What is the Sun how hot, how big, and 
 how distant ? whence comes its heat ? What is the Moon ? What 
 scenery do its landscapes show ? how does the moon move ? how is 
 it related to the earth ? What of the planets are they globes like 
 the earth ? how large are they, and how far off ? What do we 
 know of the satellites of Jupiter and of the rings of Saturn ? 
 What was the memorable discovery of Uranus ? and what was the 
 supreme intellectual triumph which brought the planet Neptune 
 to light? Then, as to the other bodies of our system, what 
 are we to say of those mysterious objects, the comets? can we 
 perceive order to reign in their seemingly capricious movements ? 
 do we know anything of their nature and of the marvellous tails 
 with which they are often decorated? What can be told about 
 the familiar shooting-star which so often dashes into our atmo- 
 sphere to perish in a streak of splendour ? What do we know of 
 those constellations which have been from all antiquity, and of 
 the myriad hosts of smaller stars which our telescopes disclose? 
 B 
 
2 THE STORY OF THE HEAVENS. 
 
 Can it be true that these countless orbs are really majestic suns, 
 sunk to an appalling depth in the abyss of unfathomable space? 
 What have we to tell of all the different varieties of stars of 
 coloured stars, of variable stars, of double stars, of multiple stars 4 
 of stars that move, and of stars that seem at rest ? What of 
 those most supremely glorious objects, the great star clusters? 
 What of the milky way ? And lastly, what can we tell of those 
 marvellous nebulae which our telescopes disclose, poised at an im- 
 measurable distance on the very confines of the universe? Such 
 are a few of the questions which occur, when we ponder on the 
 mysteries of the heavens. 
 
 The history of Astronomy is, in one respect, only too like many 
 other histories. The earliest part of it is completely and hopelessly 
 unknown. The stars had been studied; and some great astronomical 
 discoveries had been made, untold ages before those to which our 
 earliest historical records extend. For example, the perception of 
 the apparent movements, of the sun and of the moon, and the recog- 
 nition of the planets by their movements, are both to be classed 
 among these discoveries of the pre-historic ages. Nor is it to be 
 said that these achievements were all of a very obvious or ele- 
 mentary character. To us of the present day who have been 
 familiar with such truths from childhood, they may now seem 
 simple and rudimentary; but in the infancy of science, the first man 
 who arose to demonstrate one of these great doctrines was indeed 
 a most sagacious philosopher. 
 
 Of all the phenomena of Astronomy, the first and the most 
 obvious is that of the rising and the setting of the sun. We may 
 fairly conjecture, that in the dawn of the growth of the human 
 intellect this was probably one of the very first problems to engage 
 the attention of those whose thoughts rose above the animal 
 anxieties of everyday existence. A sun sets and disappears in the 
 west ; that sun is obviously a very brilliant body, and the simplest 
 reflection suggests that it is a body of -very considerable importance. 
 The following morning a sun arises in the east, moves across the 
 heavens, and it too disappears in the west ; the same process happens 
 every day. To us it is obvious that the sun, which appears each day, 
 
EARLIEST IDEAS OF THE STARS. 3 
 
 is the same sun; but this would not be an obvious truth to one who 
 thought his senses showed him that the earth was a flat plane of 
 indefinite extent, and that around the inhabited regions on all sides 
 extended, to vast distances, either desert wastes or trackless oceans. 
 How could the sun, which plunged into the ocean at a fabulous 
 distance in the west, reappear the next morning at an equally 
 great distance to the east ? The old mythological account asserted 
 that after the sun had dipped in the western ocean at sunset (the 
 Iberians, and other ancient nations, actually imagined that they 
 could hear the hissing of the waters when the glowing globe was 
 plunged therein), he was seized by Vulcan and placed in a golden 
 goblet, and thus navigated the ocean round by the north, so as to 
 reach the east again in time for sunrise the following morning. 
 Even the more sober physicists of old, as we are told by Aristotle, 
 believed that in some manner the sun was conveyed round over the 
 earth's surface by the north, and that the darkness of night arose 
 from the elevation of the northern lands, which cut off the sun's 
 light during his midnight voyage. 
 
 Even in very early times it was found more rational to sup- 
 pose that the sun actually pursued his course down below the solid 
 earth during the darkness of night. The earliest astronomers had, 
 moreover, learned to recognise the fixed stars. It was seen that, 
 like the sun, many of these stars rose and set in the course of the 
 diurnal movement, while the moon obviously followed the same Jaw. 
 It thus became plain that the various heavenly bodies possessed 
 the power of actually going below the solid earth. Once it was 
 realised that the whole contents of the heavens performed these 
 movements, it became possible to take a very important step in the 
 knowledge of the constitution of the universe. It was clear that 
 the earth could not be a plane extending to an indefinitely great 
 distance. It was also obvious that there must be a finite depth to 
 the earth below our feet. Nay, more, it became certain that what- 
 ever be the shape of the earth, it was at all events something 
 detached from all other bodies, and poised without visible support 
 in space. When first presented to the mind of man, this must have 
 appeared a very startling truth. It was surely difficult to realise 
 B 2 
 
4 THE STORY OF THE HEAVENS. 
 
 that the solid earth on which we stand reposed on nothing- ! What 
 is to keep it from falling- ? How can it be poised, like the legendary 
 coffin of Mahomet, without tangible support ? But difficult as it 
 may have been to receive this doctrine, yet its necessary truth 
 commanded assent, and the first great step in Astronomy had been 
 made. 
 
 The changes of the seasons and the recurrence of seed-time and 
 of harvest must, from the earliest times, have been associated with 
 certain changes in the position of the sun. In the summer at mid- 
 day the sun rises high in the heavens, in the winter the sun is 
 always low. The sun, therefore, hnd an annual movement up and 
 down in the heavens, combined with the diurnal movement of rising 
 and setting 1 . But besides these movements of the sun there was 
 another of no less importance, which was not quite so obvious, 
 though still capable of being detected by the simplest observations, 
 when combined with a philosophical habit of reflection. The very 
 earliest observers of the stars can hardly fail to have noticed, that 
 the constellations visible at night varied with the season of the 
 year. For instance, the constellation of Orion, which is so well 
 seen during the winter nights, becomes invisible in the summer, 
 and the place it occupied is then taken by quite different stars. 
 So it is with other constellations; and, indeed, in ancient days, 
 the time for commencing the cycle of agricultural occupations 
 was sometimes indicated by the position of the constellations in 
 the evening. 
 
 Reflection on this subject must have demonstrated in very 
 early times the apparent annual movement of the sun. It was 
 seen that the places of the stars, relatively to each other, did not 
 alter appreciably, and there could be no explanation of the changes 
 in the constellations with the seasons, except by supposing that the 
 place of the sun was altering, so as to make the complete circuit of 
 the heavens in the course of the year. The same conclusion is 
 easily confirmed by looking from time to time at the west after 
 sunset, and watching the stars. As the season progresses, it will 
 be noticed that each evening the western constellations sink lower 
 and lower towards the sun, until at length they come so near the 
 
ANCIENT DISCOVERIES. 5 
 
 sun that they set at the same time as he does. This is simply 
 explained by the supposition that the sun is gradually but con- 
 tinually rising- up from the west to meet the stars. This motion 
 is of course not to be confounded with the ordinary diurnal 
 motion, in which all the heavenly bodies alike participate; inas- 
 much as besides this motion of the whole heavens, the sun has a slow 
 motion in the opposite direction ; so that while the sun and a star 
 may set to-day at the same time, by to-morrow the sun will have 
 moved a little towards the east, relatively to the star, and thus the 
 star will set a few minutes before the sun.* 
 
 The patient watchings of the early astronomers enabled the sun's 
 track through the heavens to be ascertained, and it was found that 
 in its annual circuit the sun invariably pursued the same path and 
 traversed the same constellations. The belt of constellations thus 
 specially distinguished is known by the name of the zodiac, while 
 the circle traversed by the sun is called the ecliptic. The zodiac 
 was divided into twelve equal portions or " signs," and thus the 
 stages on the sun's great journey were conveniently indicated. In 
 the very earliest ages, also, it seems that the duration of the year, or 
 the period required by the sun to run its course around the heavens, 
 became accurately known. The skill of the ancient geometers was 
 also demonstrated by the accurate measures they succeeded in 
 making of the position of the ecliptic with regard to the equator, 
 and in measuring the angle between these two most important 
 circles on the heavens. 
 
 The principal phenomena presented by the motion of the moon 
 have also been understood from an antiquity beyond all historical 
 record. The slightest attention reveals the important truth that 
 the moon does not occupy a fixed position in the starry heavens. 
 Indeed, the motion of the moon among the stars is a phenomenon 
 much more easy to recognise than that of the sun among the stars, 
 as during the course of a single night the movement of the moon 
 from west to east across the heavens can be perceived with but 
 very moderate attention. It is most probable that the motion of 
 
 * It may, however, be remarked that a star is never seen to set, as owing to our 
 atmosphere it ceases to be visible before it reaches the horizon. 
 
6 THE STORY OF THE HEAVENS. 
 
 the moon among the stars was a discovery prior to that of the 
 annual motion of the sun, inasmuch as it depends upon simple 
 observation, and involves but little exercise of any intellectual 
 power. The time of revolution of the moon had also been dis- 
 covered, and the phases of the moon had been correctly attributed 
 to the varying aspect under which the sun-illuminated side of the 
 moon is turned towards the earth. 
 
 But even this does not exhaust the list of great discoveries 
 which have come down to us from prehistoric times. The striking 
 phenomenon of a lunar eclipse, in which the brilliant surface 
 is plunged temporarily into darkness, and the still more imposing 
 spectacle of a solar eclipse, in which the sun himself undergoes 
 a partial, or even a total obscuration, had also been correctly 
 explained. Then, too, the acuteness of the early astronomers 
 had detected the five wandering stars or planets : they had traced 
 the movements of Mercury and Venus, Mars, Jupiter, and Saturn. 
 They had observed with awe the various configurations of these 
 planets; and just as the sun, and in a lesser degree the moon, 
 were intimately associated with the affairs of daily life, so in 
 the imagination of these early investigators the movements of 
 the planets were thought to be pregnant with human weal or 
 human woe. At length a certain degree of order was perceived to 
 govern the capricious movements of the planets. It was found 
 that they obeyed certain laws. The cultivation of the science of 
 geometry went hand in hand with the study of astronomy; and as 
 we emerge from the dim pre- historic ages into the historical period, 
 we find that a theory possessing some degree of coherence had 
 been established, to explain the phenomena of the heavens. 
 
 Although the Ptolemaic doctrine is now known to be framed on 
 an utterly extravagant estimate of the true place of the earth in 
 the scheme of the heavens, yet the apparent movements of the 
 celestial bodies are accounted for by the theory with considerable 
 accuracy. This theory is described in the great work of Ptolemy, 
 known as the "Almagest," which was written in the second century 
 of our era, and for fourteen centuries was regarded as the final 
 authority on all questions of astronomy. 
 
COPERNICUS. 7 
 
 Ptolemy saw that the shape of the earth was globular, and he 
 demonstrated this by the arguments which we employ at the 
 present day. He also saw how this mighty globe was poised, in 
 what he believed to be the centre of the universe. He admitted 
 that the diurnal movement of the whole heavens could be accounted 
 for by the revolution of the earth upon its axis, but he assigned 
 reasons for the deliberate rejection of this view. The earth 
 according to him was a fixed body; it possessed neither rotation nor 
 translation, but remained constantly at rest at the centre of the. 
 universe. The sun and the moon he supposed to move in circular 
 orbits around the earth in the centre. The movements of the 
 planets were more complicated, as it was necessary to account for 
 the occasional retrograde motions as well as for the direct motions. 
 The ancient geometry refused to admit that any movement, except 
 circular, could be perfect, and accordingly a contrivance was devised 
 by which each planet revolved in a circle, while the centre of 
 that circle described another circle around the earth. It must be 
 admitted that this scheme, though so widely divergent from what 
 is now known to be the truth, did really present a fairly accurate 
 account of the movements of the planets. 
 
 Such was the system of Astronomy which prevailed during the 
 Middle Ages, and which was only finally overturned by the great 
 work to which Copernicus devoted his lifetime. The discovery of 
 the true system of the universe was nearly simultaneous with the 
 discovery of the New World by Columbus. The first principles 
 which were established by the labours of Copernicus, stated that 
 the diurnal movement of the heavens was really due to the rotation 
 of the earth on its axis. He showed the difference between real 
 motions and apparent motions ; he proved that all the appearances 
 of the daily rising and setting of the sun, and the stars could be 
 just as well accounted for by the supposition that the earth rotated, 
 as by the more cumbrous supposition of Ptolemy. He showed that 
 the latter supposition would attribute an almost infinite velocity 
 to the stars, and that the rotation of the entire universe around the 
 earth was really a preposterous supposition. The second great 
 point, which it is the immortal glory of Copernicus to have 
 
8 THE STORY OF THE HEAVENS. 
 
 demonstrated, assigned to the earth its true position in the fabric 
 of the universe. He transferred the centre, about which all the 
 planets revolve, from the earth to the sun ; and he established the 
 somewhat humiliating truth, that our earth is after all merely 
 one of the system of planets revolving around the sun, and 
 pursuing a track between the paths of Venus and of Mars. 
 
 Such was, in brief outline, the great revolution which swept 
 from astronomy those distorted views of the earth's importance, 
 arising from the fact that we are domiciled on that particular 
 planet. The achievement of Copernicus was soon to be followed by 
 the invention of the telescopy that wondrous instrument by which 
 the modern science of astronomy has been created. To the con- 
 sideration of this most important subject we may well devote 
 the first chapter of our book. 
 
PLATE H. 
 
 A TYPICAL SUN-SPOT. 
 
 (AFTER UANQLEYJ 
 
CHAPTER I. 
 
 THE ASTRONOMICAL OBSERVATORY. 
 
 Early Astronomical Observations The Observatory of Tycho Brahe The Pupil 
 of the Eye Vision of Faint Objects The Telgscope The Object-Glass 
 Advantages of Large Telescopes The Equatorial The Observatory The 
 Power of a Telescope Reflecting Telescopes Lord Rosse's Great Reflector 
 at Parsonslown How the mighty Telescope is used The Instruments of 
 Precision The Meridian Circle The Spider Lines Delicacy of pointing a 
 Telescope The Precautions necessary in making Observations The Ideal 
 Instrument and the Practical one The Elimination of Error The ordinary 
 Opera-Glass as an Astronomical Instrument The Great Bear Counting the 
 Stars in the Constellation How to become an Observer. 
 
 THE earliest traces of the Astronomical Observatory are as little 
 known, as the earliest discoveries in astronomy itself. Probably 
 the first application of instruments to the observations of the 
 heavenly bodies, consisted in the extremely simple operation of 
 measuring the length of the shadow cast by the sun at noonday. 
 The variations in the length of this shadow from day to day, and 
 its periodical maxima and minima, furnished valuable information 
 in the early attempts to investigate the movements of the sun. But 
 even in very early times there were astronomical instruments em- 
 ployed which possessed considerable complexity, and showed no small 
 amount of astronomical knowledge. 
 
 The first great advance in this subject was made by the cele- 
 brated Tycho Brahe, who wa^ born in 1546, three years after the 
 death of Copernicus. His attention seems first to have been 
 directed to "astronomy by the eclipse of the sun which occurred on 
 the 21st August, 1560. It amazed his reflective spirit to find 
 that so surprising a phenomenon admitted of actual prediction, and 
 he determined to devote his life to the study of a science possessed 
 of such wonderful precision. In the year 1576 the King of 
 
10 THE STOEY OF THE HEAVENS. 
 
 Denmark had established Tycho Brahe on the island of Huen, and 
 had furnished him with the splendid observatory of Uraniberg. It 
 was here that Tycho assiduously observed the places of the heavenly 
 bodies for some twenty years, and accumulated the observations 
 which were destined, in the hands of Kepler, to lead to the great 
 discovery of the planetary movements. Compared with our modern 
 astronomical equipment the great instruments of Tycho are but 
 quaint and primitive apparatus. In his days the telescope had not 
 yet been invented, and he could only determine the places of the 
 heavenly bodies in a comparatively crude manner ; but his skill and 
 patience in a great degree compensated for the imperfection of his 
 instruments, and with him it may be said that the epoch of accurate 
 astronomical observation commences. 
 
 The application of the telescope by Galileo gave a most wonderful 
 impulse to the study of the heavenly bodies. This extraordinary 
 man stands out prominently in the history of astronomy, not alone 
 for his connection with this supreme invention, but for his achieve- 
 ments in the more abstract parts of astronomy. It was Galileo 
 who first laid with any solidity the foundation of the science of 
 Dynamics, of which astronomy is the most splendid illustration ; 
 and it was he who expounded and upheld the great doctrine of 
 Copernicus, and thereby drew down upon himself the penalties of 
 the Inquisition. 
 
 The structure of the eye itself, and more particularly the 
 exquisite adaptation of the pupil, presents us with an apt illustra- 
 tion of the principle of the telescope. To see an object, it is 
 necessary that the light from that object should enter the eye. 
 The portal through which the light enters the eye is the pupil. 
 In daytime, when the light is abundant, the iris gradually 
 decreases the size of the pupil, and as the portal is thus con- 
 tracted, less light can enter. At night, on the other hand, when 
 the light is scarce, the eye requires to grasp all it can. The 
 pupil then expands, more and more light is admitted according 
 as the pupil grows larger, until at. length the pupil is dilated to 
 its utmost extent. The admission of light is thus controlled in 
 the most perfect manner. 
 
THE ASTRONOMICAL OBSERVATORY. 
 
 11 
 
 Rays of light 
 from the Star 
 
 Objective 
 
 The stars send us their feeole rays of light, and those rays form 
 an image on the retina ; but, even with the most widely-opened pupil, 
 it may happen that the image is still not bright enough to excite 
 the sensation of vision. Here the telescope comes to our aid : it 
 catches all the rays in a beam of dimensions far too large to enter 
 the pupil, and concentrates those rays into a small 
 beam which can enter the pupil. We thus have 
 the image on the retina intensified in brilliancy ; 
 in fact, it is illuminated with nearly as much 
 light as would be obtained through a pupil as 
 large as the object-glass of the telescope. 
 
 In our astronomical observatories we find two 
 entirely different classes of telescopes. The more 
 familiar forms are those known as refractors, in 
 which the operation of condensing the rays of 
 light is effected by refraction. The same object 
 can, however, be attained in a wholly different 
 manner by the aid of the laws of reflection, and 
 accordingly many telescopes, including the most 
 gigantic instruments yet erected, are known as 
 reflectors. The character of the refractor is shown 
 in Fig. 1. The rays from the star fall upon the 
 object-glass which is at the end of the telescope, 
 and after passing through it they are refracted 
 into a converging beam, so that all intersect at 
 the focus. Diverging from thence, they encounter 
 the eye-piece, which has the effect of again re- 
 ducing them to parallelism. The large cylindrical 
 beam which poured down on the object-glass is 
 thus concentrated into a small one, which can enter the pupil. The 
 composite nature of light requires a more complex form of object- 
 glass than the simple lens here shown. In modern telescopes we 
 employ what is known as the achromatic object-glass, which con- 
 sists of one lens of flint glass and one of crown glass, combined 
 together. 
 
 It will thus be apparent, that the larger the object-glass, the 
 
 (Focus> 
 
 piece : 
 
 U! 
 
 To the 
 Eye 
 
 Fig. 1. Principle 
 of theKefracting 
 Telescope. 
 
12 
 
 THE STORY OF THE HEAVENS. 
 
 greater the quantity of light grasped, and the greater will be the 
 success of the telescope in revealing very faint objects. Hence it 
 is that in the efforts to increase the powers of their telescopes, 
 
 Fig. 2. The Dome of the South Equatorial at Dunsink Observatory, Co. Dublin. 
 
 each succeeding race of astronomers has sought to obtain larger 
 object-glasses than those which were used by their predecessors. 
 
 The appearance of an astronomical observatory, built to hold an 
 instrument of moderate dimensions, is shown in the adjoining 
 figures. The first (Fig. 2) represents the dome erected at Dunsink 
 
THE ASTRONOMICAL OBSERVATORY. 
 
 13 
 
 Observatory for the equatorial telescope, the object-glass of which 
 was presented to the Board of Trinity College, Dublin, by the late 
 Sir James South. The main part of the building is a circular wall, 
 on the top of which reposes a hemispherical roof. In this roof 
 is a shutter, which can be opened so as to allow the telescope in 
 
 Fig. 3. Section of the Dome of Dunsink Observatory. 
 
 the interior to be directed towards the heavens. The whole struc- 
 ture revolves, so that the opening may be pointed to any part of 
 the sky which it is desired to examine. The next view (Fig. 3) 
 exhibits a section of the roof, showing the machinery by which 
 the attendant causes it to revolve, as well as the telescope itself. 
 The eye of the observer is at the eye-piece, and he is in the act of 
 turning a handle, which has the power of slowly moving the 
 telescope,, in order to direct the instrument towards any point that 
 
14 THE STORY OF THE HEAVENS. 
 
 may be desired. A telescope mounted in the manner here shown, 
 is called an equatorial. The convenience of the equatorial form of 
 mounting lies in the ease with which the telescope can be moved 
 so as to follow any celestial object in its journey around the sky. 
 The necessary movements are given by clockwork, so that, once 
 the instrument has been correctly pointed, and the clockwork 
 started, the star will remain in the observer's field of view not- 
 withstanding the apparent diurnal movement. The two lenses 
 which together form the object-glass are in this case twelve inches 
 in diameter, and it is on the correctness of the objective that the 
 good performance of the telescope mainly depends. The eye-piece 
 consists merely of one or two small lenses ; various eye-pieces can 
 be employed, according to the magnifying power which may be 
 desired. It is to be observed that for many purposes of astronomy 
 highly magnifying powers are not desirable. The object-glass can 
 only grasp a certain quantity of light, and if the magnifying power 
 be too great, the light will be thinly dispersed over a large surface, 
 and the result will be unsatisfactory. 
 
 The power of a refracting telescope so far as the expression has 
 a definite meaning is measured by the diameter of its object-glass. 
 There has, indeed, been some degree of rivalry between the various 
 civilised nations as to which should possess the greatest refracting 
 telescope. Among the largest telescopes of this type the world 
 has yet seen, is that recently constructed by Mr. Howard Grubb, 
 of Dublin, for the splendid observatory at Vienna. This great 
 instrument is represented in Fig. 4. The dimensions of it may 
 be estimated from the fact that the objest-glass is two feet and 
 three inches in diameter. Many ingenious contrivances obviate 
 the inconveniences incident to the use of an instrument of such 
 vast proportions. We may here only notice the method by 
 which the graduated circles attached to the telescope are brought 
 within easy view of the observer. These circles are situated at 
 parts of the instrument very remote from the eye-piece at which 
 the observer is stationed. They can, however, be readily seen 
 by small auxiliary telescope tubes (shown in the figure, close to 
 the eye-piece), which, by suitable reflectors, conduct the rays of 
 
Fig. 4. The Great Vienna Telescope. 
 
16 
 
 THE STORY OF THE HEAVENS. 
 
 light from the illuminated circles to the eye of the observer. 
 The clock movement of this great instrument is also noteworthy, 
 as it is controlled by electricity, so that the mighty tube follows 
 the star with almost mathematical precision. 
 
 Numerous refracting telescopes of exquisite perfection have been 
 
 produced by Messrs. Alvan Clark, of 
 Cambridgeport, Boston, Mass. The 
 size of their instruments has been 
 gradually increasing, and they have 
 recently completed a gigantic telescope 
 with an object-glass of no less than 
 thirty inches in diameter for the Rus- 
 sian astronomers. 
 
 Can refracting telescopes be con- 
 structed of still greater dimensions ? 
 The present limit to the size of the 
 refractor chiefly lies in the material of 
 the object-glass. Glass manufacturers 
 experience great difficulties in any 
 attempts to form large discs of opti- 
 cal glass pure enough and uniform 
 enough to be suitable for telescopes. 
 These difficulties increase with every 
 increase in the size of the instrument, 
 and at the present moment this is the 
 chief impediment to the construction 
 of refracting telescopes of the largest 
 dimensions. 
 
 There is, however, the alternative 
 method of constructing a telescope, in which this difficulty does not 
 arise. The simplest form of reflector is that shown in Fig. 5, which 
 represents the Herschelian instrument. The rays from the star 
 fall on a beautifully polished and carefully shaped mirror, so 
 that, after the reflection, they proceed to a focus, and diverging 
 thence, fall on the eye-piece, from which they emerge, reduced to 
 parallelism and fitted for reception by the eye. It is essentially on 
 
 sr 
 
 e 
 
 F 
 fr 
 
 ays o 
 om th 
 
 I 
 
 Ugh 
 e Sts 
 
 ; 
 r 
 
 I 
 
 \ 
 
 \ 
 
 \ 
 
 
 
 
 
 
 \ 
 
 I 
 
 ', 
 
 
 
 ; 
 
 
 \ 
 
 I 
 
 
 ] 
 
 
 ^| 
 
 
 Mirror 
 
 Fig. 5. Principle of Herschel's 
 Reflecting Telescope. 
 

18 THE STORY OF THE HEAVENS. 
 
 this principle, though with an additional reflection, that the 
 mightiest telescope in existence has been constructed. This 
 renowned instrument, known wherever science is known, was built, 
 forty years ago, by the late Earl of Rosse at Parsonstown. The 
 colossal dimensions of this instrument have never been surpassed ; 
 they have, indeed, never been rivalled. The reflector in this case is 
 a thick metallic disc, consisting of an alloy of two parts of copper 
 to one of tin, forming a hard and brittle metal intractable for 
 mechanical operations, but admitting of a brilliant polish, and of 
 receiving and retaining an accurate figure. The great reflector 
 six feet in diameter reposes at the end of a tube sixty feet 
 long. This tube is mounted between two castellated walls of 
 masonry, which form an imposing feature on the lawn at Birr 
 Castle, as represented in Fig. 6. This instrument does not admit 
 of being directed towards any part of the sky like the equatorials 
 we have recently been considering. The great reflector is only 
 capable of an up and down movement along the meridian, and 
 of a small lateral movement east and west of the meridian. 
 A little consideration will, however, show that, though the 
 telescope cannot at any moment be directed to any particular star, 
 yet that each star visible in the latitude of Parsonstown can be 
 observed when looked for at the right time. 
 
 As the object is approaching the meridian, be it planet or 
 comet, star or nebula, the telescope is raised to the right height. 
 This is accomplished by a chain passing from the mouth of 
 the instrument to a windlass at the northern end of the walls. 
 By this windlass the telescope can be raised or lowered, and an 
 ingenious system of counterpoises renders the movement equally 
 easy at all altitudes. The observer then takes his station in the 
 lofty gallery which gives access to the eye-piece; and when the 
 right moment has arrived, the object enters the field of view. 
 A vast clockwork mechanism at the lower end of the tube gives 
 movement to the great instrument, so that the object can be 
 followed by the observer until he has made his measurements, or 
 finished his drawing. 
 
 It will thus be seen that, notwithstanding the stupendous size 
 
THE ASTRONOMICAL OBSERVATORY. 
 
 19 
 
 of this telescope (the tube is large enough for a tall man to walk 
 through without stooping), it is comparatively easy to observe with. 
 It must not, however, be assumed that for all the purposes of 
 
 Fig. 7. Meridian Circle. 
 
 astronomy an instrument so colossal is the most suitable. The 
 mighty reflector is chiefly of use where very faint objects 
 are to be sought for; but where accurate measurements are 
 required of objects not unusually faint, telescopes of smaller di- 
 mensions and of different construction are more suitable. Among 
 the other great reflectors, we may mention that constructed by 
 c 2 
 
20 THE STORY OF THE HEAVENS. 
 
 Mr. Common, of Baling, three feet in aperture, which possesses 
 great optical perfection and has done excellent astronomical 
 work. 
 
 The fundamental truths of the movements of the heavenly 
 bodies have been chiefly learned from the work of instruments of 
 comparatively moderate telescopic power, specially arranged to 
 enable precise measures of position to be secured. Indeed, in the 
 early stages of astronomy, important observations of position were 
 obtained by contrivances which showed the direction of the object 
 without any telescopic aid. 
 
 In our modern observatories the most important measurements 
 are those obtained by that most accurate of all instruments of pre- 
 cision, known as the meridian circle. It would be out of place 
 to attempt to give here any minute description of this instrument, 
 even in any of its multitudinous forms. It is, however, equally 
 impossible, in any adequate account of the Story of the Heavens, 
 to avoid some reference to this fundamental instrument; and 
 therefore we shall give a very brief account of one of the 
 simpler forms, choosing for this purpose a great instrument in 
 the Paris Observatory, which is represented in the illustration 
 (Fig. 7). 
 
 The telescope is attached at its centre to an axis at right 
 angles to its length. The pivots at the extremities of this axis 
 rotate in fixed bearings, so that the movements of the teles- 
 cope are completely restricted to the plane of the meridian. In- 
 side the eye-piece of the telescope extremely fine vertical lines are 
 stretched. The observer watches the moon, or star, or planet, 
 or whatever may be the object, enter the field of view; and he 
 notes the second, or fraction of a second, by the clock, as the star 
 passes over each of the lines. The circle attached to the teles- 
 cope is divided into degrees and subdivisions of a degree, and this 
 circle, which moves with the telescope, will indicate the eleva- 
 tion at which the telescope is pointed. For the accurate reading 
 of the circle, microscopes are used. These microscopes are shown 
 in the sketch, each one being fixed into an aperture in the wall 
 which supports one of the pivots. At the opposite side is a lamp, 
 
THE ASTRONOMICAL OBSERVATORY. 21 
 
 the light from which passes through the perforated pivot and is 
 thence deflected to illuminate the lines at the focus. 
 
 The lines, which the observer sees stretched over the field of 
 view of the telescope, demand a few words of explanation. We 
 require for this purpose a line which shall be very fine and durable, 
 elastic, and of little or no weight. These conditions cannot be 
 completely fulfilled by any metallic wire, but they are most 
 exquisitely fulfilled in the beautiful thread which is spun by the 
 spider. These gossamer threads are stretched with nice skill 
 across the field of view of the telescope, and secured in their 
 proper places. With instruments so furnished, it is easy to under- 
 stand the precision of modern observations. The telescope is 
 directed towards a star, and the image of the star is a minute 
 point of light. When that point is made to coincide with the 
 intersection of the two central spider lines, the telescope is properly 
 sighted. 
 
 We use the word sighted designedly, because we wish to suggest 
 a comparison between the sighting of a rifle at the target and the 
 sighting of a telescope at a star. Instead of the large bull's-eye 
 of a rifle-target, suppose that the target only contained an ordinary 
 watch-dial ; the rifleman would not be able to sight the dial. 
 But with the telescope of the meridian circle we could easily see 
 the watch-dial at the distance of a mile. The meridian circle has, 
 indeed, such delicacy as a sighting instrument, that it could be 
 pointed separately to each of two stars, which subtend at the eye 
 an angle no greater than that subtended by an adjoining pair of 
 the sixty minute dots around the circumference of a watch dial a 
 mile away. 
 
 This delicacy of sighting would be of little use were it not 
 combined with arrangements by which, when the telescope has 
 been pointed correctly, its position can be ascertained and recorded. 
 One element is secured by the astronomical clock, which gives the 
 moment when the object crosses the central vertical wire ; the other 
 element is given by the graduated circle which reads the zenith 
 distance. 
 
 Superb meridian instruments adorn our great observatories, and 
 
22 THE STORY OF THE HEAVENS. 
 
 are nightly devoted to those measurements upon which the great 
 truths of astronomy are mainly based. These instruments are 
 made with every refinement of skill; but it is the duty of the 
 painstaking astronomer to distrust the accuracy of his instrument 
 in every conceivable way. The great tube may be as rigid a 
 structure as mechanical engineers can produce; the divisions on 
 the circle may have been engraved by the most perfect mechanical 
 contrivance ; but the conscientious astronomer will not rely upon 
 mechanical precision. That meridian circle which, to the unin- 
 itiated, seems a marvellous piece of workmanship possessing 
 almost illimitable accuracy, is presented in a different light to the 
 astronomer who makes use of it. No one can appreciate, indeed, 
 so fully as he, the skill of the artist who has made it, and the 
 numerous beautiful contrivances for illumination and reading off, 
 which give to the instrument its perfection ; but while he 
 recognises the beauty of the actual machine he is using, the 
 astronomer has always before his mind's eye an ideal instrument 
 of absolute perfection, to which the actual meridian circle only 
 makes an approximation. Contrasted with this ideal instrument 
 the best meridian circle is little more than a mass of imperfections. 
 The ideal tube is perfectly rigid, the actual tube is flexible ; 
 the ideal divisions of the circle are all perfectly uniform; the 
 actual divisions are not uniform. The ideal instrument is a 
 geometrical embodiment of perfect circles, perfect straight lines, 
 and perfect right angles ; the actual instrument can only give 
 us approximate circles, approximate straight lines, and approxi- 
 mate right angles. Perhaps the spider's part of the work is on the 
 whole the best ; he gives us the nearest mechanical approach to a 
 perfectly straight line ; but we mar his work by not being able to 
 put in his beautiful threads with perfect uniformity, while our 
 attempts to stretch two of them across the field of view at right 
 angles, do not succeed in producing an angle of exactly ninety 
 degrees. Nor are the difficulties encountered by the meridian 
 observer solely due to his instrument. He has to contend with his 
 own want of skill ; he has often to allow for personal peculiarities 
 of an unexpected nature ; the troubles that the atmosphere can give 
 
THE ASTRONOMICAL OBSERVATORY. 23 
 
 him are notorious ; while the levelling of his instrument tells him 
 that he cannot even rely on the solid earth itself. The meridian 
 circle shows that the earthquakes, which sometimes startle us, are 
 merely the more conspicuous instances of incessant and universal 
 movements in the earth, which every night in the year derange 
 the delicacy of the instrument. 
 
 When the existence of these errors has been recognised, the first 
 great step has been taken. By an alliance between the astronomer 
 and the mathematician it is possible to measure the differences and 
 the irregularities which separate the ideal meridian circle from the 
 actual meridian circle. Once this has been done, it is possible 
 to estimate the effect which all the irregularities can produce on 
 the observations, and finally, to purge the observations from the 
 grosser errors with which they are contaminated. We thus have 
 observations, not indeed mathematically accurate, but still close 
 approximations to those which would be obtained by a perfect 
 observer, using an ideal instrument of geometrical accuracy, standing 
 on an earth of absolute rigidity, and viewing the heavens without 
 the intervention of the atmosphere. 
 
 It is not, however, necessary to use such great instruments as 
 those just described in order to obtain some idea of the aid the 
 telescope will afford in showing the celestial glories. The most 
 suitable instrument for commencing astronomical studies is within 
 ordinary reach. It is the ordinary binocular that a captain uses on 
 board ship ; or if that cannot be had, then the common opera-glass 
 will answer nearly as well. This is, no doubt, not nearly so 
 powerful as a large telescope, but it has some compensating advan- 
 tages which the telescope does not possess. The opera-glass will 
 survey a large region of the sky at once, while a telescope only 
 looks at a small part of the sky. Let us suppose that the observer 
 is provided with an opera-glass and is about to commence his 
 astronomical studies. 
 
 The first step is to become acquainted with the very renowned 
 group of seven stars which is represented in Fig. 8. It is 
 often called the Plough, but astronomers prefer to regard it 
 as a portion of the constellation of the Great Bear (Ursa Major). 
 
24 THE STOltY OF THE HEAVENS. 
 
 There are many features of interest in this constellation, and the 
 beginner should learn as soon as possible to identify the seven 
 remarkable stars. Of these the two stars, a and /3, at the head of 
 the bear are generally called the " pointers/' They are of special 
 use in astronomy, because they enable us to find out the most im- 
 portant star in the whole sky, which is known as the fc pole star." 
 We shall return in a later chapter to the study of the different 
 constellations. Our present object is a simpler one ; it is merely 
 to employ the Great Bear as a means of teaching us how vast is 
 
 Fig. 8. The Great Bear. 
 
 the richness of the heavens in stars. Each student of astro- 
 nomy is recommended to make one very simple observation on the 
 Great Bear, which will give a wondrous conception of what the 
 telescope can do, and will also reveal in a very impressive manner 
 the glories of the starry heavens. 
 
 Fix the attention on that region in the Great Bear bounded by 
 the four stars a ft 7 S. They form a sort of rectangle, of which 
 the stars named are the corners. The next fine night try and 
 count how many stars are visible within that rectangle. There 
 are no really bright stars, but there are two or three sufficiently 
 bright to be easily seen. On a very fine night, when there is no 
 moon, perhaps a dozen might be perceived, or even more, according 
 to the keenness of the eyesight.. But when the opera-glass is 
 directed to the same region, a most interesting, and indeed astonish- 
 
THE ASTRONOMICAL OBSERVATORY. 25 
 
 ing sight will be witnessed. Instead of the few stars which 
 were seen before with difficulty, a hundred stars or more can now 
 be seen with the greatest ease. The opera-glass will, indeed, easily 
 disclose ten times as many stars as could be seen with the unaided 
 eye. 
 
 But even the opera-glass will not show nearly all the stars in 
 this region. Any good telescope will reveal hundreds of stars too 
 faint for the opera-glass. The greater the telescope, the more 
 numerous the stars ; so that in one of the colossal instruments 
 this region would be found studded with thousands of stars. 
 
 We have chosen the Great Bear for the purpose of this illustra- 
 tion, because it is more generally known than any other constellation. 
 But the Great Bear is not exceptionally rich in stars ; any other 
 part of the sky would equally well have demonstrated the grand 
 truth, that the stars which our unaided eyes disclose, are only an 
 exceedingly small fraction of the entire number with which the 
 whole heaven is teeming. To tell the number of the stars is a task 
 which no man has accomplished ; but various estimates have been 
 made. Our great telescopes can probably show at least 50,000,000 
 stars. There would be a star apiece for every man, woman, and 
 child in the United Kingdom, and there would still remain a liberal 
 margin for distribution elsewhere. 
 
 The student of the heavens who uses a good refracting teles- 
 cope, having an object-glass about three inches in diameter, will 
 find ample and delightful occupation for many a fine evening. He 
 should also be provided with an atlas of the stars, while a copy of 
 the " Nautical Almanac/' and of Webb's " Celestial Objects for 
 Common Telescopes/' will form a sufficiently complete astronomical 
 equipment for much interesting occupation, 
 
CHAPTER II. 
 
 THE SUN. 
 
 The vast Size of the. Sun Hotter than Fusing Platinum Is the Sun the Source of 
 Heat for the Earth ? The Sun is 92,700,000 miles distant How to realise 
 the magnitude of this distance Day and Night Luminous and Non-Luminous 
 Bodies Contrast between the Sun and the Stars The Sun a Star The Spots on 
 the Sun Changes in the Form of a Spot They are Depressions on the Surface 
 The Kotation of the Sun on its Axis The Size and Weight of the Sun Is the 
 Sun a Solid Body ? View of a Typical Sun-Spot Periodicity of the Sun-Spots 
 Connection between the Sun-Spots Terrestrial Magnetism The Faculae 
 The Granulated Appearance of the Sun The Prominences surrounding the 
 Sun Total Eclipse of the Sun Size and Movement of the Prominences- 
 Drawings of the Objects, coloured The Corona Surrounding the Sun The 
 Heat of the Sun. 
 
 IN commencing our examination of the orbs which surround us, 
 we naturally begin with our peerless sun. llis splendid brilliance 
 gives him the pre-eminence over all other celestial bodies. The 
 proportions of the sun are commensurate with his importance. 
 
 Astronomers are actually able to measure the sun; and they 
 find that his dimensions are so great as to tax our imagina- 
 tion to realise them. The diameter of the sun, or the length of 
 the axis, passing through the centre from one side to the other, 
 is 865,000 miles. Yet this bare statement of the dimensions 
 of the great globe fails to awaken an adequate idea of its vast- 
 ness. If a railway were laid round the sun, and if we were to 
 start in an express train moving sixty miles an hour, we should 
 have to travel night and day for five years without intermission 
 before we had accomplished our journey. 
 
 If the sun be compared with the size of the earth, its stupendous 
 bulk becomes still more apparent. Suppose his globe were cut up 
 into one million parts : each of these parts would appreciably exceed 
 the bulk of our earth. Were the sun placed in one pan of a 
 
THE SUN. 27 
 
 mighty weighing balance, and were 300,000 bodies as heavy as our 
 earth placed in the other, the sun would still turn the scale. Fig. 9 
 exhibits a large white circle and a very small one. These circles 
 are drawn to exhibit the comparative size of the earth and the sun, 
 the small circle being the earth and the large one the sun. 
 
 The temperature of the sun has an intensity far surpassing the 
 greatest temperature we can artificially produce. In our labora- 
 tories we send a galvanic current through a piece of platinum 
 
 Fig. 9. Comparative Sizes of the Earth, and the Sun. 
 
 wire. The wire first becomes red-hot, then white-hot; then it is 
 almost of dazzling brilliancy, until it fuses and breaks. The 
 temperature of the melting platinum wire could hardly be sur- 
 passed in the most elaborate furnaces, but it falls far short of the 
 temperature of the sun. 
 
 It must, however, be admitted that there is one seeming dis- 
 crepancy between the fact of the sun's high temperature and a 
 well-known physical fact. " If the sun were hot/' it has been 
 said, " then the nearer we get to the sun the hotter we should be ; 
 yet this is not the case. On the top of a high mountain we are 
 nearer to the sun, and yet everybody knows that it is much colder 
 
28 THE STORY OF THE HEAVEXS. 
 
 on the top of the mountain than in the valley at its foot. If the 
 mountain be as high as Mont Blanc, then we are two or three miles 
 nearer the sun; yet up there, instead of additional warmth, we 
 find eternal snow/'' A simple illustration will dispose of this diffi- 
 culty. Go into a greenhouse on a sunshiny day, and we find the 
 temperature much hotter there than outside. The glass will allow 
 the hot sunbeams to enter, but it refuses to allow them out again 
 with equal freedom, and consequently the temperature rises. Our 
 whole earth is in this way to be likened to a greenhouse, only, 
 instead of the panes of glass, we are enveloped by an enormous 
 coating of atmosphere. When we are on the earth's surface, we 
 are, as it were, inside the greenhouse, and we benefit by the 
 interposition of the atmosphere ; but when we begin to climb very 
 high mountains, we gradually get through the atmosphere, and 
 then we suffer from the cold. If we could imagine the earth 
 to be stripped of its coat of air, then eternal frost would reign over 
 the whole earth as well as on the tops of the mountains. 
 
 The actual distance of the sun from the earth is about 
 92,700,000 miles; but merely reciting the figures does not give 
 a vivid impression of the real magnitude. 92,700,000 is a very 
 large quantity. Try to count it. It would be necessary to count 
 as quickly as possible for three days and three nights before one 
 million was completed ; yet this would have to be repeated nearly 
 ninety-three times before we had even counted all the miles 
 between the earth and the sun. 
 
 Every clear night we see a vast host of stars scattered over the 
 sky. Some are bright, some are faint, some are grouped into 
 remarkable forms. With regard to this vast host we can now ask 
 an important question. Are they bodies which shine by theii 
 own light like the sun, or do they only shine with light borrowed 
 from the sun ? The answer is easily stated. Most of these bodies 
 shine by their own light, and they are properly called stars. 
 If, then, our sun and the multitude of stars, properly so called, 
 are each and all self-luminous brilliant bodies, what is the great 
 distinction between the sun and the stars ? There is, of course, a 
 vast and obvious difference between the unrivalled splendour of the 
 
THE SUN. 29 
 
 sun and the feeble twinkle of the stars. Yet this distinction does 
 not necessarily indicate that the sun has an intrinsic splendour 
 superior to that of the stars. The fact is that we are nestled up 
 comparatively close to the sun for the benefit of his warmth and 
 light, while we are separated from even the nearest of the stars by 
 a mighty abyss. If the sun were gradually to retreat from the 
 earth his light would decrease, so that when he had penetrated the 
 depths of space to a distance comparable with that by which we are 
 separated from the stars, his glory would have utterly departed. 
 No longer would the sun be the majestic orb with which we are 
 familiar. No longer would he be a source of genial heat, or a 
 luminary to dispel the darkness of night. Our great sun would 
 have shrunk to the unimportance of a star, not so bright as many 
 of those which we see every night. 
 
 Momentous indeed is the conclusion to which we are now led. 
 That myriad host of stars which stud our sky every night, has 
 sprung into vast importance. Each one of those stars is itself 
 a mighty sun, actually rivalling, and in many cases surpassing, 
 the splendour of our own luminary. We thus open up a majestic 
 conception of the vast dimensions of space, and of the dignity and 
 splendour of the myriad globes by which that space is tenanted. 
 
 There is another aspect of the picture not without its utility. 
 We must from henceforth remember that our sun is only a star, 
 and not by any means an important star. If the sun and the 
 earth, and all which it contains, were to vanish, the effect in the 
 universe would merely be, that a tiny star had ceased its twinkling. 
 Viewed merely as a star, the sun thus assumes a place of insig- 
 nificance in the mighty fabric of the universe. But it is not as a 
 star that we have to deal with the sun. To us his proximity gives 
 him an importance incalculably transcending that of all the other 
 stars. We receded from the sun to obtain his true perspective in 
 the universe ; let us now draw near, and give to him that attention 
 which his importance requires. 
 
 To the unaided eye the sun appears to be a flat circle. If, 
 however, it be examined with the telescope, taking care of course 
 to employ a piece of deep-coloured glass, or some similar pre- 
 
30 THE STOEY OF THE HEAVENS. 
 
 caution to screen the eye from injury, it will then be seen that the 
 sun is really not a flat surface but a glowing globe, of which one 
 hemisphere is presented to us. The first question which we must 
 attempt to answer is with reference to the constitution of that 
 globe ; and the first branch of that question is, whether the glowing 
 
 Pig. 10. The Sun, photographed on September 22, 1870. 
 
 matter which forms the globe is a solid mass, and, if it be not 
 solid, whether it is liquid or gaseous ? At the first glance we 
 might think that the sun must certainly be solid. We have all 
 seen white-hot iron, and we might naturally think that the sun 
 was a stupendous ball of solid white-hot substance, or something 
 analogous thereto. But this view could not be correct ; and our 
 first task will be to show that the sun is certainly not a solid 
 body so far as we can see it. 
 
THE SUN. 31 
 
 Look at the general view of the sun shown by a telescope of 
 moderate dimensions. It is represented in Fig. 10.* We observe 
 the circular outline and the general bright surface, but the 
 brilliancy of the surface is not quite uninterrupted. There are, 
 here and there on the surface, small, dark objects called spots, 
 which can be made to render a great deal of information with 
 respect to the sun. These spots vary both as to size and as to 
 number indeed, the sun seems sometimes almost devoid of spots. 
 In the early days of telescopes the discoverers of the sun's spots 
 were laughed at. They were told that the sun was far too perfect 
 to have any blemishes, and that it was absurd to suppose that 
 ' ' the eye of the universe could suffer from ophthalmia." 
 
 The general character of a sun spot, as seen in a moderate 
 telescope under ordinary circumstances, is illustrated in Fig. 11, 
 
 Tig. 11. An Ordinary Sun-Spot. 
 
 in which the dark central part is seen well contrasted with 
 the lighter margin. Various theories have been propounded 
 as to the nature of these curious features, and one of the early 
 suppositions was that they were merely objects situated between 
 the earth and the sun, and thus projected on the sun as a back- 
 ground. It is easy to prove that this cannot be the case, by 
 carefully watching the same spot for a few days. Look first at 
 the spot marked A in Fig. 12, which exhibits a small portion 
 of the sun and a small part of its edge as seen in a good 
 telescope; A represents a spot of an ordinary type on the sun. 
 The central portion of the spot seems black by contrast with the 
 brilliant background in which the spot is shown. Around the 
 black centre we have a shaded region of a somewhat lighter hue. 
 We carefully observe the spot and note how far off it appears 
 
 * This picture is a copy of a very beautiful photograph of the sun, taken by 
 Mr. Euthcrfurd at New York on the 22nd September. 1870. 
 
32 THE STORY OF THE HEAVENS. 
 
 to be from the edge of the sun. The next day we repeat the 
 observation, and find that the spot is no longer in its original 
 position. It has travelled nearer the edge of the sun, to the place 
 marked B, Repeat the same observation the third day, and it will 
 be found that the spot has attained the position c, again nearer to 
 the edge of the sun. It will also be noticed that the appearance 
 of the spot has changed. The shaded portion at one side has 
 diminished and, indeed, disappeared. Day after day the spot 
 gradually approaches the edge of the sun until, finally, it is on 
 
 Fig. 12. Successive Appearances of a Sun-Spot. 
 
 rare occasions actually seen on the sun's edge, though it is not 
 so represented in this drawing. 
 
 It is by such observations we learn that the spot cannot be any 
 body floating aloft above the surface of the sun, for then an 
 interval between the spot and the sun would be seen after the spot 
 attained the edge. Yet, though we must admit that the spots are 
 really on the surface of the sun, we cannot agree with those old 
 philosophers who said that these spots are blemishes which detract 
 from the perfection of the eye of the heavens. We ought rather, 
 in these days of scientific activity, to feel grateful to the spots ; 
 for they teach us much about the glories of the sun of which 
 we otherwise must have remained in ignorance. 
 
 If an artist were to view the changes in the appearance of the 
 spot as it approached the sun's edge, he might think that the 
 apparent alterations of the form of the spot were chiefly due to 
 
THE SUN. 
 
 33 
 
 what he would call the effect of fore-shortening-, and he would 
 draw the conclusion that the spot was really a cavity in the surface 
 of the sun. If the interior parts of the sun were much darker than 
 the exterior, and if the spots were really basin-shaped apertures 
 through the outer portions of the sun, then some of the changes in 
 
 Fig. 13. Schemer's Observations on Sun-Spots. 
 
 the appearances of the spots could be readily explained. When the 
 region is turned directly towards the observer he sees the bottom of 
 the basin which exposes the dark interior of the sun. The view of 
 the spot is then represented by A. As the spot is carried nearer to 
 the limb of the sun, one side of the basin becomes much fore- 
 shortened, and the appearance of the spot is represented at B, in 
 which we observe that the shaded edge of the spot on the left is 
 narrower than that on the other side. Finally, when the spot is 
 
34 TEE STORY OF THE HEAVENS 
 
 extremely close to the limb, then one side of the basin would be 
 entirely hid from view, and only a glimpse be had of the dark 
 interior; while the opposite side of the basin is distorted into 
 undue prominence. It cannot, however, be regarded as proved 
 that the sun-spots are really depressions in the surface; indeed 
 many astronomers hold different views on the subject. 
 
 The progress of the spots over the face of the sun is well illus- 
 trated in the drawing shown in Fig. 13, which was made more than 
 250 years ago. On the 2nd March, 1627, the skilful astronomer 
 Scheiner observed a spot in the position marked 2, just on the edge 
 ; of the sun. On the next day, the spot had moved to the position 
 ' marked 3. It appears that Scheiner was favoured by a succession 
 of fine days, for day after day he followed the spot until the 
 eleventh, when his observations were interrupted. On this day, 
 therefore, he marked a blank on his drawing, in the position which 
 he reasonably conjectured to have been that which the spot 
 occupied. The following day he renewed his observation. The 
 13th was again cloudy, but 011 the 14th another and final view of 
 the spot was obtained, just as it was approaching the edge of the 
 sun and before its disappearance. In the same month it so hap- 
 pened that another conspicuous spot was visible on the sun, and 
 the faithful Scheiner recorded its place also; and again we find 
 interruptions due to the clouds on the llth and 13th. In each 
 case the spot travelled in the same direction and crossed the face 
 of the sun in a period of about twelve or thirteen days. 
 
 It is invariably found that these objects move across the sun in 
 the same direction. It is -also noticed that when the spots disappear 
 at the edge of the sun, and remain invisible for twelve or thirteen 
 days, the same spots often reappear at the other edge. It is there- 
 fore obvious that the spots must move round the back of the 
 sun in about the same time that they occupy in crossing his face. 
 Further inquiries on this subject have enabled the movements of 
 the spots^to be measured with accuracy, and it has been shown that 
 each spot accomplishes a complete revolution around the sun in 
 about twenty-five days and five hours. 
 
 So remarkable a characteristic of the movements of the spots 
 
THE SUN. 35 
 
 demands satisfactory explanation. How does it come to pass that 
 the spots both large and small,, regular or irregular all accom- 
 plish their revolutions in nearly the same time ? A very simple 
 explanation of the phenomenon conducts at once to a very important 
 discovery. 
 
 We know that the sun is a globe, and that our earth .is also a 
 globe. We also know that the earth performs a daily rotation on 
 its axis, and it is natural to ask, whether it may not be likewise pos- 
 sible for the sun to perform a rotation. If the sun slowly rotated in 
 a period of about twenty-five days and five hours, then the hemi- 
 sphere of the sun directed towards the earth would be completely 
 turned to the other side in a fortnight, and we should at once 
 be able to ' account for the apparent movement of the spots. This 
 explanation is so simple, and so satisfactory it is confirmed by so 
 many other lines of reasoning that no doubt can any longer be 
 attached to it ; and hence we have, as the first-fruits of the study 
 of the spots, the very interesting and remarkable discovery of the 
 rotation of the sun on its axis. It has, however, been shown that 
 the time of rotation of the sun-spot varies slightly with the posi- 
 tion of the spot. Observations made with spots at the Equator 
 give for the period of rotation a value of 25 J days, while, judg- 
 ing from spots at the latitude of 30, the period of rotation of the 
 sun is a day longer. It thus follows that we cannot state the 
 period of the sun's rotation with the same accuracy that we can 
 that of the earth. It can only be said to lie somewhere between 
 the two extremes of about 25 and 26 J days. 
 
 It must not, however, be imagined that the only changes in the 
 spots are those variations of perspective which arise from the 
 rotation of the sun. In this movement all spots alike participate ; 
 but there are other movements and changes constantly going on in 
 the individual spots. Some of these objects may last for days, 
 for weeks, or for months, but they are in no sense permanent ; and 
 after an existence of greater or less duration, the spots on one part 
 of the sun will disappear, while as frequently fresh spots will 
 become visible in other places. The inference from these various 
 facts is irresistible. It tells us that the visible surface of the sun 
 D 2 
 
36 THE STORY OF THE HEAVENS. 
 
 is not a solid mass is not even a liquid mass but that the sun, 
 as far as we can see it, consists of matter in the gaseous or vaporous 
 condition. 
 
 The sun-spots are confined to certain limited regions of the sur- 
 face. They are nearly always found in two zones on each side of the 
 Equator,, and between the latitudes of 10 and 30 degrees. Spots are 
 comparatively rare on the Equator,, E E' (Fig. 14) and very few are 
 found beyond the latitude of 35 degrees, while we have the authority 
 of Professor Young for the statement that there is only a single 
 recorded instance of a spot more than 45 degrees from the Solar 
 
 Fig. 14. Zones on the Sun's surface in which Spots appear. 
 
 Equator one observed in 1846 by Dr. Peters at Naples. The 
 average duration of a sun-spot is about two or three months, and 
 the longest life of a spot that has been recorded is one which in 
 1840 and 1841 lasted for eighteen months. There are, however, 
 some spots which last only for a day or two, and some only for a. 
 few hours. 
 
 It should also be observed that the sun-spots usually appear in 
 groups, and very often a large spot is attended or followed by a 
 number of smaller ones, more or less imperfect. It often happens 
 that a large spot divides into two or more smaller spots, and these 
 parts have been sometimes seen to fly apart, with a velocity in some- 
 cases not less than a thousand miles an hour. On rare occasions a 
 phenomenon of the most surprising character has been witnessed in 
 connection with the sun-spots, where patches of intense brightness 
 suddenly break out, remain visible for a few minutes, and trave) 
 
THE SUN. 37 
 
 with a velocity over 100 miles a second. One of these events has 
 become celebrated for the extraordinary character of the phenomena, 
 as well as for the fortunate circumstance that it has been 
 authenticated by the independent testimony of two skilled wit- 
 nesses. On the forenoon of the 1st September, 1859, two well- 
 known observers of the sun, Mr. Carrington and Mr. Hodgson, 
 were both engaged in observation. Mr. Carrington was employed 
 at his self-imposed daily task of observing the positions, the 
 configuration, and the size of the spots by means of an image of 
 the sun upon a screen. Mr. Hodgson, many miles away, was at 
 the same moment sketching some details of sun-spot structure. 
 They saw simultaneously two luminous objects, shaped something 
 like two new moons, each about eight thousand miles long and 
 two thousand miles wide, at a distance of about twelve thousand 
 miles apart : these suddenly burst into view near the edge of a 
 great sun-spot, with a brightness at least five or six times that 
 of the neighbouring parts of the sun, and travelled eastward over 
 the spot in parallel lines, growing smaller and fainter, until in 
 about five minutes they disappeared, after a journey of about thirty- 
 six thousand miles. 
 
 We have still to note one very extraordinary feature, which 
 points to an intimate connection between the phenomena of sun- 
 spots, and the purely terrestrial phenomena of magnetism. It has 
 been noticed that the occurrence of the maximum of sun-spots 
 occurs simultaneously with an unusual amount of disturbance of 
 the magnetic needle. The latter are well known to be connected 
 with the phenomena of the aurora borealis, inasmuch as an unusual 
 aurora seems to be invariably accompanied by a great magnetic 
 disturbance. It has also been shown that there is an almost perfect 
 parallelism between the intensity of auroral phenomena and the 
 abundance of sun-spots. Besides these general coincidences, there 
 have been also special cases in which a peculiar outbreak on the sun 
 has been associated with remarkable auroral or magnetic phe- 
 nomena. Thus, the occurrence cited above as witnessed by Mr, 
 Carrington and Mr. Hodgson in 1859, was immediately followed 
 by a magnetic storm of unusual intensity, as well as by splendid 
 
S3 THE STORY OF THE HEAVENS. 
 
 auroras, not only in Europe and America, but even in the Southern 
 Hemisphere. A very interesting instance of a similar kind is 
 recorded by Professor Young, who, when observing at Sherman on 
 the 3rd August, 1872, perceived a very violent disturbance of the 
 sun's surface. He was told the same day by the photographer of 
 the party, who was engaged in magnetic observations, and whc 
 was quite in ignorance of what Professor Young had seen, that he 
 had been obliged to desist from the magnetic observations, in con- 
 
 Fig. 15. The Texture of the Sun and a small Spot. 
 
 sequence of the violent fluctuations of the needle. Subsequent 
 inquiry showed that in England on the same day a magnetic storm 
 was also witnessed. 
 
 These observations demonstrate that there is some connection 
 between solar phenomena and terrestrial magnetism, but what the 
 nature of that connection may be is quite unknown, and will form 
 a problem of deep interest for the future labours of astronomers 
 and physicists. 
 
 Another mysterious law governs the sun-spots. Their number 
 fluctuates from year to year, but it would seem that the epochs of 
 maximum sun-spots succeed each other with a certain degree of 
 regularity. The observations of sun-spots for nearly three cen- 
 
THE SUN. 39 
 
 furies show that the recurrence of a maximum takes place, on an 
 average, every eleven years. The course of one of these cycles is 
 somewhat as follows : For two or three years the sun-spots are 
 both larger and more numerous than on the average ; then they 
 begin to diminish, until in about five or six years from the maximum 
 
 Fig. 16. Dr. Huggins' drawing of a remarkable arrangement of Solar Granules. 
 
 they reach a minimum ; then the spots begin to increase, and in 
 another five or six years the maximum is once more attained. The 
 cause of this periodicity is a question of the most profound interest, 
 but at present the answer must be regarded as unknown. It has, 
 indeed, to be admitted that the real nature of sun-spots is still a 
 matter of uncertainty. No theory yet proposed will account in 
 a thoroughly satisfactory manner for all the phenomena which 
 
40 THE STORY OF THE HEAVENS. 
 
 they present, when viewed with the telescope and the spectroscope, 
 as well as for their peculiar distribution over the sun, and the 
 marvellous phenomena of periodicity. 
 
 When the atmosphere will allow of very good vision, we can see 
 that the sun's surface is mottled in a remarkable manner. This 
 is well shown in Fig. ] 5, in which we perceive that the spot in the 
 central part of the picture is merely an enlargement of one of the 
 minute pores with which the surface is marked. A very remark- 
 able instance of the granulated appearance which the sun often 
 presents is shown in a drawing made by the accurate pencil of 
 Dr. W. Huggins (Fig. 16). This curious arrangement has also 
 
 Fig. 17. The Willow-leaf texture of the Sun's surface. 
 
 been witnessed by many other observers. Indeed, photographs 
 have been taken in which these brilliant granules seem disposed to 
 arrange themselves in patterns of marvellous regularity. 
 
 It would thus appear as if the luminous surface of the sun was com- 
 posed of intensely bright clouds suspended in a darker atmosphere. 
 Some observers have thought that these floating objects are, occa- 
 sionally at all events, of a characteristic size and shape, variously 
 known as " willow leaves" or "rice grains." In Fig. 17, the 
 curious willow-leaf texture is shown surrounding a sun-spot. But 
 the spot itself seldom fails to give the impression of violent dis- 
 turbance, as is well shown in Professor Langley's fine drawing 
 (Plate II.) of a spot which he observed on December 23-24, 1873. 
 
 Near the edge of the sun, as represented in Plate III., will be seen 
 some of those brighter streaks or patches which are called faculse 
 

o 
 
 Vg 2 
 
 3 " 
 
 o < 
 
 < 
 
THE SUN. 41 
 
 (little torches). They are often of enormous dimensions, covering 
 areas vastly larger than any of our continents. 
 
 The margin of the sun is fringed with objects of very great 
 interest. They are so faint that in the full blaze of sunlight 
 they cannot be seen. They are invisible for the same reason that 
 stars are invisible in daylight. We see the stars at night, when 
 the sun is gone, and so we can see the fringe surrounding the sun 
 when the brilliant central portion is obscured by the rare occur- 
 rence of a total eclipse. 
 
 For an eclipse of the sun to occur, the moon must actually come 
 between the earth and the sun. The occurrence of an eclipse will 
 
 Fig. 18. Prominences seen in Total Eclipse. 
 
 be more fully considered later on. For the present it will be 
 sufficient to observe that by the movement of the moon it may so 
 happen that the moon completely hides the sun, and thus for a few 
 minutes produces what we call a total eclipse. The few minutes 
 during which a total eclipse lasts are of the most priceless value to 
 the astronomer. Darkness reigns over the earth, and in that 
 darkness rare and beautiful sights can be witnessed. 
 
 We have in Fig. 18, a view of a total eclipse, showing some of 
 those remarkable objects known as prominences, (a, l y c, d, e), which 
 project from the surface of the sun. Objects of this character 
 surround the sun at other times as well as at eclipses, but their 
 light is so faint that the great light of the sun renders them 
 invisible. With the obscurity which surrounds the sun during 
 
42 THE STOEY OF THE HEAVENS. 
 
 a total eclipse as a background, the phenomenon starts into 
 brilliancy. 
 
 It has been demonstrated that these very curious objects are, 
 as their appearance indicates, really mighty glowing masses of gas ; 
 and a most beautiful arrangement has been discovered by which it 
 has been made possible to view the prominences without waiting 
 for the aid of an eclipse. It would be anticipating what we shall 
 have to say in a future chapter were we at this point to give any 
 detailed explanation of the ingenious contrivance by which these 
 objects can be seen in the full blaze of sunlight. Suffice it now to 
 observe that the principle of the method depends upon the peculiar 
 character of the light from the prominences, which the spectroscope 
 enables us to isolate from the glare produced by the ordinary solar 
 beams. It gives to astronomers the great advantage of looking at 
 the prominences for hours together, instead of being limited to the 
 few minutes during which an eclipse lasts. The prominences appear 
 to be merely protuberant portions of a layer of red incandescent 
 gas surrounding the sun. This gas has been shown to consist of 
 hydrogen and probably other substances. 
 
 Majestic indeed are the proportions of some of those mighty 
 flames which leap from the surface of the sun ; yet these flames 
 flicker as do our terrestrial flames, when we allow them time com- 
 parable to their gigantic dimensions. Drawings of the same 
 prominence often show great changes in a few hours, or even less. 
 The magnitude of the changes could not be less than many 
 thousands of miles, and the actual velocity with which such 
 masses move is often not less than 100 miles a second. Still 
 more violent are the solar convulsions which some observers have 
 been so fortunate as to behold, when from the sun's surface, as 
 from a mighty furnace, vast incandescent masses are projected 
 upwards. All indications point to the surface of the sun as the 
 seat of the most frightful storms and tempests, in which the winds 
 sweep along incandescent vapours. 
 
 The remarkable power which the spectroscope places at our 
 disposal of enabling the prominences to be seen without a total 
 eclipse has been largely availed of in making drawings of these 
 
THE SUN. 43 
 
 objects. Plate IV. gives a very beautiful view of a number of 
 them as seen by Trouvelot with the great telescope at Cambridge, 
 U.S. These drawings show the red colour of the flame-like 
 objects, not very happily described as prominences ; and they also 
 show, in the different pictures, the wondrous variety of aspect which 
 these objects assume. The dimensions of the prominences may 
 be inferred from the scale appended to the plate. The largest of 
 them is fully 80,000 miles high; but many observers have re- 
 corded prominences of much greater altitude. The rapid changes 
 of these objects is well illustrated in the two sketches on the left 
 of the lowest line, which were drawn on April 27th, 1872. These 
 are both drawings of the same prominence taken at an interval no 
 greater than twenty minutes. This mighty flame is so vast that 
 its length is ten times as great as the diameter of the earth, yet 
 in this brief period it has completely changed its aspect; the 
 upper part of the flame has, indeed, broken away, and is now 
 shown in that part of the drawing between the two figures on the 
 line above. The drawings also show various instances of the re- 
 markable spike-like prominences, taken at different times and on 
 different parts of the sun. These spikes usually attain altitudes 
 not greater than 20,000 miles, but sometimes they stretch up to 
 stupendous distances. We may quote one special object of this 
 kind, whose remarkable history has been chronicled by Professor 
 Young,* the well-known authority in this department of as- 
 tronomy. On October 7th, 1880, a prominence was seen, at 
 about 10.30 a.m., on the south-east limb of the sun. It was 
 then an object of no unusual appearance, being about 40,000 
 miles high, and attracted no special attention ; but half an hour 
 witnessed a marvellous transformation. During that brief interval 
 the prominence became very brilliant, and doubled its length. 
 For another hour the mighty flame still soared upwards, until it 
 attained the unprecedented elevation of 350,000 miles a distance 
 more than one-third of the diameter of the sun. Here the energy 
 
 * During a visit to the United States in the autumn of 1884, the author was 
 fortunate enough, by the kindness of Professor Young, to observe several solar 
 prominences with the superb instruments at Princeton, New Jersey. 
 
44 THE STORY OF THE HEAVENS. 
 
 of the mighty outbreak seems to have expended itself : the flame 
 broke up into filaments, and by 12.30 an interval of only two 
 hours from the time when it was first noticed the huge promi- 
 nence had completely faded away. 
 
 The facts we have recorded give a surprising indication of the 
 violence of those fiery storms by which the surface of the sun is 
 occasionally disturbed. No doubt this vast prominence was excep- 
 tional in its magnitude,, and in the vastness of the changes of 
 which it was an indication ; but we may, at all events, take it as 
 the basis of an estimate of the maximum changes which the surface 
 of the sun witnesses. The velocity must have been 200,000 miles 
 an hour a rate which must have more than averaged fifty miles 
 a second. This mighty flame leaped up from the sun with a velo- 
 city more than 100 times as great as that of the swiftest bullet 
 that was ever fired from a rifle. 
 
 The most striking feature of a total eclipse of the sun is 
 unquestionably the Corona, or aureole of light which is then seen to 
 surround the sun. On such an occasion, when the sky is clear, the 
 moon appears of an inky darkness, not like a flat screen, but like 
 the huge black ball that it really is. " From behind it (I quote 
 Professor Young) stream out on all sides radiant filaments, beams, 
 and sheets of pearly light, which reach to a distance sometimes of 
 several degrees from the solar surface, forming an irregular stellate 
 halo with the black globe of the moon in its apparent centre. The 
 portion nearest the sun is of dazzling brightness, but still less 
 brilliant than the prominences which blaze through it like car- 
 buncles. Generally this inner corona has a pretty uniform height, 
 forming a ring three or four minutes of arc in width, separated by 
 a somewhat definite outline from the outer corona, which reaches to 
 a much greater distance, and is far more irregular in form; usually 
 there are several " rifts/' as they have been called, like narrow 
 beams of darkness, extending from the very edge of the sun to 
 the outer night, and much resembling the cloud shadows which 
 radiate from the sun before a thunder shower. But the edges of 
 these rifts are frequently curved, showing them to be something 
 else than real shadows ; sometimes there are narrow bright streamers 
 
PLATE IV. 
 
 Scale of English Miles. 
 
 10 20 30 40 50 60 70 80 90 100,000 
 
 1 I I I I I I I I I I 
 
 SOLAR PROMINENCES. 
 (DRAWN BY TROUVELOT AT HARVARD COLLEGE, CAMBRIDGE, U.S., IN 1872.) 
 
THE SUN. 45 
 
 as long as the rifts, or longer. These are often inclined, oc- 
 casionally are even nearly tangential to the solar surface, and 
 frequently curved. On the whole, the corona is usually less exten- 
 sive and brilliant on the solar poles, and there is a recognisable 
 tendency to accumulation above the middle latitudes or spot zones, 
 
 Fig. 19. View of the Corona in a total eclipse. 
 
 so that roughly speaking the corona shows a disposition to assume 
 the form of a quadrilateral or four-rayed star, though in almost 
 every individual case this form is greatly modified by abnormal 
 streamers at some point or other/ 1 ' Fig. 19 represents a view of 
 the corona during a total eclipse. 
 
 We further present, in Plate V., the drawing made by Professor 
 W. Harkness, which represents the corona as obtained from a 
 
46 THE STORY OF THE HEAVENS. 
 
 comparison of a large number of photographs taken at different 
 places in the United States during the total eclipse of July 29th, 
 1878. 
 
 As to the precise nature of this wonderful appendage, we must 
 for the present be content to wait for further light. Probably when 
 we understand the streamers of the aurora borealis and the tails 
 of comets, we shall have learned something of those substances, 
 well nigh spiritual in texture, which constitute the solar corona. 
 
 A remarkable appendage to the sun, which extends to a distance 
 much greater than that of the corona, produces the phenomenon 
 of the zodiacal light. A pearly glow is sometimes seen to spread 
 over a part of the sky in the vicinity of the point where the sun 
 has disappeared after sunset. The same spectacle may also be 
 witnessed before sunrise, and it would seem as if the material 
 producing the zodiacal light, whatever it may be, had a lens-shaped 
 form with the sun in the centre. The nature of this object is still 
 a matter of great uncertainty. We have represented in Fig. 20, 
 a view of this mysterious phenomenon. 
 
 In all directions the sun pours forth, with the most prodigal 
 liberality, its torrents of light and of heat. The greater part of that 
 light and heat seems quite wasted in the depths of space. Our earth 
 intercepts only the merest fraction, less than the 2,000,000,000th 
 part of the whole. Our fellow planets and the moon also intercept 
 a trifle ; but what portion of the mighty flood can they utilise ? 
 The sip that a flying swallow takes from a river is as far from ex- 
 hausting the water in the river as are the planets from using all the 
 heat which streams from the sun. Were the radiation of the sun 
 to be intercepted, all life on this earth must cease. An immovable 
 atmosphere would brood over an ocean which, if not actually frozen, 
 could be only disturbed by the sullen undulations of the tides, and 
 the silence of death over the surflce of the earth would only be 
 broken by the occasional groans of a volcano. 
 
 We must postpone to a future chapter the important question 
 of the source of the sun's heat. Let us simply terminate this 
 chapter by a brief recital of what we at present enjoy by the benign 
 influence of the sun. His gracious beams supply the magic power 
 
48 THE STORY OF THE HEAVENS. 
 
 that enables our corn to grow and ripen. It is the heat of the sun 
 which raises water from the ocean in the form of vapour, and then 
 sends down that vapour as rain to refresh the earth and to fill the 
 rivers, which bear our ships down to the ocean. It is the heat 
 of the sun beating on the large continents, which gives rise to the 
 breezes and winds that waft our vessels across the deep; and 
 when on a winter's evening we draw around the fire and feel its 
 invigorating rays, we are really only enjoying sunbeams which 
 shone on the earth countless ages ago. The heat in those ancient 
 sunbeams developed the mighty vegetation of the coal epoch, and 
 in the form of coal that heat has slumbered for millions of years, 
 till we now call it again into activity. It is the power of the sun 
 stored up in coal that urges on our steam-engines. It is the light 
 of the sun stored up in coal that beams from every gas-light in 
 our cities. 
 
 For our power to live and move, for the plenty with which we 
 are surrounded, for the beauty with which nature is adorned, we are 
 immediately indebted to one body in the countless hosts of space ; 
 and that body is the sun. 
 
PLATE V. 
 
 TOTAL SOLAR ECLIPSE, JULY 29TH, 1878. 
 
 THE CORONA FROM THE PHOTOGRAPHS. 
 
 (HARKNESS.) 
 
CHAPTER TTL 
 
 THE MOON. 
 
 The Moon and the Tides The Use of the Moon in Navigation The Changes of 
 the Moon The Moon and the Poets Whence the Light of the Moon ? Sizes 
 of the Earth and the Moon Weight of the Moon Changes in Apparent Size 
 Variations in its Distance Influence of the Earth on the Moon The Path 
 of the Moon Explanation of the Moon's Phases Lunar Eclipses Eclipses of 
 the Sun, how produced Visibility of the Moon in a Total Eclipse How 
 Eclipses are Predicted Uses of the Moon in finding Longitude The Moon 
 not connected with the Weather Topography of the Moon Nasmyth's 
 Drawing of Triesnecker Volcanoes on the Moon Normal Lunar Crater 
 Plato The Shadows of Lunar Mountains The Micrometer Lunar Heights 
 Former Activity on the Moon Nasmyth's View of the Formation of Craters 
 Gravitation on the Moon Varied Sizes of the Lunar Craters Other features 
 of the Moon Is there Life on the Moon? Absence of Water and of Air 
 Explanation of the Rugged Character of Lunar Scenery Possibility of Life 
 on Distant Bodies in Space. 
 
 IF the moon were suddenly to be struck out of existence, we should 
 be immediately apprised of the fact by a wail from every seaport 
 in the kingdom. From London, from Liverpool, from Bristol, 
 we should hear the same story the rise and fall of the tide had 
 almost ceased. The ships in dock could not get out; the ships 
 outside could not get in ; and the maritime commerce of the world 
 would be thrown into dire confusion. 
 
 It is the moon which, principally, causes the daily ebb and flow 
 of the tide, and this is the most important work which the moon 
 has to do. Fleets of fishing boats around our coasts time their 
 daily movements by the tide, and are largely indebted to the moon 
 for bringing them in and out of harbour. Experienced sailors assure 
 us that the tides are of the utmost service to navigation. The 
 question how the moon causes the tides, must be postponed to a 
 future chapter, where we shall also sketch the marvellous part which 
 the tides seem to have played in the past history of our earth. 
 
 E 
 
50 TEE STORY OF THE HEAVENS. 
 
 Who is there who has not watched, with admiration, the beau- 
 tiful series of changes through which the moon passes every month ? 
 ; We first see her as an exquisite crescent of pale light in the 
 western sky after sunset. Night after night she moves further and 
 further to^he east, until she becomes full, and rises about the same 
 time that the sun sets. From the time of full moon the disc of 
 light begins to diminish until the last quarter is reached. Then 
 it is that the moon is seen high in the heavens in the morning. 
 As the days pass by, the crescent shape is again assumed. The 
 crescent wanes thinner and thinner as the moon draws closer 
 to the sun. Finally she becomes lost in the overpowering light 
 of the sun, again to emerge as the new moon, and again to go- 
 through the same cycle of changes. 
 
 The brilliancy of the moon arises solely from the light of the 
 sun, which falls on the dark or not self-luminous substance of 
 the moon. Out of the vast flood of light, which the sun pours- 
 forth with such prodigality into space, the dark body of the moon 
 intercepts a little, and of that little it reflects a small fraction to- 
 illuminate the earth. The moon sheds so much light, and seems sc* 
 bright, that it is often difficult at night to remember that the moon 
 has no light except what falls on it from the sun. Nevertheless,, 
 the actual surface of the brightest full moon is perhaps not much 
 brighter than the streets of London on a clear sunshiny day. A very 
 simple observation will suffice to show that the moon's light is only 
 sunlight. Look some morning at the moon in daylight, and com- 
 pare the moon with the clouds. The brightness of the moon and 
 of the clouds are directly comparable, and then it is seen plainly 
 that the sun which illuminates the clouds has also illumined the- 
 moon. The attempt has been made to measure the relative bright- 
 ness of the sun and the full moon. If 600,000 full moons were 
 shining at once, their collective brilliancy would be equal to that of 
 the sun. 
 
 The beautiful crescent moon has furnished a theme for many 
 a poet. Indeed, if we may venture on the most gentle criticism, it 
 would seem that some poets have forgotten that the moon is not 
 to be seen every night. A poetical description of evening is almost 
 
THE MOON. 51 
 
 certain to be associated with a description of the moon in some 
 phase or other. We may cite one notable instance in which a 
 poet, describing- an historical event, has enshrined in exquisite 
 verse a statement which cannot have been correct. Every child 
 that speaks our language has been taught that the burial of Sir 
 John Moore took place 
 
 "By the struggling moonbeams' misty light." 
 
 There is an appearance of detail in this statement which wears 
 the garb of truth. We are not going to doubt that the night 
 was really misty, and inquire whether the moonbeams really had 
 to struggle into visibility, for the question is a much more 
 fundamental one. We do not know who was the first to raise the 
 point as to whether the moonbeams really shone on that memorable 
 event at all or not ; but the question having been raised, the Nautical 
 Almanac immediately supplies an answer. From it we learn in 
 language, whose truthfulness constitutes its only claim to be poetry, 
 that the moon was new on the 16th January, 1809, at one o'clock 
 in the morning of the day of the battle of Corunna. It is evidently 
 implied in the ballad that the funeral took place on the night 
 following the battle. We are therefore assured that the moon can 
 hardly have been a day old when the hero was consigned to his 
 grave. But the moon in such a case is practically invisible, and 
 yields no appreciable moonbeams at all, misty or otherwise. In- 
 deed, if the funeral took place at the " dead of night/' as the poet 
 tells us it did, the moon must have been far below the horizon at 
 the time.* 
 
 In alluding to this and similar instances, Mr. Nasmyth gives a 
 word of advice to authors or to artists who desire to bring the moon 
 on a scene without knowing as a matter of fact that the moon was 
 actually present. He recommends them to follow the example of 
 Bottom in Midsummer-Night's Dream, and consult "a, calendar, 
 
 * Some ungainly critic has observed that the poet himself seems to have felt a 
 doubt on the matter, because he has supplemented the dubious moonbeams by the 
 "lantern dimly burning." The more generous, if somewhat sanguine remark, has 
 been also made, that "the time will come when the evidence of this poem will 
 prevail over any astronomical calculations." 
 
 E 2 
 
52 
 
 THE STORY OF THE HEAVENS. 
 
 a calendar! look in the almanac; find out moonshine, find out 
 moonshine ! " 
 
 Among the countless host of celestial bodies the sun, the moon, 
 the planets, and the stars the moon enjoys one special claim on 
 our attention. The moon is our nearest neighbour. It is just 
 possible that a comet may occasionally dart past the earth at a 
 smaller distance than the moon, but with this exception the other 
 
 Fig. 21. Comparative sizes of the Earth and Moon. 
 
 bodies are all hundreds or thousands, or even many millions of 
 times as far off as the moon. 
 
 The moon is really one of the smallest objects visible to us 
 which the heavens contain. Every one of the thousands of stars 
 visible with the unaided eye is enormously larger than the moon. 
 The brilliancy and apparent size of the moon arises from the fact 
 that she is only 240,000 miles away, which is a distance almost 
 immeasurably small when compared with the distances of the stars 
 or other great bodies of the universe. 
 
 Fig. 21 exhibits the relative sizes of the earth and the 
 moon. The small globe represents the moon, while the larger 
 
THE MOON. 53 
 
 globe represents the earth. When we measure the actual dia- 
 meters of the two globes, we find that of the earth to be 
 7,918 miles, and of the moon 2,160 miles, so that the diameter 
 of the earth is nearly four times as great as the diameter of 
 the moon. If the earth were cut into fifty pieces, all equally 
 large, then one of these pieces rolled into a globe would equal 
 the size of the moon. The superficial extent of the moon is equal 
 to about one-thirteenth part of the surface of the earth. The 
 hemisphere of the moon turned towards us exhibits at any moment 
 an area equal to about one twenty-seventh part of the area of the 
 earth. This, to speak approximately, is about half the area of 
 Europe. The materials of the earth are, however, much heavier 
 than those contained in the moon. It would take more than 
 eighty globes, each as ponderous as the moon, to weigh down the 
 earth. 
 
 Amid the incessant changes which the moon presents to us, 
 one obvious fact stands forth prominently. Whether the moon be 
 new or full, at first quarter or at last, whether it be high in the 
 heavens or low near the horizon, whether it be in process of eclipse 
 by the sun, or whether the sun himself is being eclipsed by the 
 moon, one feature remains invariable the apparent size of the 
 moon is nearly constant. We can express the matter numerically. 
 A globe one foot in diameter, at a distance of 110 feet from 
 the observer, would under ordinary circumstances be just sufficient 
 to hide the disk of the moon ; occasionally, however, the globe 
 would have to be brought in to a distance of only 101 feet, or 
 occasionally it might have to be moved out to as much as 1 19 feet 
 if the moon is to be exactly hidden. It is unusual for either of 
 these limits to be approached, and the distance at which the globe 
 must be situated so as to exactly cover the moon is usually more 
 than 105 feet, and less than 115 feet. These fluctuations in the 
 apparent size of the moon are contained within such narrow limits, 
 that in the first glance at the subject they may be overlooked. 
 It will be easily seen that the apparent size of the moon must be 
 connected with its real distance from the earth. Suppose, for the 
 sake of illustration, that the moon were to recede into space, its 
 
54 THE STOEY OF THE HEAVENS. 
 
 size would seem to dwindle, and long ere it had reached the dis- 
 tance of even the very nearest of the other celestial bodies, it would 
 have shrunk into insignificance. On the other hand, if the moon 
 were to come nearer to the earth, its apparent size would gradually 
 increase, until, when close to the earth, it would seem like a mighty 
 continent stretching over the sky. We find that the apparent size 
 of the moon is nearly constant, and hence we infer that the real 
 distance of the moon is also constant. The average value of that 
 distance is 240,000 miles. It may approach under rare circum- 
 stances to a distance but little more than 220,000 miles; it may 
 recede under rare circumstances to a distance hardly less than 
 260,000 miles, but the ordinary fluctuations do not exceed more 
 than about 13,000 miles on either side of its mean value. 
 
 From the moon's incessant changes we perceive that she is in 
 constant motion, and we now further see that whatever these move- 
 ments may be, the earth and the moon must still always remp.in at 
 nearly the same distance apart. If we further add that the path 
 pursued by the moon around the heavens lies in a plane, then we 
 are forced to the conclusion that the moon must be revolving in 
 a nearly circular path around the earth at the centre. It can, 
 indeed, be shown that the constant distance of the two bodies 
 involves as a necessary condition the revolution of the moon around 
 the earth. The attraction between the moon and the earth tends 
 to bring the two bodies together. The only way by which such a 
 catastrophe can be permanently avoided is by causing the moon 
 to revolve around the earth. The attraction between the earth and 
 the moon still exists, but its effect is not then shown in bringing 
 the moon in towards the earth. The attraction has now to exert 
 its whole power in keeping the moon in its circular path ; were the 
 attraction to cease the moon would start off in a straight line, and 
 recede never to return. 
 
 The fact of the moon's revolution around the earth is easily 
 demonstrated by observations of the stars. The rising and setting 
 of the moon is of course due to the rotation of the earth, and this 
 apparent diurnal movement the moon has in common with the sun 
 and with the stars. It will, however, be noticed that the moon is 
 
THE MOON. 55 
 
 continually changing its place among the stars. Even in the 
 course of a single night, the displacement of the moon will be con- 
 spicuous to a careful observer, without the aid of a telescope. The 
 moon completes its revolution in 27*3 days. 
 
 In Fig. 22 we have a view of the relative positions of the 
 earth, the sun, and the moon, but it is to be observed that the 
 
 Above the Plane 
 
 Intersection of paths 
 of Earth and Moon 
 
 ;New 
 
 X. quarter.. '' 
 
 Below the Plane 
 
 Fig. 22. The Moon's path around the Earth. 
 
 distance of the sun is really much greater than can possibly be 
 represented in the figure. That half of the moon which is turned 
 towards the sun is brilliantly lighted up, and according as we see 
 more or less of that brilliant half we say that the moon is more or 
 less full, all the " phases " being visible in succession as shown by 
 
 ) I f < ( 
 
 Fig. 23. The Phases of the Moon. 
 
 the numbers in Fig. 23. A beginner sometimes finds a difficulty 
 in understanding how a full moon at night is really lighted by 
 the sun. "Is not/' he will say, "the earth in the way? and 
 must it not cut off the sunlight from every object on the other 
 side of the earth to the sun ?" A study of Fig. 22 will explain 
 the difficulty. The plane in which the moon revolves does not 
 coincide with the plane in which the earth revolves around the 
 sun. The line in which the plane of the earth's motion is in- 
 tersected by that of the moon divides the moon's path into two 
 semicircles. We must imagine the moon's path to be tilted a 
 
56 THE STORY OF THE HEAVENS. 
 
 little, so that the upper semicircle is somewhat above the plane of 
 the paper, and the other semicircle below. It thus follows that 
 when the moon is in the position marked full, under the cir- 
 cumstances shown in the figure, the moon will ( be just above 
 the line joining the earth and sun ; the sunlight will thus pass 
 over the earth to the moon, and the moon will be illuminated. 
 At new moon the moon will be under the line joining the earth 
 and the sun. 
 
 As the relative positions of the earth and the sun are changing, 
 it sometimes happens that the sun does come exactly into the posi- 
 tion of the line of intersection. When this is the case, the earth, at 
 the time of full moon, lies directly between the moon and the sun ; 
 the moon is thus plunged into the shadow of the earth, the light from 
 the sun is intercepted, and we say that the moon is eclipsed. The 
 moon sometimes only partially enters the earth's shadow, in which 
 case the eclipse is a partial one. When, on the other hand, the sun 
 is situated on the line of intersection at the time of new moon, the 
 moon lies directly between the earth and the sun, and the dark body 
 of the moon then cuts off the sunlight from the earth, producing a 
 solar eclipse. Usually only a part of the sun is thus obscured, form- 
 ing the well-known partial eclipse ; if, however, the moon pass cen- 
 trally over the sun, then we may have either of two very remarkable 
 kinds of eclipse. Sometimes the moon entirety blots out the sun, 
 and then we have the sublime spectacle of a total eclipse, which tells 
 us so much as to the nature of the sun, and to which we have already 
 referred in the last chapter. Occasionally, however, even when the 
 moon is placed centrally over the sun, a thin rim of sunlight is seen 
 round the margin of the moon. We then have what is known as 
 an annular eclipse. It is very remarkable that the moon is some- 
 times able to completely hide the sun, and sometimes fails to do 
 so. It happens, curiously enough, that the average apparent size 
 of the moon is equal to the apparent size of the sun, but owing to 
 the fluctuations in their distances, the actual apparent sizes of 
 both bodies undergo certain changes. It may happen that the 
 apparent size of the moon is greater than that of the sun. In this 
 case a central passage produces a total eclipse ; but it may also 
 
THE MOON. 
 
 57 
 
 happen that the apparent size of the sun exceeds that of the moon, 
 in which case a central passage can only produce an annular eclipse. 
 
 There are hardly any more interesting ce- 
 lestial phenomena than the different descrip- 
 tions of eclipses. The almanac will always 
 give timely notice of the occurrence, and the 
 more striking features can be observed with- 
 out a telescope. In an eclipse of the moon 
 (Fig. 24) it is interesting to note the moment 
 when the black shadow is first detected, to 
 watch its gradual encroachment over the 
 bright surface of the moon, to follow it, in 
 case the eclipse is total, until there is only a 
 thin crescent of moonlight left, and to watch 
 the final extinction of that crescent when the 
 whole moon is plunged into the shadow. But 
 now a spectacle of great interest and beauty 
 is often manifested ; for though the moon is 
 so hidden behind the earth that not a single 
 direct ray of the sunlight could reach the 
 surface, yet it is often found that the moon 
 remains visible, and indeed actually glows 
 with a copper hue bright enough to permit 
 several of the markings on the surface to 
 be seen. Whence this light ? It is due to 
 the sunbeams which have just grazed the 
 edge of the earth. In doing so they have ill 
 
 become bent by the refraction of the atmo- 
 
 /' H * ' 
 sphere, and thus turned inwards into the ll'iiis'ill 
 
 shadow. Such beams have passed through 
 a prodigious thickness of the earth's atmo- 
 sphere, and in this long journey through 
 hundreds of miles of air they have become tinged with a ruddy or 
 copper hue. Nor is this property of our atmosphere an unfamiliar 
 one. Does not the sun at sunrise or at sunset glow with a light 
 much more ruddy than the beams it dispenses at noonday? But 
 
58 THE 8 TORY OF TEE HEAVENS. 
 
 at sunset or at ,sunrise the rays have a vastly greater mass of 
 atmosphere to penetrate than they have at noon, and accordingly 
 the atmosphere imparts to the rays the ruddy colour which is a cha- 
 racteristic feature of sunset. In the case of the eclipsed moon, the 
 sunbeams have an atmospheric journey double as great as that 
 at sunset, and hence the ruddy glow of the moon is accounted for. 
 
 The almanacs give the full particulars of each eclipse that 
 happens in the corresponding year. They are able to do this 
 because astronomers have been carefully observing the moon for 
 ages, and have learned from these observations not only how the 
 moon moves at the present, but also how it will move for ages to 
 come. The actual calculations are very troublesome and com- 
 plicated, but there is one leading principle about eclipses which is 
 so simple that we must refer to it. The eclipses occurring this 
 year have no very obvious relation to the eclipses that occurred, 
 last year, or to those that will occur next year. Yet, when we 
 take a more extended view of the sequence of eclipses, a very defi- 
 nite principle becomes manifest. If we observe all the eclipses in 
 a period of eighteen years, or nineteen years, then we can predict 
 future eclipses for a long time. It is only necessary to recollect 
 that in 6,5 85 i days after one eclipse, a nearly similar eclipse follows. 
 For instance, a beautiful eclipse of the moon occurred on the 5th 
 of December, 1881. If we count back 6,585 days from that 
 date, or, that is, eighteen years and eleven days, we come to 
 November 24th, 1863, and a similar eclipse of the moon took 
 place then. Again, there were four eclipses in the year 1881. If 
 we add 6,585J days to the date of each eclipse, it will give the 
 dates of all the four eclipses in the year 1899. It was this rule 
 which enabled the ancient astronomers to predict the recurrence of 
 eclipses, before they understood the motions of the moon nearly 
 as well as we do now. 
 
 During a long voyage, and perhaps under critical circum- 
 stances, the moon will often render the sailor the most invalu- 
 able information. To navigate a ship, suppose from Liverpool 
 to China, the captain must frequently determine the precise 
 position which his ship then occupies. If he could not do this, 
 
THE MOON. 59 
 
 he would never find his way across the trackless ocean. It is in 
 the first place by observations of the sun that the place of the 
 ship is found ; but in addition to these observations, which tell him 
 his local time, the captain requires to know the Greenwich time 
 before he can place his ringer at a point of the chart and say, 
 <f My ship is here/'' To ascertain the Greenwich time, the ship 
 carries a chronometer, which has been carefully rated before start- 
 ing, and, as a precaution, two or three or more chronometers are 
 usually employed ; for an unknown error in the chronometer will 
 be a very serious or dangerous matter. Every minute that the 
 chronometer is wrong may perhaps put the captain fifteen miles 
 out of his reckoning. It is therefore sometimes of importance 
 to have the means of testing the chronometer; and it would be 
 a great convenience if every captain, when he wished, could actu- 
 ally consult some infallible standard of Greenwich time. We 
 want, in fact, a Greenwich clock which may be visible over the 
 whole globe. There is such a clock ; and, like any other clock, it 
 has a face on which certain marks are made, and a hand which 
 travels round that face. The great clock at Westminster shrinks 
 into insignificance when compared with the mighty clock which the 
 captain uses for setting his chronometer. The face of this stu- 
 pendous dial is the surface of the heavens. The numbers engraved 
 on the face of a clock are replaced by the twinkling stars ; while 
 the hand which moves over the dial is the beautiful moon herself. 
 When the captain desires to test his chronometer, he measures 
 the distance of the moon from a neighbouring star. For example, 
 he may see that the moon is three degrees from the star Regulus. 
 In the Nautical Almanac he finds the Greenwich time at which the 
 moon was three degrees from Regulus. Comparing this with the 
 indications of the chronometer, he finds the required correction. 
 
 We owe much to the moon. We hope, indeed, in a subsequent 
 chapter, to point out that we owe a great deal more to her than 
 was formerly suspected; but there is one widely-credited myth 
 about the moon which must be regarded as devoid of real founda- 
 tion. The idea that the moon and the weather are connected has 
 no doubt been entertained by high authority, but careful com- 
 
60 
 
 THE STORY OF THE HEAVENS. 
 
 parison has shown that there is no definite connection between 
 the two. 
 
 We often notice large blank spaces on maps of Africa and 
 of Australia, which indicate our ignorance of the interior of those 
 
 w 
 
 Fig. 25. Key to Chart of the Moon (Plate VI.) 
 
 great continents. We can find no such blank spaces in the map 
 of the moon. Astronomers know the surface of the moon better 
 than the geographers know the interior of Africa. Every spot on 
 the face of the moon which is as large as an English parish, has 
 been surveyed and mapped, and in many cases even christened. 
 
 The map of the moon shown in Plate VI., and based upon draw- 
 ings made with small telescopes, gives a general view of that side 
 
PLATE VI. 
 
 CHART OF THE MOON'S SURFACE. 
 
THE MOON. 
 
 61 
 
 of the moon which is presented to the earth. We can see from 
 it the most characteristic features of lunar scenery. Those dark 
 regions so well known in the ordinary full moon are easily recog- 
 nised on the map. They were originally thought to be seas, and 
 indeed they still retain the name, though it is obvious that they 
 contain no water. The map also shows certain ridges or elevated 
 portions,, and when we apply measurement to these objects we 
 learn that they are mighty mountain ranges. But the most 
 striking features in the map are those ring-like objects which are 
 scattered over the surface in profusion. These are known as the 
 lunar craters. 
 
 To facilitate reference to the different points of interest we have 
 arranged an index map, which, upon comparison with the plate 
 of the moon, will give a clue to the names of the different objects. 
 The seas are represented by capital letters ; thus, for instance, A 
 is the Mare Crisium, and H the Oceanus Procellarum. The ranges 
 of mountains are indicated by small letters; thus a on the index 
 indicates the so-called Caucasus mountains, and the Apennines are 
 denoted by c. The remarkable objects of such varied size known 
 as craters are distinguished by numbers; thus the feature on the 
 map lying under 1 on the index is the crater Posidonius. 
 
 A. Mare Crisium. 
 
 B. Foecunditatis. 
 
 C. Tranquillitatis. 
 
 D. Serenitatis. 
 
 E. Imbrium. 
 
 F. Sinus Iridium. 
 
 G. ,, Vaporum. 
 
 H. Oceanus Procellarum. 
 I. Mare Humorum. 
 J. Nubium. 
 
 K. 
 
 Nectaris. 
 
 a. Caucasus mountains. 
 
 b. Alps 
 
 c. Apennine 
 
 d. Carpathian 
 
 /. CordiUeras and D' Alem- 
 
 bert mountains. 
 g. Rook mountains. 
 h. Dcerfel ., 
 i. Leibnitz ,, 
 
 1. Posidonius. 
 
 2. Linne. 
 
 3. Aristotle. 
 
 4. Great VaUey of the 
 
 Alps. 
 
 5. Aristillus. 
 
 6. Autolycus. 
 
 7. Archimedes. 
 
 8. Plato. 
 
 9. Eratosthenes. 
 
 10. Copernicus. 
 
 11. Kepler. 
 
 12. Aristarchus. 
 
 13. Grimaldi. 
 
 14. Gassendi. 
 
 15. Schickard. 
 
 16. Wargentin. 
 
 17. Clavius. 
 
 18. Tycho. 
 
 19. Alphons. 
 
 20. Ptolemy. 
 
 21. Catharina. 
 
 22. Cyrillus. 
 
 23. Theophilus. 
 24 Petavius. 
 
 25. Hyginus. 
 
 26. Triesnecker. 
 
 Lunar objects can only be well seen when the sunlight falls 
 upon them in such a manner as to exhibit strongly-contrasted 
 
62 THE STORY OF THE HEAVENS. 
 
 light and shade. It is on this account impossible to observe the 
 moon well at the time when it is full, for then no conspicuous 
 shadows are cast. The best time for seeing- any object usually 
 coincides with the time when the boundary between light and 
 shade passes through the neighbourhood, and then the features are 
 brought out with exquisite distinctness. 
 
 Plate VII.* gives an illustration of the lunar scenery, 
 the object represented being known to astronomers by the name 
 Triesnecker. The district included is only a very small fraction of the 
 entire surface of the moon, yet the actual area is very considerable, 
 embracing as it does many hundreds of square miles. We see in 
 it various ranges of lunar mountains, while the central object in 
 the picture is one of those remarkable lunar craters which are the 
 characteristic features of lunar scenery. This crater is about twenty 
 miles in diameter, and it has a lofty mountain in the centre, the 
 peak of which is just shown, illuminated by the rising sun. 
 
 A typical view of a lunar crater is shown in Plate VIII. This 
 is no doubt a somewhat imaginary sketch. The point of view from 
 which the artist is supposed to have taken the picture is one quite 
 unattainable by terrestrial astronomers, yet there can be little doubt 
 that it is a fair representation of the objects on the moon. We 
 should, however, recollect the scale on which it is drawn. The vast 
 crater must be many miles across, and the mountain at its centre 
 must be thousands of feet high. The telescope will, even at its 
 best, only show the moon as well as we could see it with the un- 
 aided eye if it were only about 250 miles away instead of being 
 240,000. We must not, therefore, expect to see any details on 
 the moon even with the finest telescopes, unless they were coarse 
 enough to be visible at a distance of 250 miles. A view of 
 England from a distance of 250 miles would only show London 
 as a sort of coloured spot, by contrast with the general surface of 
 the country. 
 
 * This sketch has been copied by permission from the very beautiful view in 
 Messrs. Nasmyth and Carpenter's book, of which it forms Plate XI. The other 
 illustrations of lunar scenery in Plates VII., VIII., IX., are from the same. The 
 photographs were obtained from models carefully constructed to illustrate the 
 features on the moon. 
 
THE MOON. 63 
 
 We return, however, from a somewhat fancy sketch to a more 
 prosaic examination of what the telescope does actually reveal. 
 Plate IX. represents a large crater which is well known to every 
 user of a telescope, and which has been christened Plato. The floor 
 of the crater of Plato is very nearly flat, and in this case the central 
 mountain, so often seen in other craters, is wanting. 
 
 The mountain peaks on the moon throw long, sharply defined 
 shadows. These shadows are indeed characterised by a defmiteness 
 which we do not find in terrestrial shadows, the difference no doubt 
 arising from the absence of air from the moon. The air on the earth 
 diffuses a certain amount of light, which mitigates the blackness of 
 terrestrial shadows, and tends to soften their outline. No such in- 
 fluences are at work on the moon, and the sharpness of the shadows 
 is taken advantage of in our attempts to measure the heights of 
 the lunar mountains. 
 
 It is often easy to compute the altitude of a church steeple, a 
 lofty chimney, or a similar object, by measuring the length of its 
 shadow. The simplest and the most accurate process is to measure 
 at noon the number of feet from the base of the object to the end 
 of the shadow. The elevation of the sun on the day in question 
 can be obtained from the almanac, and then the length of the 
 shadow follows by a simple calculation. Indeed, if the observations 
 were made on the 4th of March or the 6th of September at London, 
 the calculation would then be unnecessary, for the noonday length 
 of the shadow is equal to the altitude of the object. In summer 
 the length of the shadow is less than the altitude ; in winter the 
 length of the shadow exceeds the altitude. At sunrise or sunset 
 the shadows are, of course, much longer than at noon, and it is 
 shadows of this kind that are measured on the moon. These 
 measures are made by that most indispensable adjunct to the 
 equatorial telescope known as the micrometer. 
 
 The micrometer, in its most ordinary form, is a small piece of 
 apparatus which can be screwed on to the eye end of the telescope. 
 The word denotes an instrument for measuring small distances. In 
 one sense the term is not a very happy one. The objects to which 
 the astronomer applies the micrometer are usually anything but 
 
64 THE STORY OF THE HEAVENS. 
 
 small objects. They are, in fact, often objects of the most trans- 
 cendent dimensions, vastly exceeding the moon or the sun, or even 
 our whole system, in bulk. Still, the name is not altogether 
 inappropriate, for vast though the objects may be, they yet seern 
 minute, even in the telescope, on account of their great distance. 
 We require, therefore, for such measures an instrument capable of 
 the very greatest nicety. Here, again, we invoke the aid of the 
 spider, to whose assistance in another department we have already 
 referred. In the filar micrometer two spider lines are parallel, and 
 one intersects them at right angles. One or both of the parallel 
 lines can be moved by means of a screw. The distance through 
 which the line has been moved is accurately indicated by noting the 
 number of revolutions and parts of a revolution of the screw. 
 Suppose, then, the two lines be first brought into coincidence, and 
 then moved apart until the apparent length of the shadow of the 
 mountain be equal to the distance of the lines : we then know the 
 number of revolutions of the micrometer screw, which is equivalent 
 to the length of the shadow. The value of the screw is known by 
 other observations, and hence the length of the shadow can be 
 determined, and its equivalent in miles can be ascertained. The 
 elevation of the sun, at the moment when the measures were 
 made, is also found, and hence the actual elevation of the 
 mountain can be calculated. By measures of this kind the 
 height of the rampart surrounding a crater on the moon can be 
 learned. 
 
 The beauty and interest of the moon as a telescopic object 
 induces us to give to the student a somewhat detailed account of 
 the more remarkable features which it presents. Most of the 
 objects we are to describe can be effectively seen with only a very 
 moderate telescopic power.. It is, however, to be observed that all 
 of them cannot be well seen at one time. The student must 
 remember that the region most distinctly shown is the boundary 
 between light and darkness. He will, therefore, select for obser- 
 vation such objects as may happen to lie on or near that boundary 
 at the time when he is observing We shall here follow the 
 numbering on the key to the map. 
 
THE MOON. 65 
 
 1. Posidonius. The diameter of this large crater is nearly 60 
 miles. Although its surrounding wall is comparatively narrow, it is 
 so distinctly marked as to make the object very conspicuous. As so 
 frequently happens in lunar volcanoes, the bottom of the crater is 
 below the level of the surrounding plain, in the present instance 
 to the extent of nearly 2,500 feet. Towards the close of last 
 century, Schroeter, the industrious Hanoverian observer, fancied he 
 saw traces of activity in the little crater on the floor of Posidonius. 
 It is remarkable that he described it as being only grey in the 
 interior, at a time when it ought to have been full of black shadow. 
 This is particularly interesting in connection with what has been 
 observed in the next object. 
 
 2. Li nne. This small crater lies right out in the Mare Sereni- 
 tatis. Fifty or sixty years ago it was described as about 6| miles 
 in diameter, and otherwise so conspicuous as to be used by two 
 astronomers as a fundamental point in the survey of the moon. In 
 1866 Schmidt, of Athens, announced that the crater was obscured, 
 apparently by cloud. Since then an exceedingly small crater has 
 been visible, but the whole object is now so inconspicuous that it 
 would hardly be chosen as a landmark. On the whole, this is the 
 most clearly proved case of present change in a lunar object. 
 
 3. Aristotle. This great philosopher's name is attached to a 
 grand crater 50 miles in diameter, the interior of which, although 
 very hilly, shows no decidedly marked central cone. But the lofty 
 wall of the crater, exceeding 10,500 feet in height, overshadows 
 the bottom so long that its irregularities are never seen to ad- 
 vantage. 
 
 4. The Great Valley of the Alps. Right through the lunar 
 Alps runs this wonderfully straight valley, with a widths ranging 
 from 3^ to 6 miles. It is, according to Madler, at least 11,500 
 feet deep, and over 80 miles in length. A few low ridges run 
 parallel to its sides, possibly the result of landslips. 
 
 5. Aristillus. Under favourable conditions Lord Rosse's great 
 telescope has shown the exterior of this magnificent crater to be 
 marked over with deep gullies, radiating from its centre. It is 
 about 34 miles wide, and about 10,000 feet in depth. 
 
 F 
 
66 THE STOUT OF TILE HEAVENS. 
 
 6. Autolycus is somewhat smaller than the foregoing, to 
 which it forms a companion in accordance with what Madler 
 thought a well defined relation amongst lunar craters, by which 
 they frequently occurred in pairs, with the smaller one more usually 
 to the south. This does indeed occur repeatedly, but towards the 
 edge the arrangement is generally rather apparent than real, and 
 is merely a result of foreshortening. 
 
 7. Archimedes. This large plain, about 50 miles in diameter, 
 has its vast smooth interior divided into seven distinct zones run- 
 ning east and west. There is no central mountain or other obvious 
 internal sign of former activity, but its irregular wall rises into 
 abrupt towers, and is marked outside by decided terraces. 
 
 8. Plato. In the northern part of the moon this extensive 
 " steel-grey " plain is noticeable with the smallest telescope. It is 
 admirably represented in Plate IX. The average height of the 
 rampart is about 3,800 feet on the eastern side ; the western 
 side is somewhat lower, but there is one peak rising to the 
 height of nearly 7,300 feet. The plain girdled by this vast 
 rampart is of ample proportions. It is a somewhat irregular 
 circle, about sixty miles in diameter, and containing an area of 
 2,700 square miles. On its floor the shadows of the western 
 wall are beautifully shown, as also three small craters, of which 
 a large number have 'been detected by persevering observers. 
 The narrow sharp line leading from the crater to the left is one of 
 those remarkable " clefts " which traverse the moon in so many 
 directions. Another may be seen further to the left. Above Plato 
 are several detached mountains, the loftiest of which is Pico, 
 about 8,000 feet in height. Its long and pointed shadow would 
 at first sight lead one to suppose that it must be very steep ; but 
 Schmidt, who specially studied the inclination of lunar slopes, is 
 of opinion that it is not nearly so steep as many of the Swiss 
 mountains that are frequently ascended. To give some idea of 
 Schmidt's amazing industry in lunar researches, it may be men- 
 tioned that in six years he made nearly 57,000 individual settings 
 of his micrometer in the measurement of lunar altitudes. His great 
 chart of the mountains in the moon is based on no less than 2,731 
 
THE MOON. 67 
 
 drawings and sketches, if those are counted twice that may have 
 been used for two divisions ^f the map. 
 
 9. Eratosthenes. This profound crater, upwards of 37 miles 
 in diameter, lies at the end of the gigantic range of the Apennines, 
 and not improbably, in accordance with Madler's suggestion, it once 
 formed the outlet of the stupendous forces that elevated those 
 comparatively craterless peaks. 
 
 10. Copernicus. Of all the lunar craters this is one of the 
 grandest and best known. It is particularly well known through 
 Sir John HerschePs drawing, so beautifully reproduced in the 
 many editions of the " Outlines of Astronomy." The region to 
 the west is dotted over with innumerable minute craterlets. It 
 has a central many-peaked mountain about 2,400 feet in height. 
 There is good reason to believe that the terracing shown in its 
 interior is mainly due to the repeated alternate rise, partial con- 
 gealation and retreat of a vast sea of lava. At full moon it is 
 surrounded by radiating streaks. 
 
 11. Kepler. Although the internal depth of this crater is 
 scarcely less than 10,000 feet, it has but a very low surrounding 
 wall, which is remarkable for being covered with the same glisten- 
 ing substance that also forms a system of bright rays, not unlike 
 those surrounding the last object. It has been pointed out that 
 this is the only instance in which these mysterious bright rays 
 occur in a level part of the moon. 
 
 12. Aristarchus is the most brilliant crater in the moon, being 
 specially vivid with a low power in a large telescope. So bright is 
 it, indeed, that it has often been seen in the dark side of the moon, 
 just after new moon, and has thus given rise to marvellous stories 
 of active lunar volcanoes. To the south-east is another smaller 
 crater, Herodotus, north of which is a narrow deep valley, nowhere 
 more than 2J miles broad, which makes a remarkable zigzag. It 
 is one of the largest of the lunar " clefts/' 
 
 13. Grimaldi calls for notice as the darkest object of its size in 
 the moon. Under very exceptional circumstances it has been seen 
 with the naked eye, and as its area has been estimated at nearly 
 14,000 square miles, it gives an idea of how little unaided vision 
 
 F 2 
 
68 THE STORY OF THE HEAVENS. 
 
 can discern in the moon ; but it must be added that we always see 
 Grimaldi considerably foreshortened. 
 
 14. The great crater Gassendi has been very frequently mapped 
 on account of its very elaborate system of " clefts." At its northern 
 end it communicates with a smaller but much deeper crater, that is 
 often still filled with black shadow, after the whole floor of Gassendi 
 is illuminated. 
 
 15. Schickard is another large crater being very little less 
 in area than the foregoing. Within its vast expanse Madler 
 detected 23 minor craters. It was with regard to this crater that 
 Chacornac first pointed out that owing to the curvature of the 
 surface of the moon, a spectator at the centre of the floor " would 
 think himself in a boundless desert," the surrounding wall, 
 although in part over 10,000 feet high, being entirely beneath the 
 horizon. 
 
 16. Close to the foregoing is Wargentin. There can be little 
 doubt that this is really a huge crater filled almost to the very 
 brim with congealed lava. 
 
 17. Clavius. Near the 60th parallel of lunar south latitude, 
 lies this enormous enclosure, whose area is not less than, 16,500 
 square miles. Both in its interior and on its walls are many peaks 
 and secondary craters. The telescopic view of a sunrise upon the 
 surface of Clavius is truly said by Madler to be indescribably mag- 
 nificent. One of the peaks rises not less than 24,000 feet above 
 the bottom of one of the included craters. Madler even expressed 
 the opinion that in this wild neighbourhood there are craters so 
 profound that no ray of sunlight ever penetrated their lowest depths, 
 while, as if to compensate for this, there are peaks whose summits 
 enjoy a mean day almost twice as long as their night. 
 
 18. If the full moon be viewed through even an opera-glass or the 
 smallest hand-telescope, one crater is immediately seen to be con- 
 spicuous beyond all others, by reason of the brilliant rays or streaks 
 that radiate from it. This is the majestic Tycho, 17,000 feet in depth 
 and 50 miles in diameter (Plate X.). A peak 6,000 feet in height 
 rises in the centre of its floor, while a series of terraces diversify its 
 interior slopes, but it is the wonderful and profoundly mysterious 
 
PLATE VH. 
 
 TRIESNECKER. 
 
 (AFTER NASMYTH.) 
 
THE MOON. 69 
 
 bright rays that chiefly call forth our admiration and surprise. 
 When the sun rises on Tycho, these streaks are utterly invisible in 
 its neighbourhood, so much so indeed that it then requires a 
 practised eye to recognise Tycho amidst its mountainous surround- 
 ings. But as soon as the sun has attained a height of about 25 to 
 SO above the horizon, the rays emerge from their obscurity, and 
 gradually increase in brightness until full moon, when they become 
 the most conspicuous objects on her surface. As yet no satisfactory 
 explanation has been given of the origin of these objects. They 
 are of all lengths, from a few hundred to two, or in one instance 
 nearly three thousand miles in length. They extend themselves 
 with superb indifference across vast plains, into the deepest craters, 
 or over the highest opposing ridges. We know of nothing on 
 our earth to which they can be compared. 
 
 Near the centre of the moon's disc is a fine range of craters, 
 always full open to our view under all illuminations, of these two 
 may be mentioned Alphons (19), the floor of which is strangely 
 marked by two bright and several dark markings not due to irregu- 
 larities in the surface. Ptolemy (20). Besides several small inclosed 
 craters, its floor is crossed by very numerous low ridges, visible 
 when the sun is rising or setting. 
 
 21, 22, 23. When the moon is five or six days old a beautiful 
 group of three craters will be readily found on the boundary line 
 between night and day. These are Catharina, Cyrillus, and 
 Theophilus. Catharina, the most southerly of the group, is more 
 than 16,000 feet deep, and connected with Cyrillus by a wide 
 valley; but between Cyrillus and Theophilus there is no such 
 connection. Indeed Cyrillus looks as if its huge surrounding 
 ramparts, as high as Mount Blanc, had been completely finished 
 when the volcanic forces commenced the formation of Theophilus, 
 the rampart of which encroaches considerably on its older neigh- 
 bour. Theophilus stands as a well-defined round crater about 64 
 miles in diameter, with an internal depth of 14,000 to 18,000 feet, 
 and a beautiful central group of mountains, one-third of that height 
 on its floor. This proves that the last eruptive efforts in this part 
 of the moon fully equalled in intensity those that had preceded 
 
70 THE STORY OF THE HEAVENS. 
 
 them. Although Theophilus is on the whole the deepest crater we 
 can see in the moon, it has received little or no deformation by 
 secondary eruptions, while the floor and wall of Catharina show 
 complete sequences of lesser craters of various sizes that have broken 
 in upon and partly destroyed each other. In the spring of the year 
 somewhat before the first quarter, this instructive group of extinct 
 volcanoes can be seen to great advantage at a convenient hour in 
 the evening. 
 
 24. Pelavius is remarkable not only for its great size, but also 
 for the rare feature of having a double rampart. It is a beautiful 
 object in the very new moon, or just after full moon, but dis- 
 appears absolutely when the sun is more than 45 above its 
 horizon. The crater floor is remarkably convex, culminating in a 
 central group of hills intersected by a deep cleft. 
 
 25. Hyginus is a small crater near the neighbourhood just 
 mentioned. One of the largest of the lunar chasms passes right 
 through it, making an abrupt turn as it does so. It is not difficult 
 to find under any illumination. 
 
 26. Triesnecker. This fine crater has been already described on 
 p. 62, but is again alluded to in order to draw attention to the 
 elaborate system of chasms so conspicuously shown in Plate VII. 
 That these chasms are depressions is abundantly evident by the 
 shadows inside them. Very often their margins are appreciably 
 raised. They seem to be fractures in the moon's surface. 
 
 Of the various mountains that are occasionally seen as pro- 
 jections on the actual edge of the moon, those called after Leibnitz 
 (i) seem to be the highest. Schmidt found the highest peak 
 upwards of 41,900 feet above a neighbouring valley. In comparing 
 these altitudes with those of mountains on our earth, we must for 
 the latter add the depth of the sea to the height of the land. 
 Reckoned in this way, our highest mountains are still higher than 
 any we know of in the moon. 
 
 We must now discuss the very important question as to the 
 origin of these remarkable features on the surface of the moon. 
 We shall admit at the outset that our evidence on this subject 
 is only indirect. To establish by unimpeachable evidence the 
 
THE MOON. 71 
 
 volcanic origin of the remarkable lunar craters, it would be almost 
 necessary that volcanic outbursts should have been witnessed on 
 the moon through our telescopes, and that such outbursts should 
 have been seen to result in the formation of the well-known ring, 
 with or without the mountain rising from the centre of the 
 plain that the ring surrounds. Have any such phenomena been 
 witnessed by astronomers ? To say that nothing of the kind has 
 ever been witnessed would be rather too emphatic a statement. On 
 certain occasions careful observers have reported the occurrence of 
 minute local changes on the moon. As we have already remarked, a 
 crater named Linne, of dimensions respectable, no doubt, to a lunar 
 inhabitant, but forming a very inconsiderable telescopic object, was 
 thought to have undergone some change. On another occasion a 
 minute crater was thought to have arisen near to the well-known 
 crater named Hyginus ; but the mere enumeration of such instances 
 gives real emphasis to the statement that there is at the present 
 time no appreciable source of disturbance of the moon's surface. 
 Even were these trifling cases of suspected change really established 
 and this is perhaps rather farther than most astronomers would 
 be willing to go they still are utterly insignificant when compared 
 with the mighty phenomena that gave rise to the host of great 
 craters which cover so large a portion of the moon's surface. 
 
 We are led inevitably to the conclusion that the surface of the 
 moon once possessed much greater activity than is at present the 
 case. We can also give a reasonable, or, at all events, a plausible, 
 explanation of the cessation of that activity in recent times. Let us 
 glance at two other bodies of our system, the earth and the sun, 
 and compare them with the moon. Of the three bodies, the sun 
 is enormously the greatest, while the moon is much less than the 
 earth. We have also seen that the sun is heated to an enormous 
 temperature, and further on in this work the reasons will be given 
 for believing that the sun is gradually parting with its heat. The 
 surface of the earth, formed as it is of solid rocks and clay, or 
 covered as it is in great part by the vast expanse of ocean, bears 
 but few obvious traces of a high temperature. Nevertheless, it is 
 highly probable from ordinary volcanic phenomena that the interior 
 
72 THE STORY OF THE HEAVENS. 
 
 of the earth still possesses an enormously high temperature. The 
 argument is, then, as follows. A large "body takes a longer time to 
 cool than a small body. A large iron casting will take days to 
 cool ; a small casting will become cold in a few hours. Whatever 
 may have been the original source of heat in our system a question 
 which need not now be discussed it seems sufficiently plain that 
 the different bodies in the system were all originally heated, and 
 have now for ages been gradually cooling. The sun is so vast that 
 he has not yet had time to cool ; the earth, of intermediate bulk, 
 has had time to cool on the outside, while still retaining vast stores 
 of internal heat ; while the moon, least of all, has had time to cool 
 to such an extent that changes of importance on its surface can no 
 longer be originated by internal fires. 
 
 We are thus led to refer the origin of the lunar craters to some 
 ancient epoch in the moon's history. How ancient that epoch is to 
 be we have no means of knowing, but it is possible to form a 
 surmise that, in all probability, the antiquity of the lunar craters 
 is enormously great. At the time when the moon was sufficiently 
 heated to have these vast volcanic convulsions, of which the mighty 
 craters are the survivals, the earth must have been very much hotter 
 than it is at present. It is not, indeed, at all unreasonable to 
 believe that when the moon was hot enough for its volcanoes to 
 be active the earth was so hot that life was impossible on its 
 surface. This supposition would point to an antiquity for the 
 moon's craters far too great to be estimated by the centuries and 
 the thousands of years which are adequate for the lapse of time 
 as recognised by the history of human events. It seems not un- 
 likely that millions of years may have elapsed since the mighty 
 craters of Plato or of Copernicus consolidated into their present form. 
 
 It will now be possible for us to attempt to account for the 
 formation of the lunar craters. The most probable views on the 
 subject are certainly those adopted by Mr. Nasmyth, though it 
 musb be admitted that they are by no means free from difficulty. 
 We can explain the way in which the rampart around the lunar 
 crater is formed, and the great mountain which so often adorns the 
 centre of the plain. The view in Fig. 26 contains an imaginary 
 
THE MOON. 
 
 73 
 
 sketch of a volcanic vent on the moon in the days when the craters 
 were active. The eruption is here in the full flush of its energy, 
 
 Fig. 26. Volcano in Activity. 
 
 Fig. 27. Subsequent Feeble Activity. 
 
 when the internal forces are hurling forth a fountain of ashes or 
 stones, which fall down at a considerable distance from the vent ; 
 and these accumulations constitute the rampart surrounding the 
 crater. 
 
74 
 
 THE STORY OF THE HEAVENS. 
 
 The second picture depicts the crater in a later stage of its 
 history. The prodigious explosive power has now been ex- 
 hausted, and has perhaps been intermitted for some time. Again J 
 the volcano bursts into activity, but this time with only a small 
 part of its original energy. A feeble jet now issues from the same 
 vent and deposits the materials close around the orifice, and thus 
 gradually raises a mountain in the centre. Finally, when all the 
 activity has subsided, and the volcano is silent and still, we find 
 the remains of the early energy testified to by the rampart which 
 surrounds the ancient crater, and the mountain which rises in its 
 
 Fig. 28. Formation of the Level Floor by Lava. 
 
 interior. The flat floor which is found in some of the craters may 
 not improbably be due to an outflow of lava which has afterwards 
 consolidated. A sketch of this is shown in Fig. 28. 
 
 One of the principal difficulties attending this method of 
 accounting for the structure of a crater, arises from the great size 
 which some of the craters attain. There are craters on the moon 
 forty, fifty, or more miles in diameter ; indeed, there is one well- 
 formed ring, with a mountain rising in the centre, the diameter of 
 which is no less than seventy-eight miles (Petavius) . It seems difficult 
 to conceive how a blowing cone at the centre could convey the 
 materials to such a distance as the thirty-nine miles between the 
 centre of Petavius and the rampart. The difficulty is, however, 
 greatly smoothed away when it is borne in mind that the force 
 of gravitation is much less on the moon than on the earth. 
 Have we not already seen that the moon is so much smaller than 
 
THE MOON. 75 
 
 the earth, that eighty moons rolled into one would not weigh as 
 much as the earth ? On the earth an ounce weighs an ounce and 
 a pound weighs a pound ; but owing to the small size of the moon 
 its gravitation is much less than that on the surface of the earth. 
 A weight of six ounces here would only weigh one ounce on the 
 moon, and a weight of six pounds here would only weigh one 
 pound on the moon. A labourer who can carry one sack of corn 
 on the earth could, with the same exertion, carry six sacks of 
 corn in the moon. A cricketer who can throw a ball 100 yards 
 on the earth could with precisely the same exertion throw the 
 same ball 600 yards on the moon. Hiawatha could shoot ten 
 arrows into the air one after the other before the first reached the 
 ground ; on the moon he might have shot his whole quiver full. 
 The volcano, which on the moon drove projectiles to the distance 
 of thirty- nine miles, need only possess the same explosive power 
 as would have been sufficient to drive the missiles six or seven 
 miles 011 the earth. A modern cannon properly elevated would 
 achieve this feat. 
 
 It must also be borne in mind that there are innumerable 
 craters on the moon of the same general type, but of the most 
 varied dimensions ; from a tiny telescopic object two or three miles 
 in diameter, we can proceed iii gradually ascending stages until 
 we reach the mighty Petavius just considered. With regard to 
 the smaller craters, there is obviously little or no difficulty in at- 
 tributing them to a volcanic source, and as the continuity from the 
 smallest to the largest craters is unbroken, it seems quite reasonable 
 to suppose that even the greatest has had a similar origin. 
 
 But we must not linger any longer over these most interesting 
 features of the moon. Attractive and beautiful as is the spectacle 
 of our nearest neighbour in the heavens, we have yet so long a task 
 to accomplish that we must dismiss any further discussions of the 
 subject. Even a single spot on the moon is capable of affording a 
 diligent observer materials for many a long night's study. All 
 such details we must pass over, and can only devote the brief 
 space which remains to a few features of more general interest. 
 
 Why are these lunar landscapes so excessively weird and so 
 
76 . THE STORY OF THE HEAVENS. 
 
 rugged ? Why do they always remind us of sterile deserts, and 
 why do we see no grassy plains or green forests such as we are 
 familiar with on the earth ? In some respects the moon is not very 
 differently circumstanced to the earth. Like it, the rnoon has 
 the pleasing alternations of day and night, though the day in the 
 moon is fourteen of our days, and the night of the moon is as long 
 as fourteen of our nights. We are warmed by the rays of the sun 
 by day, so is the moon; but, whatever may be the temperature 
 during the long day on the moon, it seems certain that the cold 
 of the lunar night would transcend that known in the bleakest 
 regions of our earth. 
 
 Even our largest telescopes can tell nothing directly as to 
 whether life can exist on the moon. The mammoth trees of 
 California might be growing on the lunar mountains, and elephants 
 might be walking about on the plains, but our telescopes could 
 not show them. The smallest object that we can see on the moon 
 must be about as large as a good sized cathedral, so that organised 
 beings, if they existed, could not make themselves visible as teles- 
 copic objects. 
 
 We are therefore compelled to resort to indirect evidence as to 
 whether life would be possible on the moon. We may say at once 
 that astronomers believe that life, as we know it, could not exist. 
 Among the necessary conditions of life water is one of the first. 
 Take every form of vegetable life, from the lichen which grows on 
 the rock up to the giant tree of the forest, and we find the sub- 
 stance of every plant contains water, and could not exist without 
 water. Nor is water less necessary for the existence of animal 
 life. Without water animal life, the life of man himself, is incon- 
 ceivable. 
 
 Unless, therefore, water be present in the moon we shall be 
 bound to conclude that life, as we know it, is impossible. The 
 great question then is, can we tell whether there is water in 
 the moon or not ? If any one stationed on the moon were to look 
 at the earth through a telescope, would he be able to see any water 
 here ? Most undoubtedly he would. He would see the clouds and 
 he would notice their incessant changes, and the clouds alone would 
 
THE MOON. 77 
 
 be almost conclusive evidence of the existence of water. An 
 astronomer on the moon would also see our oceans as smooth 
 coloured surfaces, remarkably contrasted with the land. In fact, 
 considering that much more than half of our globe is covered with 
 oceans, and that most of the remainder is liable to be obscured by 
 clouds, the lunar astronomer in looking at our earth would often 
 see hardly anything but water in one form or other, and very likely 
 he would come to the conclusion that our globe was quite unfitted 
 to be a residence except for amphibious animals. ' 
 
 But when we look at the moon with our telescopes, do we see 
 any traces of water? There are no doubt many large districts 
 which at a first glance seem like oceans, and were indeed termed 
 " seas " by the old astronomers, a name which they still absurdly 
 retain. Closer inspection shows that the so-called lunar seas are 
 deserts, often marked over with small craters and with rocks. The 
 telescope reveals no seas and no oceans, no lakes and no rivers. 
 Nor is the grandeur of the moon's scenery ever impaired by clouds 
 over her surface. Whenever the moon is above the horizon, and 
 terrestrial clouds are out of the way, we can see the features of her 
 surface with distinctness. There are no clouds in the moon ; there 
 are not even the mists or the vapours which invariably arise 
 wherever water is present, and therefore astronomers have been 
 led to the conclusion that our satellite is a sterile and a waterless 
 desert. 
 
 There is another equally essential element of life absent from 
 the moon. Our globe is surrounded with a deep clothing of air 
 resting on the surface, and extending above our heads to the height 
 of about 200 or 300 miles. We need hardly say how necessary air 
 is to life, and therefore we turn with interest to the question as to 
 whether the moon is surrounded with an atmosphere. Let us 
 clearly understand the problem we are about to consider. Imagine 
 that a traveller started from the earth on a journey to the moon ; 
 as he proceeded the air would gradually get rarer and rarer, 
 until at length, when he was a few hundred miles above the 
 earth's surface, he would have left the last perceptible traces of 
 the earth's atmosphere behind him ; but even by the time he was 
 
78 THE STORY OF THE HEAVENS. 
 
 completely free from the earth's atmosphere he would only have 
 advanced a very small fraction of the whole journey of 240,000 
 miles, and there is still a vast void to be traversed before the moon 
 is reached. If the moon were furnished with an atmosphere like 
 the earth, as the traveller got near the end of his journey, and 
 within a few hundred miles of the moon's surface, he would expect 
 to meet again with traces of an atmosphere, which would gradually 
 increase in density until he arrived at the moon's surface. The 
 traveller would thus have passed through one atmosphere at the 
 beginning of his journey, and another at the end, while the main 
 portion of the journey would have been through space more void 
 than the receiver of an air-pump. 
 
 Such would be the case if the moon were coated with an atmos- 
 phere like that surrounding our earth. But what are the facts ? 
 The traveller as he drew near the moon would look in vain for an 
 atmosphere at all resembling ours. It is possible indeed it is 
 probable that when he was close to the surface he might meet with 
 faint traces of some gaseous covering surrounding the moon, but it 
 would not be a fractional part of the ample clothing which the 
 earth now enjoys. For all purposes of respiration, as we under- 
 stand the term, we may say that there is no air on the moon, 
 and an inhabitant of our earth transferred to the moon would 
 be as certainly suffocated there as he would be in the middle of 
 space. 
 
 It may, however, be asked how we learn this. Is not air 
 transparent, and how, therefore, could our telescopes be expected to 
 show whether the moon really possessed such an envelope? The 
 fact is that it is by indirect methods of observation that we learn 
 the nakedness of the moon. There are various arguments that 
 can be adduced ; but the most conclusive is that obtained on the 
 occurrence of what is called the " occultation " of a star. It some- 
 times happens that the moon comes directly between the earth and 
 a star, and the temporary extinction of the star is an " occultation." 
 We can observe the moment when the occultation takes place, and 
 the suddenness of the extinction of the star is extremely remark- 
 able. If the moon had a copious atmosphere, the gradual inter- 
 
THE MOON. 79 
 
 position of this atmosphere by the movement of the moon would 
 produce a gradual extinction of the star, and not the sudden pheno- 
 menon usually observed. 
 
 The absence of air and of water from the moon, explains the 
 peculiar and weird mggedness of the lunar scenery. We know 
 that on the earth the action of wind, and of rain, of frost, and of 
 snow, is constantly tending to wear down our mountains and 
 reduce their asperities, but no such agents are at work on the moon. 
 Volcanoes sculptured the moon into its present condition, and, 
 though the volcanoes have been silent for ages, the traces of their 
 handiwork seem nearly as fresh to-day as they were when the mighty 
 fires were extinguished. 
 
 " The cloud-capped towers, the gorgeous palaces, the solemn 
 temples " have but a brief career on earth. It is chiefly the inces- 
 sant action of water and of air that makes them vanish like the 
 "baseless fabric of a vision/' On the moon these causes of disin- 
 tegration and of decay are all absent, though perhaps the changes 
 of temperature in the transition from lunar day to lunar night 
 would be attended with expansions and contractions that might 
 compensate in some degree for the absence of more potent agents 
 of dissolution. 
 
 It seems probable that a building on the moon would remain 
 for century after century just as it was left by the builders. There 
 need be no glass in the windows, for there is no wind and no rain 
 to keep out. There need not be fireplaces in the rooms, for fuel 
 cannot burn without air. Dwellers in a city in the moon would 
 find that no dust can rise, no odours be perceived, no sounds be 
 heard. 
 
 Man is a creature adapted for life in circumstances which are 
 very narrowly limited. A few degrees of temperature, more or 
 less; a slight variation in the composition of air; the precise 
 suitability of food ; make all the difference between health and 
 sickness, between life and death. Looking beyond the moon, into 
 the length and breadth of the universe, we find countless celestial 
 globes with every conceivable variety of temperature and of con- 
 stitution. Amid this vast number of worlds with which space 
 
80 THE STORY OF THE HEAVENS. 
 
 teems, are there any inhabited by living beings? To this great 
 question science can make no response, save this ; we cannot tell. 
 Yet it is impossible to resist a conjecture. We find our earth 
 teeming with life in every part. We find life under the most 
 varied conditions that can be conceived. We have life under 
 the burning heat of the tropics ; we have life in the everlasting 
 frost at the poles. We have life in caves where not a ray of 
 light ever penetrates. We have life in the depths of the ocean, at 
 the pressure of tons on the square inch. Whatever be the external 
 circumstances, some form of life can generally be found to which 
 those circumstances are congenial. 
 
 It is not at all likely that even among the million spheres of 
 the universe there is a single one exactly like our earth ; like it 
 in the possession of air and of water, like it in size and in composi- 
 tion. It does not seem probable that a man could live for one hour 
 on any body in the universe except the earth, or that an oak-tree 
 could live in any other sphere for a single season. Men can dwell 
 on the earth, and oak-trees can thrive there, because the constitutions 
 of the man and of the oak are specially adapted to the particular 
 circumstances of the earth. 
 
 Could we obtain a closer view of some of the celestial bodies, we 
 should probably find that they, too, teem with life, but with life 
 specially adapted to the environment. Life in forms strange and 
 weird ; life far stranger to us than Columbus found it to be in the 
 New World when he first landed there. Life, it may be, stranger 
 than ever Dante described, or Dore drew. Intelligence may yet 
 have a home among those spheres no less than on the earth. 
 There are globes greater and globes less atmospheres greater and 
 atmospheres less. And the truest philosophy on this subject is 
 crystallised in the exquisite language of Tennyson : 
 
 " This truth within thy mind rehearse, 
 That in a boundless universe 
 Is boundless better, boundless worse. 
 
 Think you this mould of hopes and fears 
 Could find no statelier than his peers 
 In yonder hundred million spheres ? " 
 
0. E 
 
 H 
 
CHAPTER IV. 
 
 THE SOLAR SYSTEM. 
 
 Exceptional Importance of the Sun and Moon The Course to be pursued The 
 Order of Distance The Neighbouring Orbs How are they to be discrimi- 
 nated? The Planets Venus and Jupiter attract notice by their brilliancy 
 Sirius not a neighbour The Planets Saturn and Mercury Telescopic Planets 
 The Criterion as to whether a Body is to be ranked as a neighbour Meaning 
 of the word Planet Uranus and Neptune Comets The Planets are Illumi- 
 nated by the Sun The Stars are not Ths Earth is really a Planet The four 
 Inner Planets, Mercury, Venus, the Earth, and Mars Velocity of the Earth 
 The Outer Planets, Jupiter, Saturn, Uranus, Neptune Light and Heat 
 received by the Planets from the Sun Comparative Sizes of the Planets The 
 Minor Planets The Planets all Revolve in the same direction The Solar 
 System An Island Group in Space. 
 
 IN the two preceding chapters of this work we have endeavoured 
 to describe the heavenly bodies in the order of their relative 
 importance to mankind. Passing on from the preliminary 
 chapters, which principally discussed the means by which we 
 were enabled to observe the heavenly bodies, we then proceeded to 
 describe those objects themselves. Could we hesitate for a moment 
 as to which of the bodies in the universe should be the first to 
 receive our attention ? We do not now allude to the intrinsic 
 significance of the sun when compared with other bodies or groups 
 of bodies scattered through space. It may be that the sun has 
 many orbs rivalling it in real splendour, in bulk, and in mass. 
 We shall, in fact, show later on in this volume that this is the case ; 
 and we shall afterwards attempt to indicate the true rank of the 
 sun amid the countless host of heaven. But whatever may be the 
 importance of the sun, viewed merely as one of the bodies which 
 teem through space, there can be no hesitation for a moment as to 
 how immeasurably his influence on the earth surpasses that of all 
 these bodies in the universe put together. It was therefore natural 
 indeed inevitable that our first excursion into the abyss of 
 G 
 
82 THE STORY OF THE HEAVENS. 
 
 space should be to explore that mighty body which is the source of 
 our life itself. 
 
 Nor could there be very much hesitation as to the second step 
 which ought to be taken. The intrinsic importance of the moon, 
 when compared with other celestial bodies,, may be small; it is, 
 indeed, we shall afterwards see, almost infinitesimal. But in the- 
 economy of our earth the moon still plays, and has played, a part 
 second only in importance to that of the sun himself. The moon 
 is so close to us that her brilliant rays pale to invisibility countless- 
 orbs of a size and an intrinsic splendour incomparably greater. 
 The moon also occupies quite an exceptional position in the history 
 of astronomy ; for the greatest discovery that science has yet wit- 
 nessed was accomplished by means of her motion. It was there- 
 fore natural that an early chapter in our Story of the Heavens- 
 should be devoted to a body whose interest approximated so closely 
 to that of the sun himself. 
 
 But the sun and the moon partly described (we shall afterwards 
 find it necessary to return to both of them), some hesitation is- 
 natural in the choice of the next step. The two great luminaries 
 being abstracted from our view, there is no other celestial body of 
 such exceptional interest and significance as to make it quite clear 
 what course to pursue, in the attempt to unfold the story of the 
 heavens in the most natural manner. Did we attempt to describe 
 the celestial bodies in the order of their actual magnitude, our 
 ignorance must at once pronounce the attempt to be impossible. 
 We cannot even make a conjecture as to which body in the heavens- 
 is to stand first on the list. Even if that mightiest body be within, 
 reach of our telescopes (in itself a highly improbable supposition),, 
 we have not the least idea in what part of the heavens it is to be 
 sought. And even if this were possible if we were able to arrange 
 all the visible bodies rank by rank in the order of their magni- 
 tude and their splendour still the scheme would be impracticable,, 
 for of most of them we know little or nothing. 
 
 We are therefore compelled to adopt a different method of 
 procedure, and the simplest, as well as the most natural, will be to- 
 follow as far as possible the order of distance of the different 
 
TEE SOLAR SYSTEM. 83 
 
 bodies. We have already spoken of the moon as the nearest neigh- 
 bour to the earth, we shall next consider some of the other celestial 
 bodies which are comparatively near to us ; then, as the work pro- 
 ceeds, we shall discuss the objects further and further away, until 
 towards the close of the volume we shall be engaged in considering 
 the most distant bodies in the universe which the telescope has yet 
 revealed to us. 
 
 Even when we have decided on this principle, our course is still 
 not free from doubt. Many of the bodies in the heavens are 
 moving so that their distances from the earth change, but this is 
 a difficulty which need not really detain us. We shall make no 
 attempt to adhere closely to the principle in all its details. It will 
 be sufficient if we first describe those great bodies not a very 
 numerous class which are, comparatively speaking, in our vicinity, 
 though still at vast and varied distances; and then we shall pass 
 on to the almost infinitely numerous bodies which are separated 
 from us by distances so stupendous that the imagination is baffled 
 in the attempt to realise them. 
 
 Let us, then, scan the heavens, to choose therefrom those 
 neighbouring orbs which are to form the material for our earliest 
 consideration. The sun has set, the moon has not risen ; a cloud- 
 less sky discloses a heaven glittering with countless gems of light. 
 Some are grouped together into well-marked constellations ; others 
 scattered promiscuously hither and thither, with every degree 
 of lustre, from the very brightest down to the faintest point that 
 the eye can just discover. Amid all this host of objects, how are 
 we to identify those which are nearest to the earth ? Look to the 
 west: the sun has but lately set, and over the spot where his 
 departing beams still linger we see the lovely evening star shining 
 forth. This is the planet Venus a beauteous orb, twin-sister to 
 the earth. The brilliancy of this planet, its rapid changes both 
 in position and in lustre, would suggest at once that it was 
 much nearer to the earth than other star-like objects. This pre- 
 sumption has been amply confirmed by careful measurements, and 
 therefore Venus is to be included in the list of the orbs which 
 constitute our neighbours. 
 G 2 
 
84 THE STORY OF THE HEAVENS. 
 
 Another conspicuous planet almost rivalling Venus in lustre, 
 and vastly surpassing Venus in the magnificence of its proportions 
 and its retinue has borne from all antiquity the majestic name of 
 Jupiter. No doubt Jupiter is much more distant from us than 
 Venus. Indeed, he is always at least twice as far away, while 
 he is sometimes ten times as far. But still we must include 
 Jupiter among our neighbours. Compared with the host of stars 
 with which the heavens glitter, Jupiter must be regarded as quite 
 near. The distance of Jupiter can only be expressed in hundreds 
 of millions of miles ; yet, vast as that distance is, it would have 
 to be multiplied by tens of thousands, or hundreds of thousands, 
 before it would be large enough to span the stupendous abyss 
 which intervenes between the earth and the stars which form the 
 constellations. 
 
 Venus and Jupiter have invited our attention by their far 
 superior brilliancy. To them must now be added a few other 
 bodies, in which brilliancy can hardly any longer be a reliable 
 indication. An observer unacquainted with astronomy might ih*t 
 improbably point to the Dog Star or Sirius, as astronomers more 
 generally know it as an object whose exceptional lustre showed it 
 to be one of our neighbours, but this is not the case. We shall after- 
 wards have occasion to refer to this gem of our northern skies with 
 considerable detail, and then it will appear that Sirius is really a 
 mighty globe far transcending in splendour our own sun, but 
 plunged into the depths of space to such an appalling distance that 
 his enfeebled rays, when they reach the earth, give us the im- 
 pression, not of a mighty sun, but only of a brilliant star. 
 
 The principle of selection which we are to adopt will be ex- 
 plained presently ; in the meantime, it will be sufficient to observe 
 that our list is to be augmented first, by the addition of the 
 unique object known as Saturn, whose brightness is, however, far 
 surpassed by that of Sirius, as well as by a few other stars. Then 
 we add Mars, an object which occasionally draws in close to the 
 earth, and shines with a fiery radiance which would hardly prepare 
 us for the truth that Mars is intrinsically one of the smallest of the 
 celestial bodies. Besides the objects we have mentioned, the ancient 
 
THE SOLAR SYSTEM. 85 
 
 astronomers had detected a fifth, known as Mercury a planet 
 which is usually invisible amid the light surrounding the sun. 
 Mercury, however, occasionally wanders far enough from the sun 
 to he seen before sunrise or after sunset, and was thus discovered 
 ages ago. These five Mercury, Venus, Mars, Jupiter, and Saturn 
 exhaust the list of planets so far as the ancient astronomers 
 knew them. 
 
 We can, however, now extend the list somewhat further, by 
 adding to it the telescopic objects which have in modern times been 
 found to be justly entitled to be included among our neighbours. 
 Here we must no longer postpone the introduction of the criterion 
 by which we are to decide whether a body is near the earth or not. 
 The planets can be to some extent recognised by the steady radiance 
 of their light as contrasted with the incessant twinkling of the 
 stars. A very small amount of attention devoted to any of the 
 bodies we have named will, however, point out a more definite 
 contrast between them and the stars. We observe, for instance, 
 Jupiter, on any clear night when the stars can be well seen, 
 and note carefully his position with regard to the stars in his 
 neighbourhood how he is to the right of this star, or to the 
 left of that ; directly between this pair of stars, or directly pointed 
 to by that. We then either mark down the place of Jupiter on 
 a map or a globe, or make a sketch of the stars in the neigh- 
 bourhood showing the position of the planet. After a month or 
 two, when the observations are to be repeated, the place of Jupiter 
 is again to be compared with those stars by which it was before 
 defined. It will be found that, while the stars have preserved 
 their mutual positions, the place of Jupiter has changed. Hence 
 it is that Jupiter is with propriety called a planet, or a wanderer, 
 incessantly moving from one part of the starry heavens to another. 
 By similar comparisons it can be shown that the other bodies we 
 have mentioned Venus and Mercury, Saturn and Mars are also 
 wanderers, and belong to that group of heavenly bodies known as 
 planets. Here, then, we have the simple criterion by which the 
 earth's neighbours are to be readily discriminated from the stars. 
 Each of the bodies near the earth is a planet, or a wanderer, and 
 
86 THE STORY OF THE HEAVENS. 
 
 the mere fact that a body is a wanderer is alone sufficient to prove 
 it to be one of the class which we are now studying. 
 
 Provided with this test, we can at once make a considerable 
 addition to our list. Amid the myriad orbs which the telescope 
 reveals,, here and there is found one which is a wanderer. Two 
 other mighty planets, known as Uranus and as Neptune, must 
 thus be added to the five already mentioned, making in all a group 
 of seven great planets. A vastly greater number may also be 
 reckoned when we admit to our view bodies which not only seem 
 minute telescopic objects, but really are minute globes when com- 
 pared with the mighty bulk of our earth. These lesser planets, to 
 the number of between two and three hundred, are also among the 
 earth's neighbours. 
 
 We should, at this point, remark that a class of heavenly 
 bodies widely differing from the planets must also be included 
 among our neighbours. The bodies we refer to are the comets, and, 
 indeed, it may happen that a comet will sometimes draw nearer to 
 the earth than even the closest approach ever made by a planet. 
 These remarkable and mysterious visitors will necessarily engage a 
 good deal of our attention later on ; but for the present we confine 
 our attention to those more substantial globes, whether large or 
 small, which are always termed planets. 
 
 In some of the lighthouses, surrounding our coasts, which 
 exhibit flashing or intermittent lights, the cutting off the light is 
 accomplished by two opaque semi-cylinders which clasp the central 
 flame at stated intervals, and thus produce the obscurity. Imagine 
 for a moment that some similar obscuring apparatus could be 
 clasped around our sun so that all his beams were intercepted. 
 That our earth would be plunged into the darkness of midnight is 
 of course an obvious consequence. A moment's reflection will show 
 that the moon, shining as it does only with the reflected rays of 
 the sun, would become totally invisible. But would this extinction 
 of the sunlight have any other effect ? Would it, for instance, 
 have any effect on the countless brilliant points that stud the 
 heavens at midnight? On a winter's night, as the sun is far 
 below the horizon, no beams could in any case illuminate the sky, 
 
THE SOLAR SYSTEM. 87 
 
 and consequently, whether the sun be really obscured or not, could 
 make no difference in the darkness o a moonless winter night. 
 But yet the sudden obscurity of the sun would produce a remark- 
 able effect, which a little attention would disclose. The stars, no 
 doubt, would not exhibit the slightest change in brilliancy. Each 
 star shines by its own light and is not indebted to the sun. The 
 constellations would thus twinkle on as before, but a wonderful 
 change would come over the planets. Were the sun to be obscured, 
 instantly the planets would disappear from view. The midnight 
 sky would thus witness the sudden extinction of the planets, while 
 all the stars would remain unaltered. It seems hard at first to 
 understand how the brilliancy of Venus, or the lustre of Jupiter 
 can be really only due to the beams which fall upon these bodies 
 from the distant sun. The evidence is, however, conclusive on 
 the question ; and it will be placed before the reader more fully 
 when we come to discuss the several planets in detail. 
 
 Another objection may be here anticipated. Suppose that we 
 are looking at Jupiter high in mid-heavens on a winter's night, it 
 might be reasonably contended that as the earth lies between Jupiter 
 and the sun, the rays of the sun therefore could not fall on Jupiter. 
 This is, perhaps, not an unnatural view for an inhabitant of this 
 earth to adopt until he has become acquainted with the sizes of the 
 various bodies concerned, and with the distances by which those 
 bodies are separated. But the question would appear in a widely 
 different form to an inhabitant of the planet Jupiter. If such a 
 being were asked whether he suffered much inconvenience by the 
 intrusion of the earth between himself and the sun, his answer 
 would be something of this kind : " No doubt such an event as the 
 passage of the earth between me and the sun is possible, and has 
 occurred on rare occasions separated by long intervals ; but so far 
 from it being the cause of any inconvenience, the whole earth, of 
 which you think so much, is really so minute, as compared with the 
 sun, that when the earth did come in front of the sun it was merely 
 seen as a minute telescopic point, and the amount of sunlight which 
 it intercepted was quite inappreciable/' 
 
 The fact that the planets shine by the sun's light, points at once 
 
88 
 
 THE STORY OF THE HEAVENS. 
 
 to the similarity between them and our earth. We are thus led to 
 regard our sun as a central glowing globe associated with a number 
 of other much smaller globes, each of which, being a dark body, is 
 indebted to the sun for its light and its heat. 
 
 That was, indeed, a grand step in astronomy which demonstrated 
 the nature of the solar system. The discovery that our earth, of 
 
 Mars 
 
 /Venus *'/ Mercury 
 
 Sun 
 
 225'davs" 
 
 365'days 
 
 687 days 
 Fig. 29. The Orbits of the four interior Tlanets. 
 
 whose stupendous bulk we are so conscious, really formed a globe 
 poised freely in space, was in itself a mighty exertion of human 
 intellect ; but when it came to be recognised that this globe was 
 but one of a whole group of globes, some smaller, no doubt, 
 but others very much larger, and that this mighty series was 
 subordinated to the supreme control of the sun, we have a chain of 
 discoveries of an importance and a magnificence which knows no 
 parallel in nature. 
 
 We thus see that the sun presides over a numerous family. 
 The members of that family are dependent upon the sun, and have 
 
PLATE X. 
 
 TYOHO AND ITS SURROUNDINGS. 
 
 (AFTER NASMYTH.) 
 
THE SOLAR SYSTEM. 89 
 
 a size suitably proportioned to their subordinate position. Even 
 Jupiter, the largest member of that family, does not contain one- 
 thousandth part of the material which forms the vast bulk of the 
 sun Yet Jupiter alone exceeds all the rest of the planets together. 
 
 In the centre of the diagram in Fig. 29 we have represented the 
 sun not, indeed, in his true proportions as to bulk, for it would 
 be impossible to maintain the true proportions in a figure of 
 reasonable dimensions. Around this sun we have four nearly 
 circular paths, indicated by dotted lines to denote the orbits in 
 which the different bodies move. The innermost of these four paths 
 is the orbit of the planet Mercury. The planet moves around the 
 sun in this path, and regains the place from which it started in 
 eighty- eight days. 
 
 The next orbit, proceeding outwards from the sun, is that of the 
 planet Venus, which we have already referred to as the well-known 
 Evening Star. Venus moves round its path in a period of 225 
 days. One step further from the sun and we come to- th- : orbit, 
 of another planet. This planet is almost the same size as Venus- - 
 it is much larger than Mercury. The planet now under considera- 
 tion accomplishes its circle in a period of 365 days. This period 
 sounds familiar to our ears. It is the length of the year ; and the 
 planet is the earth on which we stand. There is an impressive way 
 in which to realise the enormous course which the earth pursues in 
 each annual journey. Everyone knows that the circumference of a 
 circle is about three and one-seventh times the diameter of the 
 circle ; so that, taking the distance from the earth to the sun to 
 be 92,700,000 miles, the diameter of the circle which the earth 
 describes around the sun will be 185,400,000 miles, and conse- 
 quently the circumference of the mighty circle in which the earth 
 moves round the sun is about 583,000,000 miles. Over this 
 distance the earth has to travel in a year. It will be merely a sum 
 in division to find from this how far the earth must travel each 
 second in order to accomplish this long journey within a year. 
 It will appear that the earth must actually move about eighteen 
 miles every second, as otherwise it would not complete its journey 
 in the allotted time. 
 
90 THE STORY OF THE HEAVENS. 
 
 Pause for a moment to think what a velocity of eighteen miles 
 a second really implies. Can we realise a speed so tremendous ? 
 Let us compare it with our ordinary types of rapid movement. 
 Look at that express train how it crashes under the bridge, how, 
 in another moment, it is lost to view ! Can any velocity be greater 
 than that? Let us try it by figures. The train moves a mile a 
 minute ; multiply that velocity by eighteen and it becomes 
 eighteen miles a minute, but we must further multiply it by sixty 
 to make it eighteen miles a second. The velocity of the express 
 train is not even the thousandth part of the velocity of the earth. 
 Let us take another illustration. We stand at the rifle ranges to see 
 a rifle fired at a target 1,000 feet away, and we find that a second 
 or two is sufficient to carry the bullet over that distance. Can any 
 
 Fig. 30. The Earth's movement. 
 
 moving body exceed in speed the rifle-bullet ? The earth moves 
 nearly one hundred times as fast. Yet, viewed in another way, the 
 stupendous speed of the earth does not seem so inconceivable. The 
 earth is a mighty globe, so great indeed that even moving at this 
 speed it takes almost eight minutes to pass over its own diameter. 
 The speed is thus not so enormously great when the great bulk of 
 the earth is considered. To illustrate this we show here a view of 
 the progress made by the earth. The distance between the centres 
 of these circles is about six times the diameter ; and, accordingly, if 
 these two circles represent the earth, the time taken to pass from 
 one position to the other is about forty-eight minutes. 
 
 Still one more step from the earth, and we come to the orbit 
 of the fourth planet Mars, which requires 687 days, or nearly two 
 years, to complete its circuit of the heavens. "With our arrival at 
 Mars we have gained the limit to the inner portion of the solar 
 system. 
 
 The four planets we have mentioned form a group in them- 
 selves, distinguished by their comparative nearness to the sun, and 
 
THE SOLAR SYSTEM. 91 
 
 also because they are bodies of moderate dimensions. In our 
 system Venus and the Earth are globes of about the same size. 
 Mercury and Mars are smaller globes intermediate between the 
 bulk of the earth and that of the moon. The four planets which 
 come nearest to the sun are, however, vastly surpassed in bulk and 
 weight by the giant bodies of our system the stately group of 
 Jupiter and Saturn, Uranus and Neptune. 
 
 -' r Neptune 
 
 -^Uranus 
 
 Saturn 
 
 Mars 
 
 Fig. 31. The Orbits of the four giant Planets. 
 
 These giant planets enjoy the sun's guidance equally with their 
 weaker brethren. On the above diagram parts of the orbits of the 
 great outer planets are represented. The sun, as before, presides 
 at the centre, but the inner planets would now be so close to the sun 
 that it is only possible to represent the orbit of Mars. After the 
 orbit of Mars comes a considerable interval, not, however, devoid 
 of planetary activity, and then follow the orbits of Jupiter and of 
 Saturn; further still, we have Uranus, a great globe on the verge 
 of unassisted vision; and, lastly, the whole system is bounded 
 
92 
 
 THE STORY OF THE HEAVENS. 
 
 by the grand orbit of Neptune a planet of which we shall have 
 a marvellous story to narrate. 
 
 The various circles in Fig. 32 show the apparent sizes of the 
 sun as seen from the different planets. Taking the circle corres- 
 
 Fig. 32. Comparative apparent size of the Sun as seen from the various Planets. 
 
 ponding to the earth to represent the amount of heat and light 
 which the earth derives from the sun, then the other circles denote 
 the heat and the light enjoyed by the corresponding planets. The 
 next outer planet to the earth is Mars, whose share of solar bless- 
 ings is not so very inferior to that of the earth ; but when we look 
 
THE SOLAR SYSTEM. 
 
 93 
 
 at Jupiter, at Uranus, or at Neptune, we fail to see how bodies 
 so remote from the sun can enjoy climates at all comparable with 
 those of the planets which are more favourably situated. 
 
 Fig. 33. Comparative sizes of the Planets. 
 
 Fig. 33 shows a picture of the whole family of planets sur- 
 rounding- the sun represented on the same scale, for exhibiting 
 their comparative sizes. Jupiter, it will be seen, is not only the 
 greatest planet, but he is much greater than all the other 
 planets put together. Measured by bulk, Jupiter is more than 
 1,200 times as great as the earth, so that it would take at least 
 
94 THE STORY OF THE HEAVENS. 
 
 1,200 earths rolled into one to form a globe equal to the globe of 
 Jupiter. Measured by weight, the disparity between the earth and 
 Jupiter, though still enormous, is not quite so great ; but this is a 
 matter to be discussed more fully in a later chapter. 
 
 Even in this preliminary survey of the solar system we must 
 not omit to refer to the vast host of planets which attract our 
 attention, not by their bulk but by their multitude. In the ample 
 zone, bounded on the inside by the orbit of Mars, and on the 
 outside by the orbit of Jupiter, it was thought at one time that no 
 planet revolved. Modern research has shown that this vast area 
 of space is tenanted, not by one planet, but by hundreds of planets. 
 The discovery of these planets is a charge undertaken by many 
 diligent modern astronomers, while the discussion of their move- 
 ments affords labour to many others. We shall find much to learn 
 in the study of these tiny bodies, to which a chapter will be devoted 
 further on in the course of this work. 
 
 But we do not propose to enter deeply into the mere statistics 
 of the planetary system at present. Were such our intention, the 
 tables at the end of this volume will show that ample materials 
 are available. Astronomers have taken a more or less complete 
 inventory of every one of the planets. They have measured their 
 distances, the shapes of their orbits and the positions of those 
 orbits, their times of revolution, and, in the case of all the larger 
 planets, their sizes and their weights. Such results are of interest 
 for many purposes in astronomy; but it is the more general 
 features of the science which at present claim our attention. 
 
 Let us, in conclusion, note one or two important truths with 
 reference to this beautiful planetary system. We have seen that 
 the planets all revolve in nearly circular paths around the sun. 
 We have now to supplement this by another statement of very 
 great importance. Each of the planets pursues its path in the 
 same direction. It may thus happen that one planet may overtake 
 another, but it can never happen that two planets pass by each 
 other as do the trains on adjacent lines of railway. We shall 
 subsequently find that the whole welfare of our system, nay, its 
 continuous existence, is dependent upon this remarkable uniformity. 
 
THE SOLAR SYSTEM. 95 
 
 Such is our solar system ; a mighty organised group of planets 
 circulating round a common sun, poised in space, and completely 
 isolated from all external interference. No star, no constellation, 
 has any appreciable influence on our solar system. We constitute 
 a little island group, separated from the nearest stars by the most 
 appalling distances. It may be that as the other stars are suns, so 
 they too may have systems of planets circulating around them ; but 
 of this we know nothing. Of the stars we can only say that they 
 are points of light, and if they had hosts of planets those planets 
 must for ever remain invisible to us, even if they were many times 
 as large as Jupiter. 
 
 At this limitation to our possible knowledge we need not repine, 
 for just as we find in the solar system all that is necessary for our 
 daily bodily wants, so shall we find ample occupation for whatever 
 faculties we may possess, in endeavouring to understand those 
 mysteries of the heavens which lie within our reach. 
 
CHAPTER V. 
 
 THE LAW OF GRAVITATION. 
 
 Gravitation The Falling of a Stone to the Ground All Bodies fall Equally 
 Sixteen Feet in a Second Is this True at Great Heights ? Fall of a Body at 
 a height of a Quarter of a Million Miles How Newton obtained an Answer 
 from the Moon His great Disco very Statement of the Law of Gravitation 
 Illustrations of the Law How is it that all the Bodies in the Universe do not 
 rush together ? The Effect of Motion How a Circular Path can he produced 
 by Attraction General Account of the Moon's Motion Is Gravitation a Force 
 of great Intensity ? Two Weights of 50 Ibs. Two Iron Globes, 53 yards in 
 diameter, and a mile apart, attract with a force of 1 Ib. Characteristics of 
 Gravitation Orbits of the Planets not strictly Circles The Discoveries of 
 Kepler Construction of an Ellipse Kepler's First Law Does a Planet move 
 uniformly ? Law of the Changes of Velocity Kepler's Second Law The 
 Relation between the Distances and the Periodic Times Kepler's Third Law 
 Kepler's Laws and the Law of Gravitation Movement in a straight line A 
 Body unacted on by disturbing Forces would move in a straight line with 
 constant Velocity Application to the Earth and the Planets The Law of 
 Gravitation deduced from Kepler's Laws Universal Gravitation. 
 
 OUR narrative of the heavenly bodies must undergo a slight and 
 temporary interruption, while we now enunciate and describe with 
 appropriate detail the extremely important principle known as 
 the law of gravitation,, which underlies the whole of astronomy. 
 To the law of gravitation must be ascribed the movements of the 
 moon around the earth, and of the planets around the sun. Those 
 movements can be completely accounted for when once the law of 
 gravitation has been admitted. It is accordingly incumbent upon 
 us to explain that law, before we proceed to the more detailed 
 account of the separate planets. We shall find, too, that the law 
 of gravitation opens up vast chapters in the history of the stars 
 situated at the most stupendous distances in space, while it also 
 affords the key by which we are enabled to cast a retrospective 
 glance into the vistas of time past, and trace with plausibility, if 
 not with certainty, early phases in the history of our system. 
 
THE LAW OF GRAVITATION. 97 
 
 The sun and the moon, the planets and the comets, the stars and 
 the nebulae, all alike are subject to this universal law, which is now 
 to engage our attention. 
 
 What is more common than the fact that when a stone is 
 dropped it will fall to the ground ? so common is it that no one 
 at first thinks it worthy of remark. People are often surprised at 
 seeing a piece of iron drawn to a magnet. Yet the fall of a stone 
 to the ground is the manifestation of a force quite as interesting as 
 the force of magnetism. It is the earth which draws the stone, just 
 as the magnet draws the iron. In each case the force is one of 
 attraction ; but while the magnetic attraction is confined to a few 
 substances, and is of comparatively limited importance, the attrac- 
 tion of gravitation extends far and wide throughout the whole 
 universe. 
 
 Let us commence with a few very simple experiments upon the 
 force of gravitation. Take in the hand a small piece of lead, and 
 allow it to fall upon a cushion. The lead requires a certain time to 
 move from the fingers to the cushion, but that time is always 
 the same when the height is the same. Take now another and 
 larger piece of lead, and hold one in each hand at the same height. 
 If both are released at the same moment, they will both reach the 
 cushion at the same moment. It might have been thought that the 
 heavy body would fall more quickly than the light body; but when 
 the experiment is tried it is seen that this is not the case. Repeat 
 the experiment with various other substances. Try an ordinary 
 marble. It also will fall in the same time as the piece of lead. 
 With a piece of cork we again try the experiment, and again 
 obtain the same result. At first it seems to fail when we compare 
 a feather with the piece of lead; but that is solely on account 
 of the air, which resists the feather more than it resists the 
 lead. If, however, the feather be placed upon the top of a penny, 
 and the penny be horizontal when dropped, it will clear the air out 
 of the way of the feather in its descent, and then the feather will 
 fall as quickly as the penny, as quickly as the marble, or as quickly 
 as the piece of lead. 
 
 If the observer were in a gallery when trying these experi- 
 H 
 
98 THE STOR? OF THE HEAVENS. 
 
 merits, and if the cushion were sixteen feet below his hands, then 
 the time the marble would take to fall through the sixteen feet 
 would be one second. The time occupied by the cork or by the 
 lead would be the same, and even the feather itself would fall 
 through sixteen feet in one second, if it could be screened from the 
 interference of the air. Try this experiment where we like, in 
 London, or in any other city, in any island or continent, on board a 
 ship at sea, at the North Pole, or the South Pole, or the equator, 
 it will always be found that any body of any size, or any material, 
 will fall about sixteen feet in one second of time. 
 
 Lest any erroneous impression should arise, we may just men- 
 tion that the distance traversed in one second does vary slightly at 
 different parts of the earth, but from causes which need not at this 
 moment detain us. We shall for the present regard sixteen feet as 
 the distance through which any body, free from interference, would 
 fall in one second at any part of the earth's surface. But now let 
 us extend our view above the earth's surface, and inquire how far 
 this law of sixteen feet in a second may find obedience elsewhere. 
 Let us, for instance, ascend to the top of a mountain and try the 
 experiment there. It would be found that at the top of the 
 mountain a marble would take a little longer to fall through six- 
 teen feet than the same marble would if let fall at its base. The 
 difference would be very small; but yet it would be measurable, 
 and sufficient to show that the power of the earth to pull the marble 
 down to the ground was somewhat weakened when we ascend high 
 above the earth's surface. Yet no matter how high we ascend, 
 either to the top of a high mountain, or to the still greater heights 
 that have been reached in balloon ascents, we shall find that 
 the tendency of bodies to fall to the ground remains, though 
 no doubt the higher we go the more is that tendency weakened. 
 It would be of the greatest interest to find how far this power 
 of the earth to draw bodies towards it extends. We cannot get 
 more than about five or six miles above the earth's surface in a 
 balloon ; yet we want to know what would happen if we could go 
 500 miles, or 5,000 miles, or still further, into the depths of space. 
 
 Conceive that a traveller were endowed with some means of 
 
 
THE LAW OF GRAVITATION. 99 
 
 soaring aloft for miles and thousands of miles,, still up and up, until 
 at length he had attained the awful height of nearly a quarter of a 
 million of miles above the earth. Glancing down at the surface of 
 that earth, which is at such a stupendous depth beneath, he would 
 be able to see the whole world, as it were, in a bird's-eye view. 
 He would lose, no doubt, the details of towns and villages ; the 
 features in such a landscape would be whole continents and whole 
 oceans, in so far as the openings of the clouds far beneath would 
 permit the earth's surface to be exposed. 
 
 At this stupendous height he could try one of the most interesting 
 experiments that was ever in the power of a philosopher. He could 
 test whether the earth's attraction attained to such a height, and he 
 could measure the amount of that attraction. Take for the experi- 
 ment a cork, a marble, or any other object, large or small ; hold it 
 between the fingers, and let it go. Every one knows what would 
 happen in such a case down here ; but it required Sir Isaac Newton 
 to tell us what would happen in such a case up there. Newton says 
 that the power of the earth to draw bodies towards it extends even 
 up to this great height, and that the marble will fall. This is the 
 doctrine that we can now test. We are ready for the experiment. 
 The marble is released, and, lo, our first exclamation is one of 
 wonder. The moment the marble was released, instead of dropping 
 instantly, it appears to be suspended. We are on the point of 
 exclaiming that we are beyond the earth's attraction, and that 
 Newton is wrong, when our attention is arrested ; the marble is be- 
 ginning to move, so slowly that at first we have to watch it carefully. 
 Then it moves more rapidly, so that its motion is beyond all doubt, 
 until, gradually acquiring more and more velocity, the marble speeds 
 on its long journey of a quarter of a million of miles to the earth. 
 
 But surely, it will be said, such an experiment must be entirely 
 impossible; and no doubt it cannot be performed in the way 
 described. The bold idea occurred to Newton of making use of the 
 moon itself, which is almost a quarter of a million of miles above 
 the earth, for the purpose of answering the question. Never was 
 the moon put to such noble use before. The moon is actually 
 at each moment fallino- in towards the earth. We can calculate 
 
 O 
 
 ii 2 
 
100 THE STORY OF THE HEAVENS. 
 
 how much the moon is deflected in each second towards the earth, 
 and thus obtain the measure of the earth's attractive power. From 
 the moon Newton was able to learn that a body released at the 
 distance of 240,000 miles above the surface of the earth would still 
 be attracted by the earth, that in virtue of the attraction the body 
 would commence to move off towards the earth not, indeed, with 
 the velocity with which a body falls in experiments on the surface 
 but with a very much lesser speed. At the distance of the 
 moon a body, so far from falling a distance of sixteen feet in a 
 second of time, would commence its long journey so slowly that a 
 minute, instead of a second, would have elapsed before the distance 
 of sixteen feet had been accomplished.* 
 
 It was by pondering on information thus won from the moon 
 that Newton made his immortal discovery. The gravitation of 
 the earth is a force which extends far and wide through space. 
 The more distant the body, the weaker is the gravitation; here 
 Newton found the means of determining the great problem as to the 
 law according to which the intensity of the gravitation decreased. 
 The information derived from the moon, that a body 240,000 miles 
 away requires a minute to fall through a space equal to that 
 through which it would fall in a second down here, was of unspeak- 
 able importance. In the first place, it shows that the attractive 
 power of the earth, by which it draws all bodies earthward, becomes 
 weaker at a distance. This might, indeed, have been anticipated. 
 It is as reasonable to suppose that as we retreated further and further 
 into the depths of space the power of attraction should diminish, as 
 that the lustre of light should diminish as we recede from it ; and it 
 is remarkable that the law according to which the attraction of gra- 
 vitation decreases with the increase of distance, is precisely the same 
 as the law according to which the brilliancy of a light decreases as 
 its distance increases. The law of nature, stated in its simplest 
 form, asserts that the intensity of gravitation varies inversely as the 
 
 * The space described by a falling body is proportional to the product of the 
 force and the square of the time. The force varies inversely as the square of the 
 distance from the earth, so that the space will vary as the square of the time, and 
 inversely as the square of the distance. If, therefore, the distance be increased 
 sixty-fold, the time must also be increased sixty-fold, if the space fallen through is 
 to remain the same. 
 
THE LAW OF GRAVITATION. 101 
 
 square of the distance. Let us endeavour to elucidate this some- 
 what abstract statement by one or two simple illustrations. Suppose 
 a body were raised above the surface of the earth to a height of 
 nearly 4,000 miles, so as to be at an altitude equal to the radius of 
 the earth. In other words, a body so situated would be twice as far 
 from the centre of the earth as a body which was on the surface. 
 The law of gravitation says that the intensity of the attraction is 
 then to be decreased to one-fourth part, so that the pull of the 
 earth on a body, 4,000 miles up, is only one quarter of the pull of 
 the earth on that body so long as it lies on the surface. We may 
 imagine the effect of this pull to be shown in different ways. Allow 
 the body to fall, and in the interval of one second it will only drop 
 through four feet, a mere quarter of the distance that gravity ac- 
 complishes near the earth's surface. To put the matter in another 
 way ; suppose that the pull of the earth is to be measured by one of 
 those little weighing machines known as a spring balance. If a 
 weight of four pounds be hung on a spring balance, at the earth's 
 surface, the index of course shows a weight of four pounds ; but 
 conceive this balance still bearing the weight appended thereto were 
 to be carried up and up, the indicated strain would become less and 
 less, until, by the time the balance reached 4,000 miles high, where 
 it was twice as far away from the earth's centre as at first, the indi- 
 cated strain would be reduced to the fourth part, and the balance 
 would only show one pound. If we could imagine the mighty voyage 
 prolonged still further into the depths of space, the reading on the 
 scale of the balance would still continue to decrease. By the time the 
 apparatus had reached a distance of 8,000 miles high, being then three 
 times as far from the earth's centre as at first, the law of gravitation 
 tells us that the attraction must have decreased to one-ninth part. The 
 strain thus shown on the balance would be only the ninth part of 
 four pounds, or less than half a pound. But let the voyage be once 
 again resumed, and let not a halt be made this time until the 
 balance and its four-pound weight has retreated to that orbit which 
 the moon traverses in its monthly course around the earth. The 
 distance thus attained is about sixty times the radius of the earth, 
 and consequently the attraction of gravitation is diminished in the 
 
102 THE STORY OF THE HEAVENS. 
 
 proportion of one to the square of sixty ; the balance will then only 
 be strained by the inappreciable fraction of l-3,600th part of four 
 pounds ; and we also see that a weight which on the earth weighed 
 a ton and a half would, if raised 240,000 miles, weigh less than a 
 pound. But even at this mighty distance we are not to halt ; imagine 
 that we retreat still further and further, the strain shown by the 
 balance will ever decrease, but it will still exist no matter how far 
 we go. Astronomy appears to teach us that the attraction of gravi- 
 tation can extend, with suitably enfeebled intensity, across the most 
 profound gulfs of space. 
 
 The principle of gravitation is of far wider scope than we have 
 yet indicated. We have spoken merely of the attraction of the 
 earth, and we have stated that its attraction extends throughout 
 space. But the law of gravitation is not so limited. Not only 
 does the earth attract every other body, and every other body 
 attract the earth, but each of these bodies attracts each other; so 
 that in its more complete shape the law of gravitation announces 
 that " every body in the universe attracts every other body with a 
 force which varies inversely as the square of the distance/'' 
 
 It is impossible for us to over-estimate the importance of this 
 law. It supplies the clue by which we can unravel the complicated 
 movements of the planets. It has led to marvellous discoveries, 
 in which the law of gravitation has enabled us to anticipate the 
 telescope, and, indeed, actually to feel the existence of bodies before 
 those bodies have even been seen. 
 
 An objection which may be raised at this point must first be 
 dealt with. The objection is, indeed, a plausible one. If the 
 earth attracts the moon, why does not the moon tumble down on 
 the earth ? If the earth is attracted by the sun, why does it not 
 tumble into the sun ? If the sun is attracted by other stars, why 
 do they not rush together with a frightful collision ? It may not 
 unreasonably be urged that if all these bodies in the heavens are 
 attracting each other, it would seem that they must all rush together 
 in consequence of that attraction, and thus weld the whole material 
 universe into a single mighty mass. We know, as a matter of fact, 
 that these collisions do not often happen, and that there is extremely 
 
THE LAW OF GRAVITATION. 103 
 
 little likelihood o their taking place. "We see that although our 
 earth is said to have been attracted by the sun for countless ages, 
 yet that the earth is just as far from the sun as ever it was. Is 
 not this in conflict with the doctrine of universal gravitation ? In 
 the early days of astronomy such objections would be regarded, and 
 doubtless were regarded, as well nigh insuperable; even still we 
 occasionally hear them raised, and it is therefore the more incum- 
 bent on us to explain how it happens that the solar system has 
 been able to escape from the catastrophe by which it seems to have 
 been threatened. 
 
 There can be no doubt that if the moon and the earth had been 
 initially placed at rest, they would have been drawn together by the 
 attraction. So, too, if the system of planets surrounding the sun 
 had been left initially at rest they would have dashed into the 
 sun and the system would have been annihilated. It is the fact 
 that the planets are moping, and that the moon is moving, which 
 has enabled these bodies successfully to resist the attraction in so 
 far, at least, as that they are not drawn thereby to total destruction. 
 
 It is so desirable that the student should understand clearly 
 how a central attraction is compatible with revolution in a nearly 
 circular path, that we give an illustration to show how the moon 
 pursues its monthly orbit under the guidance and the control of the 
 attracting earth. 
 
 The imaginary sketch in Fig. 34 denotes a section of the earth 
 with a high mountain thereon. If a cannon were stationed 
 on the top of the mountain at C, and if the cannon-ball were 
 fired off in the direction C E with a small charge of powder, 
 the ball would move down along the first curved path. If 
 it be fired a second time with a still greater charge, the 
 path will be along the second curved line, and the ball would 
 again fall into the sea. But let us try next time with a stronger 
 charge, and, indeed, with a far stronger cannon than any piece 
 of ordnance ever yet made. The velocity of the projectile must 
 now be assumed to be some miles per second, but we can con- 
 ceive that the velocity shall be so adjusted, that the ball shall 
 move along the path C D, always at the same height above the sea, 
 
104 THE STORY OF TEE HEAVENS. 
 
 though still curving-, as every projectile must curve, from the hori- 
 zontal line in which it moved at the first moment. Arrived at D, 
 the ball will still be at the same height above the surface, and its 
 velocity will be unabated. It will therefore directly start off 
 again and move round another arc of the circle without getting 
 nearer to the surface. In this manner the projectile will travel 
 completely round the whole globe, coming back again to C and 
 then taking another start in the same path. If we could then 
 E c 
 
 Imaginary Section 
 of the Earth 
 
 Fig. 34. Illustration of the Moon's motion. 
 
 abolish the mountain and the cannon at the top, we should have 
 a body revolving for ever around the earth in consequence of the 
 attraction of gravitation. 
 
 Make now a bold stretch of the imagination. Conceive a 
 mighty cannon which could fire off a round bullet no less than 
 2,000 miles in diameter. Discharge this enormous bullet with a 
 velocity of about 3,000 feet per second, which is two or three times 
 as great as the velocity actually attainable in modern artillery. Let 
 this mighty bullet be fired horizontally from some station nearly 
 a quarter of a million miles above the surface of the earth. That 
 stupendous bullet would sweep right round the earth in a nearly 
 circular orbit, and return to where it started in about four weeks. 
 
THE LAW OF GRAVITATION. 105 
 
 It would then commenca another revolution, four weeks more would 
 find it again at the starting point, and this motion would go on 
 for ages. 
 
 Do not suppose that we are entirely romancing. We cannot 
 show the mighty cannon, but we can show the mighty bullet. We 
 see it every month ; it is the beautiful moon herself. No one 
 asserts that the moon was ever shot from such a cannon; but it 
 must be admitted that she moves as if she were. In a later 
 chapter we shall inquire into the history of the moon, and show 
 how she came to revolve in this wonderful manner. 
 
 As with the moon around the earth, so with the earth around 
 the sun. The illustration shows that a circular or nearly circular 
 motion harmonises with the conception of the law of universal 
 gravitation. 
 
 We are accustomed to regard gravitation as a force of stu- 
 pendous magnitude. Does not gravitation control the moon in its 
 revolution around the earth ? Is not even the mighty earth itself 
 retained in its path around the sun by the surpassing power of the 
 sun's attraction ? No doubt the actual force which keeps the earth 
 in its path, as well as that which retains the moon in our neighbour- 
 hood, is of vast intensity, but that is because gravitation is in such 
 cases associated with bodies of most stupendous mass. No one can 
 deny that all bodies accessible to our observation appear to attract 
 each other in accordance with the law of gravitation ; but it must 
 be confessed that, unless one or both of the attracting bodies possess 
 enormously great mass, the intensity is almost immeasurably small. 
 Let us attempt to illustrate how feeble is the gravitation between 
 masses of easily manageable dimensions. Take for instance two 
 iron weights, each weighing about 501b., and separated by a dis- 
 tance of one foot from centre to centre. There is a certain attrac- 
 tion of gravitation between these weights. The two weights are 
 drawn together, yet they do not move. The attraction between 
 them, though it certainly exists, is an extremely minute force. We 
 are not here dealing with a force at all comparable in its intensity 
 with magnetic attraction. Every one knows that a magnet will 
 draw a piece of iron with considerable force, but the intensity of 
 
106 THE 8TOEY OF THE HEAVENS. 
 
 gravitation is very much less on masses o equal amount. The 
 attraction between these two 501b. weights is less than the one- 
 ten-millionth part of a single pound. Such a force is utterly 
 infinitesimal in comparison with the friction between the weights 
 and the table on which they stand, and hence there is no response 
 to the attraction by even the slightest movement. Yet, if we can 
 conceive each of these weights mounted on wheels absolutely devoid 
 of friction, and running on absolutely perfect horizontal rails, then 
 there is no doubt that the weights would slowly commence to draw 
 together, and in the course of time would arrive in actual contact. 
 
 If we desire to conceive gravitation as a force of measurable 
 intensity, we must employ masses immensely more ponderous than 
 those 501b. weights. Imagine a pair of globes, each composed of 
 417,000 tons of cast iron, and each, if solid, being about 53 yards 
 in diameter. Imagine these globes placed at a distance of one 
 mile apart. Each globe attracts the other by the force of gravita- 
 tion. It does not matter that buildings and obstacles of every 
 description intervene; gravitation will pass through such impedi- 
 ments as easily as light passes through glass. No screen can be 
 devised dense enough to intercept the passage of gravitation. Each 
 of these iron globes will therefore under all circumstances attract 
 the other ; but, notwithstanding their ample proportions, the inten- 
 sity of that attraction is still very small, though appreciable. The 
 attraction between these two globes is not more than a force equal 
 to the pressure exerted by a single pound weight. A child could hold 
 back one of these massive globes from its attraction by the other. 
 Suppose that all was clear, and that friction could be so neutral- 
 ised as to permit the globes to follow the impulse of their mutual 
 attractions. The two globes will then commence to approach, 
 but the masses are so large, while the attraction is so small, 
 that the speed will be accelerated very slowly. A microscope 
 would be necessary to show when the motion has actually com- 
 menced. An hour and a half must elapse before the distance is 
 diminished by a single foot; and although the pace improves 
 subsequently, yet three or four days must elapse before the two 
 globes will come together. 
 
THE LAW OF GRAVITATION. 107 
 
 The most remarkable characteristic of the force of gravita- 
 tion must be here specially explained. The intensity appears to 
 depend only on the magnitudes of the masses, and not at all on 
 the nature of the substances of which these masses are com- 
 posed. We have described the two globes as made of cast iron, 
 but if either or both were composed of lead or copper, of wood 
 or stone, of air or water, the attractive power would still be the 
 same, provided only that the masses remained unaltered. In 
 this we observe a profound difference between the attraction of 
 gravitation and magnetic attraction. In the latter case the 
 attraction is not perceptible at all in the great majority of sub- 
 stances, and is only considerable in the case of iron. 
 
 In our account of the solar system we have represented the 
 moon as revolving around the earth in a nearly circular path, and 
 the planets as revolving around the sun in orbits which are also 
 approximately circular. It is now our duty to give a more minute 
 description of these remarkable paths ; and, instead of dismissing 
 them as being nearly circles, we must ascertain precisely in what 
 respects they differ from circles. 
 
 If a planet revolved around the sun in a truly circular path, of 
 which the sun was always at the centre, it would then be obvious 
 that the distance from the sun to the planet, being always equal to 
 the radius of the circle, will be of constant magnitude. Now, there 
 can be no doubt that the distance from the sun to each planet is 
 approximately constant ; but when accurate observations are made, 
 it becomes clear that the distance is not absolutely constant. The 
 variations in distance may amount to many millions of miles, but, 
 even in extreme cases, the variation in the distance of the planet is 
 only a small fraction usually a very small fraction of the total 
 amount of that distance. The circumstances vary in the case of 
 each of the planets. The orbit of the earth itself is such that the 
 distance from the earth to the sun departs but little from its mean 
 value. Venus makes even a closer approach to perfectly circular 
 movement ; while, on the other hand, the path of Mars, and much 
 more the path of Mercury, show considerable relative fluctuations 
 in the distance from the planet to the sun. 
 
108 THE STORY OF THE HEAVENS. 
 
 It has often been noticed that many of the great discoveries in 
 science have their origin in the nice observation and explanation of 
 minute departures from some law approximately true. We have 
 in this department of astronomy an excellent illustration of this 
 principle. The orbits of the planets are nearly circles, but they 
 are not exactly circles. Now, why is this ? There must be some 
 natural reason. That reason has been ascertained, and it has led to 
 several of the grandest discoveries that the mind of man has ever 
 achieved in the realms of Nature. 
 
 In the first place, let us see what we are to infer from the fact 
 that the distance of a planet from the sun is not constant. The 
 motion in a circle is one of such beauty and simplicity, that we are 
 reluctant to abandon it, unless the necessity for doing so be made 
 clearly apparent. Can we not devise any way by which the circular 
 motion might be preserved, and yet be compatible with the fluctua- 
 tions in the distance from the planet to the sun ? This is clearly 
 impossible with the sun at the centre of the circle. But suppose the 
 sun were not at the centre of the circle, while the planet, as before, 
 revolved around the sun. The distance between the sun and the 
 planet would then fluctuate. The more eccentric the position of 
 the sun, the larger would be the proportionate fluctuation in the 
 distance of the planet when at the different parts of its orbit. It 
 might further be supposed that by placing a series of circles around 
 the sun the various planetary orbits could be thus accounted for. 
 The centre of the circle belonging to Venus is to coincide very 
 nearly with the centre of the sun, and the centres of the orbits of all 
 the other planets are to be placed at such suitable distances from 
 the sun, as will render a satisfactory explanation of the gradual 
 increase and decrease of the distance between the two bodies. 
 
 There can be no doubt that the movements of the moon and of 
 the planets would be, to a large extent, explained by such a system 
 of circular orbits; but the spirit of astronomical inquiry is not 
 satisfied with approximate results. Again and again the planets 
 are observed, and again and again are the observations compared 
 with the places which the planets would have if they moved in 
 accordance with the system here indicated. The centres of the 
 
 
THE LAW OF GRAVITATION. 109 
 
 circles are moved hither and thither, their radii are adjusted with 
 greater care ; but it is all of no avail. The observations of the 
 planets are minutely examined to see if they can be in error ; but 
 of errors there are none at all sufficient to account for the dis- 
 crepancies. The conclusion was thus inevitable astronomers were 
 forced to abandon the circular motion, which was thought to possess 
 such unrivalled symmetry and beauty, and were compelled to admit 
 that the orbits of the planets were not circular. 
 
 Then if these orbits be not circles, what are they ? Such was the 
 great problem which Kepler proposed to solve, and which, to his 
 immortal glory, he succeeded in solving and in proving to demon- 
 stration. The great discovery of the true shape of the planetary 
 orbits stands out as one of the most conspicuous events in the 
 history of astronomy. It may, in fact, be doubted whether any 
 other discovery in the whole range of science has led to results 
 of such far-reaching interest. 
 
 We must here adventure for a while into the field of science 
 known as geometry, and study therein the nature of that curve 
 which the discovery of Kepler has raised to such unparalleled 
 importance. The subject, no doubt, is a difficult one, and to 
 pursue it with any detail would involve us in many abstruse 
 calculations which would be out of place in this volume; but a 
 general sketch of the subject is indispensable, and we must attempt 
 to render it such justice as may be compatible with our limits. 
 
 The curve which represents with perfect fidelity the movements 
 of a planet in its revolution around the sun, belongs to that well- 
 known group of curves which mathematicians describe as the 
 conic sections. The particular form of conic section which denotes 
 the orbit of a planet is known by the name of the ellipse : it is 
 spoken of somewhat less accurately as an oval. The ellipse is 
 a curve which can be readily constructed. There is no simpler 
 method of doing so than that which is familiar to draughtsmen, and 
 which we shall here briefly describe. 
 
 We represent on the next page (Fig. 35) two pins passing 
 through a sheet of paper. A loop of twine passes over the two 
 pins in the manner here indicated, and is stretched by the point 
 
110 
 
 THE STORY OF THE HEAVENS. 
 
 of a pencil. With a little care the pencil can be guided so as 
 to keep the string stretched, and its point will then describe a 
 curve completely around the pins, returning to the point from 
 which it started. We thus produce that celebrated geometrical 
 curve which is called an ellipse. It will be very instructive to 
 draw a number of ellipses, varying in each case the circumstances 
 under which they are formed. If, for instance, the pins remain 
 placed as before, while the length of the loop is increased, 
 so that the pencil is farther away from the pins, then it will be 
 
 Fig. 35. Drawing an Ellipse. 
 
 observed that the ellipse has lost some of its elongation, and 
 approaches more closely to a circle. On the other hand, if the 
 length of the cord in the loop be lessened, while the pins remain as 
 before, the ellipse will be found more oval, or, as a mathematician 
 would say, its eccentricity is increased. It is also useful to study 
 the changes which the form of the ellipse undergoes when one of 
 the pins is altered, while the length of the loop remains unchanged. 
 If the two pins be brought nearer together the eccentricity will 
 decrease, and the ellipse will approximate more closely to the shape 
 of a circle. If the pins be separated more widely the eccentricity 
 of the ellipse will be increased. That the circle is really a form of 
 ellipse will be evident, if we suppose the two pins to draw in so 
 close together that they become coincident ; the point will then 
 simply trace out a circle, as the pencil moves round the figure. 
 
THE LAW OF GRAVITATION'. Ill 
 
 It will be obvious that the points marked by the pins possess 
 very remarkable relations with respect to the curve. Each one of 
 these points is called a focus of the ellipse, and an ellipse can only 
 have one pair of foci. There is in fact only one pair of positions 
 possible for the two pins, when an ellipse of specified size and 
 shape is to be constructed. 
 
 The ellipse is thus a curve differing principally from a circle, 
 inasmuch as it possesses variety of form. We can have large and 
 small ellipses just as we can have large and small circles, but we 
 can also have ellipses of greater or less eccentricity. If the ellipse 
 has not the perfect simplicity of the circle, it has at least the charm 
 of variety which the circle has not ; while the ellipse has also the 
 beauty which can be derived from an outline of perfect grace, and 
 an association with the most noble conceptions. 
 
 The ancient geometricians had studied the ellipse ; they had 
 noticed its foci ; they had become acquainted with its properties ; 
 and thus Kepler was familiar with the ellipse at the time when he 
 undertook his ever-memorable researches on the movements of the 
 planets. He had found, as we have already indicated, that the 
 movements of the planets could not be reconciled with circular 
 orbits. What shape of orbit should next be tried ? The ellipse 
 was ready to hand, its properties were known, and the comparison 
 could be made ; memorable, indeed, was the consequence of this com- 
 parison, Kepler found that the movement of the planets could be 
 explained, by supposing that the path in which each one revolved 
 was an ellipse. This in itself was a discovery of the most com- 
 manding importance. On the one hand, it reduced to order the 
 movements of the great globes which circulate round the sun; 
 while on the other, it took that beautiful class of curves which had 
 exercised the highest geometrical powers of the ancients, and at 
 once gave to those curves the dignity of defining the great high- 
 ways of the universe. 
 
 But we have as yet only partly enunciated the first discovery of 
 Kepler. We have seen that a planet revolves in an ellipse around 
 the sun, and that the sun is therefore at some point in the interior 
 of the ellipse but at what point? Interesting indeed is the 
 
112 THE STOET OF THE HEAVENS. 
 
 answer to this question ; for have we not pointed out how the foci 
 possess a significance which no other points enjoy? Kepler showed 
 that the sun must be situated in one of the foci in the ellipse in 
 which each planet revolves. We can thus enunciate the first of 
 Kepler's laws of planetary motion in the following- words : 
 
 Each planet revolves around the sun in an elliptic path, having 
 the sun at one of the foci. 
 
 We are now enabled to form a clear picture of the orbits of the 
 planets, be they ever so numerous, as they revolve around the sun. 
 In the first place we observe that the ellipse is eminently a plane 
 curve ; that is to say, each planet must, in the course of its long 
 journey, confine its movements to one plane. Each planet has thus 
 a certain plane appropriated to it. It is true that all these planes 
 are very nearly coincident, at least in so far as the great planets 
 are concerned ; but still they are distinct, and the only feature in 
 which they all agree, is that each one of them passes through the 
 sun. All the elliptic orbits of the planets have one focus in com- 
 mon, and that focus lies at the centre of the sun. 
 
 It is well to illustrate this remarkable law by considering the 
 circumstances of two or three different planets. Take first the case 
 of the earth, where the path, though really an ellipse, is very 
 nearly circular. In fact, if it were drawn accurately to scale on 
 a sheet of paper, the difference between the elliptic orbit and the 
 circle would hardly be detected without careful measurement. In 
 the case of Venus the ellipse is still more nearly a circle, and the 
 two foci of the ellipse are very nearly coincident with the centre 
 of the circle. On the other hand, in the case of Mercury, we have 
 an ellipse which departs from the circle to a very marked extent, 
 while in the orbits of some of the lesser planets the eccentricity is 
 still greater. It is extremely remarkable that every planet, no 
 matter how far from the sun, should be found to move in an ellipse 
 of some shape or other. We shall presently show that necessity 
 compels each planet to pursue an elliptic path, and that no other 
 form of path is possible. 
 
 Started on its elliptic path, the planet pursues its stately course, 
 and after a certain interval known as the periodic time, regains the 
 
THE LAW OF GRAVITATION. 113 
 
 position from which it started. Again the planet traces out anew 
 the elliptic path which it formerly pursued, and thus, revolution 
 after revolution, the same track is followed around the sun. Let 
 us now attempt to follow the planet in its course, and observe the 
 history of its motion during the time requisite for a complete 
 circuit of the sun. The dimensions of a planetary orbit are so 
 stupendous that the planet must run its course very rapidly in order 
 to -complete the journey within the allotted time. The earth, as 
 we have already seen, would have to move about eighteen miles 
 
 Mean Velocity 
 
 Mean Veloc'it"/ 
 Fig. 36. Varying velocity of elliptic motion. 
 
 a second to complete a voyage round the sun in the course of 
 36 5i days. The question then arises, as to whether the rate at 
 which a planet moves is uniform or not. Does the earth, for 
 instance, actually move at all times with the velocity of eighteen 
 miles a second, or does our planet sometimes move more rapidly and 
 sometimes more slowly, so that the average of eighteen miles a 
 second is still maintained ? This is a question of very great im- 
 portance, and we are able to answer it in the clearest and most 
 emphatic manner. The velocity of a planet is not uniform, and the 
 variations of that velocity can be explained by the adjoining figure. 
 Let us imagine the planet first of all to be situated at that part of 
 its path most distant from the sun towards the right of the figure. 
 In this position the velocity is at its lowest value ; as the planet 
 begins to approach the sun the speed gradually improves until it 
 I 
 
114 THE STORY OF THE HEAVENS. 
 
 attains its mean value. After this point has been passed, and the planet 
 is now rapidly hurrying on towards the sun, the velocity with which 
 it moves becomes gradually greater and greater, until at length, 
 as it dashes round the sun, its velocity reaches the highest point. 
 After passing the sun the distance of the planet from the sun in- 
 creases, and the velocity of the motion begins to abate; gradually it 
 sinks down until the mean value is again reached, and then it falls 
 still lower, until the planet recedes to its greatest distance from the 
 sun, by which time the velocity has abated to the poiut from 
 which we supposed it to commence. We thus observe that the 
 nearer the planet is to the sun the quicker it moves. We can, 
 
 A 
 Fig. 37. Equal areas in equal times. 
 
 however, give numerical defmiteness to the law, according to which 
 the velocity of the planet varies. The adjoining figure gives a 
 planetary orbit with, of course, the sun at the focus, S. We have 
 taken two portions, A B and C D, round the ellipse ; we have 
 joined their extremities to the focus; and we have marked the 
 two nearly triangular areas by shading. Then Kepler's second law 
 may be stated in these words : 
 
 " Every planet moves round the sun with such a velocity at every 
 point, that a straight line drawn from it to the sun passes over equal 
 areas in equal times. 33 
 
 If, therefore, the two shaded portions, A B S and DCS, are 
 equal in area, then the times occupied by the planet in travelling 
 over the portions of the ellipse, A B and C D, are equal. If the one 
 area be greater than the other, then the time occupied is greater 
 in the proportion of the areas. 
 
THE LAW OF GRAVITATION. 115 
 
 This law being admitted, the reason of the increase in the 
 planet's velocity when it approaches the sun is at once apparent. 
 To accomplish a definite area when near the sun, a larger arc is 
 obviously necessary than at other parts of the path ; while at the 
 opposite extremity of the path, a small arc suffices for a large area, 
 and the velocity is accordingly less. 
 
 These two laws completely enunciate the motion of a planet 
 round the sun. The first defines the path which the planet 
 pursues ; the second describes how the velocity of the planet varies 
 at different points along its path. But Kepler added to these a 
 third law, which enables us to compare the movements of two 
 different planets revolving round the same sun. Before stating 
 this law, it is necessary to explain exactly what is meant by the 
 mean distance of a planet. In its elliptic path the distance from 
 the sun to the planet is constantly changing; but it is nevertheless 
 easy to attach a distinct meaning to that distance which is an 
 average of all the distances. This average is called the mean 
 distance. The simplest way of finding the mean distance is to 
 add the greatest distance to the least distance, and take half the 
 sum. We have already defined the periodic time of the planet; it 
 is the number of days which the planet requires for the com- 
 pletion of a journey round its path. Kepler's third law establishes 
 a relation between the mean distance and the periodic time. That 
 relation is stated in the following words : 
 
 " The squares of the periodic times are proportional to the cubes 
 of the mean distances" 
 
 Kepler saw that the different planets had different periodic 
 times ; he also saw that the greater the mean distance of the planet 
 the greater was its periodic time, and he was determined to find 
 out the connection between the two. It was easily seen, that it 
 would not be true to say that the periodic time is merely propor- 
 tional to the mean distance. Were this the case, if one planet had 
 a distance twice as great as another, the periodic time in the 
 former case would have been double that in the latter ; but obser- 
 vations showed that though the periodic time in the one case 
 exceeded that in the other, yet that it was less than twice as much. 
 i 2 
 
116 THE STORY OF THE HEAVENS. 
 
 By repeated trials, which would have exhausted the patience of one 
 less confident in himself, and less assured of the accuracy of the 
 observations which he sought to interpret, Kepler at length dis- 
 covered the true law, and expressed it in the form we have stated. 
 
 To illustrate the nature of this law, we shall take for comparison 
 the earth and the planet Venus. If we denote the mean distance 
 of the earth from the sun by unity, then the mean distance of 
 Venus from the sun is 0*7233. If we omit decimals beyond the 
 first place, we can represent the periodic time of the earth as 365*3 
 days, and the periodic time of Venus as 224' 7 days. Now the law 
 which Kepler asserts is that the square of 365*3 is to the square of 
 224*7 in the same proportion as unity is to the cube of 0*7233. 
 The reader can easily verify the truth of this identity by actual 
 multiplication. It is, however, to be remembered that, as only four 
 figures have been retained in the expressions of the periodic times, 
 so only four figures are to be considered significant in making 
 the calculations. Perhaps, however, the most striking manner of 
 making the verification will be to regard the time of the revolution 
 of Venus as an unknown quantity, and deduce it from the known 
 revolution of the earth and the mean distance of Venus. In this 
 way, by assuming Kepler's law, we deduce the cube of the periodic 
 time by a simple proportion, and the resulting value of 2:24*7 days 
 can then be obtained. As a matter of fact, in the calculations of 
 astronomy, the distances of the planets are usually ascertained from 
 Kepler's law. The periodic time of the planet is an element which 
 can be measured with great accuracy; and once it is known, then 
 the square of the mean distance, and consequently the mean distance 
 itself, is determined. 
 
 Such are the three celebrated laws of Planetary Motion, which 
 have always been associated with the name of their discoverer. 
 The profound skill by which these laws were elicited from the 
 masses of observations, the intrinsic beauty of the laws them- 
 selves and their absolute truthfulness, their wide-spread generality, 
 and the bond of union which they have established between the 
 various members of the solar system, have given these laws quite 
 an exceptional position in astronomy. 
 
THE LAW OF GRAVITATION. 117 
 
 As established by Kepler, these planetary laws were merely the 
 results of observation. It was found,, as a matter of fact, that the 
 planets did move in ellipses, but Kepler assigned no reason why 
 they should move in ellipses rather than in any other curve. Still 
 less was he able to give a reason why they should sweep over equal 
 areas in equal times, or why that third law was invariably obeyed. 
 The laws as they came from Kepler's hands stood out as three 
 independent truths ; thoroughly established, but wholly unsupported 
 by any explanations as to why these movements rather than any 
 other movements should be those appropriate for the revolutions of 
 the planets. 
 
 It was the crowning triumph of the great law of universal 
 gravitation to remove from Kepler 's laws this empirical character. 
 Newton's grand discovery bound together the three isolated laws of 
 Kepler into one beautiful doctrine. He showed not only that those 
 laws are true, but he showed why they must be true, and why no 
 other laws could have been true. He proved to demonstration 
 in his famous work, the " Principia/' that the true explanation of 
 the laws of Kepler was to be sought in the attraction of gravi- 
 tation. He showed that in the sun resided a power of attraction, 
 and how it was a necessary consequence of that attraction that 
 every planet should revolve in an elliptic orbit round the sun, 
 having the sun as one focus; that the radius of the planet's orbit 
 should sweep over equal areas in equal times ; and that in comparing 
 the movements of two planets, it became necessary to have the 
 squares of the periodic times proportional to the cubes of the mean 
 distances. 
 
 As this is not a mathematical treatise, it will be impossible for 
 us to discuss the proofs which Newton has given, and which have 
 commanded the immediate and universal acquiescence of all who 
 have taken the trouble to understand them. We must here only 
 confine ourselves to a very brief and general survey of the subject, 
 which will indicate the character of the reasoning employed, 
 without introducing details of a technical character. 
 
 Let us, in the first place, endeavour to think of a globe freely 
 poised in space and completely isolated from the influence of every 
 
118 THE STORY OF THE HEAVENS. 
 
 other body in the universe. Let us imagine that this globe is set 
 in motion by some impulse which starts it forward like a mighty 
 bullet through the realms of space. When the impulse ceases the 
 globe is in motion, and it continues to move onwards. But what 
 will be the path which it will pursue ? We are so accustomed to 
 see a stone thrown into the air moving in a curved path, that we 
 might naturally think a body projected into free space will also 
 move in a curve. A little consideration will, however, show that 
 the cases are very different. In the realms of free space we 
 find no conception of upwards or downwards ; all paths are alike ; 
 there is no reason why the body should swerve to the right or to the 
 left ; and hence we are led to surmise that under these circumstances 
 a body, once started and freed from all interference, would move in 
 a straight line. It is true that this statement is one 'which can 
 never be submitted to the test of direct experiment. Circumstanced 
 as we are on the surface of the earth, we have no means of isolating 
 a body from external forces. The resistance of the air, as well as fric- 
 tion in various other forms, no less than the gravitation towards the 
 earth itself, interfere with our experiments. A stone thrown along 
 a sheet of ice will be exposed to but little interference, and in this case 
 we see that the stone will take a straight course along the frozen 
 surface. A stone similarly cast into empty space would pursue a 
 course absolutely rectilinear. This we demonstrate, not by any 
 attempts at an experiment which would necessarily be futile, but 
 by indirect reasoning. The truth of this principle can never for a 
 moment be doubted by one who has duly weighed the arguments 
 which have been produced in its behalf. 
 
 Admitting, then, the rectilinear path of the body, the next 
 question which arises relates to the velocity with which that 
 movement is performed. The stone gliding over the smooth ice on 
 a frozen lake will, as everyone has observed, travel a long distance 
 before it comes to rest. There is but little friction between the ice 
 and the stone, but there is some friction; and as that friction 
 always tends to stop the motion, it will at length happen that the 
 stone is brought to rest. In a voyage through the solitudes of 
 space a body experiences no friction ; there is no tendency for the 
 
THE LAW OF GRAVITATION. 119 
 
 velocity to be reduced, and consequently we believe that the body 
 could journey on for ever with unabated speed. No doubt such a 
 statement seems at variance with our ordinary experience. A 
 sailing ship comes to rest on the sea when the wind dies away. 
 A train will gradually lose its velocity when the steam has been 
 turned off. A humming-top will slowly expend its rotation and 
 come to rest. In these instances we seem to have proof that when 
 the force which has imparted motion has ceased,, the motion itself 
 will gradually wane and ultimately cease entirely. But in all 
 these cases it will be found, on reflection, that the decline of the 
 motion is to be attributed to the action of resisting forces. The 
 sailing ship is retarded by the rubbing of the water on its sides; 
 the train is retarded by the friction of the wheels, and by the fact 
 that it has to force its way through the air ; and the resistance of 
 the air is mainly the cause of the stopping of the humming-top, 
 for if the air be withdrawn, by making the experiment in a vacuum, 
 the top will continue to spin for a greatly lengthened period. 
 When we duly weigh these considerations we shall find it possible 
 to admit that a body, once projected freely in space and acted upon 
 by no external resistance, will continue to move on for ever in a 
 straight line, and will preserve unabated to the end of time the 
 velocity with which it originally started. This principle is known 
 as i}iQ first law of motion. 
 
 Let us apply this great principle to the important question of 
 the movement of the planets. Take, for instance, the case of our 
 earth, and discuss the consequences of the first law of motion. 
 Our earth is every moment moving with a velocity of about 
 eighteen miles a second, and the first law of motion assures us that 
 if the earth were submitted to no external force, it would for ever 
 pursue a straight track through the universe, and never depart from 
 the precise velocity which it has at the present moment. But is 
 the earth moving in this manner? Obviously not. We have 
 already found that the earth is moving round the sun, and the 
 beautiful laws of Kepler have given to that motion the most 
 perfect distinctness and precision. The consequence is irresistible. 
 The earth cannot be free from external force. Some potent 
 
120 THE STORY OF THE HEAVENS. 
 
 influence on the earth must be in ceaseless action. That influence, 
 whatever it may be, constantly deflects the earth from the rec- 
 tilinear path which it tends to pursue, and constrains the earth to 
 trace out an ellipse instead of a straight line. 
 
 The great problem to be solved is now easily stated. There 
 must be some constant influence on the earth. What is that 
 influence, from whence does it proceed, and to what law is it 
 submitted ? Nor is the question confined to the earth alone. 
 Mercury and Venus, Mars, Jupiter, and Saturn proclaim aloud that, 
 as they are not moving in rectilinear paths, they must be exposed 
 to some force. What is this force which guides the planets in 
 their paths? Before the time of Newton this question might 
 have been asked in vain. It was the mighty genius of Newton 
 which supplied the answer, and thus revolutionised the whole of 
 modern science. 
 
 Where lie the data from which the answer to the question is 
 to be elicited? W T e have here no problem which can be solved 
 by mere mathematical meditation. Mathematics is no doubt a 
 useful, indeed an indispensable instrument in the inquiry ; but we 
 must not attribute to mathematics a potency which it does not 
 possess. In a case of this kind, all that mathematics can do is to 
 interpret the results obtained by observation. The data, then, 
 from which Newton proceeded, were the observed facts in the 
 movement of the earth and the other planets. Those facts had 
 found a most beautiful expression by the aid of Kepler's laws. 
 It was, accordingly, the laws of Kepler which Newton took as the 
 basis of his labours, and it was for the interpretation of Kepler's 
 laws that Newton invoked the aid of that celebrated mathematical 
 reasoning which he created. 
 
 The question is then to be approached in this way : A planet 
 being subject to some external influence, we have to determine what 
 that influence is, from our knowledge that the path of each planet 
 is an ellipse, and that each planet sweeps round the sun over equal 
 areas in equal times. The influence on each planet is what a 
 mathematician would call a force, and a force must have a line of 
 direction. The most simple conception of a force is that of a pull 
 
THE LAW OF GRAVITATION. ]21 
 
 communicated along a rope, and the direction of the rope is in this 
 case the direction of the force. Let us imagine that the force 
 exerted on each planet is imparted by an invisible rope. What do 
 Kepler's laws tell us with regard to the direction of this rope and 
 to the intensity of the strain which is transmitted along it ? 
 
 The mathematical analysis of Kepler's laws would be beyond 
 the scope of this volume. We must, therefore, confine ourselves 
 to the results obtained by them, passing by the details of the 
 reasoning. Newton first took the law which asserted that the 
 planet moved over equal areas in equal times, and he showed by 
 unimpeachable logic that this at once gave the direction in which 
 the force acted on the planet. If for the sake of illustration we 
 regard as before the force to be exerted by the medium of a rope, 
 Newton showed that that rope must be invariably directed towards 
 the sun. In other words, that the force exerted on each planet was 
 at all times directed exactly from the planet towards the sun. 
 
 It still remained to explain the intensity of the force, and to 
 show how the intensity of that force varied when the planet w r as at 
 different points of its path. Kepler's first law enables this question 
 to be answered. If the planet's path be elliptic, and if the force 
 be always directed towards the sun at one focus of that ellipse, then 
 mathematical analysis obliges us to say, that the intensity of the 
 force must vary inversely as the square of the distance from the 
 planet to the sun. 
 
 The movements of the planets in conformity with Kepler's laws 
 would thus be accounted for even in their minutest details, if we 
 admit that an attractive power draws the planet towards the sun, 
 and that the intensity of this attraction varies inversely as the square 
 of the distance. Can we hesitate to say that such an attraction 
 does exist ? We have seen how the earth attracts a falling body ; 
 we have seen how the earth's attraction extends to the moon, and 
 explains the revolution of the moon around the earth. We have 
 now learned that the movement of the planets round the sun can 
 he also explained to be the consequence of this law of attraction. 
 But the evidence in support of the law of universal gravitation 
 is, in truth, much stronger than any we have yet presented. We 
 
122 THE STOEY OF THE HEAVENS. 
 
 shall have occasion to dwell on this matter further on. We shall 
 show not only how the sun attracts the planets, but how the 
 planets attract each other; and we shall find how tin's mutual 
 attraction of the planets has led to remarkable discoveries which 
 have raised the truth of the law of gravitation beyond the possi- 
 bility of doubt. 
 
 Admitting the law of gravitation, we can then show that the 
 planets must revolve around the sun in elliptic paths with the sun 
 in the focus. We can show that they must sweep over equal 
 areas in equal times. We can prove that the squares of the 
 periodic times must be proportional to the cubes of their mean 
 distances. Still further we can show how the mysterious move- 
 ments of comets can be accounted for. By the same great law 
 we can explain the revolutions of the satellites. We can account 
 for the tides, and for numerous other details throughout the Solar 
 System. Finally, we shall show that when we extend our view 
 beyond the limits of our Solar System to the beautiful starry 
 systems scattered through space we find even there evidence of 
 the great law of universal gravitation. 
 
CHAPTER VI. 
 
 THE PLANET OP ROMANCE. 
 
 Outline of the Subject Is Mercury the Planet nearest the Sun ? Transit of an 
 Interior Planet across the Sun Has a Transit of Vulcan ever been seen? 
 Visibility of Planets during a Total Eclipse of the Sun Professor Watson's 
 Researches in 1878. 
 
 PROVIDED with a general survey of the Solar System, and with such 
 an outline of the law of universal gravitation as the last chapter 
 has afforded us, we commence the more detailed examination of 
 the planets and their satellites. We shall begin with the planets 
 nearest to the sun, and then we shall gradually proceed outwards to 
 one planet after another, until we reach the confines of the system. 
 We shall find much to occupy our attention. Each planet is itself 
 a globe, and it will be our duty to describe what is known of that 
 globe. The satellites by which so many of the planets are accom- 
 panied possess many points of interest. The circumstances of 
 their discovery, their sizes, their movements, and their distances, 
 must all be duly considered. Then, too, it will be found that the 
 movements of the planets present much matter for reflection and 
 examination. We shall have occasion to show how the planets 
 mutually disturb each other, and what remarkable consequences 
 have arisen from these disturbances. We must also occasionally 
 refer to the important problems of celestial measuring and celestial 
 weighing. We must show how the sizes, the weights, and the 
 distances of the various members of our system are to be dis- 
 covered. A great part of our task will lead us over ground which 
 is thoroughly certain, and where the results have been confirmed by 
 frequent observation. It happens, however, that at the very outset 
 of our course we are obliged to deal with observations which are far 
 
124 THE STORY OF THE HEAVENS. 
 
 from certain. The existence of a planet much closer than those 
 hitherto known has been asserted by competent authority. The 
 question is still quite unsettled, and the planet cannot with cer- 
 tainty be pointed out. Hence it is that we have called the subject 
 of this chapter, The Planet of Romance. 
 
 It had often been thought that Mercury, long 1 supposed to be 
 the nearest planet to the sun, was perhaps not really the body 
 entitled to that distinction. Mercury revolves round the sun at 
 an average distance of about 36,000,000 miles. In the interval 
 between it and the sun there might have been one or many other 
 planets. There might have been one revolving at ten million 
 miles, another at fifteen, and so on. But did such planets exist? 
 Did even one planet revolve inside the orbit of Mercury ? There 
 were certain reasons for believing in such a planet. In the move- 
 ments of Mercury indications were perceptible of an influence that 
 could have been accounted for by the supposition of an interior 
 planet. But there was necessarily a great difficulty about seeing this 
 object. It must always be close to the sun, and even in the best 
 telescope it is generally impossible to see a starlike point in that 
 position.* Nor could such a planet be seen after sunset, for at the 
 best it would set almost immediately after the sun, and a like 
 difficulty would make it invisible at sunrise. 
 
 Our ordinary means of observing a planet have therefore com- 
 pletely failed. We are compelled to resort to extraordinary 
 methods, if we would seek to settle the great question as to the 
 existence of the intra-Mercurial planets. There are at least two 
 extraordinary methods of observation which might be expected 
 occasionally to answer our purpose. 
 
 The first of these methods would arise when it so happened that 
 the unknown planet came directly between the earth and the sun. 
 In the adjoining diagram we have the sun at the centre ; the 
 internal orbit denotes that of the unknown planet, which has 
 received the name of Vulcan before even its very existence has been 
 at all satisfactorily established. The outer orbit denotes that of 
 the earth. As Vulcan moves more rapidly than the earth, it will 
 frequently happen that the planet will overtake the earth, so that 
 
THE PLANET OF ROMANCE. 125 
 
 the three bodies will have the positions represented in the diagram. 
 It would not, however, necessarily follow that Vulcan was exactly 
 between the earth and the sun. The path of the planet may be 
 tilted up so that, as seen from the earth, Vulcan would be over the 
 sun, or under the sun, according to circumstances. If, however^ 
 Vulcan really do exist, we can be assured that sometimes the three 
 bodies will be directly in line, and this would then give the desired 
 opportunity of making the telescopic discovery of the planet. \Ve 
 
 Earth 
 
 Fig. 38. The Transit of the Planet of Romance. 
 
 should expect on such an occasion to see the planet as a dark spot, 
 moving slowly across the face of the sun. The two other planets 
 interior to the earth, namely, Mercury and Venus, are occasionally 
 seen in the act of transit ; and there cannot be a doubt that if 
 Vulcan exist, its transits across the sun must be more numerous 
 than those of Mercury, and far more numerous than those of 
 Venus. On the other hand, it may reasonably be anticipated that 
 Vulcan is a small globe, and as it will be much more distant from 
 us than even Mercury at the time of transit, we could not expect 
 that the transit of Vulcan would be a spectacle at all comparable in 
 importance with the transit of either of the two other planets. 
 
 The question then arises, as to whether telescopic research has 
 ever shown anything which can be regarded as a transit of Vulcan. 
 
126 THE 8TORY OF THE HEAVENS. 
 
 On this point it is not possible to speak with any degree of 
 certainty. It has, on more than one occasion, been asserted by 
 observers that a spot has been seen rapidly traversing the sun, and 
 from its shape and general appearance they have presumed it to 
 have been an intra -Mercurial planet. But a close examination of 
 the circumstances under which such observations have been made 
 has not tended to increase confidence in this presumption. Such 
 discoveries have usually been made by persons little familiar with 
 telescopic observations. It is certainly a significant fact that, 
 notwithstanding the diligent scrutiny to which the sun has been 
 exposed during the past century by astronomers who have specially 
 devoted themselves to this branch of research, no telescopic dis- 
 covery of Vulcan has been in this way made by any really expe- 
 rienced astronomer. From an examination of the whole subject, we 
 are inclined to believe that there is not at this moment any reliable 
 telescopic evidence of the transit of an mtra-Mercurial planet over 
 the face of the sun. 
 
 But there is still another method by which we might reasonably 
 hope to detect such planets if they really exist. This method is 
 one of great rarity, and requires observations possessing no small 
 degree of skill and delicacy. Its application is only possible when 
 the sun is obscured by a total eclipse. 
 
 When the moon is placed directly between the earth and the 
 sun, the brightness of day is temporarily exchanged for the gloom 
 of night. If the sky be free from clouds, the stars spring forth, and 
 can be seen close up to where the corona shows the obscured sun to 
 be situated. Even if a planet were quite close to the sun, it would 
 be visible on such an occasion. Careful preparation is necessary 
 when it is proposed to make a trial of this kind. The danger to be 
 specially avoided is the risk of confounding the planet with the 
 ordinary stars, which it will probably resemble. The late dis- 
 tinguished American astronomer, Professor Watson, specially pre- 
 pared to devote himself to this research during the great total 
 eclipse in 1878. The duration of this total eclipse was very brief; 
 it lasted only two or three minutes, and all the work had to be 
 compressed into this very short interval. Professor Watson had 
 
THE PLANET OF ROMANCE. 127 
 
 previously carefully studied the stars in the neighbourhood of tbe 
 sun. When the eclipse occurred, the light of the sun vanished, and 
 the stars burst forth. The practised eye of Professor Watson at once 
 identified the stars, of which he had formed a map, not merely on 
 paper, but also engraved on his memory ; and among them he saw an 
 object, which certainly seemed to be the long-sought inter-Mercurial 
 planet. We should indeed add, that the same observer seems to 
 have also seen a second planet on the same occasion. To a certain 
 extent Mr. Watson's observation was confirmed by another ob- 
 server, Mr. Swift. It is, however, right to add that Vulcan 
 has not been observed, though specially looked for, during the 
 eclipses which have occurred since 1878. We cannot, however, 
 believe it possible that so experienced an astronomer as Mr. Watson 
 was mistaken. He has been one of the most successful dis- 
 coverers of minor planets, and, not improbably, posterity will 
 have to admit, when the inter-Mercurial planet or planets become 
 better known, that the first reliable observation on this subject was 
 made by Watson. 
 
CHAPTER VII. 
 
 MERCURY. 
 
 The Ancient Astronomical Discoveries How Mercury was first found ? Not easily 
 seen Mercury was known in 265 B.C. Skill necessary in the Discovery The 
 Distinction of Mercury from a Star Mercury in the East and in the West 
 The Prediction How to Ohserve Mercury Its Telescopic Appearance Diffi- 
 culty of Observing its Appearance Orbit of Mercury Velocity of the Planet 
 Can there he Life on the Planet ? Changes in its Temperature Transit of 
 Mercury over the Sun Gassendi's Observation Atmosphere around Mercury 
 The Weight of Mercury. 
 
 LONG and glorious is the record of astronomical discovery. The 
 discoveries of modern days have succeeded each other with such 
 rapidity, they have so often dazzled our imaginations with their 
 brilliancy, that we are sometimes apt to think that astronomical 
 discovery is a purely modern product. But no error could be more 
 fundamentally wrong 1 . While v r e appreciate to the utmost the 
 achievements of modern times, let us endeavour to do justice to 
 the labours of the astronomers of antiquity. 
 
 And when we speak of the astronomers of antiquity, let us 
 understand clearly what is meant. Astronomy is now growing 
 so rapidly that each century witnesses a surprising advance ; each 
 generation, each decade, each year, has its own rewards for those 
 diligent astronomers by whom the heavens are so carefully scanned. 
 We must, however, project our glance to a remote epoch in time 
 past, if we would view the memorable discovery of Mercury. Com- 
 pared with it, the discoveries of Newton are to be regarded as very 
 modern achievements ; even the enunciation of the Copernican 
 system of the heavens is itself a recent event in comparison with 
 the discovery of the planet Mercury. 
 
 By whom was this great discovery made ? Let us see if this 
 question can be answered by examination of astronomical records. 
 
MEEOUEY. 129 
 
 At the close of the memorable life of the great Copernicus he was 
 heard to express his sincere regret that his eyes had never shown 
 him the planet Mercury. Often had he tried to see this planet, 
 whose movements were in such a marked way illustrative of the 
 great theory of the celestial motions which it was his immortal 
 glory to have established; but he had never been successful. 
 Mercury is not easily to be seen, and it may well have been 
 that the vapours from the Vistula obscured the horizon at Frauen- 
 burg where Copernicus dwelt, and that thus his opportunities of 
 seeing Mercury were probably even rarer than they are at other 
 places. But one circumstance is plain. The existence of the planet 
 was quite familiar to Copernicus, and therefore we must look to 
 some earlier epoch for its discovery. In the scanty astronomical 
 literature of the Middle Ages we find occasional references to 
 the existence of this planet. We can trace observations of Mer- 
 cury back through the early centuries to the commencement of 
 our era. Back earlier still it can be followed, until at length we 
 come on the first record of an observation which has des- 
 cended to us, being one made in the year 265 before the Christian 
 era. It is not pretended, however, that this observation records 
 the discovery of the planet. It is merely an observation of an 
 object already well-known, while the earlier observations and any 
 account of the discovery seem to have totally perished. It does not 
 appear in the least degree likely that the discovery was even then 
 a recent one. It may have been that the planet was independently 
 discovered in two or more localities, but all records of such discoveries 
 are totally wanting ; and we are ignorant alike of the name of the 
 discoverer, of the nation to which he belonged, and of the epoch in 
 which his great discovery was made. 
 
 Although this discovery is of such vast antiquity, although it 
 was made before correct notions were entertained as to the true 
 system of the universe, and, it is needless to add, long before the 
 invention of the telescope, yet it must not be assumed that the 
 discovery of Mercury was by any means a simple or obvious matter. 
 This will be manifest when we try to conceive the manner in which 
 the discovery must have been first accomplished. 
 
130 THE STORY OF THE HEAVENS. 
 
 Some primaeval astronomer, long familiar with the heavens, 
 had learned to recognise the various stars and constellations. Ex- 
 perience had impressed upon him the permanence of these objects ; he 
 had seen that Sirius invariably appeared at the same seasons of the 
 year ; and he had noticed how it was placed with regard to Orion 
 and the other neighbouring constellations. In the same manner 
 each of the other bright stars was to him a familiar object always 
 to be found in a particular region of the heavens. He saw how the 
 heavens, as a whole, rose and set in such a way, that though each star 
 appeared to move, yet the relative positions of the stars were in- 
 capable of alteration. No doubt this ancient astronomer was ac- 
 quainted with Venus ; he knew it as the evening star ; he knew it 
 as the morning star ; and he was accustomed to regard Venus as a 
 body which oscillated from one side of the sun to the other. We 
 can easily imagine how, in the clear skies of an Eastern desert, 
 the discovery of Mercury was made. The sun has set, the brief 
 twilight has almost cea sed, when lo, near that part of the horizon 
 where the glow of the setting sun still illuminates the sky, a bright 
 star is seen. But surely, the careless observer might say, there is 
 nothing wonderful here. Is not the whole heaven spangled with 
 stars ? why should there not be a star in this locality also ? But the 
 primeval astronomer will not accept this explanation. He knows 
 that there is no bright star at this place in the heavens. If the 
 object of his attention be not a star, what then can it be ? Eager 
 to examine this question, the heavens are watched next night, and 
 there again, higher up in the heavens, and more brilliant still, is 
 the object seen the night before. Each successive night the 
 object grows more and more brilliant, until at length it becomes 
 a conspicuous gem. Perhaps it will rise still higher and higher ; 
 perhaps it will increase till it attains the brilliancy of Venus itself. 
 Such were the surmises not improbably made by those who first 
 watched this object ; but they were not realised. After a few 
 nights of exceptional brilliancy the lustre of this mysterious orb 
 declines. The planet again draws near the horizon at sunset until at 
 length it sets so soon after the sun that it has become invisible. 
 Is it lost for ever? Years may pass away before another good 
 
MERCURY. 131 
 
 opportunity of observing the object after sunset happens ; but then 
 again it will be seen to run through the same series of changes, 
 though, perhaps, under very different circumstances. The greatest 
 height above the horizon, and the greatest brilliancy, both vary 
 enormously. It was not until after long and careful observations had 
 been made, that the primeval astronomer could assure himself that 
 the various appearances could be all attributed to a single body. 
 In the Eastern deserts the phenomena of sunrise must have been 
 nearly as familiar as those of sunset, and in the clear skies, at 
 the point where the sunbeams were commencing to dawn above the 
 horizon, a bright starlike point might sometimes be perceived. Each 
 successive day this object rose higher and higher above the horizon 
 before the moment of sunrise, and its lustre increased with the 
 distance ; then again it would draw in towards the sun, and return 
 for a while to invisibility. Such were the data which were pre- 
 sented to the mind of the primitive astronomer. One body was 
 seen after sunset, another body w r as seen before sunrise. To us it 
 may seem an obvious inference from the observed facts, that the two 
 bodies were identical. The inference is a correct one, but it is in 
 no sense an obvious one. Long and patient observation established 
 the remarkable law that one of these bodies was never seen until 
 the other had disappeared. Hence it was inferred that the 
 phenomena, both at sunrise and at sunset, were due to the same 
 body, which oscillated to and fro about the sun. 
 
 We can easily imagine that the announcement of the identity 
 of these two bodies was one which would have to be carefully 
 tested before it could be accepted. How are the tests to be applied 
 in a case of this kind ? There can hardly be a doubt that the most 
 complete and convincing demonstration of scientific truth is found 
 in the fulfilment of prediction. When Mercury had been observed 
 for years, a certain regularity in the recurrence of its visibility was 
 noticed. Once this regularity had been fully established, prediction 
 became possible. The time when Mercury would be seen after 
 sunset, the time when it would be seen before sunrise, could be 
 foretold with accuracy ! When it was found that these predictions 
 were obeyed to the letter that the planet was always seen when 
 J 2 
 
132 THE STORY OF THE HEAVENS. 
 
 looked for in accordance with the predictions it was impossible to 
 refuse assent to the hypothesis on which these predictions were 
 based. Underlying that hypothesis was the assumption that all 
 the various appearances arose from the oscillations of a single body, 
 and hence the discovery of Mercury was established on a basis as 
 firm as the discovery of Jupiter or of Venus. 
 
 In the latitudes of the British Islands it is generally possible to 
 see Mercury some time during the course of the year. It is not 
 practicable to lay down, within reasonable limits, any general rule 
 for finding the dates at which the search should be made ; but the 
 
 Fig. 39. The Movement of Mercury, showing the variations in Phase and in 
 
 apparent size. 
 
 student who is determined to see Mercury will generally succeed 
 with a little patience. He must first consult an almanac which 
 gives the planetary positions, and select an occasion when Mercury 
 is stated to be an evening or a morning star. Such an occasion 
 during the spring months is especially suitable, as the elevation of 
 Mercury above the horizon is usually greater then than at other 
 seasons; and in the evening twilight, about three-quarters of an 
 hour after sunset, a view of this shy but beautiful object will 
 reward the observer's attention. 
 
 To those astronomers who are provided with equatorial telescopes 
 such instructions are unnecessary. To enjoy a telescopic view of 
 Mercury, we first turn to the nautical almanac, and find the 
 position in which the planet lies. If that position be above our 
 
MERCURY. 133 
 
 horizon, we can at once direct the telescope to the planet, and even 
 in broad daylight the planet will very often be seen. The tele- 
 scopic appearance of Mercury is, however, not unfrequently disap- 
 pointing. Though Mercury is really much larger than the moon, 
 yet it is so very far off that its telescopic appearance is insignificant. 
 There is, however, one feature in a telescopic view which would 
 immediately attract attention. Mercury is then seen, not usually 
 as a circular object, but more or less crescent-shaped, like a minia- 
 ture moon. The phases of Mercury are also to be accounted for 
 
 Fig. 40. Mercury as a Crescent. 
 
 on exactly the same principles as the phases of the moon. Mercury 
 is a globe composed, like our earth, of materials possessing in them- 
 selves no source of illumination, but one-half of Mercury must 
 always be turned towards the sun, and this half is accordingly 
 lighted up brilliantly by the rays of the sun. When we look at 
 Mercury we see nothing of the non-illuminated side, and the 
 crescent is due to the fore-shortened view which we obtain of the 
 illuminated half. Mercury is such a small object that, in the 
 glitter of the naked- eye view, the shape of the luminous body 
 cannot be defined. Indeed, even in the much larger crescent of 
 Venus, the aid of the telescope has to be invoked before the crescent 
 form can be observed. Beyond, however, the fact that Mercury is 
 a crescent, and that it undergoes varying phases in correspondence 
 
134 THE STORY OF THE HEAVENS 
 
 with the changes in its relative position to the earth and the suri, 
 we cannot see much of the planet. It is too small and too bright 
 to admit of the delineation of details on its surface. No doubt 
 attempts have been made, and observations have been recorded as to 
 certain physical features on the planet. It has been supposed that 
 traces of mighty mountains have been seen, and that the existence 
 of an atmosphere surrounding the planet has been indicated. But 
 such statements must be received with very great hesitation, if not 
 with actual discredit, and beyond this mere allusion they need not 
 further engage our attention. 
 
 The facts that are thoroughly established with regard to 
 Mercury are mainly the numerical statements with regard to the 
 path it describes around the sun. The time taken by the planet to 
 complete a revolution is very nearly eighty-eight days. The 
 average distance from the sun is about 36,000,000 miles, and the 
 average velocity with which the planet moves is over twenty-nine 
 miles a second. We have already alluded to the most characteristic 
 and remarkable feature of the orbit of Mercury. That orbit differs 
 from the paths of all the other large planets by its much greater 
 departure from the circular form. In the majority of cases the 
 planetary orbits are so little elliptic that a diagram of the orbit 
 drawn accurately to scale would not be perceived to differ from a 
 circle unless careful measures were made. In the case of Mercury 
 the circumstances are different. The elliptic form of the path 
 would be quite unmistakeable by the most casual observer. The 
 distance from the sun to the planet fluctuates between very con- 
 siderable limits. The lowest value it can attain is about 30,000,000 
 miles; the highest value is about 43,000,000 miles. In accordance 
 with Kepler's second law, the velocity of the planet must exhibit 
 corresponding changes. It must sweep rapidly around that part of 
 his path near the sun, and more slowly round the remote parts of 
 his path. The greatest velocity is about thirty- five miles a second, 
 and the least is twenty-three miles a second. 
 
 For an adequate perception of the movements of Mercury, we 
 ought not to dissociate the magnitude of the velocity from the vast 
 dimensions of the body by which that velocity is performed. No 
 
MERCURY. 135 
 
 doubt a velocity of twenty-nine miles in a second is enormously 
 great when compared with ordinary velocities. The velocity of 
 Mercury is not less than a hundred times as great as the velocity of 
 the rifle-bullet. But when we compare the sizes of the bodies with 
 their velocities the velocity of Mercury seems relatively much less 
 than that of the bullet. A rifle-bullet traverses a distance equal 
 to its own diameter many thousands of times in a second. But 
 even though Mercury is moving so much faster, yet the diameter of 
 the planet is so considerable that a period of two minutes will be 
 required for it to move through a space equal to its diameter. 
 Viewing the globe of the planet as a whole, the velocity of its 
 movement is but a stately and dignified progress appropriate to 
 its dimensions. 
 
 As we can learn little or nothing of the true surface of Mercury, 
 it is utterly impossible for us to say whether life can exist on the 
 surface of that planet. We may, however, not unreasonably 
 conclude that there can hardly be life on Mercury at all analogous 
 to the life which we know on the earth. The heat and the light of 
 the sun beat down on Mercury with an intensity many-fold greater 
 than we experience on the earth. When Mercury is at its longest 
 distance from the sun, the intensity of solar heat is even then more 
 than four times as great as the greatest heat which ever reaches the 
 earth. But when Mercury, in the course of its remarkable changes 
 of distance, draws in to the warmest part of its orbit, it is exposed to 
 an appalling scorching. The intensity of the sun's heat must then 
 be not less than nine times as great as the greatest radiation to which 
 we are exposed. These changes succeed each other much more 
 rapidly than the variations of our seasons. On Mercury the interval 
 between midsummer to midwinter is only forty-four days, while the 
 whole year is only eighty-eight days. These rapid and tremendous 
 changes in solar heat must in themselves exercise a profound effect 
 on the habitability of Mercury. Mr. Ledger well remarks, in his 
 most interesting work,* that if there be inhabitants on Mercury the 
 words " perihelion " and " aphelion/' which are here often regarded 
 as expressing ideas of an intricate or recondite character, must, on the 
 * "The Sun: its Planets, and their Satellites." London: 1882 (page 147). 
 
136 THE STORY OF THE HEAVENS. 
 
 surface of that planet, be familiar to everybody. The words imply, 
 " near the sun/' and " away from the sun ; " but we do not associate 
 these expressions with any obvious phenomena, because the changes 
 in the distance from the earth to the sun are so inconsiderable. 
 But in Mercury, where in six weeks the sun rises to more than 
 double his apparent size, and gives more than double the quantity 
 of light and of heat, such changes must be familiar to everyone. 
 Perihelion and aphelion will embody ideas obviously and intimately 
 connected with the whole economy of the planet. 
 
 It is nevertheless rash to found any inferences as to climate 
 merely upon the proximity or the remoteness of the sun. Climate 
 depends upon other matters besides the sun's distance. The 
 atmosphere surrounding the earth has a profound influence on our 
 climate, and if Mercury have an atmosphere as has often been 
 supposed its climate may be thereby modified to an enormous extent. 
 It seems, however, hardly possible to suppose that any atmosphere 
 could form an adequate protection for the inhabitants from the 
 violent and enormous fluctuations of solar radiation. All we can 
 say is, that the problem of life in Mercury belongs to the class of 
 unsolved, and perhaps unsolvable, mysteries. 
 
 It was in the year 1627 that Kepler made an important announce- 
 ment of impending astronomical events. Kepler had been study- 
 ing profoundly the movements of the planets. He had examined 
 the former observations which had been made ; and from his study 
 of the past he had ventured to predict the future. Kepler announced 
 that in the year 1631 the planets Venus and Mercury would both 
 make a transit across the sun, and he assigned the dates to be 
 November 7th for Mercury, and December 6th for Venus. This 
 was at the time a very remarkable prediction. We are so ac- 
 customed to turn to our almanacs and learn from thence all the 
 astronomical phenomena which are anticipated during the year, 
 that we are apt to forget that in early times this was impossible. 
 It has only been by slow degrees that astronomy has been rendered 
 so perfect as to enable us to predict, with accuracy, the occurrence 
 of the more delicate phenomena. The announcement of those 
 transits by Kepler, some years before they occurred, was justly 
 
MERCURY. 137 
 
 regarded at the time as a most remarkable achievement. The 
 illustrious Gassendi prepared to apply the test of actual observation 
 to the predictions of Kepler. We can now indicate the time of 
 the transit accurately to a few minutes, but in those early attempts 
 equal precision was not to be thought of. Gassendi considered it 
 necessary to commence watching for the transit of Mercury two 
 whole days before the time indicated by Kepler, and he had 
 arranged a most ingenious plan for studying the transit. The 
 light of the sun was admitted into a darkened room through a hole 
 in the shutter, and by a lens an image of the sun was formed on a 
 white screen. This is, indeed, an admirable and a very pleasing 
 way of studying the surface of the sun, and even at the present 
 day, with our best telescopes, one of the best-known methods 
 of viewing the sun is founded on the same principle. Gassendi 
 commenced his watch on the 5th of November, and carefully studied 
 the sun's image at every available opportunity. It was not, how- 
 ever, until five hours after the time assigned by Kepler that the 
 transit of Mercury actually commenced. Gassendi's preparations 
 had been made with all the resources which he could command, 
 but these resources seem very imperfect when compared with the 
 appliances of our modern observatories. He was anxious to note 
 the time when the planet appeared, and for this purpose he had 
 stationed an assistant in the room beneath, who was to observe the 
 altitude of the sun at the moment indicated by Gassendi. The 
 signal to the assistant was to be conveyed by a very primitive 
 apparatus. Gassendi was to stamp on the floor when the critical 
 moment had arrived. In spite of the long delay, which exhausted 
 the patience of the assistant, some valuable observations were 
 obtained, and thus the first transit of a planet over the sun was 
 observed. 
 
 The transits of Mercury are chiefly of importance on account of 
 the accuracy which their observation infuses into our calculations of 
 the movements of the planet. It has often been hoped that the 
 observations during a transit would present to us reliable informa- 
 tion as to the physical character of the globe of Mercury. To some 
 extent but not, unhappily, to any large extent these hopes have 
 
138 THE STORY OF THE HEAVENS. 
 
 been realised. Skilful observers have described the appearance of 
 the planet in transit as that of a round, dark spot, surrounded by a 
 luminous margin of a depth variously estimated on different occa- 
 sions at from one-third to two-thirds of the planet's diameter. 
 There can be hardly a doubt that a dense atmosphere surrounding- 
 Mercury would be capable of producing an appearance resembling 
 that which has been described, and, therefore, the probability that 
 such an atmosphere really exists must be admitted. On the other 
 hand, the measurements of the intensity of light from Mercury 
 seem to make the existence of an atmosphere somewhat doubtful. 
 
 Here we take leave of the planet Mercury an interesting and 
 beautiful object which stimulates our intellectual curiosity, while 
 at the same time eluding all our attempts to obtain more complete 
 knowledge. There is, however, one point of attainable knowledge 
 which we have not touched on in this chapter. It is a difficult, 
 but not by any means an impossible task to weigh Mercury in 
 the celestial balance, and determine his mass in comparison with 
 the other globes of our system. This is a delicate operation, but 
 it leads us through some of the most interesting paths of astro- 
 nomical discovery. The weight of the planet, as recently deter- 
 mined by Von Asten, is about one-twenty-fourth part of the weight 
 of the earth. 
 
CHAPTER VIII. 
 
 VENUS. 
 
 Interest attaching to this Planet The Unexpectedness of its Appearance The 
 Evening Star Visibility in Daylight Only Lighted by the Sun The Phases 
 of Venus Why the Crescent is not Visible to the Unaided Eye Variations in 
 the Apparent Size of the Planet Eesemblance of Venus to the Earth The 
 Transit of Venus Why of such especial Interest The Scale of the Solar 
 System Orbits of the Earth and Venus not in the same Plane Recurrence of 
 the Transits in Pairs Appearance of Venus in Transit Transits of 1874 and 
 1882 The Early Transits of 1631 and 1639 The Observations of Horrocks 
 and Crabtree The Announcement of Halley How the Track of the Planet 
 differs from different places Illustrations of Parallax Voyage to Otaheite 
 The result of Encke Probable Value of the Sun's Distance Observations of 
 the recent Transit of Venus at Dunsink The Question of an Atmosphere to 
 Venus Dr. Copeland's Observations Utility of such Researches Other De- 
 terminations of the Sun's Distance Statistics about Venus. 
 
 IT might, for one reason, be not inappropriate to have commenced 
 our review of the planetary system by the description of the 
 planet Venus. This planet is not especially remarkable for its size, 
 for there are other planets hundreds of times larger. The orbit 
 of Venus is no doubt larger than that of Mercury, but it is much 
 smaller than that of the outer planets. Venus has not even the 
 splendid retinue of minor attendants which give such dignity and 
 such interest to the mighty planets of our system. Yet the fact 
 still remains that Venus is peerless among the planetary host. We 
 speak not now of spectacles only seen in the telescope, we refer to 
 the ordinary observation which detected Venus ages before tele- 
 scopes were invented. Who has not been delighted with the view of 
 this glorious object ? It is not to be seen at all times. For months 
 together the beauties of Venus are hidden from mortal gaze. Its 
 beauties are even enhanced by the caprice and the uncertainty that 
 attends its appearance. We do not say that there is any caprice in 
 the movements of Venus, as known to those who diligently consult 
 
140 THE STORY OF THE HEAVENS. 
 
 their almanacs. The movements of the lovely planet are there 
 prescribed with a prosaic detail hardly in correspondence with the 
 character of the goddess of love. But to those who do not 
 devote particular attention to the stars, the very unexpectedness 
 of the appearance of Venus is one of its greatest charms. Venus 
 has not been noticed, not been thought of, for many months. It 
 is a beautifully clear evening ; the sun has just set. The lover of 
 nature turns to admire the sunset, as every lover of nature will. In 
 the golden glory of the west a beauteous gem is seen to glitter ; it 
 is the evening star the planet Venus. A week or two later 
 another beautiful sunset is seen, and now the planet is no longer 
 merely a glittering point low down ; it has risen high above the 
 horizon, and continues a brilliant object long after the shades 
 of night have descended. Again, a little longer, and Venus has 
 gained its full brilliancy and splendour. All the heavenly host 
 even Sirius and even Jupiter must pale before the splendid lustre 
 of Venus, the unrivalled queen of the firmament. 
 
 After weeks of splendour the height of Venus at sunset 
 diminishes, and its lustre begins gradually to decline. It sinks to 
 invisibility, and is forgotten by the great majority of mankind ; 
 but the capricious goddess has only moved from the west to the 
 east. Ere the sun rises the morning star will be seen in the east. 
 Its splendour gradually augments until it rivals the beauty of the 
 evening star. Then again the planet draws near to the sun, and 
 remains lost to view for many months, until the same cycle of 
 changes recommences, after an interval of a year and seven 
 months. 
 
 When Venus is at its brightest, it can be easily seen in broad 
 daylight with the unaided eye. This striking spectacle proclaims 
 in an unmistakable manner the unrivalled supremacy of Venus as 
 compared with the other planets and with the fixed stars. In- 
 deed, at this time Venus is from forty to sixty times as bright as 
 the brightest star in the northern heavens. 
 
 The beautiful evening star is often such a very brilliant object, 
 that it may seem difficult at first to realise that Venus is not 
 self-luminous. Yet it is impossible to doubt that the planet is 
 
VENUS. 141 
 
 really only a dark globe, and to that extent resembling our own 
 earth. The brilliancy of the planet is not so very much greater than 
 the brilliancy of the earth on a sunshiny day, and the splendour 
 of Venus entirely arises from the reflected light of the sun, in the 
 manner already explained with respect to the moon. 
 
 We cannot, however, distinguish the beautiful crescent shape of 
 the planet by the unaided eye, which merely shows a brilliant point 
 too small to possess sensible form. This is to be explained on 
 physiological grounds. The optical contrivances in the eye form 
 an image of the planet on the retina ; this image is very small. 
 Even when Venus is nearest to the earth, the diameter of the planet 
 subtends an angle not much more than one minute of arc. On the 
 retina of the eye a picture of Venus is thus drawn about one six- 
 thousandth part of an inch in diameter. Great as may be the 
 delicacy of the retina, it is not adequate to the perception of 
 form in a picture so minute. The nervous structure, which has 
 been described as the source of vision, forms too coarse a canvas for 
 the reception of the details of this tiny picture. Hence it is that 
 to the unaided eye the brilliant Venus appears merely as a bright 
 spot. The unaided eye cannot tell what shape it has ; still less can 
 it reveal the true beauty of the crescent. If the diameter of Venus 
 were several times as great as it actually is ; were Venus, for 
 instance, as large as Jupiter or some of the other great planets, 
 then its crescent could be readily discerned by the unaided eye. It 
 is curious to speculate on what might have been the history of 
 astronomy had Venus only been as large as Jupiter. Were every 
 one able to see the crescent form without a telescope, it would then 
 have been an elementary and almost obvious truth that Venus was 
 a dark body revolving round the sun. The analogy between Venus 
 and our earth would have been at once perceived ; and the great 
 theory which was left to be discovered by Copernicus in compara- 
 tively modern times, might not improbably have been handed down 
 to us with the other discoveries which have come from the ancient 
 nations of the East. 
 
 In Fig. 41 we have three views of Venus under different aspects. 
 The planet is so much closer to the earth when the crescent is seen. 
 
142 THE STORY OF THE HEAVENS. 
 
 that it appears to be part of a much larger circle than that made 
 by Venus when more nearly full. This drawing shows the different 
 aspects of the planet in their true relative proportions. It is very 
 difficult to perceive distinctly any markings on the excessively 
 brilliant surface. Sometimes observers have seen spots or other 
 features, and occasionally the pointed extremities of the horns 
 have been irregular, as if to show that the surface of Venus is 
 not smooth. Attempts have even been made to prove from such 
 
 Fig. 41. Different Aspects of Venus in the Telescope. 
 
 observations that there must be lofty mountains in Venus, but we 
 cannot place much confidence in the results. 
 
 It so happens that our earth and Venus are very nearly equal 
 in bulk. The difference is hardly perceptible, but the earth has a 
 diameter a few miles greater than that of Venus. It is almost 
 equally remarkable that the time of rotation of Venus on its axis 
 seems to be very nearly equal to the time of rotation of the earth 
 on its axis. The earth rotates once in a day, and Venus in about 
 half an hour less than one of our days. There are also indications 
 of the existence of an atmosphere around Venus, but we have no 
 means of knowing at present what the gases may be of which that 
 atmosphere is composed. 
 
 If there be oxygen in the atmosphere of Venus, then it would 
 
VENUS. 143 
 
 seem possible that there might be life on Venus which was not of a 
 very different character from life on the earth. No doubt the sun's 
 heat on Venus is greatly in excess of the sun's heat with which we 
 are acquainted, but this is a difficulty not insuperable. We see at 
 present on the earth, life in very hot regions and life in very cold 
 regions. Indeed, as we go into the tropics we find life more and 
 more exuberant, so that should water be present on the surface of 
 Venus and oxygen in its atmosphere, we might expect to find 
 in that planet a luxuriant tropical life, of a kind perhaps analogous 
 in some respects to life on the earth. 
 
 In our account of the planet Mercury, as well as in our brief 
 description of the hypothetical planet Vulcan, it has been necessary 
 to allude to the phenomena presented by the transit of a planet over 
 the face of the sun. This phenomenon, always of interest and 
 always meriting the attention of astronomers, is especially notice- 
 able in the case of Venus. The transit of Venus rises in fact, to, 
 an importance hardly surpassed by any other phenomenon in our 
 system, and hence it will necessarily engage our attention in the 
 present chapter. We have in recent years had the opportunity of 
 witnessing two of these rare occurrences. No future transit can 
 occur till after this generation shall have passed away; not, 
 indeed, until the June of A.D. 2004. It is hardly too much to assert 
 that the recent transit of 1882 and the previous one of 1874 have 
 received a degree of attention never before accorded to any astro- 
 nomical phenomenon. 
 
 The transit of Venus is hardly to be described as a very striking 
 or beautiful spectacle. It is not nearly so fine a sight as a great 
 comet or a shower of shooting stars Why is it, then, that the 
 transit of Venus is regarded as of such great scientific importance ? 
 It is because the transit enables us to solve one of the greatest 
 problems which has ever engaged the mind of man. It is bv thp 
 transit of Venus that we attempt to determine the scale on which 
 our solar system is constructed. Truly this is a noble problem. 
 Let us dwell upon it for a moment. In the centre of our 
 system we have the sun a majestic globe more than a million 
 times as large as the earth. Circling round the sun we have 
 
144 THE STORY OF THE HEAVENS. 
 
 the planets, of which our earth is but one. There are hundreds of 
 small planets. There are a few comparable with our earth ; there 
 are others vastly surpassing the earth. Besides the planets there 
 are other bodies in our system. Many of the planets are accom- 
 panied by systems of revolving- moons. There are hundreds,, per- 
 haps thousands, of comets, while the minor bodies of our system 
 exist in countless millions. Each member of this stupendous host 
 moves in a prescribed orbit around the sun, and collectively they 
 form the solar system. 
 
 It is comparatively easy to learn the shape of this system, to 
 measure the relative distances of the planets from the sun, and even 
 the relative sizes of the planets themselves. Peculiar difficulties 
 are, however, experienced when we seek to ascertain the actual size 
 of the system, as well as its shape. It is this latter question which 
 the transit of Venus enables us to solve. 
 
 Look, for instance, at an ordinary map of Europe. We see the 
 various countries laid down with precision ; we can tell the courses 
 of the rivers ; we can say that France is larger than England, 
 and Russia larger than France ; but no matter how perfectly the 
 map may be constructed, something else is necessary before we can 
 have a complete conception of the dimensions of the country. We 
 must know the scale on which the map is drawn. The map contains 
 a reference line with certain marks upon it. This line is to give 
 the scale of the map. Its duty is to tell us that an inch on the 
 map corresponds with so many miles on the actual surface. 
 Without being supplemented by the scale, the map would for many 
 purposes be quite useless. Suppose that we consulted a map in 
 order to choose a route from London to Vienna, the map at once 
 points out the direction to be taken and the various towns and 
 countries to be traversed ; but unless we consult the little scale in 
 the corner, the map will not tell how many miles long the journey 
 is to be. 
 
 A map of the solar system can be readily constructed. We can 
 draw on it the orbits of some of the planets and of their satellites, 
 and we can include many of the comets. We can give to the sun 
 and to the planets their proper bulk. But to render the map quite 
 
VENUS. 145 
 
 efficient something more is necessary. We must have the scale 
 which is to tell us how many millions of miles on the heavens 
 will correspond to one inch of the map. It is at this point we 
 encounter a difficulty. It is comparatively easy to have all the 
 relative sizes of the orbits of the different bodies correct very 
 simple observations suffice for this purpose but it is not at all 
 easy to assign, with accuracy, the correct scale of the celestial 
 map. There are, however, several ways of solving the problem, 
 though they are all difficult and laborious. The most celebrated 
 method is that presented on an occasion of the transit of Venus. 
 Herein, then, lies the importance of the transit of Venus. It is 
 one of the best known means of finding the actual scale on which 
 our system is constructed. Observe the full importance of the 
 problem. Once the transit of Venus has given us the scale, then 
 all is known. We know the size of the sun; we know his distance; 
 we know the bulk of Jupiter, and the distances at which his 
 satellites revolve ; we know the dimensions of the comets, and the 
 number of miles to which they recede in their wanderings; we 
 know the velocity of the shooting stars ; and we learn the important 
 lesson that our earth is but one of the minor members of the sun's 
 majestic family. 
 
 As the path of Venus lies inside that of the earth, and as Venus 
 moves more quickly than the earth, it follows that the earth is 
 frequently passed by Venus, and just at the moment of passing it 
 will sometimes happen that the earth, the planet, and the sun lie 
 in the same straight line. We can then see Venus on the face of 
 the sun, and this is the phenomenon which we call the transit of 
 Venus. It is, indeed, quite plain that if the three bodies were 
 exactly in a line, an observer on the earth, looking at the planet, 
 will see it brought out vividly against the brilliant background 
 of the sun. 
 
 Considering that the earth is overtaken by Venus once every 
 nineteen months, it might be thought that the transits of Venus 
 should occur with corresponding frequency. This is not the case ; 
 the transit of Venus is an exceedingly rare occurrence, and a hun- 
 dred years or more will often elapse without a single transit taking 
 K 
 
146 THE 8TOEY OF THE HEAVENS. 
 
 place. This rarity of the transits arises from the fact that the 
 path of the planet is inclined to the plane of the earth's orbit ; 
 so that while in half of its path Venus is above the plane 
 of the earth's orbit, in the other half it is below. When 
 Venus overtakes the earth, the line from the earth to Venus 
 will therefore usually pass over or under the sun. If, however, 
 it should happen that Venus overtakes the earth at or near either 
 of the points in which the plane of the orbit of Venus passes 
 through that of the earth, then the three bodies will be in line, 
 and a transit of Venus will be the consequence. The rarity of the 
 occurrence of a transit need no longer be a mystery. The earth 
 passes through one of the critical parts every December, and 
 through the other every June. If, therefore, it so happens that the 
 conjunction of Venus occurs at, or close to, June 6th or December 
 7th, then a transit of Venus will occur at that conjunction, but 
 under no other circumstances. 
 
 The most remarkable law with reference to the recurrence of 
 the phenomenon is the well-known eight-year interval. The tran- 
 sits may be all grouped together into pairs ; the two transits of any 
 single pair being separated by an interval of eight years. For 
 instance, a transit of Venus took place in 1761, and again in 1769. 
 No further transits occurred until those recently witnessed in 1874 
 and in 1882. Then, again, comes a long interval, for another 
 transit will not occur until 2004, but then it will be followed by 
 another in 20] 2. 
 
 This recurrence of the transits in pairs admits of a very simple 
 explanation. It so happens that the periodic time of Venus bears a 
 remarkable relation to the periodic time of the earth. Venus ac- 
 complishes thirteen revolutions around the sun in very nearly the 
 same time that the earth requires for eight revolutions. If, there- 
 fore, in 1874, Venus and the earth were in line with the sun, then 
 in eight years more the earth will again be found in the same place; 
 and so will Venus, for it has just been able to accomplish thirteen 
 revolutions. A transit of Venus having occurred on the first occa- 
 sion, a transit must also occur on the second. 
 
 It is not, however, to be supposed that every eight years the 
 
VENUS. 147 
 
 planets will again regain the same position with sufficient precision 
 for a regular eight-year transit interval. For it is only approximately 
 true that thirteen revolutions of Venus are coincident with eight 
 revolutions of the earth. Each conjunction after an interval of 
 eight years takes place at a slightly different position of the planets, 
 
 Fig. 42. Venus on the Sun at the Transit of 1874. 
 
 so that when the two planets come together again in the year 1890 
 the point of conjunction will be so far removed from the critical 
 point that the line from the earth to Venus will not intersect the 
 sun, and thus, although Venus will pass very near the sun, yet no 
 transit will take place. 
 
 Fig. 42 represents the transit of Venus in 1874. It is taken 
 from a photograph obtained, during the occurrence of the transit, 
 K 2 
 
H8 TEE STORY OF THE HEAVENS. 
 
 by M. Janssen. His telescope was directed towards the sun during 
 the eventful minutes, and thus an image of the sun was depicted on 
 the photographic plate placed in the telescope. The circular margin 
 represents the disc of the sun. On that disc we see the round sharp 
 image of the planet Venus, showing the appearance of the planet 
 during the progress of the transit in 1874. The only other 
 features to be noticed are a few spots on the sun rather dimly 
 shown, and a network of lines which were stretched across the 
 field of view of the telescope to facilitate the measurements. It 
 might be supposed that the appearance of Venus in front of the 
 sun could be mistaken for one of the spots in the sun, which are 
 often large and round, and occasionally have even simulated the 
 appearance of a planet. But this view will not bear examination. 
 The occurrence of the transit at the predicted moment, and at the 
 precise point of the sun's margin which the calculations had indi- 
 cated, the sharpness of the planet's shape, and the circumstances of 
 its motion, all discriminate the planet as something totally distinct 
 from an ordinary sun spot. 
 
 The adjoining sketch exhibits the course which the planet 
 pursued in its course across the sun on the two occasions in 1874 and 
 1882. Our generation has had the good fortune to witness the 
 two occurrences indicated on this picture. The white circle denotes 
 the disc of the sun ; the planet enters on the white surface, and at 
 first is like a bite out of the sun's margin. Gradually the black spot 
 steals in front of the sun, until, after nearly half an hour, the black 
 disc is entirely visible. Slowly the planet wends its way across, 
 followed by hundreds of telescopes from every accessible part of 
 the globe whence the phenomenon is visible, until at length, in the 
 course of a few hours, the planet emerges from the other side. 
 
 It will be useful to take a brief retrospect of the different tran- 
 sits of Venus of which there is any historical record. They are not 
 numerous. Doubtless hundreds of transits have occurred since man 
 first came on the earth. It was not until the approach of the year 
 1631 that attention began to be directed to the matter, though the 
 transit which undoubtedly occurred in that year was not, so far as 
 we know, observed by any one. 
 
VENUS. 
 
 149 
 
 The success of Gassendi, in observing the transit of Mercury, to 
 which we referred in the last chapter, led him to hope that he would 
 be equally fortunate in observing the transit of Venus, which 
 Kepler had also foretold. Gassendi looked at the sun on the 4th, 
 5th, and 6th December. He looked at it again on the 7th, but he 
 saw no sign of the planet. We now know the reason. The transit 
 
 Fig. 43. The path of Venus across the Sun in the Transits of 1874 and 1882. 
 
 of Venus took place during the night, between the 6th and the 7th, 
 and would therefore have been invisible to European observers. 
 
 Kepler had supposed that, after the transit of 163], there would 
 not be another until 1761, but in this the usual acutenessof Kepler 
 seems to have deserted him. He appears not to have fully appre- 
 ciated the remarkable eight-year period, which necessitates that the 
 transit of 1631 would be followed by another in 1639. This transit 
 of 1639 is the one with which the history of the subject may be 
 said to commence. It was the first occasion on which the transit 
 was ever actually witnessed; nor was it then seen by many. So fat- 
 as is known, that transit was only witnessed by two persons. 
 
150 THE STORY OF THE HEAVENS. 
 
 A young- and ardent English astronomer, named Horrocks, 
 had undertaken some computations ahout the motions of Venus. 
 Horrocks made the discovery that the transit of Venus would be 
 repeated in 1639,, and he prepared to observe it. The sun rose on 
 the morning 1 of the eventful day which happened to be a Sunday. 
 The clerical profession, which Horrocks followed, here came into 
 collision with his desires as an astronomer. Horrocks decided that 
 his clerical duties must be performed, but that every spare moment 
 of the day should be devoted to studying the sun. At nine, he 
 says, he was called away by business of the highest importance 
 referring, no doubt, to his official duties; but the service was 
 quickly performed, and a little before ten he was again on the 
 watch, only to find the brilliant face of the sun without any 
 unusual feature. It was marked with a spot, but nothing that 
 could be mistaken for a planet. Again, at noon, came an interrup- 
 tion ; he went to church, but he was back by one. Nor were these 
 the only interruptions to his observations. The sun was also more 
 or less clouded over during part of the day. However, at a quarter 
 past three in the afternoon his clerical duties were over ; the clouds 
 had dispersed, and he once more resumed his observations. To his 
 incredible delight he then saw on the sun the round dark spot, 
 which he at once identified as the planet Venus. The observations 
 could not last long ; it was the depth of winter, and the sun was 
 rapidly setting. Only half an hour was available, but he had made 
 such careful preparations beforehand that half an hour sufficed to 
 enable him to secure careful and exact measurements. 
 
 Horrccks had previously acquainted his friend, William Crab- 
 tree, with the impending occurrence. Crabtree was therefore on the 
 watch, and succeeded in seeing the transit. But to no one else 
 had Horrocks communicated the intelligence ; as he says, " I hope 
 to be excused for not informing other of my friends of the 
 expected phenomenon, but most of them care little for trifles of 
 this kind, rather preferring their hawks and hounds, to say no 
 worse; and although England is not without votaries of astronomy, 
 with some of whom I am acquainted, I was unable to convey to 
 them the agreeable tidings, having myself had so little notice." 
 
VENUS. 151 
 
 It was not till long afterwards that the full importance of the 
 transit of Venus was perceived. Nearly a century had rolled away 
 when the great astronomer, Halley (1656 1741), drew attention 
 to the subject. The next transit was to occur in 1761, and forty- 
 five years before that event Halley explained his celebrated method 
 of finding the distance of the sun by means of the transit of Venus. 
 Halley was then a man sixty years of age; he could have no 
 expectation that he would live to witness the transit ; but in noble 
 language he commends the problem to the notice of the learned, 
 and thus addresses the Koyal Society of London : " And this is 
 what I am now desirous to lay before this illustrious Society, which 
 I foretell will continue for ages, that I may explain beforehand to 
 young astronomers, who may perhaps live to observe these things, 
 a method by which the immense distance of the sun may be truly 
 obtained. ... I recommend it, therefore, again and again to 
 those curious astronomers who, when I am dead, will have an 
 opportunity of observing these things, that they would remember 
 this my admonition, and diligently apply themselves with all their 
 might in making the observations, and I earnestly wish them all 
 imaginable success in the first place, that they may not by the 
 unseasonable obscurity of a cloudy sky be deprived of this most 
 desirable sight, and then that, having ascertained with more exact- 
 ness the magnitudes of the planetary orbits, it may redound to their 
 immortal fame and glory." Halley lived to a good old age, but 
 he died nineteen years before the transit occurred. 
 
 The student of astronomy who desires to learn how the transit 
 of Venus will tell the distance from the sun must prepare to en- 
 counter a geometrical problem of no little complexity. We cannot 
 give to the subject the detail that would be requisite for a full 
 explanation. All we can attempt is to render a general account of 
 the method, sufficient to enable the reader to see that the transit of 
 Venus really does contain all the elements necessary for the solution 
 of the problem. 
 
 We must first explain clearly the conception which is known to 
 astronomers by the name of parallax; for it is by parallax that the 
 distance of the sun, or, indeed, the distance of any other celestial 
 
152 THE STORY OF THE HEAVENS. 
 
 body, must be determined. Let us take a simple illustration. 
 Stand near a window from whence you can look at buildings, or 
 the trees, the clouds, or any distant objects. Place on the glass 
 a thin strip of paper vertically in the middle of one of the panes. 
 Close the right eye, and note with the left eye the position of the 
 strip of paper relatively to the objects in the background. Then, 
 while still remaining in the same position, close the left eye and 
 again observe the position of the strip of paper with the right eye. 
 You will find that the position of the paper on the background has 
 changed. As I sit in my study and look out of the window I see 
 a strip of paper, with my right eye, in front of a certain bough on 
 a tree a couple of hundred yards away ; with my left eye the paper 
 is no longer in front of that bough ; it has moved to a position near 
 the edge of the tree. This apparent displacement of the strip of 
 paper, relatively to the distant background, is what is called parallax. 
 
 Move closer to the window, and repeat the observation, and you 
 find that the apparent displacement of the strip increases. Move 
 away from the window, and the displacement decreases. Move to 
 the other side of the room, the displacement is much less, though 
 probably still visible. We thus see that the change in the apparent 
 place of the strip of paper, as viewed with the right eye or the left 
 eye, varies in amount as the distance changes ; but it varies in the 
 opposite way to the distance, for as either becomes greater the 
 other becomes less. We can thus associate with each particular 
 distance a corresponding particular displacement. From this it will 
 be easy to infer, that if we have the means of measuring the amount 
 of displacement, then we have the means of calculating the distance 
 from the observer to the window. 
 
 It is this principle, applied on a gigantic scale, which enables us 
 to measure the distances of the heavenly bodies. Look, for instance, 
 at the planet Venus ; let this correspond to the strip of paper, and 
 let the sun, on which Venus is seen in the act of transit, be the 
 background. Instead of the two eyes of the observer, we now place 
 two observatories in distant regions of the earth ; we look at Venus 
 from one observatory, we look at it from the other; we measure 
 the amount of the displacement, and from that we calculate the 
 
VENUS. 153 
 
 distance of the planet. All depends, then, on the means which we 
 have of measuring the displacement of Venus as viewed from the 
 two different stations. There are various ways of accomplishing 
 this, but the most simple is that originally proposed by Halley. 
 
 From the observatory at A Venus seems to pursue the upper of 
 the two tracks shown in the adjoining figure. From the observa- 
 tory at B it follows th^ lower track, and it is for us to measure the 
 
 Fig. 44. To illustrate the observation of the Transit of Venus from two localities, 
 A and B, on the Earth. 
 
 distance between the two tracks. This can be accomplished in several 
 ways. Suppose the observer at A note the time that Venus has 
 occupied in crossing the disc, and that similar observations be made 
 at B. As the track seen from B is the larger, it must follow that 
 the time observed at B will be greater than that at A. When the 
 observers from the different hemispheres come together and compare 
 their observations, the. times observed will enable the lengths of 
 the tracks to be calculated. The lengths being known, their places 
 on the circular disc of the sun are determined, and hence the 
 amount of displacement of Venus in transit is ascertained. Thus 
 
154 THE STORY OF THE HEAVENS. 
 
 it is that the distance o Venus is measured, and thus the scale of 
 the solar system is known. 
 
 The two transits to which Halley's memorable researches referred 
 occurred in the years 1761 and 1769. The results of the first were 
 not very successful, in spite of the arduous labours of those who 
 undertook the observations ; but the transit of 1769 will be for ever 
 memorable, not only on account of the determination of the sun's 
 distance, but as giving- rise to the first of the celebrated voyages 
 of Captain Cook. It was to see the transit of Venus, that Captain 
 Cook was commissioned to sail to Otaheite, and there, on the 3rd 
 of June, on a splendid day in that most exquisite climate, the 
 transit of Venus was carefully observed and measured by different 
 observers. Simultaneously with these observations others were 
 obtained in Europe, and from the combination of the two the first 
 accurate knowledge of the sun's distance was gained. The most 
 complete discussion of these observations did not, however, take 
 place for some time. It was not until the year 1824 that the 
 illustrious Encke computed the distance of the sun, and gave as the 
 definite result 95,000,000 miles. 
 
 For many years this number was invariably adopted, and most 
 of the present generation will remember how they were taught in 
 their school-days that the sun was 95,000,000 miles away. At 
 length doubts began to be whispered as to the accuracy of this 
 result. The doubts arose in different quarters, and were presented 
 with different degrees of importance ; but they all pointed in one 
 direction, they all indicated that the distance of the sun was not 
 really so great as the result which Encke had obtained. It must 
 be remembered that there are several ways of finding the distance 
 of the sun, and it will be our duty to allude to some other methods 
 later on. It has been ascertained that the result obtained by Encke 
 was too great, and that the distance of the sun may probably be 
 now stated at 92,700,000 miles. 
 
 I venture to record our personal experience of the last transit 
 of Venus, which we had the good fortune to view from Dunsink 
 Observatory on the afternoon of the 6th of December, 188:2. 
 
 The morning of the eventful day appeared to be about as 
 
VENUS. 155 
 
 unfavourable for a grand astronomical spectacle as could well be 
 imagined. Snow, a couple of inches thick, covered the ground, and 
 more was falling, with but little intermission, all the forenoon. It 
 seemed almost hopeless that a view of the great event could be 
 obtained from this observatory ; but it is well in such cases to bear 
 in mind the injunction given to the observers on a celebrated eclipse 
 expedition. They were instructed, no matter what the day should 
 be like, that they were to make all their preparations precisely as 
 they would have done were the sun shining in undimrned splendour. 
 By this advice no doubt many observers have profited ; and we acted 
 upon it here with very considerable success. 
 
 We have at this observatory two equatorials, one of them an 
 old, but tolerably good instrument, of about six inches aperture, 
 the other the great South equatorial of twelve inches aperture 
 already referred to. At eleven o'clock the day looked worse than 
 ever ; but we at once proceeded to make all ready. I stationed 
 Mr. Rambaut at the small equatorial, while I myself took charge 
 of the South instrument. The snow was still falling when the 
 domes were opened : but, according to our pre-arranged scheme, 
 the telescopes were directed, not indeed upon the sun, but to the 
 place where we knew the sun was, and the clockwork was set 
 in motion, which carried round the telescopes, still constantly 
 pointing towards the invisible sun. The predicted time of the 
 transit had not yet arrived. Mr. Hind, the distinguished super- 
 intendent of the " Nautical Almanac/' had kindly sent us his 
 computations, showing that, viewed from Dunsink, the transit 
 ought to commence at 1 h. 35 min. 48 sees, mean time at Dublin, 
 and that the point on the sun's disc where the planet would enter 
 was 147 from the north point of the sun round by east. 
 This timely intimation was of twofold advantage. It told us, in 
 the first place, the precise moment when the event was to be 
 expected ; and, what was perhaps quite as useful, it told us the exact 
 point of the sun to which the attention was to be directed. This is 
 a matter of very considerable importance, for in a large telescope it 
 is only possible to see a part of the sun at once, and therefore, 
 unless the proper part of the sun be placed in the field of view, the 
 phenomenon may be entirely missed. 
 
156 THE STORY OF THE HEAVENS. 
 
 The eye-piece employed on the South equatorial may also 
 receive a brief notice. It will, of course, be obvious that the full 
 glare of the sun must be greatly mitigated before the eye can 
 view it with impunity. The light from the sun falls upon a 
 piece of transparent glass inclined at a certain angle, and the 
 chief portion of the sun's heat, as well as a certain amount 
 of its light, pass through the glass and are lost. A certain 
 fraction of the light is, however, reflected from the glass, and 
 enters the eye-piece. This light is already much reduced in 
 intensity, but it undergoes as much further reduction as we please 
 by an ingenious contrivance. The glass which reflects the light 
 does so at what is called the polarising angle, and between the eye- 
 piece and the eye is a plate of tourmaline. This plate of tourmaline 
 can be turned round by the observer. In one position it hardly 
 interferes with the light at all, while in the position at right angles 
 thereto it cuts off nearly the whole of the light. By simply 
 adjusting the position of the tourmaline, the observer has it in his 
 power to render the image of any brightness that may be con- 
 venient, and thus the observations of the sun can be conducted with 
 the appropriate degree of illumination. 
 
 But such appliances seemed on this occasion to be a mere 
 mockery. The tourmaline was all ready, but up to one o'clock 
 not a trace of the sun could be seen. Shortly after one o'clock, 
 however, we noticed that the day was getting lighter ; and, on 
 looking to the north, from whence the wind and the snow were 
 coming, we saw, to our inexpressible delight, that the clouds were 
 breaking. At length the sky towards the south began to improve, 
 and at last, as the critical moment approached, we could detect the 
 spot where the sun was becoming visible through the clouds. But 
 Mr. Hind's predicted moment arrived and passed, and still the sun 
 had not broken through the clouds, though every moment the 
 certainty that it would do so became more apparent. The external 
 contact was therefore missed. We tried to console ourselves by the 
 reflection that this was not, after all, a very important phase, and 
 hoped that the internal contact would be more successful. 
 
 At length the struggling sun pierced the clouds, and I saw 
 
VENUS. 157 
 
 the round, sharp disc of the sun in the finder, and eagerly 
 glanced at the point on which attention was concentrated. Some 
 minutes had now elapsed since Mr. Hind's predicted moment 
 of first contact, and, to my delight, I saw the small dark bite 
 out of the sun showing that the transit of Venus had commenced, 
 and that the planet was then one-third on the sun. But the 
 most critical moment had not yet arrived. By the expression 
 t( first internal contact"" we are to understand the moment 
 when the planet is just exactly on the sun. This first contact 
 was timed to occur twenty-one minutes later than the external 
 contact already referred to. But again the clouds disappointed our 
 hope of seeing the internal contact. While steadily looking at the 
 exquisitely beautiful sight of the gradual advance of the planet, I 
 became aware that there were other objects besides Venus between 
 me and the sun. These objects were snowflakes, which again began 
 to fall rapidly. They were, I must admit, most singularly beautiful. 
 The telescopic effect of a snowstorm with the sun as a background 
 I had never before seen. It was a most remarkable sight, and 
 reminded me of the golden rain which is sometimes seen during 
 pyrotechnic displays ; but I would gladly have dispensed with the 
 spectacle, for it necessarily followed that the sun and Venus again 
 disappeared from view. The clouds gathered, the snowstorm 
 descended as heavily as ever, and we hardly dared to hope that we 
 should see anything more. 1 h. 57 min. came and passed, the first 
 internal contact was over, and Venus had fully entered on the sun. 
 We had only obtained a brief view, and we had been able to make 
 no measures or other observations that could be of service. Still, 
 to have seen even a part of a transit of Venus is an event to re- 
 member for a lifetime, and we felt more delight than can be easily 
 expressed at even this slight gleam of success. 
 
 But better things were in store. My assistant came over 
 to report to me that he had also been successful in seeing 
 Venus in the same phase as I had. We both resumed our posts, 
 and at half-past two the clouds began to disperse, and the 
 prospect of seeing the sun began to improve. It was now no 
 question of the observations of contact. Venus by this time 
 
158 THE STOEY OF THE HEAVENS. 
 
 was well on the sun, and we therefore prepared to make obser- 
 vations with the micrometer attached to the eye-piece. The 
 clouds at length dispersed, and at this time Venus had so com- 
 pletely entered on the sun that the distance from the edsje of the 
 planet to the edge of the sun was about twice the diameter of the 
 planet. We measured the distance of the inner edge of Venus from 
 the nearest limb of the sun. These observations were repeated as fre- 
 quently as possible, but it should be added that they were only made 
 with very considerable difficulty. The sun was now very low, and 
 the edges of the sun and of Venus were by no means of that steady 
 character which is suitable for micrometrical measurement. The 
 edge of the sun was "boiling/' and Venus, though it no doubt was 
 sometimes circular, was very often distorted to such a degree as to 
 make the measures very uncertain. 
 
 We succeeded in obtaining sixteen measures altogether; but 
 the sun was now getting low, the clouds began again to inter- 
 fere, and we saw that the pursuit of the transit must be left to 
 the thousands of astronomers in happier climes who hrd been 
 eagerly awaiting it. But before the phenomena had ceased 
 I spared a few minutes from the somewhat mechanical work 
 at the micrometer to take a view of the transit in the more 
 picturesque form which the large field of the finder presented. 
 Truly it was a most exquisite and memorable sight. The sun was 
 already beginning to put on the ruddy hues of sunset, and there, far 
 in on its face, was the sharp, round, black disc of Venus. It was 
 then easy to sympathise with the supreme joy of Horrocks when, in 
 1639, he for the first time witnessed this spectacle. The intrinsic 
 beauty of the phenomenon, its rarity, the fulfilment of the predic- 
 tion, the noble problem which the transit of Venus enables us to 
 solve, are all present to our thoughts when we look at this pleasing 
 picture, the like of which will not occur again until the flowers 
 are blooming in the June of A.D. 2004. 
 
 The occasion of a transit of Venus also affords an opportunity of 
 studying the physical nature of the planet, and we may here briefly 
 indicate the results that have been obtained. In the first place, a 
 transit will throw some light on the question as to whether Venus 
 
VENUS. 159 
 
 is accompanied by a satellite. If Venus were attended by a small 
 body in close proximity, it would be conceivable that under ordi- 
 nary circumstances the brilliancy of the planet would obliterate 
 the feeble beam of rays from the minute companion, and thus the 
 satellite would remain undiscovered. It was therefore a matter of 
 great interest to scrutinise the vicinity of the planet while in the 
 act of transit over the sun. If a satellite existed and the exist- 
 ence of a satellite has often been suspected then it would be 
 capable of detection against the brilliant background of the sun. 
 Special attention was directed to this point during the recent 
 transits, but no satellite of Venus was to be found. It seems, 
 therefore, to be very unlikely that Venus can be attended by any 
 satellite of appreciable dimensions. 
 
 The observations directed to the investigation of the atmosphere 
 surrounding Venus have been more successful. If the planet were 
 devoid of an atmosphere, then it would be totally invisible just 
 before commencing to enter on the sun, and would relapse into 
 total invisibility as soon as it had left the sun. The observations 
 made during the transits are not in conformity with such supposi- 
 tions. Special attention has been directed to this point during the 
 recent transits. The result has been very remarkable, and has 
 proved in the most conclusive manner the existence of an atmos- 
 phere around Venus. As the planet gradually moved off the sun 
 the circular edge of the planet extending out into the darkness was 
 seen to be bounded by a circular arc of light, and Dr. Copeland, 
 who observed this transit under exceptionally favourable circum- 
 stances, was actually able to follow the planet until it had passed 
 entirely away from the sun, at which time the globe, though itself 
 invisible, was distinctly marked by the girdle of light by which it 
 was surrounded. This luminous circle is inexplicable save by the 
 supposition that the globe of Venus is surrounded by an atmos- 
 pheric shell in the same way as the earth. 
 
 It may be asked, what is the advantage of devoting so much 
 time and labour to a celestial phenomenon like the transit of 
 Venus which has so little bearing on practical affairs ? What does 
 it matter whether the sun be 95,000,000 miles off, or whether it be 
 
160 THE STORY OF THE HEAVENS. 
 
 only 93,000,000, or any other distance ? We must admit at once 
 that the inquiry has but a slender bearing on matters of practical 
 utility. No doubt a fanciful person might contend, that to compute 
 our Nautical Almanacs with perfect accuracy we require to know the 
 distance of the sun with accuracy. Our mighty commerce depends 
 on skilful navigation, and one factor in successful navigation is the 
 reliability of the Nautical Almanac. The increased perfection of the 
 almanac must therefore bear some relation to increased perfection in 
 navigation. Now, as good authorities tell us that in running for 
 a harbour on a tempestuous night, or in other critical emergen- 
 cies, even a yard of sea-room is often of great consequence, so it 
 may conceivably happen that to the infinitesimal influence of the 
 transit of Venus on the Nautical Almanac is due the safety of a 
 gallant vessel. 
 
 But the time, the labour, and the money expended in observing the 
 transit of Venus are really to be defended on quite different grounds. 
 We see in it a fruitful source of information. It tells us the dis- 
 tance of the sun, which is the foundation of all the great measure- 
 ments of the universe. It gratifies the intellectual curiosity of 
 man by a view of the true dimensions of the majestic solar system, 
 in which the earth is seen to play a dignified, though still sub- 
 ordinate, part ; and it leads us to a conception of the stupendous 
 scale on which the great universe is constructed. 
 
 It is not possible for us, with a due regard to the limits of this 
 volume, to linger any longer over the consideration of the transit of 
 Venus. When we begin to study the details of the observations, 
 we are immediately confronted with a multitude of technical and 
 intricate matters. On the occasion of a transit, it has first to 
 be decided where the observations are to be made in itself a 
 question that has led to no little discussion. Then the instru- 
 ments that are to be used, and the description of observations to be 
 made, have to be investigated with considerable complexity. The 
 observers must be specially trained for the work, for even Methuselah 
 himself could hardly have lived long enough to have had much 
 practice in the observations of transits of Venus. To compensate 
 for the inevitable want of experience, the observers had to be pre- 
 
VENUS. 161 
 
 pared by a special course of instruction, in which a fictitious transit 
 was observed. Then, too, the interpretation o the observations 
 involves many thorny and many controverted questions. To pursue 
 all these matters so as to render them intelligible would lead us into 
 great detail, and therefore we do not make the attempt. This 
 course is the more advisable when it is remembered that the transit 
 of Venus is only one of the methods of finding the sun's distance 
 a celebrated one, no doubt, but not perhaps the most reliable. It 
 seems not unlikely that the final determination of the sun's distance 
 will be obtained in quite a different manner. This will be explained 
 in Chapter XI., and hence we feel the less reluctance in passing 
 away from the consideration of the transit of Venus as a method 
 of celestial surveying. 
 
 We must now close our description of this lovely planet ; but 
 before doing so, let us add or in some cases repeat a few sta- 
 tistical facts as to the size and the dimensions of the planet and 
 its orbit. 
 
 The diameter of Venus is about 7,660 miles, and the planet 
 shows no measurable departure from the globular form, though we 
 can hardly doubt that its polar diameter must really be somewhat 
 shorter than the equatorial diameter. This diameter is only about 
 258 miles less than that of the earth. The mass of Venus is about 
 three-quarters of the mass of the earth ; or if, as is more usual, we 
 compare the mass of Venus with the sun, it is to be represented by 
 the fraction 1 divided by 425,000. It is to be observed that the 
 mass of Venus is not quite so great in comparison with its bulk as 
 might have been expected. The density of Venus is about 0*850 of 
 that of the earth. Venus would weigh 4' 81 times as much as a 
 globe of water of equal size. The gravitation at the surface of 
 Venus will, to a slight extent, be less than the gravitation at the 
 surface of the earth. A body here falls sixteen feet in a 
 second ; a body let fall at the surface of Venus would fall about 
 three feet less. The time of rotation of Venus is an element 
 about which there is still considerable uncertainty. It is sup- 
 posed to occupy about twenty- three hours and twenty -one 
 minutes. 
 I. 
 
162 THE STORY OF THE HEAVENS. 
 
 The orbit of Venus is remarkable for the close approach which 
 it makes to a circle. The greatest distance of Venus from the sun 
 does not exceed by 1 per cent, the least distance. Its mean dis- 
 tance from the sun is about 67,000,000 miles, and the movement 
 in the orbit has a mean velocity of nearly 22 miles per second, the 
 entire journey being accomplished in 221*70 days. 
 
 
CHAPTER IX. 
 
 THE EARTH. 
 
 The Earth is a great Globe How the Size of the Earth is Measured The Base 
 Line The Latitude found by the Elevation of the Pole A Degree of the 
 Meridian The Earth not a Sphere The Pendulum Experiment Is the Motion 
 of the Earth slow or fast ? Coincidence of the Axis of Rotation and the Axis 
 of Figure The Existence of Heat in the Earth The Earth once in a Soft 
 Condition Effects of Centrifugal Force Comparison with the Sun and Jupiter 
 The Protuberance at the Equator The Weighing of the Earth Comparison 
 between the Weight of the Earth and an equal Globe of Water Comparison 
 of the Earth with a Leaden Globe The Pendulum Use of the Pendulum in 
 Measuring the Intensity of Gravitation The Principle of Isochronism Shape 
 of the Earth Measured by the Pendulum. 
 
 * 
 
 THAT the earth is a round body is a truth immediately suggested 
 to us by the most simple astronomical considerations. The sun is 
 round, the moon is round, telescopes show that the planets are 
 round. No doubt comets are not round, but then a comet is not 
 a solid body at all. We can see right through a comet, and its 
 weight is too small for our measures to appreciate. If, then, all 
 the solid bodies we can see are round globes, is it not likely that 
 the earth is a globe also ? Eut we have far more direct informa- 
 tion than mere surmise. 
 
 There is no better way of actually seeing that the surface of 
 the sea is curved than by watching on a clear day a distant 
 ship. When the ship is a long way off and is still receding, 
 its hull will gradually disappear, while the masts will remain 
 visible. On a fine summer's day we can often see with, or 
 indeed without an opera-glass, the top of the funnel of a steamer 
 appearing above the sea, while the body of the steamer is below. 
 If the sea were perfectly flat, there is nothing to obscure the body 
 of the vessel, and it therefore would be visible as long as the 
 funnel remains visible ; but if the sea be really curved the pro- 
 L 2 
 
164 THE STORY OF THE HEAVENS. 
 
 tuberant part intercepts the view of the hull, while leaving the 
 funnel visible. 
 
 We thus learn how the sea is curved at every part, and thus 
 it is natural to suppose that the earth is a globe. When we make 
 more careful measurements we find that the globe is not perfectly 
 round. It is flattened to some extent at each of the poles. This 
 may be easily illustrated by an indiarubber globe, which can be 
 compressed on two opposite sides so as to bulge out at the centre. 
 The earth is similarly flattened at the poles, and bulged out at the 
 equator. The divergence of the earth from the truly globular 
 form is, however, not very great, and would hardly be noticed 
 without very careful measurements. 
 
 The determination of the size of the earth involves operations 
 of no little delicacy. Very much skill and very much labour have 
 been devoted to the work, and the dimensions of the earth are 
 known with a high degree of accuracy, though perhaps not with 
 all the precision that we may ultimately hope to attain. The 
 scientific importance of an accurate measurement of the earth can 
 hardly be over-estimated. The radius of the earth is itself the unit 
 in which astronomical magnitudes are generally expressed. For 
 example, when observations are made with the view of finding the 
 distance of the moon, the observations, when discussed and reduced, 
 tell us that the distance of the moon is equal to fifty-nine times 
 the equatorial radius of the earth. If we want to find the distance 
 of the moon in miles, we require to know the number of miles in 
 the earth's radius. 
 
 A level part of the earth's surface having been chosen, a line a 
 few miles long is measured. This is called the base line, and as all 
 the subsequent measures depend on the base line, it is indispens- 
 able that this measurement be made with scrupulous accuracy. To 
 measure a line four or five miles long with such precision as to 
 exclude any errors greater than a few inches, demands the most 
 minute precautions. We do not now enter upon a description of 
 the operations that are necessary. It is a most laborious piece of 
 work, and many ponderous volumes have been devoted to the 
 discussion of the results. But when a few base lines have been 
 
 
THE EARTH. 165 
 
 measured in different places on the earth's surface, the measuring 
 rods are to be laid aside, and the subsequent task of the survey of 
 the earth is to be done by the measurement of angles and by tri- 
 gonometrical calculations based thereon. Starting from a base 
 line a few miles long, distances of greater length are measured, 
 until at length stretches of 100 miles long, or even more, can 
 be accomplished. It is thus possible to measure a long line 
 running due north and south. 
 
 So far the work has been merely that of the terrestrial surveyor. 
 The distance thus measured is then handed over to the astronomer 
 to deduce from it the dimensions of the earth. The astronomer 
 fixes his observatory at the northern end of the long line, and pro- 
 ceeds to determine his latitude by observation. There are various 
 ways by which the latitude is to be found. They will be found 
 fully described in works on practical astronomy. We shall here 
 only indicate in a very brief manner the principle on which such 
 observations are to be made. 
 
 Everyone ought to be familiar with the Pole Star, which, though 
 by no means the most brilliant, is probably the most important 
 star in the whole heavens. In these latitudes we are accustomed to 
 find the Pole Star at a considerable elevation, and there, night after 
 night, we see it, always in the same place in the northern sky. 
 But suppose we start on a voyage to the southern hemisphere : as 
 we approach the equator we find, night after night, the Pole Star is 
 closer to the horizon, till at the equator the Pole Star will be on the 
 horizon ; while if we cross the line, we find on entering the southern 
 hemisphere that the Pole Star has become invisible. 
 
 On the other hand, a traveller leaving England for Norway 
 will find that the Pole Star is every night higher up in the heavens 
 than he has been accustomed to see it. If he extend his journey 
 farther north, the Pole Star will gradually rise higher and higher, 
 until at length, when approaching the pole of the earth, the Pole 
 Star is high up over his head. We are thus led to perceive that 
 the higher our latitude, the higher, in general, is the elevation of 
 the Pole Star e But we cannot use precise language until we replace 
 the Pole Star by the pole of the heavens itself. The pole of the 
 
166 THE STORY OF THE HEAVENS. 
 
 heavens is near the Pole Star, and the Pole Star revolves around 
 the pole of the heavens, as all the other stars do, once every day ; 
 the circle described by the Pole Star is however so small that, 
 unless we pay special attention to it, the motion is not perceived. 
 The true pole is not a visible point, but it is capable of being 
 accurately denned, and it enables us to state with the utmost pre- 
 cision the relation between the pole and the latitude. The statement 
 is, that the elevation of the pole above the horizon is equal to the 
 latitude of the place. 
 
 The astronomer stationed at one end of the long line measures 
 the elevation of the pole above the horizon. This is an operation 
 of some delicacy. In the first place, as the pole is invisible, he has 
 to measure, instead of it, the height of the Pole Star when that 
 height is greatest, and repeat the operation twelve hours later, 
 when the height of the Pole Star is least ; the mean between the 
 two gives the height of the pole, but this has -to be corrected in 
 various ways which it is not necessary for us here to discuss. 
 Suffice it to say that by such operations the latitude of one end of 
 the line is determined. The astronomer then, with all his equip- 
 ment of instruments, moves to the other end of the line. He there 
 repeats the operations, and he finds that the pole has now a different 
 elevation, corresponding to the different latitude. The difference of 
 the two elevations thus gives him an accurate measure of the 
 number of degrees and fractional parts of a degree between the 
 latitudes of the two stations. This can now be compared with the 
 actual distance in miles between the two stations, which has been 
 ascertained by the trigonometrical survey. A simple calculation 
 will then give the number of miles and fractional parts of a mile 
 corresponding to one degree of latitude or, as it is more usually 
 expressed, the length of a degree of the meridian. 
 
 This operation is repeated in different parts of the earth in 
 the northern hemisphere and in the southern, in high latitudes and 
 in low latitudes. If the sea-level over the entire earth were a 
 perfect globe, an important consequence would follow the length 
 of a degree of the meridian would be the same everywhere. It 
 would be the same in Peru as in Sweden, the same in India as in 
 
THE EARTH. 167 
 
 England. But the lengths of the degrees are not all the same, and 
 hence we learn that our earth is not really a sphere. The measured 
 lengths of the degrees enable us to see in what way the shape of the 
 earth departs from a perfect sphere. Near the pole the length of a 
 degree is longer than near the equator. This shows that the earth 
 is flattened at the poles and protuberant at the equator, and it pro- 
 vides us with the means of actually calculating the length of the 
 polar and the equatorial axes. 
 
 The polar axis of the earth may be defined to be the shortest 
 diameter of the earth. This axis intersects the surface at the north 
 and south poles. Around this axis the earth performs one rota- 
 tion every sidereal day. The sidereal day is a little shorter than 
 the ordinary day, being only 23 hours, 56 minutes, and 4 seconds. 
 The rotation is performed just as if a rigid axis passed through 
 the centre of the earth; or, to use the old and homely illustration, 
 the earth rotates just as a ball of worsted may be made to rotate 
 around a knitting-needle thrust through its centre. The rotation 
 of the earth upon its axis admits of being demonstrated in a very 
 remarkable manner by actual experiment. If the pole of the earth 
 were an attainable point, then the experiment would have a degree 
 of simplicity that can only be partially realised at other stations. 
 For the purpose of describing the principle of this experiment we 
 may suppose that the observer is actually situated at the pole, and 
 that over the point on the earth where the polar axis intersects the 
 surface a lofty dome has been erected. From the summit of the 
 dome a long wire descends nearly to the ground, and to this a 
 ponderous weight is attached. The weight is to be drawn aside 
 and then released. Slowly it will swing from side to side, but the 
 plane in which it moves will remain invariable. If the building 
 were also at rest, then the position of the plane of oscillation would 
 remain in the same position relatively to the surrounding walls. 
 But the building rotates with the earth, and will turn completely 
 round in the sidereal day. There will accordingly be a change in 
 the place of the plane of oscillation relatively to the building, 
 which will appear to rotate at such a rate that it would complete a 
 revolution in the sidereal day. If the building be situated at lower 
 
168 THE STORY OF THE HEAVENS. 
 
 latitude the apparent change in the plane of oscillation of the 
 penduLim is not so rapid, yet the movement of the plane is still 
 sufficient to be perfectly observed, and the measured amount of dis- 
 placement of the plane has been found to agree with the amount 
 calculated on the supposition that the earth rotates. This is known 
 as Foucaulf s celebrated pendulum experiment. 
 
 Regarding the earth as a spinning body, it might be thought 
 that the rotation, being only once in four minutes less than a day of 
 twenty-four hours, was a very slow movement. A wheel of ordinary 
 dimensions would be turning very slowly at this rate; only, indeed, 
 about half the speed of the hour-hand of a clock. When, however, 
 the size of the earth is taken into consideration, our conception of 
 the speed will be somewhat modified. The radius of the earth is 
 so great that a point on the equator will have to dash along at the 
 rate of a thousand miles per hour in order to complete its circuit 
 in the allotted time. 
 
 It is a circumstance worthy of the very closest attention, that 
 the axis about which the earth rotates is identical with the 
 shortest diameter of the earth as found by actual surveying. This 
 is a coincidence which would be utterly inconceivable if the shape 
 of the earth was not in some way found to be connected with the 
 fact that the earth is rotating. What connection can then be 
 traced ? Let us inquire into the subject, and we shall find that the 
 shape of the earth is a consequence of its rotation. 
 
 The earth at the present time is subject, at various localities, to 
 occasional volcanic outbreaks. The phenomena of volcanoes, the 
 allied phenomena of earthquakes, the well-known fact that the heat 
 increases the deeper we descend into the earth, the existence of hot 
 springs, the geysers found in Iceland and elsewhere, all testify to 
 the fact that heat exists in the interior of the earth. Whether that 
 heat be, as some suppose, universal in the interior of the earth, or 
 whether it be merely local at the several places where its manifesta- 
 tions are felt, is a matter which need not now concern us. All that 
 is necessary for our present purpose is the admission that heat is 
 present to some extent. This internal heat, be it much or little, 
 has obviously a different origin to the heat which we know on the 
 
TEE EARTH. 169 
 
 surface. The heat we enjoy is derived from the sun. The Internal 
 heat cannot have been derived from the sun ; its intensity is far 
 too great, and there are other insuperable difficulties attending the 
 supposition that it has come from the sun. Where then has this 
 heat come from ? This is a question which at present we can 
 hardly answer nor, indeed, does it much concern our argument 
 that we should answer it. The fact that the heat is there 
 being admitted, all that we require is to apply one or two of 
 the well-known laws of heat to the interpretation of the facts. 
 We have first to consider the well-known law by which heat 
 tends to diffuse itself and spread away from its original source. 
 The heat, deep-seated in the interior of the earth, tends to pene- 
 trate through the superincumbent rocks, and slowly reaches the 
 surface. It is true that the rocks and materials with which 
 our earth is covered are not good conductors of heat; most of 
 them are, indeed, extremely bad conductors of heat; but good or 
 bad, they nevertheless are conductors, and through them the heat 
 must creep to the surface. It cannot be urged against this con- 
 clusion that we do not feel this heat. A few feet of brickwork will 
 so keep in the heat of a mighty blast furnace that but little will 
 escape through the bricks; but some heat does escape, and the 
 bricks have never been made, and so far as we know, never could be 
 made, which would absolutely intercept all the heat. If a few feet 
 of brickwork can thus nearly mask the heat of a furnace, cannot 
 some scores of miles of rock nearly mask the heat in the depths 
 of the earth, even though that heat were seven times hotter 
 than the mightiest furnace that ever existed? The heat would 
 escape slowly, and perhaps imperceptibly, but unless all our know- 
 ledge of nature is a delusion, no rocks, however thick, can prevent, 
 in the course of time, the leakage of the heat to the surface. 
 When this heat arrives at the surface of the earth it must, in 
 virtue of another law of heat, gradually radiate away and be lost. 
 
 It would, perhaps, lead us too far to discuss some of the objec- 
 tions which may be raised against what we have here stated. It is 
 often said that the heat in the interior of the earth is being 
 produced by chemical combination or by mechanical process, and 
 
170 THE STORY OF THE HEAVENS. 
 
 thus that the heat may be constantly renewed as fast or even 
 faster than it escapes. This, however, is more a difference in 
 form than in substance. If heat be produced in the way just 
 supposed (and there can be no doubt that there may be such 
 an origin for some o the heat in the interior of the globe) there 
 must be a certain expenditure of chemical or mechanical energies 
 that produces a certain exhaustion. For every unit of heat 
 which escapes, there will either be a loss of an unit of heat from 
 the globe or, what comes nearly to the same thing, a loss of an 
 unit of heat-making power from the chemical or the mechanical 
 energies. The substantial result is the same, the heat of the earth 
 must be decreasing. It should, of course, be observed that a great 
 part of the escape of heat from the earth is of an obvious character, 
 and not dependent upon the slow processes of conduction. Each 
 outburst of a volcano discharges a stupendous quantity of heat 
 which disappears very speedily from the earth ; while in many 
 places, as in the hot springs, there is a perennial discharge of heat 
 which must attain enormous proportions. 
 
 The earth is thus losing heat; but the earth never gains any 
 heat to replace the losses of heat or of heat-making power. The 
 consequence is obvious ; the interior of the earth must be growing 
 colder. No doubt this is an extremely slow process ; the life of an 
 individual, the life of a nation, perhaps the life of the human race 
 itself has not been long enough to witness any pronounced change 
 in the store of terrestrial heat. But the law is inevitable, and 
 though the decline in heat may be slow, yet it is continuous, and 
 in the lapse of ages must necessarily produce great and important 
 effects. 
 
 It is not our present purpose to attempt to make any forecast 
 as to the future effects on the earth which may arise from this 
 process. We wish at present rather to look back into past time 
 and see to what results we are inevitably led ; and here we may at 
 once dismiss as inappreciable such intervals of time as we are familiar 
 with in ordinary life or even in ordinary history. As the earth is 
 daily losing internal heat, or the equivalent of heat, the earth must 
 have had more heat yesterday than it had to-day, more last year 
 
THE EARTH. 171 
 
 than this year, more twenty years ago than ten years ago. The 
 effect has not been appreciable in historic time ; but when we rise 
 from hundreds of years to thousands of years, from thousands 
 of years to hundreds of thousands of years, and from hundreds of 
 thousands of years to millions of years, the effect is not only 
 appreciable but even of startling magnitude. There must have 
 been a time when the earth contained much more heat than at 
 present. There must have been a time when Ihe surface of the 
 earth was sensibly hot from this source. We cannot pretend to 
 say how many thousands or millions of years ago this epoch 
 was; but of this we may be sure, that earlier still the earth 
 was hotter still, until at length we find the temperature in- 
 crease to a red heat, from a red heat we look back to a still 
 earlier age when the earth was white hot, back again till we find 
 the surface of our now solid earth was actually molten. We need 
 not push the retrospect any further at present, and still less 
 is it necessary for us to attempt to assign the probable origin 
 of that heat. This, it will be observed, is not required in our 
 argument. We find heat now, and we know that heat is being 
 lost every day. From this the conclusion that we have already 
 drawn seems inevitable, and thus we are conducted back to some 
 remote epoch in the abyss of time past when our solid earth was a 
 globe molten and soft throughout. 
 
 A dewdrop on the petal of a flower is nearly globular ; but it 
 is not quite a globe, because the gravitation presses it against the 
 flower and somewhat distorts the shape. A falling drop of rain is 
 a globe ; a drop of oil suspended in a liquid with which it does not 
 mix, forms a globe. Passing from small things to great things, 
 let us endeavour to conceive a stupendous globe of molten matter. 
 Let that globe be as large as the earth, and let its materials be so 
 soft as to obey the forces of attraction exerted by each part of the 
 globe on all the other parts. There can be no doubt as to the 
 effect of these attractions, they would tend to smooth down any 
 irregularities on the surface just in the same way as the surface of 
 the ocean is smooth when freed from the disturbing influences 
 of the wind. We might therefore expect that our molten globe, 
 
172 THE 8 TOBY OF THE HEAVENS. 
 
 isolated from all external interference, would assume the form of a 
 sphere. 
 
 But now suppose that this great sphere, which we have hitherto 
 assumed to be at rest, is made to rotate round an axis passing 
 through its centre. We need not suppose that this axis is a material 
 object, nor are we concerned with any supposition as to how the 
 velocity of rotation was caused. We can, however, easily see what 
 the consequence of the rotation would be. The sphere would 
 become deformed, the centrifugal force would make the molten 
 body bulge out at the equator and flatten down at the poles. The 
 greater the velocity of rotation the greater would be the bulging. 
 To each velocity of rotation a certain degree of bulging would be 
 appropriate. The molten earth then bulged out to an extent which 
 was dependent upon the fact that it turned round once a day. 
 Now suppose that the earth, while still rotating, commences to 
 pass from the liquid to the solid state. The form which the earth 
 would assume on consolidation would, no doubt, be very irregular 
 on the surface; it would be irregular in consequence of the 
 upheavals and the outbursts incident to the transformation of so 
 mighty a mass of matter ; but irregular though it be, we can be 
 sure that, on the whole, the form of the earth's surface would 
 coincide with the shape which it had assumed by the movement of 
 rotation. Hence we can explain the protuberant form of the 
 equator of the earth, and we can appeal to that form in corrobora- 
 tion of the view that the earth was once in a soft or molten 
 condition. 
 
 The argument may be supported and illustrated by comparing 
 the shape of our earth with the shapes of some of the other 
 celestial bodies. The sun, for instance, seems to be almost a 
 perfect globe. No measures that we can make show that the 
 polar diameter of the sun is shorter than the equatorial diameter. 
 But this is what we might have expected. No doubt the sun 
 is rotating on its axis, and as it is the rotation that causes the 
 protuberance, why should not the rotation have deformed the sun 
 like the earth ? The probability is that a difference really does 
 exist between the two diameters of the sun, but that the difference 
 
THE EARTH. 173 
 
 is too small for us to measure. It is impossible not to connect this 
 with the slowness of the sun's rotation. The sun takes twenty-five 
 days to complete a rotation, and the protuberance appropriate to 
 so low a velocity is not appreciable. 
 
 On the other hand, when we look at one of the quickly-rotating 
 planets, we obtain a very different result. Let us take the most 
 striking instance, which is presented in the great planet Jupiter. 
 Viewed in the telescope, Jupiter is seen at once not to be a globe. 
 The difference is indeed so conspicuous, that measures are not 
 necessary to show that the polar diameter of Jupiter is shorter than 
 the equatorial diameter. The departure of Jupiter from the truly 
 spherical shape is indeed much greater than the departure of the 
 earth. It is impossible not to connect this with the much more 
 rapid rotation of Jupiter. We shall presently have to devote a 
 chapter to the consideration of this splendid orb. We may, how- 
 ever, so far anticipate what we shall then say as to state that the 
 time of Jupiter's rotation is under ten hours, and this notwith- 
 standing the fact that Jupiter is more than one thousand times 
 greater than the earth. The enormously rapid rotation of Jupiter 
 has caused him to bulge out at the equator to a most remarkable 
 extent. 
 
 The survey of our earth, and the measurement of its dimensions 
 having been accomplished, the next operation for the astronomer 
 is the determination of its weight. Here, indeed, is a problem 
 which taxes the resources of science to the very uttermost. Of the 
 interior of the earth we know little I might almost say we know 
 nothing. No doubt we sink deep mines into the earth. These 
 mines enable us to penetrate half a mile, or even a whole mile, 
 into the depths of the interior. But this is, after all, only a 
 most insignificant attempt to explore the interior of the earth. 
 What is an advance of one mile in comparison with the distance to 
 the centre of the earth ? It is only about one four-thousandth part 
 of the whole. Our knowledge of the earth merely reaches to 
 an utterly insignificant depth below the surface, and we have not 
 a conception of what may be the nature of our globe only a few 
 miles below whore we are standing. Seeing, then, our almost 
 
174 THE STORY OF THE HEAVENS. 
 
 complete ignorance of the solid contents of the earth, does it 
 not seem a hopeless task to attempt to weigh the entire globe 
 and determine its weight ? Yet that problem has been solved, 
 and the weight of the earth is known not, indeed, with the 
 accuracy attained in other astronomical researches, but still with 
 a tolerable approximation. 
 
 It is needless to enunciate the weight of the earth in our ordi- 
 nary units. The enumeration of billions of tons does not convey 
 any distinct impression. It is a far more natural course to compare 
 the mass of the earth with that of an equal globe of water. We 
 should be prepared to find that our earth was heavier than an 
 an equal globe of water. The rocks which form its surface are 
 heavier, bulk for bulk, than the oceans which repose on those 
 rocks. The abundance of metals in the earth, the gradual increase 
 in the density of the earth, which must arise from the enormous 
 pressure at great depths all these considerations will prepare us 
 to learn that the earth is very much heavier than a globe of water 
 of equal size. Newton supposed that the earth was somewhere 
 between five and six times as heavy as an equal bulk of water. 
 Nor is it hard to see that such a suggestion is plausible. The 
 rocks and materials on the surface are usually about two or three 
 times as heavy as water. There is certainly a vast quantity of iron 
 in the earth. It has been supposed that down in the remote 
 depths of the earth there is a very large proportion of iron. Now 
 an iron earth would weigh about seven times as much as an equal 
 globe of water. We are thus led to see that the earth's weight 
 must be probably more than three, and probably less than seven, 
 times an equal globe of water; and hence, in fixing the density 
 between five and six, Newton adopted a result plausible at the 
 moment, and since shown to be probably correct. Several methods 
 have been proposed by which this important question can be 
 solved with accuracy. Of all these methods we shall here only 
 describe one, because it illustrates, in a very remarkable manner, 
 the law of universal gravitation. 
 
 In our chapter on gravitation, it was pointed out that the force 
 of gravitation between two masses of moderate dimensions was 
 
THE EARTH. 175 
 
 extremely minute, and the difficulty in weighing the earth arises 
 from this cause. The practical application of the process is en- 
 cumbered by multitudinous details, which it will be unnecessary 
 for us to consider at present. The principle of the process is 
 simple enough. To give definiteness to our description, let us 
 conceive a large globe about two feet in diameter ; and as it is 
 desirable to have this globe as heavy as possible, let us suppose 
 it to be made of lead. A small globe brought near the large 
 globe is attracted by the force of gravitation. The amount of this 
 attraction is extremely small, but, nevertheless, it can be measured 
 by a refined process which renders extremely small forces sensible. 
 The intensity of this attraction depends both on the masses of the 
 globes and on their distance apart, as well as on the force of 
 gravitation. We can also readily measure the attraction of the 
 earth upon the small globe. This is in fact nothing more nor less 
 than the weight of the small globe in the ordinary acceptation of 
 the word. We thus can compare the attraction exerted by the 
 leaden globe with the attraction exerted by the earth. If the 
 centre of the earth and the centre of the leaden globe were at the 
 same distance from the attracted body, then the intensity of their 
 attractions would give at once the ratio of their masses by simple 
 proportion. In this case, however, matters are not so simple ; the 
 leaden ball is only distant by a few inches from the attracted 
 ball, while the centre of the earth's attraction is nearly 4,000 
 miles away at the centre of the earth. Allowance has to be made 
 for this difference, and the attraction of the leaden sphere has to 
 be reduced to what it would be were it removed to a distance 
 of 4,000 miles. This can fortunately be effected by a simple 
 calculation depending upon the great law that the intensity of 
 gravitation varies inversely as the square of the distance. We can 
 thus, partly by calculation and partly by experiment, compare the 
 intensity of the attraction of the leaden sphere with the attraction 
 of the earth. It is known that the attractions are proportional to 
 the masses, so that thus the comparative masses of the earth and 
 of the leaden sphere have been measured ; and it has been ascer- 
 tained that the earth is about half as heavy as a globe of lead 
 
176 THE STORY OF THE HEAVENS. 
 
 of equal size would be. We may thus state finally that the 
 mass of the earth is a little more than five times as great as the 
 mass of a globe of water equal to it in bulk. 
 
 We have, in our chapter on Gravitation, mentioned the fact that 
 a body let fall near the surface of the earth moves through sixteen 
 feet in the first second. This distance varies slightly at different 
 parts of the earth. If the earth were perfectly round, then the 
 attraction would be the same at every part, and the body would 
 fall through the same distance everywhere. The earth is not 
 round, so the distance which the body falls in one second differs 
 slightly at different places. At the pole the radius of the earth 
 is shorter than at the equator, and accordingly the attraction of 
 the earth at the pole is greater than at the equator. If we 
 had the means of measuring exactly the distance a body would 
 fall in one second at the pole and at the equator, we would have 
 the means of ascertaining the actual shape of the earth. 
 
 It is, however, difficult to measure accurately the distance a 
 body will fall in one second. We have, therefore, been obliged 
 to resort to other means for measuring the force of attraction 
 of the earth at the equator and other accessible parts of its sur- 
 face. The methods adopted are founded on the pendulum, which 
 is at the same time the simplest and one of the most useful 
 of philosophical instruments. The ideal pendulum is a small and 
 heavy weight suspended from a fixed point by a fine and flexible 
 wire. If we draw the pendulum aside from its vertical position 
 and then release it, the weight will swing to and fro. 
 
 For its journey to and fro the pendulum requires a certain 
 amount of time,, but this time does not appreciably depend on the 
 length of the arc through which the pendulum swings. To 
 verify this very remarkable law, suspend another pendulum 
 beside the first, both being of the same length; draw both pen- 
 dulums aside and release them : they swing together and return 
 together. This might have been expected. But if we draw 
 one pendulum a great deal to one side, and the other only a 
 little, the two pendulums still swing sympathetically. This, 
 perhaps, would not have been expected. Try it again, with even 
 
THE EARTH. 177 
 
 a still greater difference in the arc of vibration, and still we see 
 the two weights occupy the same time for the swing. We can 
 vary the experiment in another way. Let us change the weights 
 on the pendulums, so that they are of unequal size, though both of 
 iron. Shall we find any difference in the periods of vibration? 
 We try again : the period is the same as before ; swing them 
 through different arcs, large or small, the period is still the same. 
 But it may be said that this is due to both weights being of the 
 same material. Try it again, using a leaden weight instead of one 
 of the iron weights ; the result is identical. Even with a ball of 
 wood the period of oscillation is the same as that of the ball of 
 iron, and this is true no matter what be the arc through which the 
 vibration takes place. 
 
 If, however, we change the length of the wire by which the 
 weight is supported, then the period will not remain unchanged. 
 This can be very easily illustrated. Take a short pendulum with a 
 wire only one-fourth of the length of the long pendulum ; suspend 
 the two close together, and compare the periods of vibration of the 
 short pendulum with that of the long one, and we find that the 
 short pendulum has a period only half that of the long one. We 
 may state the result generally, and say that the time of vibration 
 of a pendulum is proportional to the square root of its length. If 
 we quadruple the length of the pendulum, we double the time of its 
 vibration ; if we increase the length of the pendulum ninefold, we 
 increase its period of vibration threefold. 
 
 It is the gravitation of the earth which makes the pendulum 
 swing. The greater the gravitation, the more rapidly will the 
 pendulum oscillate. This will be easily accounted for. If the earth 
 pulls the weight down very quickly, the time will be very short ; if 
 the power o the earth's attraction be lessened, then it cannot pull 
 the weight down so quickly, and the period will be lengthened. 
 
 It is possible to determine the time of vibration of the pen- 
 dulum with great accuracy. Let it swing for 10,000 oscil- 
 lations, and measure the time that these oscillations have con- 
 sumed. The arc through which the pendulum swings may not be 
 quite constant, but, as we have seen, this does not appreciably 
 II 
 
178 THE STORY OF THE HEAVENS. 
 
 affect the time of its oscillation. Suppose, then, that an error of 
 a second is made in the determination of the time of 10,000 oscil- 
 lations ; this will only entail an error of the ten-thousandth part of 
 the second in the time of a single oscillation, and will give a corre- 
 spondingly accurate determination of the gravity. 
 
 Take a pendulum to the equator. Let it perform 10,000 oscil- 
 lations, and determine carefully the time that these oscillations 
 have required. Bring the same pendulum to another part of the 
 earth, and repeat the experiment. We have thus a means of com- 
 paring the gravitation at the two places. There are no doubt a 
 multitude of precautions to be observed which need not here con- 
 cern us. We need not here enter into details as to the manner in 
 which the motion of the pendulum is to be sustained, nor as to the 
 effects of changes of temperature in the alteration of its length. It 
 will suffice for us to see how the time of the pendulum's swing 
 can be measured accurately, and how from that measurement the 
 intensity of gravitation can be calculated. 
 
 The pendulum thus enables us to make, with the highest degree 
 of accuracy, a gravitational survey of the surface of the earth. We 
 cannot, however, infer that gravity alone affects the oscillations of 
 the pendulum. We have seen how the earth rotates on its axis, 
 and we have attributed to this rotation the bulging of the earth at 
 the equator. But the centrifugal force arising from the rotation 
 affects bodies on the surface of the earth. It has the effect of 
 decreasing the weight of bodies, and this effect is greatest at the 
 equator, and lessens gradually as we approach the poles. From 
 this cause alone the attraction of the pendulum at the equator is 
 less than elsewhere, and therefore the oscillations of the pendulum 
 there will take a longer time than at other localities. A part of 
 the apparent change in gravitation is thus merely due to the centri- 
 fugal force ; but there is, besides this, a real change in gravitation. 
 At a hasty glance it might be thought that, as there was a pro- 
 tuberance of matter at the equator, there ought to be a greater 
 attraction at the equator than elsewhere. This is not so. The 
 effect of the additional matter is more than compensated by the 
 greater distance of the pendulum from the centre of the earth. 
 
THE EARTH. 179 
 
 Indeed, a moment's reflection will show that the pendulum at the 
 pole is, on the whole, nearer to the mass of the earth generally than 
 the pendulum is at the equator. It shows, in a very marked way, 
 how the researches in different branches of science are interwoven 
 when we find that, by merely swinging- a pendulum at different 
 parts of the earth, we are enabled to determine its shape as ac- 
 curately as by the elaborate measurements of the arcs of the 
 meridian made at different parts of the earth. 
 
 We defer to a future chapter the important phenomenon of the 
 earth's movement, which is known as " precession/' In a work 
 on astronomy, it does not come within our scope to enter into 
 further detail on the subject of our planet. The surface of the 
 earth, its contour and its oceans, its mountain chains and its rivers, 
 are for the physical geographer ; while its rocks and their contents, 
 its volcanoes and its earthquakes, are to be studied by the geologists 
 and the physicists. 
 
CHAPTER X. 
 
 MAES. 
 
 Our nearer Neighbours in the Heavens Surface of Mars can be Examined in the 
 Telescope Remarkable Orbit of Mars Resemblance of Mars to a Star 
 Meaning of Opposition The Eccentricity of the Orbit of Mars Different 
 Oppositions of Mars Apparent Movements of the Planet Effect of the Earth's 
 Movement Measurement of the Distance of Mars Theoretical Investigation 
 of the Sun's Distance Drawings of the Planet Is there Snow on Mars ? 
 The Rotation of the Planet Gravitation on Mars Has Mars any Satellites ? 
 Mr. Asaph Hall's great Discovery The Revolutions of the Satellites Deimos 
 and Phobos Gulliver's Travels. 
 
 THE special relation in which we stand to one planet of our 
 system has necessitated a somewhat different treatment of that 
 planet from the treatment appropriate to the others. We discussed 
 Mercury and Venus as distant objects known chiefly by telescopic 
 research, and by calculations of which astronomical observations 
 were the foundation. Our knowledge of the earth is of a different 
 character, and attained in a different way. Yet it was necessary for 
 symmetry that we should discuss the earth after the planet Venus, 
 in order to give to the earth its true position in the solar system. 
 But now that the earth has been passed in our outward progress 
 from the sun, we come to the planet Mars ; and here again we 
 resume, though in a somewhat modified form, the methods that 
 were appropriate to Venus and to Mercury. 
 
 Venus and Mars have, from one point of view, quite peculiar 
 claims on our attention. They are our nearest planetary neigh- 
 bours, on either side. We may naturally expect to learn more 
 of them than of the other planets farther off. In the case of 
 Venus, however, this anticipation can hardly be realised, for, as we 
 have already pointed out, its dazzling brilliancy prevents us from 
 making a really satisfactory telescopic examination. W^hen we turn 
 
MAES. IBi 
 
 to our other planetary neighbour, Mars, we are enabled to learn a 
 good deal with regard to his appearance. Indeed, with the excep- 
 tion of the moon, we are better acquainted with the details of the 
 surface of Mars than with those of any other celestial body. 
 
 This very beautiful planet offers many features for consideration 
 besides those presented by its physical structure. The orbit of 
 Mars is a very remarkable one, and it was by the observations 
 of this orbit that the celebrated laws of Kepler were discovered. 
 During the occasional approaches of Mars to the earth, it has been 
 possible to measure its distance with accuracy, and thus another 
 method of finding the sun's distance has arisen, which, to say the 
 least, may compete in precision with that afforded by the transit 
 of Venus ; and it must also be observed that the greatest achieve- 
 ment in pure telescopic research which this century has witnessed 
 was that of the discovery of the satellites of Mars. 
 
 To the unaided eye, Mars generally appears like a star of the 
 first magnitude. It is usually to be distinguished by its ruddy 
 colour, but the beginner in astronomy cannot rely, for the identi- 
 fication of Mars, on its colour only. There are several stars nearly, 
 if not quite, as ruddy as Mars. This planet has often been mis- 
 taken for the bright star Aldebaran, the brightest star in the 
 constellation of the Bull. It often resembles Betelgueze, a very 
 brilliant point in the constellation of Orion. Mistakes of this 
 kind will be impossible if the beginner has first learned the principal 
 constellations and the principal bright stars. He will then find 
 great interest in tracing out the positions of the planets, and in 
 watching their ceaseless movements. The position of each planet at 
 any time can always be ascertained from the almanac. Sometimes 
 the planet will be too near the sun to be visible. It will rise with 
 the sun and set with the sun, and consequently will not be above 
 the horizon during the night. The best time for seeing one of the 
 planets exterior to the earth will be during what is called its opposi- 
 tion. The opposition of Mars occurs when the earth comes directly 
 between Mars and the sun. In this case, the distance from Mars 
 to the earth is less than at any other time. There is also another 
 advantage in viewing Mars during opposition. The planet is then 
 
182 
 
 THE STORY OF THE HEAVENS. 
 
 at one side of the earth and the sun at the opposite side, so that 
 when Mars is high in the heavens the sun is directly beneath the 
 earth ; in other words, the planet is then at its greatest elevation 
 above the horizon at midnight. Some oppositions of Mars are, 
 
 Fig. 45. The orbits of the Earth and of Mars, showing the favourable Opposition 
 
 of 1877. 
 
 however, much more favourable than others. This is distinctly 
 shown in Fig. 45, which represents the orbit of Mars and the orbit 
 of the Earth accurately drawn to scale. It will be seen that while 
 the orbit of the earth is very nearly circular, the orbit of Mars has a 
 very decided degree of eccentricity ; indeed, with the exception of the 
 orbit of Mercury, that of Mars has the greatest eccentricity of any 
 
MARS. 183 
 
 orbit of the larger planets in our system. It will thus be observed that 
 the value of an opposition of Mars for telescopic purposes will vary 
 greatly. The favourable oppositions will be those which occur as 
 near as possible to the 26th of August. The opposite extreme 
 will be found in an opposition which occurs near the 22nd of 
 February. In the latter case the distance between the planet and 
 the earth is nearly twice as great as the former. The last favourable 
 opposition occurred in the year 1877. During that opposition, Mars 
 was a magnificent object, and received much and deserved atten- 
 tion. The favourable oppositions follow each other at somewhat 
 irregular intervals ; the next will occur in the year 1892, and 
 another in the year 1909. 
 
 The apparent movements of Mars are by no means simple. We 
 can imagine the embarrassment of the early astronomer who first 
 undertook the task of observing or attempting to decipher these 
 movements. The planet is seen to be a brilliant and conspicuous 
 object. It attracts the astronomer's attention ; he looks carefully, 
 and he sees how the planet lies among the constellations with which 
 he is familiar. A few nights later he looks at the planet again ; 
 but is it exactly in the same place ? He thinks not. He notes more 
 carefully than before the place of the planet. He sees how it is 
 situated with regard to the stars. Again, in a few days, his obser- 
 vations are repeated. There is no longer a trace of doubt about the 
 matter Mars has decidedly changed his position. It is a planet, 
 a wanderer. Night after night the primitive astronomer is at his 
 post. He notes the changes of Mars. He sees that it is now moving 
 even more rapidly than it was at first. Is it going to complete the 
 circuit of the heavens ? The astronomer determines to watch the 
 planet, and see whether this surmise is justified. He pursues his task 
 night after night, and at length he begins to think that the planet 
 is not moving quite so rapidly as at first. A few nights more, and he 
 is sure of the fact : the planet is moving more slowly. Again a few 
 nights more, and he begins to surmise that the motion may cease; 
 after a short time the motion does cease, and the planet seems to 
 rest; but is it going to remain at rest for ever ? Has its long journey 
 been finished ? For many nights this seems to be the case, but at 
 
184 
 
 THE STORY OF THE HEAVENS. 
 
 length the astronomer begins to suspect that the planet must be 
 commencing to move backwards. A few nights more, and the 
 fact is confirmed beyond possibility of doubt, and the extraordinary 
 discovery of the direct and the retrograde movement of Mars has 
 been accomplished. 
 
 In the greater part of its journey around the heavens, Mars 
 seems to move steadily from the west to the east. It moves 
 
 Fig. 46. The apparent movements of Mars in 1877. 
 
 backwards, in fact, as the moon moves and as the sun moves. It 
 is only during a comparatively small part of its path that these 
 complex movements are accomplished which presented such an 
 enigma to the primitive observer. We have in the adjoining 
 picture the track of the actual journey which Mars accomplished in 
 the opposition of 1877. The figure only shows that part of its path 
 which presents the anomalous features ; the rest of the path is 
 pursued, not indeed with uniformity of velocity, but with uni- 
 formity of direction. 
 
 
MAES. 185 
 
 This complexity of the apparent movements of Mars seems, 
 at first sight, fatal to the acceptance of any really simple and 
 elementary explanation of the planetary motion. If the motion of 
 Mars were purely elliptic, how, it may well be said, could it perform 
 this extraordinary evolution ? The explanation is to be sought in 
 the fact that the earth on which we stand is moving. Even if 
 Mars were at rest, the fact that our earth is moving would make 
 Mars appear to move. The apparent movements of Mars are 
 thus combined with the real movements. This circumstance will 
 not embarrass the geometer. He is able to disentangle the true 
 movement of the planet from its association with the apparent 
 movement, and to account completely for the very complicated 
 observed movement of Mars. Could we transfer our point of view 
 from the ever-shifting earth to an immovable standpoint like the 
 sun, we should then see .that the true shape of the orbit of Mars 
 was an ellipse, performed by revolving round the sun in conformity 
 with the laws of Kepler, discovered by observations on this planet. 
 
 Under the most favourable circumstances, the planet Mars, at 
 the time of opposition, may approach the earth to a distance not 
 greater than about 35,000,000 miles. No doubt this seems an 
 enormous distance, when estimated by any standard adapted for 
 terrestrial measurements ; it is, however, hardly greater than the dis- 
 tance of Venus when nearest, and it is much less than the distance 
 from the earth to the sun. We have already explained how the form 
 of the solar system is known from Kepler's laws, and how the 
 absolute size of the system and of its various parts can be known 
 when the direct measurement of any one part has been accomplished. 
 A close approach of Mars affords a favourable opportunity for 
 measuring his distance, and thus, in a different way, solving the 
 same problem as that presented in the transit of Venus. We are 
 thus a second time led to a knowledge of the distance of the sun 
 and the distances of the planets generally, and to many other 
 numerical facts about the solar system. 
 
 A very elaborate and successful attempt was made on the 
 occasion of the opposition of Mars in 1877 to apply this refined 
 method to the solution of the great problem. It cannot be said 
 
186 THE STORY OF THE HEAVENS. 
 
 to have been the first occasion on which this method was suggested, 
 or even practically attempted. But the observations and the dis- 
 cussions of 1877 were conducted with a degree of skill and a minute 
 attention to all the necessary precautions which render them an 
 important contribution to astronomy. Mr. David Gill, now Her 
 Majesty's Astronomer at the Cape of Good Hope, undertook a 
 journey to the Island of Ascension for the purpose of observing the 
 parallax of Mars in 1877. On this occasion, Mars approached to^ 
 the earth so closely as to afford an admirable opportunity for the 
 application of the method. Mr. Gill succeeded in obtaining a most 
 valuable series of measurements, and from them he concluded the 
 distance of the sun with an accuracy not inferior to that attainable 
 by the transit of Venus. 
 
 There is yet another method by which Mars can be made to give 
 us information as to the distance of the sun. This method is one 
 of very great beauty, and is deeply interesting from its connection 
 with some of the loftiest mathematical enquiries. It was fore- 
 shadowed in the Dynamical theory of Newton, and was wrought to 
 perfection by Le Verrier. It is based upon the great law of gravita- 
 tion, and is intimately associated with the splendid discoveries in 
 planetary perturbation which form so striking a chapter in modern 
 astronomical discovery. 
 
 There is a certain relation known between two quantities which 
 at first sight seem quite independent. These quantities are the 
 mass of the earth and the distance of the sun. It follows, from 
 the measurements of the intensity of gravitation on the earth's 
 surface, and from the relative movements of the sun and of the 
 earth, that the sun's parallax is proportional to the cube root 
 of the mass of the earth. There is no uncertainty about this 
 result, and the consequence is obvious. If we have the means of 
 weighing the earth in comparison with the sun, then the distance 
 of the sun can be immediately deduced. How, then, are we to 
 place our great earth in the weighing scales ? This is the problem 
 which Le Verrier has shown us how to solve, and he does so by 
 invoking the aid of the planet Mars. 
 
 If Mars in his revolution around the sun were solely swayed by 
 
MARS. 187 
 
 the attraction of the sun, he would, in accordance with the well- 
 known laws of planetary motion, follow for ever the same elliptic 
 path. At the end of one century, or even of many centuries, the 
 shape, the size, and the position of that ellipse would remain un- 
 altered. Fortunately for our present purpose, a disturbance in the 
 orhit of Mars is produced by the earth. Although the mass of the 
 earth is so much less than that of the sun, yet the earth is still 
 large enough to exercise an appreciable attraction on Mars. The 
 ellipse described by Mars is consequently not always the same. The 
 shape of that ellipse and its position gradually change, so that the 
 position of the planet depends upon the mass of the earth. The 
 place in which the planet is found can be determined by observa- 
 tion ; the place which the planet would have had if the earth were 
 absent can be found by calculation. The difference between the 
 two is due to the attraction of the earth, and, when it has been 
 measured, the mass of the earth can be ascertained. The amount 
 of displacement increases from one century to another, but as the 
 rate of growth is small, ancient observations are necessary to enable 
 the measures to be made with accuracy. 
 
 A remarkable occurrence which took place more than two 
 centuries ago enables the place of Mars to be determined with 
 great precision at that date. On the 1st of October, 1672, three 
 independent observers witnessed the occultation of a star in 
 Aquarius by the planet Mars. The place of the star is known 
 with accuracy, and hence we are provided with the means of 
 accurately defining the point in the heavens occupied by Mars on 
 the day in question. From this result, combined with the modern 
 meridian observations, we learn that the displacement of Mars by 
 the attraction of the earth has, in the lapse of two centuries, grown 
 to about five minutes of arc (294 seconds) . It has been maintained 
 that this cannot be erroneous to the extent of more than a second, 
 and hence it would follow that the earth's mass is determined to 
 about one three-hundredth part of its amount. If no error were 
 present this would give the sun's distance to about one nine-hun- 
 dredth part. Notwithstanding the intrinsic beauty of this method, 
 and the very high auspices under which it has been introduced, it 
 
188 THE STORY OF THE HEAVENS. 
 
 will, we think, hardly be worthy of reliance in comparison with some 
 of the other methods. We make no impeachment of the fidelity 
 of the observations ; and we feel no doubt that the displacement of 
 the planet is mainly, if not entirely, due to the disturbing- effect of 
 the earth's attraction ; but it seems quite impossible to be sure that 
 some other cause, minute though it must be, may not also have 
 contributed to the result. We cannot be absolutely sure that the 
 theory is above suspicion. Interesting and beautiful though this 
 method may be, we must rather regard it as a striking confirmation 
 of the law of gravitation, than as affording an accurate means of 
 measuring the sun's distance. 
 
 Fig. 47. Relative sizes of Mars and the Earth. 
 
 The close approaches of Mars to the earth afford us opportunities 
 for making a careful telescopic scrutiny of his surface. It must 
 not be expected that the details on Mars could be delineated with the 
 same minuteness as those on the moon. Even under the most favour- 
 able circumstances, Mars is still more than a hundred times as far 
 as the moon, and, therefore, the features on Mars have to be at least 
 one hundred times as large if they are to be seen as distinctly as 
 the features on the moon. Mars is very much smaller than the 
 earth. The diameter of the planet is 4,200 miles, but little more 
 than half that of the earth. Fig. 47 shows the comparative sizes 
 of the two bodies. 
 
 We here present to the reader two drawings of the tele- 
 scopic appearance of the planet Mars. They were drawn by 
 the late Mr. Charles E. Burton,* who employed in the work a 
 
 * " Transactions of the Royal Dublin Society," 1880. 
 
MARS. 189 
 
 telescope of moderate dimensions though of great optical perfection. 
 It seems hard to decline the suggestion that these markings on 
 Mars may really correspond to the divisions of land and water on 
 that planet. There are circumstances which strongly suggest that 
 water may be present on Mars. At the poles of Mars are large 
 white regions, shown to some extent on Mr. Burton's drawings, 
 which undergo periodic changes ; and it has been surmised that 
 these are due to an accumulation of ice or of snow on the polar 
 regions of the planet. On some occasions^ indeed, an te ice-cap " 
 
 May 12, 1S71. April 11, 1873. 
 
 Fig. 48. Views of Mars by C. E. Burton. 
 
 on Mars, with its brilliancy and its sharply-defined margin, is a 
 most striking feature in the telescopic view of the planet. 
 
 In making an examination of the planet, it is to be observed 
 that it does not, like the moon, always present the same face 
 towards the telescope. Mars rotates upon an axis in exactly the same 
 manner as the earth. It is, indeed, not a little remarkable that the 
 period required by Mars for one complete rotation is only about half 
 an hour more than the period of rotation of the earth. The exact 
 period is 24 hours, 37 minutes, 22f seconds (Proctor). It, therefore, 
 follows that the aspect of the planet changes from hour to hour ; 
 the western side gradually sinks from view, the eastern side 
 gradually rises into view. In twelve hours the aspect of the planet 
 is completely changed. These changes, together with the inevitable 
 effects of foreshortening, render it often difficult to identify 
 
190 THE STOEY OF THE HEAVENS. 
 
 the objects on the planet with those on the map. The maps, it 
 must be confessed, fall very far short of the maps of the moon in 
 definiteness and in certainty ; yet there can hardly be a doubt that 
 the main features of the planet are to be regarded as thoroughly 
 established, and some astronomers have given names to a-i the 
 prominent objects. 
 
 On the question as to the possibility of life on Mars, a few 
 words may be added. If we could be certain of the existence of 
 water on Mars, then one of the most fundamental conditions would 
 be fulfilled; and even though the atmosphere on Mars had but few 
 points of resemblance in composition or in density to the atmosphere 
 of the earth, life might still be possible. Yet even if we could 
 suppose that a man would find suitable nutriment for his body and 
 suitable air for his respiration, it yet seems very doubtful whether 
 he would be able to live. Owing to the small size of Mars and 
 the smallness of its mass in comparison with the earth, the intensity 
 of the gravitation on the surface of Mars would be very different 
 to the gravitation on the surface of the earth. We have already 
 alluded to the small gravitation on the moon, and in a lesser 
 degree the same remarks will apply to Mars. A body which 
 weighs on the earth two pounds would on the surface of Mars 
 weigh only one pound. Nearly the same exertion which will raise 
 a 56 Ib. weight on the earth would lift two similar weights on 
 Mars. The effect of such changes on man would be indeed 
 remarkable. He would experience a buoyancy quite unfamiliar to 
 his terrestrial experience. His labours would no doubt be greatly 
 lightened under such circumstances. The fatigue of walking 
 would be reduced to one-half he would walk up two steps of stairs 
 at a time with the same exertion required here for one step. If 
 he could jump four feet high on the earth, he could with precisely 
 the same exertion jump. eight feet high on Mars. Yet, notwith-, 
 standing all this, it may well be doubted whether our organism 
 would be adapted to a change so radical and so complete. We 
 should imagine that the circulation of the blood and the various 
 movements which constitute life would be greatly, and probably 
 even fatally, deranged by any such changes. The effect on our system 
 
MARS. 191 
 
 which would arise from a great diminution of gravity may be 
 illustrated in the following way : In the act of jumping from a 
 chair to the ground, our body during the act of jumping is to a 
 certain extent in the condition in which it would be if we were 
 resident on a planet which possessed no power of attraction. A 
 moment's reflection will show this to be the case. There is no 
 doubt that a man could not sustain a ton weight in his hand while 
 he stood on the earth, yet, while jumping down, a ton or any 
 other weight could be held in the hand; for, as it could but 
 fall, and as he is falling simultaneously, the exertion of holding 
 it in the hand vanishes. During the act of falling, therefore, the 
 weight is held in the hand with no exertion, and this is what we 
 should find if we were standing on a planet devoid of attracting 
 power. 
 
 The earth is attended by one moon. Jupiter is attended by 
 four moons. Mars is a planet revolving between the orbits 
 of the earth and of Jupiter. It is a body of the same general 
 type as the earth and Jupiter. It is ruled by the same sun, and all 
 three planets form part of the same system ; but as the earth has 
 one moon and Jupiter four moons, why should not Mars also have 
 a moon ? No doubt Mars is a small body, less even than the earth, 
 and vastly less than Jupiter. We could not expect Mars to have 
 large moons, but why should it be unlike its two neighbours, 
 and not have any moon at all ? So reasoned astronomers, but yet 
 until recently no satellite of Mars could be found. For centuries 
 the planet has been diligently examined with this special object, and 
 as failure after failure came to be recorded, the conclusion seemed 
 almost to be justified that the chain of analogical reasoning had 
 broken down. The moonless Mars was thought to be an exception 
 to the rule, that all the great planets outside Venus were dignified 
 by an attendant retinue of satellites. 
 
 It seemed almost hopeless to begin again a research which 
 had so often been tried, and had so invariably led to disappoint- 
 ment ; yet, fortunately, the present^ generation has witnessed still 
 one more attack, conducted with consummate skill, and with the 
 most perfect equipment. This attempt has obtained the success it 
 
192 THE STORY OF THE HEAVENS. 
 
 so well merited, and the result has been the memorable discovery 
 of two satellites of Mars. 
 
 This, perhaps the most important telescopic discovery of this 
 century, was made by Professor Asaph Hall, the distinguished as- 
 tronomer,, at the observatory of Washington. Mr. Hall was pro- 
 vided with an instrument of colossal proportions and of exquisite 
 workmanship, known as the great Washington refractor. It is 
 the product of the celebrated workshop of Messrs. Alvan Clark 
 and Sons, from which so many large telescopes have proceeded, 
 and in its noble proportions far outstripped any other telescope 
 ever devoted to the same research. The object-glass measures 
 twenty-six inches in diameter, and is hardly less remarkable for the 
 perfection of its definition than for its size. But even the skill of 
 Mr. Hall, and the space-penetrating power of his telescope, would 
 not on ordinary occasions have been able to discover the satellites 
 of Mars. Advantage was prudently taken of that memorable 
 opposition of Mars in 1877, when, as we have already described, 
 the planet came unusually near the earth. 
 
 It was known that Mars could not have a large moon. Had 
 Mars been attended by a moon which was one-hundredth part of 
 the bulk of our moon, it must long ago have been discovered. 
 Mr. Hall, therefore, knew that if there were any satellites to 
 Mars, they must be extremely small, and he braced himself for a 
 severe and diligent search. The circumstances were all favourable. 
 Not only was Mars as near as it could well be to the earth ; not 
 only was the great telescope at Washington the most powerful 
 refractor then in existence; but the situation of Washington is 
 such, that Mars was seen from the observatory at a high elevation 
 in a pure air. It was while the British Association were meeting 
 at Plymouth in 1877, that a telegram flashed across the Atlantic. 
 Brilliant success had rewarded Mr. Hall's efforts. He had hoped 
 to discover one satellite. The discovery of even one would have 
 made the whole scientific world ring ; but fortune smiled on Mr, 
 Hall. He discovered first one satellite, and then he discovered 
 a second; and in connection with these satellites, he further dis- 
 covered a unique fact in the solar system. 
 
MARS. 193 
 
 The outer of the satellites revolves around the planet in the 
 period of 30 hours, 17 min., 54 sees.; it is the inner satellite which 
 has commanded the attention and curiosity of every astronomer in 
 the world. Mars turns round on his axis in a martial day, which 
 is very nearly the same length as our day of twenty-four hours. 
 The inner satellite of Mars moves round in 7 hours, 39 min., 
 14 sees ! The inner satellite, in fact, revolves three times round 
 Mars in the same time that Mars can turn round once. This 
 circumstance is unparalleled in the solar system ; indeed, as far 
 as we know, it is unparalleled in the universe. In the case of our 
 own planet, the earth rotates twenty-seven times for one revolu- 
 tion of the moon. To some extent the same may be said of 
 Jupiter and of Saturn, while in the great system of the sun him- 
 self and the planets, the sun rotates on his axis several times for 
 each revolution of the planets. There is no other known case 
 where the satellite revolves around the primary more quickly than 
 the primary rotates on its axis. The anomalous movement of the 
 satellite of Mars has, however, been accounted for. In a sub- 
 sequent chapter we shall again allude to this, as it is connected with 
 an important department of modern astronomy. 
 
 It will be interesting to endeavour to obtain some notion of the 
 probable size of these two satellites of Mars. The average light 
 of our full moon has been found, by actual measurement, to be 
 24,000 times that of Vega, a bright star of the first magnitude. 
 Were our moon removed to the distance of Mars when the planet 
 is nearest to us, due allowance being made for the enfeeblement of 
 the solar light, the moon would still appear to us more than half 
 as bright (0*5887) as a first magnitude star. But Deimos was 
 estimated to be no brighter than a star of the twelfth magni- 
 tude, of which it takes about 25,120 to give as much light 
 as Vega. From this it is evident that if Deimos is made of 
 materials similar to our moon, its face must have an area of about 
 one 15,000th part of the moon's, or it must be about 18 miles in 
 diameter. Phobos is brighter by about half a magnitude, and 
 would require a diameter of 22J miles of moon material in order 
 to reflect the light we actually receive. But Phobos is so close to 
 
194 TEE STORY OF THE HEAVENS. 
 
 Mars that, notwithstanding- its smallness, it will seem, when seen 
 overhead from the surface of the planet, to be about two-thirds the 
 size of our moon. The more distant and smaller satellite will 
 appear but about a quarter of the size of its rival. 
 
 As the satellites revolve in paths vertically above the equator of 
 their primary, the one less than 4,000 miles and the other only 
 some 14,500 miles above the surface, it follows that they can 
 never be visible from the poles of Mars ; indeed, to see Phobos, 
 the observer's planetary latitude must not be above 68f , or the 
 satellite will be hidden by the body of Mars, just as we, in the 
 British Islands, would be unable to see an object revolving round 
 the earth a few hundred miles above the equator. For the same 
 reason Phobos will only be rather more than one-third of each 
 revolution above the horizon of a spectator on the planet. 
 
 Before passing from the attractive subject of the satellites, we 
 may just mention two points of a literary character. Mr. Hall 
 consulted his classical friends as to the names which should be 
 conferred on the two satellites. Homer was referred to, and a 
 passage in the " Iliad " suggested the names Deimos and Phobos. 
 These personages were the attendants of Mars, and the lines in 
 which they occur have been thus construed by my friend Professor 
 Tyrrell : 
 
 " Mars spake, and called Dismay and Rout 
 To yoke his steeds, and he did on his harness sheen." 
 
 A curious circumstance with respect to the satellites of Mars 
 will be familiar to those who are acquainted with ' l Gulliver's 
 Travels." The astronomers on board the flying Island of Laputa 
 had, according to Gulliver, keen vision and good telescopes. 
 Gulliver announced that they had found two satellites to Mars, one 
 of which revolves round Mars in ten hours. The author has thus 
 not only made a correct guess about the number of the satellites, 
 but he actually stated the periodic time with considerable accuracy ! 
 We do not know what can have suggested the latter guess. A few 
 years ago any astronomer reading the voyage to Laputa would have 
 said this was absurd. There might be two satellites to Mars no 
 
MARS. 195 
 
 doubt ; but to say that one of them goes round in ten hours would 
 be to assert what was " impossible \" Yet the truth has been even 
 stranger than fiction. 
 
 And now we must bring to a close our account of this beautiful 
 and interesting planet. There are many additional features of it 
 over which we are tempted to linger, but there are so many other 
 bodies which claim our attention in the solar system, so many other 
 bodies which exceed Mars in size and intrinsic importance, that we 
 are obliged to desist. Our next step will not, however, at once 
 conduct us to the giant planets. We find outside Mars a host of 
 planets, small indeed, but of the greatest interest; and with these 
 we shall find abundant occupation for the following chapter. 
 
CHAPTER XL 
 
 THE MINOR PLANETS. 
 
 The Lesser Members of our System Bode's Law The Vacant Region in the 
 Planetary System The Eesearch The Discovery of Piazzi Was the small 
 Body a Planet ? The Planet hecomes Invisible Gauss undertakes the Search 
 by Mathematics The Planet Eecovered Further Discoveries Number of 
 Minor Planets now known The Region to be Searched The Construction of 
 the Chart for the Search for Small Planets How a Minor Planet is Discovered 
 Physical Nature of the Minor Planets Small Gravitation on the Minor 
 Planets The Berlin Computations How the Minor Planets tell us the 
 Distance of the Sun Accuracy of the Observations How they may be Multi- 
 pliedVictoria and Sappho The most perfect Method. 
 
 IN our chapters on the Sun and Moon, on the Earth and Venus, and 
 on Mercury and Mars, we have been usually discussing the features 
 and the movements of globes of vast dimensions. The least of 
 all these bodies is the moon, but even our moon is a ball 2,000 
 miles from one side to the other. In approaching the subject of 
 the minor planets,, we must be prepared to find planets of dimensions 
 quite inconsiderable in comparison with the great globes of our 
 system. No doubt these minor planets are all of them some few 
 miles, and some of them a great many miles, in diameter. Were 
 they close to the earth they would be conspicuous, and even 
 splendid objects ; but as they are so distant they do not, even in 
 our greatest telescopes, become very remarkable, while to the 
 unaided eye they are totally invisible. 
 
 In a diagram of the orbits of the various planets, it is 
 shown that a wide space exists between the orbit of Mars and the 
 orbit of Jupiter. It was often surmised that this wide region mus 
 be tenanted by some other planet. The presumption became mucl 
 stronger when a remarkable law was discovered which exhibited, 
 with considerable accuracy, the relative distances of the great 
 planets of our system. Take the series of numbers 0, 3, 6, 12, 24, 
 
 
THE MINOR PLANETS. 197 
 
 48, 96, whereof each number (except the second) is double of the 
 number which precedes it. If we now add four to each, we have 
 the series 4, 7, 10, 16, 28, 52, 100. With the exception of the 
 fifth of these numbers (28) they are all sensibly proportional to the 
 distances of the various planets from the sun. In fact, the dis- 
 tances are as follows: Mercury, 3*9; Venus, 7*2; Earth, 10; 
 Mars, 15'2; Jupiter, 52'9 ; Saturn, 95*4. Although we have no 
 physical reason to offer why this law known as Bode's law should 
 be true, yet the fact that it is so nearly true in the case of all the 
 known planets, tempts us to ask whether there may not also be a 
 planet revolving around the sun at the distance represented by 28. 
 
 So strongly was this felt that a large number of astronomers 
 decided, at the end of the last century, to make a united effort to 
 search for the unknown planet. It seemed certain that the planet 
 could not be large, otherwise it would have been found long 
 ago. If it should exist, then means must be found for dis- 
 criminating between the planet and the hosts of stars strewn along 
 its path. 
 
 The search for the small planet was soon rewarded by a success 
 which has rendered the evening of the first day in this century 
 memorable in astronomy. It was in the pure skies of Palermo 
 that the observatory was situated, where the memorable dis- 
 covery of the first minor planet was accomplished by Piazzi. This 
 laborious and accomplished astronomer had organised a most in- 
 genious system of exploring the heavens, which was eminently 
 calculated to discriminate a planet from among the stars. On a 
 certain night he would select a series of stars to the number of 
 fifty, more or less according to circumstances. With his meridian 
 circle he determined the places of all the fifty stars. The following 
 night, or at all events as soon as convenient, he re-observed the 
 whole fifty stars with the same instrument and in the same manner, 
 and the whole operation was repeated on two, or perhaps more, 
 nights subsequently. When the observations were compared 
 together he was in possession of some four or more places of each 
 one of the stars on four nights, and the whole series was complete. 
 He was persevering enough to carry on these observations for very 
 
193 THE STORY OF THE HEAVENS. 
 
 many groups of stars, and at length he was rewarded by a success 
 which amply compensated him for all his toil. It was on the 1st 
 of January, 1801, that he commenced for the one hundredth and 
 fifty-ninth time to observe a group of stars. Fifty stars this night 
 were viewed in his telescope, and their places were carefully 
 recorded. Of these objects the first twelve were undoubtedly stars, 
 and so to all appearance was the thirteenth, a star of the eighth 
 magnitude in the constellation of Taurus. There was nothing to 
 distinguish the telescopic appearance of this object from all the 
 stars which preceded or followed it. The following night Piazzi, 
 according to his custom, re-observed the whole fifty stars, and he 
 did the same again on the 3rd of January, and once again on the 
 4th. He then, as usual, brought together the four places he had 
 found for each of the stars on his list. When this was done it was 
 at once seen that the thirteenth object on the list was quite a 
 different body from the remainder and from all the other stars 
 which he had ever observed before. The four places of this 
 mysterious object were all different ; in other words, the object was 
 in movement, and was therefore a planet. 
 
 A few days' observation sufficed to show how this little object, 
 afterwards called Ceres, revolved around the sun, and how it circu- 
 lated in that vacant path intermediate between the path of Mars 
 and the path of Jupiter. Great, indeed, was the interest aroused 
 by this discovery, and great, indeed, is the influence which it has 
 exercised on the progress of astronomy. The majestic planets of 
 our system were now to admit a much more humble object to a 
 share of the benefits dispensed by the sun. 
 
 After Piazzi had obtained a few observations of the object, and 
 after he had fully identified its character as a planet, the season for 
 observing this part of the heavens had passed away, and the new 
 planet of course ceased to be visible. In a few months, no doubt, 
 the same part of the sky would be above the horizon after dark, and 
 the stars would of course be seen as before. The planet, how- 
 ever, was moving, and would continue to move, and by the time 
 the next season had arrived it would have passed off into some 
 distant region, and would be again confounded with the stars 
 
THE MINOR PLANETS. 199 
 
 which it so closely resembled. How, then, was the planet to be 
 pursued through its period of invisibility and identified when it 
 again came within reach of observation ? 
 
 This difficulty attracted the attention of astronomers, and 
 they sought for some method by which the place of the planet 
 could be recovered so as to prevent Piazzi's discovery from 
 falling into oblivion. A young German mathematician, whose 
 name was Gauss, opened his distinguished career by his suc- 
 cessful attempt to solve this problem. A planet, as we have 
 shown, describes an ellipse around the sun, and the sun lies at a 
 focus of that ellipse. It can be demonstrated that when three 
 positions of a planet are known, then the ellipse in which that 
 planet moves is completely determined. Piazzi had seen the planet 
 on at least three occasions, and he had on each occasion measured 
 the place which the planet occupied. This information was avail- 
 able to Gauss, and the problem which he had to solve may be thus 
 stated. Knowing the place of the planet on three nights, it is 
 required, without any further observations, to tell what the place 
 of the planet will be on a special occasion some months in the 
 future. Mathematical calculations, based on the laws of Kepler, 
 will enable this problem to be solved, and Gauss succeeded in solving 
 it. He showed that though the telescope of the astronomer was 
 unable to detect the planet during its long season of invisi- 
 bility, yet tbe pen of the mathematician could follow it with 
 unfailing certainty. When, therefore, the progress of the seasons 
 permitted the observations to be renewed, the search was recom- 
 menced. The telescope was directed to the point which Gauss's 
 calculations indicated, and there was the little Ceres. Ever since 
 its rediscovery, the planet has been so completely bound in the toils 
 of mathematical reasoning, that its place every night of the year 
 can be indicated with a fidelity approaching to that attainable with 
 the moon or with the great planets of our system. 
 
 The discovery of one minor planet was quickly followed by 
 similar discoveries, so that within seven years Pallas, Juno, and 
 Vesta were added to the Solar system. The orbits of all these 
 bodies lie in the region between the orbit of Mars and of Jupiter, 
 
200 
 
 THE STORY OF THE HEAVENS. 
 
 and for many years it seems to have been thought that our planetary 
 system was now complete. Forty years later the career of dis- 
 covery was again commenced. Planet after planet was added to 
 the list ; gradually the discoveries became a stream of increasing 
 volume, until in 1884* the total number of the known minor 
 
 Fig. 49. The Zone of Minor Planets between Mars and Jupiter. 
 
 planets exceeded 240. Their distribution in the solar system is 
 somewhat as represented in Fig. 49. 
 
 These numerous discoveries are not now accomplished exactly in 
 the same way as the original discovery of Piazzi. By the improve- 
 ment of astronomical telescopes, and by the devotion with which 
 certain astronomers have applied themselves to this interesting 
 research, a special method of observing has been created with the 
 distinct purpose of searching for these little objects. 
 
 It is known that the paths in which all the great planets move 
 
 
THE MINOR PLANETS. 201 
 
 through the heavens coincide very nearly with the path which the 
 sun appears to follow among the stars, and which is known as the 
 ecliptic. It is natural to assume that the small planets also move 
 in the same great highway, which leads them through all the signs 
 of the zodiac in succession. Some of the small planets, no doubt, 
 deviate rather widely from the track of the sun, but the great 
 majority are still tolerably near it. This consideration at once 
 simplifies the search, for the new planets. A certain zone extend- 
 ing around the heavens is to be examined, but there is little 
 advantage in pushing the examination into other parts of the sky. 
 
 The next step is to construct a map containing all the stars in 
 this region. This is a task of very great labour ; the stars visible 
 in the large telescopes are so numerous that many tens of thousands, 
 perhaps we should say hundreds of thousands, are included in the 
 region even so narrowly limited. The fact is that many of the 
 minor planets now discovered are objects of extreme minuteness ; 
 they can only be seen with very powerful telescopes, and for their 
 detection it is necessary to use charts on which even the faintest 
 stars are depicted. Many astronomers have concurred in the labour 
 of producing these charts. We cannot here mention them all. We 
 will therefore only allude to the last series which have appeared 
 they have been prepared by Professor Peters with exquisite care, and 
 as, in his hands, they have already served to enable him to discover 
 very many new planets, so now he has placed them in the hands of 
 astronomers at large who may be capable of carrying on this very 
 fascinating inquiry. 
 
 The astronomer about to seek for a new planet directs his 
 telescope towards that part of the sun's path which is on the 
 meridian at midnight; there, if anywhere, lies the chance of 
 success, because that is the region in which a planet is nearer to 
 the earth than at any other part of its course. Point by point he 
 compares his chart with the heavens, and usually he finds the stars 
 in the heavens and the stars in the chart to correspond; but 
 sometimes it will happen that a point in the heavens is missing 
 from the chart. His attention is at once arrested ; he follows the 
 object with care, and if it moves it is a planet. Still he cannot 
 
202 THE STORY OF THE HEAVENS. 
 
 be sure that he has really made a discovery ; he has found a planet 
 no doubt, but it may be one of the host of planets already known. 
 To clear up this point he must under take a further, and sometimes 
 a very laborious, inquiry ; he must trace out all the known planets 
 and see whether it is possible for one of them to have been in 
 the position on the night in question. If he can satisfy himself 
 that no known planet could have been there, he is then entitled 
 to announce to his brother astronomers the discovery of a new 
 planet. It seems certain that all the more important of the minor 
 planets have been long since discovered. The recent additions to 
 the list are generally extremely minute objects, beyond the powers 
 of small telescopes. 
 
 Of the physical nature of these bodies and of the character of 
 their surface we are entirely ignorant. It may be, for anything we 
 can tell, that these little bodies are miniature globes like our earth, 
 diversified by continents and by oceans. If there be life on such 
 bodies, whose diameter is often not very many miles, that life must 
 be something totally different to the life with which we are familiar. 
 Setting aside every other difficulty from the possible absence of 
 water and from the great improbability of an atmosphere of a 
 density and a composition suitable for respiration, the simple fact 
 of gravitation alone would present the most profound contrast to 
 the condition of life on the earth. 
 
 Let us attempt to illustrate this point, and suppose that we 
 take the case of a minor planet eight miles in diameter, or in round 
 numbers one-thousandth part of the diameter of the earth. If we 
 further suppose that the materials of the planet are the same as the 
 materials of the earth, it is easy to prove that the gravity on the 
 surface of the planet will be only one-thousandth part of the gravity 
 of the earth. It follows that the weight of an object on the earth 
 would be reduced to the thousandth part if that object were trans- 
 ferred to the planet. This would not be disclosed of course by an 
 ordinary weighing scales, where the weights are placed in one pan 
 and the body to be weighed in the other. Tested in this way a 
 body would of course weigh precisely the same anywhere ; for if 
 the gravitation of the body is altered, so is also in equal proportion 
 
 
 i 
 
THE MINOR PLANETS. 203 
 
 the gravitation of the weights which balance it. But weighed 
 with a spring balance the change would be at once evident, and the 
 effort with which a weight could be raised would be reduced to one- 
 thousandth part. A load of one thousand pounds could be lifted 
 from the surface of the planet by the same effort which would lift 
 one pound on the earth ; the effects which this would produce are 
 very remarkable. Let us instance one of them, which involves a 
 point of no little interest in some other branches of astronomy. 
 
 A stone thrown up into the air soon falls to the ground. A 
 rifle bullet fired straight up into the air will ascend higher and 
 higher, until at length its motion ceases, it begins to turn, and falls 
 down to the ground. Let us for the moment suppose that we had 
 a rifle of infinite strength and gunpowder of unlimited power. As 
 we increase the charge we find that the bullet will ascend higher 
 and higher, and each time it will take a longer period before it 
 returns to the ground. The return of the bullet to the ground 
 is, of course, due to the attraction of the earth. This attrac- 
 tion acts on the bullet at all times, and gradually lessens the 
 velocity, ultimately overcomes the upward motion, and then 
 brings the bullet back. It must be remembered that the 
 efficiency of the attraction decreases when the height is increased. 
 Consequently when the bullet has a prodigiously great initial 
 velocity, in consequence of which it ascends to an enormous height, 
 the return of the bullet is retarded by a twofold cause. In 
 the first place the distance through which it has to be recalled is 
 greatly increased, and in the second place the efficiency of gravita- 
 tion in effecting its recall has decreased. The greater the velocity, 
 the feebler must be the capacity of gravitation for bringing back 
 the body, and thus at length the velocity can be increased to that 
 point at which the gravitation, constantly declining as the body 
 ascends, is never able to entirely neutralise the velocity, and hence 
 we have the remarkable case of a body projected away never to 
 return. 
 
 It is possible to exhibit this reasoning in a numerical form, and 
 it can be shown that a velocity of six. miles a second vertically 
 upwards, would suffice to convey a body entirely away from the 
 
204 THE STORY OF TEE HEAVENS. 
 
 gravitation of the earth. This velocity is far beyond the utmost 
 limits of our artillery. It is, indeed, from twenty to thirty times as 
 swift as a cannon shot ; and even if we could produce it, the resistance 
 of the air would present an insuperable difficulty. Such reflections, 
 however, do not affect the conclusion that there is for each planet 
 a certain specific velocity appropriate to that planet, and depending 
 solely upon the size and the mass of the planet. 
 
 If we assume for the sake of simplicity that the materials of 
 the planets are similar, then the law which expresses the critical 
 velocity for each planet can be stated in an extremely simple form. 
 It is, in fact, simply proportional to the diameter of the planet. 
 For a minor planet whose diameter was one-thousandth part of 
 that of the earth, the critical velocity would be the thousandth 
 part of six miles a second that is about thirty feet per second. 
 This is a low velocity compared with ordinary standards. A child 
 can here toss a ball up fifteen or sixteen feet high, yet to carry 
 a ball up this height it must be projected with a velocity of thirty 
 feet per second. A child, standing upon a planet eight miles in 
 diameter, throws his ball vertically upwards ; up and up the ball 
 will soar to an amazing elevation. If the original velocity were 
 less than thirty feet per second the ball will at length gradually 
 cease moving, will begin to turn, will fall with gradually accele- 
 rating velocity, until at length it returns with the same velocity 
 with which it had been projected. But if the original velocity 
 had been as much or more than thirty feet per second, then the 
 ball would soar up and up never to return. In a future chapter it 
 will be necessary for us to refer again to this subject. 
 
 The atmosphere surrounding a small planet must, under 
 ordinary circumstances, be one of very great rarity. Even sup- 
 posing that the quantity of air over each square foot on the small 
 planet was as much as that over each square foot on the earth, yet 
 the difference in gravitation would cause the widest difference in 
 the density at the surface. It is the weight of the superincumbent 
 air which presses on the lower strata and gives them their density. 
 If the gravitation were reduced, then the pressure would be lessened 
 and the air would expand. We should therefore expect to find that 
 
 
TEE MINOR PLANETS. 205 
 
 the atmosphere surrounding a small planet is of extreme rarity, 
 though possibly of enormous volume. 
 
 A great increase in the number of minor planets has re- 
 warded the zeal of those astronomers who have devoted their 
 labours to this subject. Their success has entailed a vast amount 
 of labour on the computers of the "Berlin Year-book/' That 
 useful work in this respect occupies a position which has not been 
 taken by our own ' { Nautical Almanac/" nor by the similar publica- 
 tions of other countries. A skilful band of computers make it their 
 duty to provide for the "Berlin Year-book " detailed information as 
 to the movements of the minor planets. As soon as a few observa- 
 tions of a planet have been obtained the little object passes into the 
 secure grasp of the computer ; he is able to predict the career of that 
 planet for years to come, and such predictions for all the known 
 minor planets are to be found in the annual volumes of the work 
 referred to. The growth of discovery has been so rapid, that the 
 necessary labour for the preparation of such predictions is now 
 enormous. It must be confessed that many of the minor planets 
 are very faint and otherwise devoid of interest, so that astronomers 
 are sometimes tempted to concur with the suggestion that a 
 proportion of the astronomical labour now devoted to the com- 
 putation of the paths of these bodies might be more profitably 
 applied. For this it would only be necessary to cast adrift all the 
 less interesting members of the host and allow them to pursue 
 their paths unwatched by the telescope, or by the still more ceaseless 
 tables of the mathematical computer. 
 
 The sun, which controls the mighty planets of our system, does 
 not disdain to guide, with equal care, the tiny globes which form 
 the minor planets. Each revolves in an elliptic orbit, and at 
 certain times some of them approach near enough to the earth to 
 have their distances measured. The observations can be made with 
 very great precision ; they can be multiplied to any extent that may 
 be desired. Some of these little bodies have consequently a great 
 astronomical future, inasmuch as they seem destined to determine 
 more accurately than Venus or than Mars the true distance from the 
 earth to the sun. The smallest of these planets will not answer; they 
 
206 TIIE STOEY OF THE HEAVENS. 
 
 can only be seen in powerful telescopes, and they do not admit of 
 being measured with the necessary accuracy. It is also obvious that 
 the planets to be chosen must come as near the earth as possible. 
 They should have orbits possessing a high degree of eccentricity, 
 so that when in perihelion and in opposition simultaneously, the 
 approach may be made advantageously. Under favourable circum- 
 stances, some of the minor planets will approach the earth to a 
 distance little more than three-quarters of the distance of the sun. 
 These various conditions limit the number of planets available for 
 this purpose to about a dozen, of which one or two will usually be 
 suitably placed each year. 
 
 For the determination of the sun's distance, this method by the 
 minor planets offers unquestionable advantages. The planet itself 
 is a minute star-like point, and the measures are made from it to 
 the other stars which are seen near it in the telescope. A few words 
 will, perhaps, be necessary at this place as to the nature of the 
 measurements referred to. When we speak of the measures from 
 the planet to the star we do not refer to what would be perhaps the 
 most ordinary acceptation of the expression. We do not mean the 
 actual measurement of the number of miles in a straight line be- 
 tween the planet and the star. This element, even if attainable, 
 could only be the result of a protracted series of observations of a 
 nature which will be explained later on when we come to speak of 
 the distances of the stars. The measures now referred to are of a 
 very much more simple type ; they merely refer to the apparent dis- 
 tance of the objects expressed in angular measure. This angular 
 measurement is of a wholly different character to the linear 
 measurement, and the two methods may, indeed, lead to results that 
 would at first seem paradoxical. Let us give, as an illustration, the 
 case of the group of stars forming the Pleiades, and those which 
 form the Great Bear. The latter is a large group, the former is a 
 small one. But why do we think the words large and small rightly 
 applied here. Each pair of stars of the Great Bear makes a large 
 angle with the eye. Each pair of stars in the Pleiades makes a 
 small angle, and it is these angles which are the direct objects of 
 astronomical measurement. We speak of the distance of the stars, 
 
THE MINOR PLANETS. 207 
 
 meaning thereby the angle which is bounded by the two lines 
 from the eye to the two stars. This is what our instruments are 
 able to measure, and it is to be observed that there is no reference to 
 linear magnitude at all. Indeed, if we allude to linear magnitudes, 
 it is quite possible, for anything we can tell, that the Pleiades are 
 really a much larger group than the Great Bear, and that the ap- 
 parent superiority of the latter is merely due to its being closer to us. 
 The most accurate of these angular measures are obtained when two 
 stars, or two star-like points, are so close together as to enable them 
 to be included in one field of view of the telescope. There are many 
 different forms of apparatus which enable the astronomer to give 
 to such measures a precision unattainable in the measurement of 
 objects less definitely marked, or at a greater apparent distance. 
 The measures of the distance of the small star-like planet from a 
 star are characterised by great accuracy, and this is one source of the 
 accuracy of the minor planet method of finding the sun's distance. 
 But there is another and, perhaps, a weightier argument in its 
 favour, the full importance of which has lately begun to be real- 
 ized. The real strength of the minor planet method rests hardly so 
 much on the individual accuracy of the observations, as on the fact 
 that from the nature of the method a considerable number of obser- 
 vations can be concentrated on the result. It will, of course, be 
 understood that when we speak of the accuracy of an observation, it 
 is not to be presumed that the observation is entirely free from error. 
 Errors always exist, and though the errors be small, yet if the 
 total quantity to be measured is small, the error may amount to an 
 appreciable fraction of the total. The one way by which the effect 
 of errors of observation can be subdued, is by taking the mean of a 
 large number of observations. This is the real source of the value of 
 the minor planet method. We have not to wait for the occurrence 
 of rare events like the transit of Venus. Each year will witness the 
 approach of some one or more minor planets sufficiently close to the 
 earth to render the observations possible. The varied circumstances 
 attending each planet, and the great variety of the observations 
 which may be made upon it, will all tend further and further to 
 eliminate errors. 
 
208 THE STORY OF THE HEAVENS. 
 
 As the planet pursues its course through the sky, and as the 
 sky is everywhere studded over with countless myriads of minute 
 stars, it is evident that the planet, itself so like a star, will always 
 have some stars in its immediate neighbourhood. As the move- 
 ments of the planet are well known, it is possible to foretell where 
 it will be on each night that it is to be observed. It is thus pos- 
 sible to pre-arrange with observers in widely-different parts of the 
 earth as to the observations to be made on each particular night. 
 
 An attempt has only recently been made, on the suggestion of 
 Mr. Gill, to carry out this method on a scale commensurate with its 
 importance. The planets Victoria and Sappho happened, in the 
 year 1882, to approach so close to the earth that arrangements 
 were made for simultaneous measurements in both the northern 
 and the southern hemispheres. A scheme was completely drawn 
 up many months before the observations were to commence. Each 
 observer who participated in the work was thus advised before- 
 hand of the stars which were to be employed each night. Viewed 
 from any part of the earth, from the Cape of Good Hope or from 
 Great Britain, the positions of the stars are absolutely unchanged. 
 Their distance is so stupendous that a change of place on the earth 
 displaces them to no appreciable extent. But the case is different 
 with a minor planet. It is hardly one-millionth part of the dis- 
 tance of the stars, and the displacement of the planet when viewed 
 from the Cape and when viewed from Europe, is a measurable 
 quantity. 
 
 The displacement of the planet is to be elicited by comparison 
 between the observations in the northern hemisphere with those in 
 the southern hemisphere. The observations in both hemispheres 
 must be as nearly simultaneous as possible, due allowance being 
 made for the motion of the planet in whatever interval there 
 may be. Although every precaution is taken to eliminate the 
 errors of each observation, yet the fact remains that we compare 
 the measures made by observers in the northern hemisphere, with 
 those made by different observers, and of course different instru- 
 ments, in the southern hemisphere. In this respect we are at no 
 greater disadvantage than in observing the transit of Venus, yet it 
 
THE MINOR PLANETS. 209 
 
 is possible to obviate even this difficulty, and thus to give the minor 
 planet method a great advantage over its rival. The difficulty would 
 be overcome if we could conceive that an observer and his observa- 
 tory, after making a set of observations on a fine night in the 
 northern hemisphere, were by means of the lamp of Aladdin, or 
 some similar agency, to be transferred, instruments and all, to the 
 southern hemisphere, and there to repeat the observations. An 
 equivalent transformation can be effected without any miraculous 
 agency, and in it we have undoubtedly the most perfect mode of 
 measuring the sun's distance with which we are acquainted. This 
 method has already been applied with success by Mr. Gill in the 
 case of Juno, but there are other planets more favourably situated. 
 Take, for instance, one of those minor planets, which sometimes 
 approach to within 70,000,000 miles of the earth. When the 
 opposition is drawing near, a skilled observer is to be placed at 
 some suitable station near the equator. The instrument he is to 
 use should be that marvellous piece of mechanical and optical skill 
 known as the heliometer.* It can be used to measure stars at a 
 greater range than is obtainable with the filar micrometer. The 
 measurements are to be made in the evening as soon as the planet 
 has risen high enough to enable it to be seen distinctly. The 
 observer and the observatory is then to be transferred to the 
 other side of the earth. How is this to be done ? Say, rather, 
 how we could prevent it being done. Is not the earth rotating 
 on its axis, so that in the course of a few hours the observator 
 on the equator is carried bodily round for thousands of miles ? As 
 the morning approaches the observations are to be repeated. The 
 planet is found to have changed its place very considerably with 
 regard to the stars. This is partly due to its own motion, but it 
 is also largely due to the parallactic displacement arising from the 
 rotation of the earth, which may amount to as much as twenty 
 
 * The heliometer is a telescope with its object-glass cut in half along a diameter. 
 One or both of these halves can be moved transversely by a screw. Each half 
 gives a complete image of the object. The measures are effected by observing how 
 many turns of the screw convey the image of the star formed by one half of 
 the object-glass to coincidence with the image of the planet formed by the other. 
 O 
 
210 TEE STORY OF THE HEAVENS. 
 
 seconds. The measures on a single night with the heliometer 
 should not have a mean error greater than one-fifth of a second, 
 and we might reasonably expect that observations could be secured 
 on about twenty-five nights during the opposition. Four such 
 groups might be expected to give the sun's distance without any 
 uncertainty greater than the thousandth part of the total amount. 
 The chief difficulty of the process arises from the movement of the 
 planet during the interval which divides the evening from the 
 morning observations. This, it must be admitted, is a drawback 
 to the method. It can, however, be avoided by diligent and 
 repeated measurements of the place of the planet with respect to 
 the stars among which it passes. Let us hope that long ere the 
 next transit of Venus approaches, the problem of the sun's dis- 
 tance will have been satisfactorily solved by the minor planets. 
 
CHAPTER XII. 
 
 JUPITER. 
 
 The great Size of Jupiter Comparison of his Diameter with that of the Earth- 
 Dimensions of the Planet and his Orbit His flotation Comparison of his 
 Weight and Bulk with that of the Earth Relative Lightness of Jupiter How 
 explained Jupiter still probably in a Heated Condition The Belts on Jupiter 
 Spots on his Surface Time of Rotation of different Spots various Storms 
 on Jupiter Jupiter not Incandescent The Satellites Their Discovery 
 Telescopic Appearance Their Orbits The Eclipses and Occultations A Satel- 
 lite in Transit The Velocity of Light Discovered How is this Velocity to be 
 Measured experimentally? Determination of the Sun's Distance by the 
 Eclipses of Jupiter's Satellites Jupiter's Satellites demonstrating the Co- 
 pernican System. 
 
 IN our exploration of the beautiful series of bodies which form the 
 solar system, we have gradually proceeded step by step outwards 
 from the sun. In the pursuit of this method we have now come 
 to the splendid planet Jupiter, which wends its majestic way in a 
 path immediately outside those orbits of the minor planets which 
 we have just been considering. Great, indeed, is the contrast 
 between these tiny globes and the stupendous globe of Jupiter. 
 Had we adopted a somewhat different method of treatment had 
 we, for instance, discussed the various bodies of our planetary 
 system in the order of their magnitude then the minor planets 
 would have been the last to be considered, while the leader of the 
 host would be Jupiter. To this position Jupiter is entitled with- 
 out an approach to rivalry. The next greatest on the list, the 
 beautiful and interesting Saturn, comes a long distance behind. 
 Another great descent in the scale of magnitude has to be made 
 before we reach Uranus and Neptune, while still another step 
 downwards must be made before we reach that lesser group of 
 planets which includes our earth. So conspicuously does Jupiter 
 tower over the rest, that even if Saturn were to be augmented by 
 o 2 
 
212 
 
 THE STORY OF THE HEAVENS. 
 
 all the other globes of our system rolled into one, the united mass 
 would still not equal the great globe of Jupiter. 
 
 The adjoining picture shows the relative dimensions of Jupiter 
 and the earth, and it conveys to the eye a more vivid impression 
 of the enormous bulk of Jupiter than we can readily obtain by 
 merely considering the numerical statements by which his bulk is to 
 be accurately estimated. As, however, it will be necessary to place 
 
 Fig. 60. The relative dimensions of Jupiter and the Earth. 
 
 the numerical facts before our readers, we do so at the outset of 
 this chapter. 
 
 Jupiter revolves in an elliptic orbit around the sun in the focus, 
 at a mean distance of 482,000,000 miles. The path of Jupiter is 
 thus about 5*2 times as great in diameter as the path pursued by 
 the earth. The shape of Jupiter's orbit departs very appreciably 
 from a circle, the greatest distance from the sun being 5*45, while 
 the least distance is about 4'95, the earth's distance from the sun 
 being taken as unity. Under the most favourable circumstances 
 for seeing Jupiter at opposition, it must still be about four times 
 as far from the earth as the earth is from the sun. Jupiter will 
 also illustrate the law, that the more distant a planet is, the 
 
JUPITER. 213 
 
 slower is the velocity with which the orbital motion of the 
 planet is accomplished. While the earth passes over eighteen 
 miles each second, Jupiter only accomplishes eight miles. Thus 
 for a two-fold reason the time occupied by an exterior planet in 
 completing a revolution is greater than the period o the earth. 
 Not only has the outer planet to pursue a longer course than the 
 earth, but the speed is less ; it thus happens that Jupiter requires 
 4,332*6 days, or about fifty days less than twelve years, to make 
 a circuit of the heavens. 
 
 The mean diameter of the great planet is about 85,000 miles. 
 We say the mean diameter, because there is a conspicuous difference 
 in the case of Jupiter between his equatorial and his polar diameter. 
 We have already seen that there is a similar difference in the case 
 of the earth, where we find the polar diameter to be shorter than 
 the equatorial ; but the disproportion of these two diameters is very 
 much larger in Jupiter than in the earth. The equatorial diameter 
 of Jupiter is 87,500 miles, while the polar diameter is not more 
 than 82,500 miles. The ellipticity of Jupiter thus produced, is 
 sufficiently marked to be obvious without any refined measures. 
 Around this short diameter the planet spins with what must be 
 considered an enormous velocity when we reflect on the size of the 
 globe. Each rotation is completed in about 9 hrs. 55imin. We 
 may naturally contrast this with the much slower rotation of our 
 earth in twenty-four hours. The difference becomes much more 
 striking if we consider the relative speeds at which points on the 
 equator of the earth and on the equator of Jupiter actually move. As 
 the diameter of Jupiter is nearly eleven times that of the earth, it 
 will follow that the actual speed of the equator on Jupiter must be 
 about twenty-seven times as great as that on the earth. It is, no 
 doubt, to this high velocity of rotation that we must ascribe the 
 extraordinary ellipticity of Jupiter ; for the rapid rotation causes a 
 great degree of centrifugal force, and this bulges out the pliant 
 materials of which he seems to be formed. 
 
 Jupiter is, so far as we can see, not a solid body. This is a 
 very important circumstance ; and, therefore, it will be necessary to 
 discuss the matter at some little length, as we here perceive a wide 
 
214 THE STORY OF THE HEAVENS. 
 
 contrast between this great planet and the other planets which have 
 previously occupied our attention. From the measurements already 
 given it is easy to calculate the bulk or the volume of Jupiter. It 
 will be found that Jupiter is about 1,200 times as large as the 
 earth; in other words, it would take 1,200 globes, each as large as 
 our earth, all rolled into one, to form a single globe as large as 
 Jupiter. If, therefore, the materials of which Jupiter is composed 
 were of a nature analogous to the materials of the earth, we 
 might expect that the weight of Jupiter would exceed the weight 
 of the earth in something like the proportion of their volumes. 
 This is the matter now proposed to be brought to trial. Here we 
 may at once be met with the query, as to how we are to find the 
 weight of Jupiter. It is not an easy matter even to weigh the 
 earth on which we stand. How, then, can we weigh a mighty 
 planet vastly larger than the earth, and distant from the earth by 
 some hundreds of millions of miles ? Truly, this is a noble pro- 
 blem. Yet the intellectual resources of man are able to achieve 
 this mighty feat of celestial engineering. They are not, perhaps, 
 actually able to make the ponderous weighing scales in which the 
 great planet is to be cast, but they are able to divert to this purpose 
 certain natural phenomena which yield the information that is 
 required. All such investigations repose on universal gravitation 
 as their foundation. The mass of Jupiter attracts other masses in 
 the solar system. The efficiency of that attraction is more particu- 
 larly shown on the bodies which are near Jupiter. In virtue of this 
 attraction certain movements are impressed on those bodies. We 
 can observe these movements in our telescopes, we can measure 
 their amount, and from such measurements we can calculate the 
 mass of the body by which the movements have been produced. 
 This is the sole method which we possess for the investigation of 
 the masses of the planets ; and, though it may be difficult in its 
 application not only from the observations which are required, 
 but from the intricacy and the profundity of the calculations to 
 which those observations must be submitted yet in the case 
 of Jupiter, at least, there is no uncertainty about the result. The 
 task is, indeed, peculiarly simplified in the case of Jupiter by 
 
JUPITER. 215 
 
 reason of the beautiful system of four moons by which he is 
 attended. These little moons move under the guidance of Jupiter, 
 and their movements are not otherwise interfered with in a way 
 calculated to prevent their use for our present purpose. It is from 
 the observations of the satellites of Jupiter that we are enabled to 
 measure his attractive power, and thence to conclude the mass of 
 the mighty planet. 
 
 To those not specially conversant with the principles of 
 mechanics, it may seem difficult to realise the accuracy of which 
 such a method is capable. Yet there can be no doubt that his 
 moons really teach us the mass of Jupiter, and do not leave a 
 margin of inaccuracy so great as one hundredth part of the total 
 amount. If other confirmation be needed, then other confirmation 
 is forthcoming. A minor planet occasionally draws near to the orbit 
 of Jupiter and experiences his attraction ; the planet is forced to 
 swerve from its path, and the amount of its deviation can be mea- 
 sured. From that measurement the mass of Jupiter can be com- 
 puted by a calculation, of which it would be impossible here even 
 to give an outline. The mass of Jupiter, as found from the minor 
 planet, agrees with the mass obtained in a totally different manner 
 from the satellites. Nor have we here exhausted the resources of 
 astronomy in its bearing on this question. We can even discard 
 the whole planetary system entirely, and we can invite the occa- 
 sional aid of a comet which, flashing through the orbits of the 
 planets, experiences large and sometimes enormous disturbances. For 
 the present it suffices to remark, that on one or two occasions it has 
 happened that venturous comets have been near enough to Jupiter 
 to feel the effect of his ponderous body, and then to proclaim by 
 their altered movements the magnitude of the mass which has 
 affected them. The satellites of Jupiter, the minor planets, and 
 the comets, all tell us the mass of Jupiter ; and, as they all give us 
 the same result (at least within extremely narrow limits) , we can- 
 not hesitate to conclude that the mass of the greatest planet of our 
 system has been determined with accuracy. 
 
 The results of these measures must now be stated. They show, 
 of course, that Jupiter is vastly inferior to the sun that, in fact, 
 
216 THE STORY OF THE HEAVENS. 
 
 it would take about 1,048 Jupiters, all rolled into one, to form a 
 globe equal in weight to the sun. They also show us that it would 
 take 310 globes as heavy as our earth to counterbalance the 
 weight of Jupiter. 
 
 No doubt this shows Jupiter to be a body of the most majestic 
 mass; but the real point for consideration is not that Jupiter is 310 
 times as heavy as the earth, but that he is not a great deal more. 
 Have we not stated that Jupiter is 1,200 times as largess, the earth? 
 How then comes it that he is only 310 times as heavy ? This 
 points at once to some fundamental contrast between the constitu- 
 tion of Jupiter and of the earth. How are we to explain this 
 difference ? We can conceive of two explanations. In the first 
 place, it might be supposed that Jupiter is constituted of materials 
 partly or wholly unknown to us on the earth. There is, however, 
 an alternative supposition at once more philosophical and more 
 consistent with the evidence. It is true that we know little or 
 nothing of what the elementary substances on Jupiter may be, but 
 one of the great discoveries of modern astronomy had taught us some- 
 thing of the elementary bodies present in other bodies of the uni- 
 verse, and has fully demonstrated that to a large extent they are 
 identical with the elementary bodies on the earth. We shall have 
 occasion to dwell on this very important subject in a future chapter, 
 and accordingly we now merely refer to it for the purpose of 
 justifying the assumption, that the materials of Jupiter are not 
 entirely different from those on the earth. And if Jupiter be com- 
 posed of bodies resembling those on the earth, there is one way, 
 and only one, in which we can account for the disparity between 
 his size and his mass. Perhaps the best way of stating the argu- 
 ment will be found in a retrospective glance at the probable past 
 history of the earth itself, for it seems not impossible that the pre- 
 sent condition of Jupiter was itself foreshadowed by the condition 
 of our earth countless ages ago. 
 
 In a previous chapter we had occasion to point out how the 
 earth seemed to be cooling from an earlier and highly-heated con- 
 dition. The further we look back the hotter seems to have been 
 the earth, and if we project our glance back to an epoch suffi- 
 
JUPITER. 217 
 
 ciently remote, we see that it must have been so hot that life on 
 its surface would be impossible. Back still earlier, we see the 
 heat to have been such that water could not rest on the earth ; and 
 hence it seems likely that at some incredibly remote epoch all the 
 oceans now reposing on the surface of the earth, and perhaps a 
 considerable portion of its more solid materials, must have been 
 actually in vapour. Such a transformation of the globe would not 
 alter its mass, for the materials weigh the same whatever be 
 their condition as to temperature, but it would alter to the most 
 remarkable extent the size of our globe. If all our oceans were 
 transformed into vapour, our atmosphere, charged with mighty 
 clouds, would have a mass some hundreds of times greater than 
 that which it has at present, and the size of the globe would be 
 correspondingly swollen. Viewed from a distant planet, the cloud- 
 laden atmosphere would seem to indicate the size of our globe, and 
 its density would accordingly be concluded to be very much less 
 than it is at present. 
 
 From these considerations it will be manifest that the dis- 
 crepancy between the size and the weight of Jupiter, as contrasted 
 with our earth, would be completely removed if we supposed that 
 Jupiter was at the present day a highly-heated body in the 
 condition of our earth countless ages ago. Every circumstance 
 'of the planet tends to justify this reasoning. We have assigned 
 the smallness of the moon as a reason why the moon has cooled 
 sufficiently to make its volcanoes silent and still. In the 
 same way the smallness of the earth, as compared with Jupiter, 
 accounts for the fact that Jupiter still retains a large part of its 
 original heat, while the smaller earth has dissipated most of its 
 store. This argument is illustrated and strengthened when we 
 introduce other planets into the comparison. As a general 
 rule we find that the smaller planets, like the earth and Mars, 
 have a high density, indicative of a low temperature, while 
 the giant planets, like Jupiter and Saturn, have a low density, 
 suggesting that they still retain a large part of their original heat. 
 We say " original heat" for the want, perhaps, of a more correct 
 expression; it will, however, indicate that we do not in the least 
 
218 THE STORY OF THE HEAVENS. 
 
 refer to the solar heat, of which, indeed, the great outer planets 
 receive much less than those nearer the sun. Where the original 
 heat may have come from is a matter still confined to the province 
 of speculation. 
 
 A complete justification of these views with regard to Jupiter 
 is to he found when we make a minute telescopic scrutiny of its 
 surface ; and it fortunately happens that the size of the planet is 
 so great that, even at a distance of more millions of miles than 
 there are days in the year, we can still trace out on its surface 
 some of the characteristic features. 
 
 Plate XI. gives a series of four different views of the planet 
 Jupiter. It has been copied from the very admirable drawings of 
 this object made by Mr. L. Trouvelot at the Astronomical Obser- 
 vatory of Harvard College, United States. The first picture shows 
 the appearance of the planet on February 2nd, 1872, through a 
 powerful refracting telescope. We at once notice in this drawing 
 that the outline of Jupiter is distinctly elliptical. The surface of 
 the planet usually shows the remarkable series of belts here repre- 
 sented; these belts are nearly parallel to each other, and to the 
 equator of the planet. 
 
 When Jupiter is observed for an hour or two, the appearance 
 of the belts undergoes some changes. These are partly due to the 
 regular rotation of the planet on its axis, which, in a period of less 
 than five hours, will completely carry away the hemisphere we first 
 saw and replace it by the hemisphere originally at the other side. 
 But besides the changes thus arising, the belts and other features 
 on the planet are also very variable. Sometimes new belts or mark- 
 ings appear, and old ones disappear; in fact, a thorough examina- 
 tion of Jupiter will demonstrate the very remarkable fact that there 
 are no permanent marks whatever upon the planet. We are here 
 immediately reminded of the contrast between Jupiter and Mars ; 
 on the smaller planet the markings are almost entirely permanent, 
 and it has been possible to construct maps of the surface with 
 tolerably accurate detail ; a map of Jupiter is, however, an im- 
 possibility the drawing of the planet which we make to-night will 
 be different to the drawing of the same hemisphere made a few 
 
PLATE XI. 
 
 Feb. 12th. 
 
 Feb. 28th. 
 
 THE PLANET JU.PITER, 
 
 1872. 
 

JUPITER. 219 
 
 weeks hence. In this respect there is an analogy between the 
 appearance of Jupiter and the appearance of the sun. 
 
 It should, however, be noticed that spots occasionally appear on 
 the planet, which seem of a rather more permanent character than 
 the belts. We may especially mention the object known as the 
 great Red Spot, which has been a very remarkable feature upon 
 Jupiter since the year 1878, and is probably shown on some of the 
 drawings made several years earlier. 
 
 The conclusion is irresistibly forced upon us, that when we look 
 at the surface of Jupiter we are not looking at any solid body. The 
 want of permanence in the features of the planet would be intel- 
 ligible, if the surface we see be merely an atmosphere laden with 
 clouds of such density that our vision can never penetrate to the 
 interior of the planet. The belts especially support this view ; we 
 are at once reminded of the equatorial zones on our own earth, and 
 it is not at all unlikely that an observer sufficiently remote from 
 the earth to obtain a just view of its appearance would see upon its 
 surface more or less perfect cloud-belts suggestive of those on 
 Jupiter. A view of our earth would be, as it were, intermediate 
 between a view of Jupiter and of Mars. In the latter case the 
 appearance of the permanent features of the planet is but to a 
 trifling extent ever obscured by clouds floating over the surface. 
 Our earth would always be partly, and often perhaps very largely, 
 covered with cloud, while Jupiter seems at all times completely 
 enveloped. 
 
 From another kind of observation we are also taught the 
 important truth that Jupiter is not, superficially at least, a solid 
 body. The period of the rotation of Jupiter around its axis is 
 derived from the observations of certain spots, which present suffi- 
 cient definiteness and sufficient permanence to be suitable for the 
 purpose. Suppose one of these spots to lie at the centre of the 
 planet's disc; its position is carefully measured, and the time is 
 noted. As the hours pass on, the spot moves to the edge of the 
 disc, then round the other side of the planet, and back again to the 
 visible disc. When the spot regains the position originally occu- 
 pied the time is again taken, and the interval which has elapsed is 
 
220 THE STORY OF THE HEAVENS. 
 
 called the period of rotation of the planet. If Jupiter were a 
 solid, and if all these features were fixed upon its surface, then it is 
 perfectly clear that the time of rotation as found by any one spot 
 would coincide precisely with the time yielded hy any other spot ; 
 but this is not found to be the case. In fact, it would be nearer 
 the truth to say that each spot gives us its own special period. 
 Nor are the differences very minute. It has been found that the 
 time in which the red spot is carried round is five minutes longer 
 than that required by certain white spots near the equator, while 
 certain small black spots have been found to accomplish the journey 
 in even two minutes less. It may, therefore, be regarded as cer- 
 tain that the globe of Jupiter, so far as we can see it, is not a solid 
 body. It consists, on the exterior at all events, of clouds and 
 vaporous masses, which seem to be agitated by storms of the utmost 
 intensity, if we may judge from the ceaseless changes of the 
 planet's surface. 
 
 It is well known that the tempests by which the atmosphere 
 surrounding the earth is convulsed, are all ultimately to be attributed 
 to the heat of the sun. It is the heat of the sun which, striking on 
 the vast continental masses, warms the air in contact therewith. This 
 heated air becomes lighter and rises, while air to supply its place 
 must flow in along the surface. The current of air so produced 
 forms a breeze or a wind; while, under exceptional circumstances, 
 we have the phenomena of cyclones and of hurricanes, all ultimately 
 due to the sun's heat. Need we add that the rains, which so often 
 accompany the storms, have also arisen from the sun's heat, which 
 has distilled from the wide expanse of ocean the moisture by which 
 the earth is refreshed ? 
 
 The storms on Jupiter seem to be vastly greater than those on 
 the earth. Yet the intensity of the sun's heat on Jupiter is only a 
 mere fraction, less indeed than the twenty-fifth part, of the sun's 
 heat on the earth. It is incredible that the motive power of the 
 appalling tempests on the great planet can be entirely, or even 
 largely, due to the feeble influence of solar heat. We are, there- 
 fore, led to seek for some other source for these disturbances. That 
 source appears obvious when we admit that Jupiter still retains a 
 
JUPITER. 221 
 
 large proportion of its primitive internal heat. Just as the sun 
 himself is distracted by the most violent tempests in consequence of 
 his internal heat, so, in a lesser degree, do we observe the same 
 phenomena in Jupiter. It may also be noticed that the spots on 
 the sun are usually, in more or less regular zones, parallel to the 
 equator of the sun, and in this respect, not dissimilar to the belts 
 on Jupiter. 
 
 It being admitted that Jupiter still retains some of its internal 
 heat, the question remains as to how much. It is, of course, 
 obvious that the heat of Jupiter is incomparably less than the heat 
 of the sun. The brilliancy of Jupiter, which makes it an object 
 of such splendour in our midnight sky, is only derived from the 
 same source which illuminates the earth, the moon, or the other 
 planets. Jupiter, in fact, shines by reflected sunlight, and not in 
 virtue of any brilliancy of his own. A beautiful proof of this 
 truth is familiar to every user of a telescope. The little satellites 
 of Jupiter sometimes come between him and the sun, and they cast 
 a shadow on Jupiter. That shadow is black, or, at all events, it 
 seems black, relatively to the brilliant surrounding surface of the 
 planet, whence it must be obvious that Jupiter is indebted only to 
 the sun for its brilliancy. The satellites supply another interesting 
 proof of this truth. A satellite sometimes enters into the shadow 
 of Jupiter, and lo ! the satellite vanishes. It vanishes because 
 Jupiter has cut off the supply of sunlight which previously 
 rendered the satellite visible. But Jupiter is not himself able to 
 offer the satellite any light in place of the sunlight which he has 
 intercepted.* 
 
 Enough, however, has been demonstrated to enable us to pro- 
 nounce on the question as to whether Jupiter can be a body in- 
 habited by living beings, as we understand the term. Obviously it 
 cannot. The internal heat and the fearful tempests seem to preclude 
 the possibility of organic life, even were there not other arguments 
 against it. It may, however, be contended, with perhaps some 
 plausibility, that Jupiter has in the distant future the prospect of a 
 
 * It is only right to add that some observers believe that, under exceptional cir- 
 cumstances, points of Jupiter have shown some slight degree of intrinsic light. 
 
222 THE STORY OF THE HEAVENS. 
 
 glorious career as the residence of organic life. The time will 
 assuredly come when the internal heat must subside, when the 
 clouds will gradually condense into oceans. On the surface it 
 may then be that dry land will appear, and thus Jupiter may be 
 rendered habitable. 
 
 From this sketch of the planet itself we now turn to the 
 interesting and beautiful system of four satellites with which 
 Jupiter is attended. We have, indeed, already found it necessary 
 to allude more than once to these little bodies, but not to such an 
 extent as to interfere with the more formal treatment which they 
 are now to receive. 
 
 Fig. 51. Jupiter and his Four Satellites as seen in a telescope of low power. 
 
 The discovery of these satellites may be regarded as marking an 
 important epoch in the history of astronomy. They are objects 
 situated in a remarkable manner on the border line which divides 
 the objects visible to the unaided eye from those which require tele- 
 scopic aid. It has been stated frequently that these objects have 
 been seen with the unaided eye; but without entering into any 
 controversy on the matter, it is sufficient to recite the well-known 
 fact that, notwithstanding Jupiter had been familiar to the un- 
 aided eyes for countless centuries, and in the clearest skies, yet no 
 one ever discovered the satellites until Galileo turned his newly- 
 invented telescope upon them. This telescope was no doubt a very 
 feeble instrument, but the feeblest instrument is quite adequate to 
 show objects so close to the limit of visibility. 
 
 The view of the planet and its elaborate system of four satel- 
 lites, as shown in a telescope of moderate power, is represented 
 in Fig. 51. We here see the globe of the planet, and nearly in 
 
JUPITER. 223 
 
 a line with its centre lie four small objects, three on side and one 
 on the other. These little objects resemble stars, but they can be 
 distinguished from stars by their ceaseless movements around the 
 planet which they never fail to accompany during his entire circuit 
 of the heavens. There is no more pleasing spectacle for the student 
 of the heavens than to follow with his telescope the movements of 
 this beautiful system. 
 
 lydays 
 
 62. Disappearances of Jupiter's Satellites. 
 
 In Fig. 52 we have represented some of the various pheno- 
 mena which the satellites present. The long black shadow is 
 that produced by the interposition of Jupiter in the path of the 
 sun's rays. The second satellite is immersed in this shadow, 
 and consequently eclipsed. The eclipse of a satellite must not be 
 attributed to the intervention of the body of Jupiter between the 
 satellite and the earth. This occurrence is called an occupation, and 
 the third satellite is shown in this condition. The second and the 
 third satellites are thus alike invisible, but the cause of the invisi- 
 bility is quite different in the two cases. The eclipse is much the 
 
224 THE STORY OF THE HEAVENS. 
 
 more striking phenomenon of the two, because the satellite, at the 
 moment it plunges into the shadow, is still at some apparent distance 
 from the edge of the planet, and is thus seen clearly up to the 
 moment of the eclipse. In an occultation the satellite in passing 
 behind the planet is, at the time of disappearance, close to the 
 planet's bright edge, and the extinction of the light of the satellite 
 cannot be observed with the same distinctness as the occurrence of 
 an eclipse. A satellite also assumes another remarkable situation 
 when it is in the course of transit over the face of the planet. 
 The satellite itself is not always very easy to see under such 
 circumstances, but the beautiful shadow which it casts forms a 
 sharp black spot on the surface of the planet; the satellite may, 
 indeed, sometimes cast a shadow on the planet even though it 
 be not actually at the moment in front of the planet, so far as 
 the earth is concerned. A case of this kind has been already 
 exhibited in Plate XI., in which we see the small disc of the second 
 satellite near the edge of the planet while the shadow is a well- 
 marked black spot on its surface. 
 
 The periods in which the four satellites of Jupiter revolve 
 around him are respectively, 1 day 18hrs. 27 min. 34 sees, for the 
 first; 3 days 13hrs. 13min. 42 sees, for the second; 7 days 3hrs. 
 42 min. 33 sees, for the third ; and 16 days 16 hrs. 32 min. 11 sees 
 for the fourth. We thus observe that the revolutions of Jupiter's 
 satellites are much more rapid than that of our moon. Even the 
 slowest and most distant satellite on Jupiter requires for each 
 revolution less than two-thirds of an ordinary lunar month. The 
 innermost satellite, revolving as it does in less than two days, pre- 
 sents a striking series of ceaseless and rapid changes, and during 
 every revolution it becomes eclipsed. The distance from the centre 
 of Jupiter to the orbit of the innermost satellite is about a quarter 
 of a millfon miles, while the radius of the outermost is a little 
 more than a million miles. The second of the satellites proceeding 
 outwards from the planet is almost the same size as our moon ; 
 the other three satellites are larger ; the third is the greatest of all, 
 and has a diameter considerably greater than that of the moon. 
 
 Among the many interesting astronomical investigations to 
 
JUPITER. 225 
 
 which the observations of Jupiter's satellites has given rise, no 
 doubt the most celebrated is that made by Roemer, which led to 
 the discovery o the velocity of light. 
 
 The eclipses of Jupiter's satellites had been observed for many 
 years, and the times of their occurrence had been recorded. At 
 length it was perceived that a certain order reigned among the 
 eclipses of the satellites, as among all other astronomical .pheno- 
 mena. When once the laws, according to which the eclipses 
 recurred, had been perceived, the usual consequence followed. 
 It became possible to predict the time at which the eclipses 
 would occur in future. Those predictions were made, and it was 
 found that the predictions were approximately verified. Further 
 improvements in the calculations were made, and it was sought 
 to predict the time with still greater accuracy. But when it 
 came to naming the actual minute at which the eclipse should 
 occur, the predictions were not always successful. Sometimes the 
 eclipse was five or ten minutes too scon. Sometimes it was five or 
 ten minutes too late. Discrepancies of this kind always demand 
 attention. It is, indeed, by such discrepancies that discoveries are 
 often made, and one of the most interesting discoveries is that 
 now before us. 
 
 The irregularity in the recurrence of the eclipses was at length 
 perceived to observe certain rules. It was noticed that when the 
 earth was near to Jupiter the eclipse generally occurred before 
 the predicted time; while when the earth happened to be at 
 the side of its orbit away from Jupiter, the eclipse occurred 
 after the predicted time. Once this was proved, the great dis- 
 covery was quickly made. When the satellite enters the shadow 
 its light gradually decreases until it disappears. It is the last 
 ray of light from the eclipsed satellite that gives the time 
 of the eclipse ; but that ray of light has to travel from the 
 satellite to the earth and enter our telescope before we can note 
 the occurrence. It used to be thought that light travelled instan- 
 taneously, so that the moment the eclipse occurred was assumed 
 to be the moment when the eclipse was, seen in the telescope. 
 This was now perceived to be incorrect. It was found that light 
 
226 THE STORY OF THE HEAVENS. 
 
 took time to travel. "When the earth was near Jupiter the light 
 had only a short journey, the intelligence of the eclipse arrived 
 quickly, and the eclipse happened sooner than the calculations indi- 
 cated. When the earth occupied a position far from Jupiter the 
 light had a longer journey, and took more than the average time, 
 so that the eclipse was later than the prediction. This simple 
 explanation removed the difficulty attending the predictions of the 
 eclipses of the satellites. But the discovery had a significance far 
 more momentous. We learned from it that light had a measurable 
 velocity, which, according to recent researches, amounts to 185,000 
 miles per second. 
 
 One of the most celebrated methods of measuring the distance 
 of the sun is derived from a combination of experiments on the 
 velocity of light with astronomical measurements. This is a 
 method of considerable refinement and interest, and although it 
 does not satisfy all the necessary conditions of a perfectly satisfac- 
 tory method, yet it is impossible to avoid some reference to it here. 
 Notwithstanding that the velocity of light is so stupendous, it 
 has been found possible to measure that velocity by actual trial. 
 This is one of the most delicate experimental researches that has 
 ever been undertaken. If it be difficult to measure the speed of a 
 rifle bullet, what shall we say of the speed of a ray of light, which is 
 nearly a million times as great ? How shall we devise an apparatus 
 subtle enough to measure the velocity which would girdle the earth 
 at the equator no less than seven times in a single second of time ? 
 All ordinary contrivances for the measurement of velocity are here 
 futile; we have to devise an instrument of a wholly different 
 character. 
 
 In the attempt to measure the speed of a moving body we first 
 mark out a certain distance, and then measure the time which the 
 body requires to traverse that distance. We determine the velocity 
 of a railway train by the time it takes to pass from one mile post 
 to the next. We measure the speed of a rifle bullet by an ingenious 
 contrivance really founded on the same principle. The greater the 
 velocity, the more desirable is it that the distance traversed during 
 the trial shall be as large as possible. In dealing with the measure- 
 
 
JUPITEE. 227 
 
 ment of the velocity of light, we therefore choose for our measured 
 distance the greatest length that may be convenient. It is, 
 however, necessary that the two ends of the line shall be visible 
 from each other. A hill a mile or two away will form a suitable 
 site for the distant station, and the distance of the selected point 
 on the hill from the observer must be carefully measured. The 
 problem is now easily stated. A ray of light is to be sent from 
 the observer to the distant station, and the time occupied by that 
 ray in the journey is to be measured. We may suppose that 
 the observer, by a suitable contrivance, has arranged a lantern 
 from which a thin ray of light issues. Let us assume that 
 this ray of light travels all the way to the distant station, and 
 there falls upon the surface of a reflecting mirror. Instantly the 
 ray of light will be diverted by reflection into a new direction 
 depending upon the inclination of the mirror. By suitable adjust- 
 ment the latter can be so placed that the light shall fall perpen- 
 dicularly upon it, in which case the ray will of course simply return 
 along the direction in which it came. Let the mirror be fixed in 
 this position throughout the course of the experiments. It follows 
 that a ray of light starting from the lantern will be returned to the 
 lantern after it has made the journey to the distant station and 
 back again. Imagine, then, a little shutter placed in front of the 
 lantern. We open the shutter, the ray streams forth to the remote 
 reflector, and back again through the opening. But now, after 
 having allowed the ray to pass through the shutter, suppose we try 
 and close it before the ray has had time to get back again. What 
 fingers could be nimble enough to do this ? Even if the distant 
 station were ten miles away, so that the light had a journey of ten 
 miles in going to the mirror and ten miles in coming back, yet the 
 whole course would be accomplished in about the nine thousandth 
 part of a second a period so short that even were it a thousand 
 times as long it would hardly enable manual dexterity to close the 
 aperture. Yet a shutter can be constructed which shall be sufficiently 
 delicate for the purpose. 
 
 The principle of this beautiful method will be sufficiently obvious 
 from the diagram on the next page (Fig. 53). The figure exhibits 
 P 2 
 
228 THE STORY OF THE HEAVENS. 
 
 a rough view of the lantern and the observer, and in front of both 
 is a large wheel with projecting- teeth. Each tooth as it passes 
 round eclipses the beam of light emerging from the lantern, and 
 also the eye, which is of course directed to the mirror at the distant 
 station. In the position of the wheel here shown the ray from the 
 lantern will pass to the mirror and back so as to be visible to 
 the eye, but if the wheel be rotating it may so happen that the 
 beam after leaving the lantern will not have time to return before 
 the next tooth of the wheel conies in front of the eye and screens 
 it. If the wheel be urged still faster the next tooth may have 
 
 Fig. 53. Mode of Measuring the Velocity of Light 
 
 passed the eye so that the ray again becomes visible. The speed 
 at which the wheel is rotating can be measured. We can thus 
 determine the time taken by one of the teeth to pass in front of 
 the eye ; we have thus a measure of the time occupied by the ray 
 of light in the double journey, and hence we have a measurement 
 of the velocity of light. 
 
 It thus appears that we can tell the velocity of light either by 
 the observations of Jupiter's satellites or by experimental inquiry. 
 If we take the latter method, then we are entitled to deduce remark- 
 able astronomical consequences. We can, in fact, employ this 
 method for solving that great problem so often referred to the 
 distance from the earth to the sun though it cannot compete in 
 accuracy with some of the other methods. 
 
 The dimensions of the solar system are so considerable that a 
 
JUPITER. 229 
 
 sunbeam requires an appreciable interval of time to span the abyss 
 which separates the earth from the sun. Eight minutes is approxi- 
 mately the duration of the journey, so that at any moment we see 
 the sun as it appeared eight minutes earlier to an observer in its 
 immediate neighbourhood. In fact, if the sun were to be suddenly 
 blotted out it would still be seen shining brilliantly for eight 
 minutes after it had really disappeared. We can determine this 
 period of time from the eclipses of Jupiter's satellites. 
 
 So long as the satellite is shining it radiates a stream of light 
 across the vast space between Jupiter and the earth. When the 
 eclipse has commenced the little orb is no longer luminous, but 
 there is, nevertheless, a long slender stream of light on its way, 
 and until all this has poured into our telescopes we still see the 
 satellite shining as before. If we could calculate the moment 
 when the eclipse really took place, and if we could observe the 
 moment at which the eclipse is seen, the difference between the two 
 gives the time which the light occupies on the journey. This can be 
 found with some accuracy ; and, as we already know the velocity 
 of light, we can ascertain the distance of Jupiter from the earth ; 
 and hence deduce the scale of the solar system. It must, how- 
 ever, be remarked that at both extremities of the process there 
 are characteristic sources of uncertainty. The occurrence of the 
 eclipse is not an instantaneous phenomenon. The satellite is large 
 enough to require an appreciable time in crossing the boundary 
 which defines the shadow, so that the observation of an eclipse is 
 not sufficiently precise to form the basis of an important and 
 accurate measurement.' 3 *' Still greater difficulties accompany the 
 attempt to define the true moment of the occurrence of the eclipse 
 as it would be seen by an observer in the vicinity of the satellite. 
 For this we would require a far more perfect theory of the move- 
 ments of Jupiter's satellites than is at present attainable. This 
 method of finding the sun's distance holds out no prospect of a 
 
 * Professor Pickering, of Cambridge, Mass., has, however, effected the important 
 improvement of measuring the decline of light of the satellite undergoing eclipse by 
 the photometer. Much additional precision may be anticipated in the results of 
 such observations. 
 
230 TEE STORY OF THE HEAVENS. 
 
 result accurate to the one-thousandth part of its amount, and we 
 may discard it, inasmuch as the other methods available seem to 
 admit of much higher accuracy. 
 
 The satellites of Jupiter have special attractions for the mathe- 
 matician, who finds in them a most striking instance of the univer- 
 sality of the law of gravitation. The satellites are, of course, 
 mainly controlled in their movements by the attraction of the 
 great planet; but they also attract each other, and two curious 
 consequences are the result. 
 
 The mean motion of the first satellite in each day about the 
 centre of Jupiter is 203*4S90. That of the second is 101'3748, 
 and that of the third is 50 *3 17 7. These quantities are so related 
 that the following law will be found to be observed : 
 
 The mean motion of the first satellite added to twice the mean 
 motion of the third is exactly equal to three times the mean motion 
 of the second. 
 
 There is another law, which is of an analogous character, and is 
 thus expressed (the mean longitude being the angle between a 
 fixed line and the radius to the mean place of the satellite) : If to 
 the mean longitude of the first satellite we add twice the mean 
 longitude of the third, and subtract three times the mean longitude 
 of the second, the difference is always 180. 
 
 It was from observation that these laws were first discovered to 
 be true. Laplace, however, showed that if the satellites revolved 
 nearly in this way, then their mutual perturbations, in accordance 
 with the law of gravitation, would preserve them in this relative 
 position for ever. 
 
 It follows from the second law that when two of the satellites 
 are in line on one side of Jupiter the other satellite must be in the 
 same line on the opposite side of the planet. The fourth satellite 
 does not enter into consideration in these laws. 
 
 We shall conclude with the remark, that the discovery of Ju- 
 piter's satellites was really the great confirmation of the Copernican 
 theory. Copernicus had asked the world to believe that our sun 
 was a great body, and that the earth and all the other planets were 
 small bodies revolving around the great one. This doctrine, so 
 
JUPITER. 231 
 
 repugnant to the theories previously held, and to the immediate evi- 
 dence of our senses, could only be established by a refined course of 
 reasoning. The discovery of Jupiter's satellites was very opportune. 
 Here we had an exquisite ocular demonstration of a system, though, 
 of course, on a much smaller scale, precisely identical with that 
 which Copernicus had proposed. The astronomer who had watched 
 Jupiter's moons circling around their primary, who had noticed 
 their eclipses and all the interesting phenomena attendant on them, 
 saw before his eyes, in a manner wholly unmistakable, that the 
 great planet controlled these small bodies, and forced them to 
 revolve around him, and thus exhibited a miniature of the great 
 solar system itself. "As in the case of the spots on the sun, 
 Galileo's announcement of this discovery was received with incredu- 
 lity by those philosophers of the day, who believed that everything 
 in nature was described in the writings of Aristotle. One eminent 
 astronomer, Clavius, said that to see the satellites one must have 
 a telescope which would produce them ; but he changed his mind 
 as soon as he saw them himself. Another philosopher, more pru- 
 dent, refused to put his eye to the telescope lest he should see them 
 and be convinced. He died shortly afterwards. ' I hope/ said the 
 caustic Galileo , ' that he saw them while on his way to heaven/ " * 
 
 * '-Newcomb's Popular Astronomy," p 336. 
 
CHAPTER XIII. 
 
 SATURN. 
 
 The Position of Saturn in the System Saturn one of the Three most Interesting 
 Objects in the Heavens Compared with Jupiter Saturn to the Unaided Eye 
 Statistics relating to the Planet Density of Saturn Lighter than "Water 
 The Researches of Galileo What he found in Saturn A Mysterious Object 
 The Discoveries made by Huyghens half a Century later How the Existence of 
 the Ring was Demonstrated Invisibility of the Rings every Fifteen Years 
 The Rotation of the Planet The Celebrated Cypher The Explanation- 
 Drawing of Saturn Tho Dark Line "W. Herschel's Researches Is the 
 Division in the Ring really a Separation ? Possibility of Deciding the Ques- 
 tion The Ring in a Critical Position Are there other Divisions in the Ring ? 
 The Third Ring Has it appeared but recently ? Physical Nature of 
 Saturn's Rings Can they be Solid ? Can they even be Slender Rings ? A 
 Fluid Probable Nature of the Rings A Multitude of Small Satellites- 
 Analogy of the Rings of Saturn to the Group of Minor Planets Problems 
 Suggested by Saturn The Group of Satellites to Saturn The Discoveries of 
 Additional Satellites The Orbit of Saturn not the Frontier of our System. 
 
 AT a profound distance in space, which sometimes approaches to 
 nearly a thousand millions of miles, the planet Saturn performs its 
 mighty revolution around the sun in a period of over a quarter of a 
 century. This gigantic orbit formed the boundary to the planetary 
 system, so far as it was known to the ancients. 
 
 Although Saturn is not so great a body as Jupiter, yet it vastly 
 exceeds the earth in bulk and in mass, and is, indeed, much greater 
 than any one of the planets, Jupiter alone excepted. But while 
 Saturn must yield the palm to Jupiter so far as mere dimensions 
 are concerned, yet it will be generally admitted that even Jupiter, 
 with all the retinue by which he is attended, cannot compete in 
 beauty with the marvellous system of Saturn. To the present 
 writer it has always seemed that Saturn is one of the throe most 
 interesting celestial objects visible to observers in northern latitudes. 
 The two others will occupy our attention in future chapters. They 
 are the great nebula in Orion, and the star cluster in Hercules. 
 
SATURN. 233 
 
 So far as the globe of Saturn is concerned, we do not meet with 
 any features which give to the planet any exceptional interest. 
 The globe is less than that of Jupiter, and as the latter is also 
 much nearer to us, the apparent size of Saturn is in a two-fold way 
 much smaller than that of Jupiter. It should also be noticed 
 that, owing to the greater distance of Saturn from the sun, its 
 intrinsic brilliancy is less than that of Jupiter. There are, no doubt, 
 certain marks and bands often to be seen on Saturn, but they are 
 not nearly so striking nor so characteristic as the ever- variable 
 belts upon Jupiter. The telescopic appearance of the globe of 
 Saturn must also be ranked as greatly inferior in interest to that 
 of Mars. The delicacy of detail which we can see on Mars when 
 favourably placed has no parallel whatever in the dim and distant 
 Saturn. Nor has Saturn, regarded again merely as a globe, any- 
 thing like the interest of Venus. The great splendour of Venus is 
 altogether out of comparison to that of Saturn, while the brilliant 
 crescent of Venus is infinitely more pleasing than any telescopic 
 view of the globe of Saturn. Yet even while we admit all this to 
 the fullest extent, it does not invalidate the claim of Saturn to be 
 one of the most supremely beautiful and interesting objects in the 
 heavens. This interest is not due to his globe ; it is due to that 
 marvellous system of rings by which Saturn is surrounded a 
 system wonderful from every point of view, and, so far as our 
 knowledge goes, without a parallel in the wide extent of the 
 universe. 
 
 To the unaided eye Saturn usually appears like a star of the 
 first magnitude. Its light alone would hardly be sufficient to dis- 
 criminate it from many of the brighter fixed stars. Yet the ancients 
 were acquainted with Saturn, and they knew it as a planet. It was 
 included with the other four great planets Mercury, Venus, Mars, 
 and Jupiter in the group of wanderers, which were bound to no 
 fixed points of the sky like the stars. On account of the great 
 distance of Saturn, its movements are much slower than those of 
 the other planets known to the ancients. Twenty-nine years and a 
 half are required for this distant object to complete its circuit of 
 the heavens ; and, though this movement is slow compared with the 
 
234 TEE STORY OF THE HEAVENS. 
 
 incessant changes of Venus, yet it is rapid enough to attract the 
 attention of any careful observer. In a single year Saturn moves 
 through a distance of about twelve degrees, a quantity sufficiently 
 large to be conspicuous to casual observation. Even in a month 
 the planet traverses an arc of the sky which can be detected by 
 any one who will take the trouble to mark the place of the planet 
 with regard to the stars in its vicinity. Those who are privileged 
 to use accurate astronomical instruments, can readily detect the 
 motion of Saturn in a few hours. 
 
 The average distance from the sun to Saturn is about 884 
 millions of miles. The path of Saturn, as of every other planet, is 
 really an ellipse with the sun in the focus. In the case of Saturn 
 the shape of this ellipse is very appreciably different from a purely 
 circular path. Around this path Saturn moves with an average 
 velocity of 5*96 miles per second. 
 
 The mean diameter of the globe of Saturn is about 71,000 
 miles. Its equatorial diameter is about 74,000 miles, and its polar 
 diameter 68,000 miles the ratio of these numbers being approxi- 
 mately that of 10 to 9. It is thus obvious that Saturn departs 
 from the truly spherical shape to a very marked extent. The pro- 
 tuberance at its equator must, no doubt, be attributed to the high 
 velocity with which the planet is rotating. The velocity of rota- 
 tion of Saturn is more than double as fast as that of the earth, 
 though it is not quite so fast as that of Jupiter. Saturn makes 
 one complete rotation in 10 hrs. 14 min. 23*8 sees. 
 
 We have already commented upon the low density of the mate- 
 rials of Jupiter, and we have endeavoured to account for that low 
 density by the supposition that internal heat is still present in 
 Jupiter to a considerable extent. We can apply the same reasoning 
 with still greater emphasis to Saturn. The lightness of this planet 
 is such as to be wholly incompatible with the supposition that its 
 globe is constituted of solid materials at all comparable with those 
 of which the crust of our earth is composed. The satellites, which 
 surround Saturn and form a system only less interesting than 
 the renowned rings themselves, enable us to weigh the planet in 
 comparison with the sun, and hence to deduce its actual mass 
 
SATURN. 235 
 
 relatively to the earth. The result is not a little remarkable. It 
 appears that the density of the earth is eight times as great as 
 that of Saturn. In fact, the density of the latter is less than 
 that of water itself, so that a mighty globe of water, equal in 
 bulk to Saturn, would actually weigh more. If we could conceive 
 a vast ocean into which a globe equal to Saturn in size and weight 
 were cast, the great globe would not sink like our earth or like any 
 of the other planets ; it would float buoyantly at the surface with 
 one- fourth of its bulk out of the water. We thus learn with high 
 probability that what our telescopes show upon Saturn is not a solid 
 surface, but merely a vast envelope of clouds surrounding a highly 
 heated interior. It is impossible to resist the suggestion that this 
 planet, like Jupiter, has still retained its heat because its mass is 
 so large. We must, however, allude to a circumstance which perhaps 
 may seem somewhat inconsistent with the view here taken. We have 
 found that Jupiter and Saturn are, both of them, much less dense 
 than the earth. When we compare the two planets together, it 
 appears that Saturn is much less dense than Jupiter. In fact, 
 every cubic mile of Jupiter weighs nearly twice as much as each 
 cubic mile of Saturn. This would seem to point to the conclusion 
 that Saturn is the more heated of the two bodies. Yet, as Jupiter 
 is the larger, it might more reasonably have been expected to be 
 the hotter. We do not attempt to reconcile this discrepancy ; in 
 fact, in our ignorance as to the material constitution of these bodies, 
 it would be idle to discuss the question. 
 
 Even if we allow for the lightness of Saturn, as compared 
 bulk for bulk with the earth, yet the volume of Saturn is so 
 enormous that the planet weighs more than eighty times as much 
 as the earth. The adjoining view represents the relative sizes of 
 Saturn and the earth (Fig. 54) . 
 
 As the unaided eye discloses none of those marvels by which 
 Saturn is surrounded, the interest which attaches to this planet may 
 be said to commence from the time when it began to be observed 
 with the telescope. The history must be briefly alluded to, for it 
 was only step by step that the real nature of this complicated object 
 was understood. When Galileo completed his little refracting 
 
236 THE STOET OF THE HEAVENS. 
 
 telescope, which, though it only magnified thirty times, was yet an 
 enormous addition to the powers of unaided vision, he made with 
 it his memorable review of the heavens. He saw the spots on the 
 sun and the mountains on the moon ; he noticed the crescent of 
 Venus and the satellites of Jupiter. Stimulated and encouraged 
 by such brilliant discoveries, he naturally sought to examine 
 the other planets, and accordingly directed his telescope to 
 Saturn. Here, again, Galileo at once made a discovery. He 
 saw that Saturn presented a visible form like the other planets, 
 
 64. Relative Sizes of Saturn and the Earth. 
 
 but that it differed from any other telescopic object, inasmuch 
 as it appeared to him to be composed of three bodies which 
 always touched each other and always maintained the same 
 relative positions. These three bodies were in a line the central 
 one was the largest, and the two others were east and west of it. 
 There was nothing he had hitherto seen in the heavens which filled 
 his mind with such astonishment, and which seemed so wholly 
 inexplicable. 
 
 In his endeavours to study this object thoroughly, Galileo con- 
 tinued his observations during the year 1610, and, to his amazement, 
 he saw the two lesser bodies gradually become smaller and smaller 
 until, in the course of the two following years, they had entirely 
 vanished, and the planet simply appeared with a round disc like 
 
JfipRWA. 
 
 SATURN. 237 
 
 Jupiter. Here, again, was a new source of anxiety to Galileo. 
 He had at that day to contend against the advocates of the ancient 
 system of astronomy, who derided his discoveries and refused to 
 accept his theories. He had announced his observation of the com- 
 posite nature of Saturn ; he had now to tell of the gradual decline 
 and the ultimate extinction of these two auxiliary globes, and he 
 naturally feared that his opponents would seize the opportunity of 
 pronouncing that the whole of his observations were illusory.* 
 " What," he remarks, " is to be said concerning so strange a meta- 
 morphosis ? Are the two lesser stars consumed after the manner 
 of the solar spots ? Have they vanished ~and suddenly fled ? Has 
 Saturn, perhaps, devoured his own children ? Or were the appear- 
 ances indeed illusion or fraud, with which the glasses have so long 
 deceived me, as well as many others to whom I have shown them ? 
 Now, perhaps, is the time come to revive the well-nigh withered 
 hopes of those who, guided by more profound contemplations, have 
 discovered the fallacy of the new observations, and demonstrated 
 the utter impossibility of their existence. I do not know what to 
 say in a case so surprising, so unlooked for, and so novel. The 
 shortness of the time, the unexpected nature of the event, the 
 weakness of my understanding, and the fear of being mistaken, 
 have greatly confounded me." 
 
 But Galileo was not mistaken. The objects were really there 
 when he first began to observe, they really did decline, and they 
 really disappeared ; but this disappearance was only for a time 
 they again came into view. They were then subjected to ceaseless 
 examination, until gradually their nature became unfolded. With 
 increased telescopic power it was found that the two bodies which 
 Galileo had described as globes, or spheres, near Saturn, were not 
 really spherical they were rather two luminous crescents with the 
 concavity of each turned towards the great central globe. It was 
 also perceived that these objects underwent a remarkable series of 
 periodic changes. At the beginning of such a series the planet 
 was found with a truly circular disc. The appendages first appeared 
 
 See Grant, " History of Physical Astronomy," page 255. 
 
 
238 THE STORY OF THE HEAVENS. 
 
 as two arms extending directly outwards on each side of the planet; 
 then these arms gradually opened into two crescents, resembling 
 handles to the globe, and attained their maximum width after about 
 seven or eight years ; then they began to contract, until after the 
 lapse of about the same time they vanished again. 
 
 The true nature of these objects was at length discovered by 
 Huyghens in 1655, nearly half a century after Galileo had first 
 detected their appearance. He discovered the shadow thrown by 
 the ring upon the globe, and his explanation of the phenomena was 
 obtained in a very philosophical manner. He noticed that the earth, 
 the sun, and the moon, rotated upon their axes, and he therefore 
 regarded it as a general law that each one of the bodies in the 
 system rotates about an axis. No doubt at that early period no 
 observations had been made which actually showed that Saturn was 
 also rotating ; but it would be highly, nay indeed infinitely, impro- 
 bable that any planet should be devoid of rotation. All the analo- 
 gies of the system pointed to the conclusion that the velocity of 
 rotation would be considerable. One satellite of Saturn was already 
 known to revolve in a period of sixteen days, being little more than 
 half our month. Huyghens assumed and he was entitled to the 
 assumption that Saturn in all probability rotated rapidly on its 
 axis. It was also to be observed that if these remarkable appendages 
 were attached by an actual bodily connection to the planet they 
 must rotate with Saturn. If, however, the appendages were not 
 actually attached it would still be necessary that they should rotate 
 if the analogy of Saturn to other objects in the system were to be 
 in any degree preserved. We see satellites near Jupiter which 
 revolve around him. We see, nearer home, how the .moon revolves 
 around the earth. We see how all the planetary system revolves 
 around the sun. All these considerations \yere present to Huyghens 
 when he came to the conclusion that, whether the curious ap- 
 pendages were actually attached to the planet or were physically 
 free from it, they must still be in rotation. 
 
 Provided with such reasonings it soon became easy to conjecture 
 the true nature of the Saturnian system. We have seen how once 
 every fifteen years the appendages declined to invisibility and then 
 
 
SATURN. 239 
 
 gradually reappeared in the form, at first, of rectilinear arms pro- 
 jecting outwards from the planet. The progressive development 
 is a slow one, and for weeks and months, night after night, the 
 same appearance is presented with but little change. But all this 
 time both Saturn and the mysterious objects around him are rotating. 
 Whatever these may be, they present the same appearance to the 
 eye, notwithstanding their ceaseless motion of rotation. Now what 
 must be the shape of an object which satisfies the conditions here 
 implied ? It will obviously not suffice to regard the projections as 
 two spokes diverging from the planet. They would change from 
 visibility to invisibility in every rotation, and thus there would be 
 ceaseless alterations of the appearance instead of that slow and 
 gradual change which requires fifteen years for a complete period. 
 There are, indeed, other considerations which preclude the possibility 
 of the objects being anything of this character, for they are always 
 of the same length as compared with the diameter of the planet. 
 A little reflection will show that one supposition and indeed only 
 one will meet all the facts of the case. If there were a thin 
 symmetrical ring rotating in its own plane around the equator of 
 Saturn, then the persistence of the object from night to night 
 would be accounted for. This at once removes the greater part of 
 the difficulty. For the rest, it was only necessary to suppose that 
 the ring was so thin that when turned actually edgewise to the 
 earth it became invisible, and then as the illuminated side of the 
 plane became turned more and more towards the earth the appen- 
 dages to the planet gradually increased. The handle-shaped 
 appearance which the planet periodically assumed demonstrated 
 that the ring was not attached to the globe. At length 
 Huyghens found that he had the clue to the great enigma which 
 had perplexed astronomers for the last fifty years. He saw 
 that the ring was an object of the most astonishing interest, 
 unique at that time, as it is, indeed, unique still. He felt, how- 
 ever, that he had hardly demonstrated the matter with all the 
 certainty which it merited, and which he thought that by further 
 attention he could secure. Yet he was loth to hazard the loss of 
 his discovery by an undue postponement of its announcement^ 
 
240 THE STORY OF THE HEAVENS. 
 
 lest some other astronomer might intervene. How, then, was he to 
 secure his priority if the discovery should turn out correct, and at 
 the same time be enabled to perfect it at his leisure ? He adopted 
 the course not unusual at the time, of making his first announce- 
 ment in cipher, and accordingly, on March 5th, 1656, he pub- 
 lished a tract, which contained the following proposition : 
 
 aaaaaaa ccccc d eeeee g: li 
 
 iiiiiii 1111 mm mmnnnnnn 
 
 oooo pp q rr a ttttt uuuuu 
 
 Perhaps some of those curious persons whose successors now 
 devote so much labour to double acrostics, may have pondered on 
 this renowned cryptograph, and even attempted to decipher it. 
 But even if such attempts were made we do not learn that they 
 were successful. A few years of further study was thus secured 
 to Huyghens. He tested his theory in every way that he could 
 devise, and he found it verified in every detail. He therefore 
 thought that it was needless for him any longer to conceal from 
 the world his great discovery, and accordingly in the year 1659 
 about three years after the appearance of his cryptograph 
 he announced the interpretation of it. By restoring the letters 
 to their original arrangement the discovery was enunciated in 
 the following words : " Annulo cingitur, tenui, piano, nusqiiam 
 cokcerente, ad eclijpticam inclinato," which may be translated into 
 the statement : " The planet is surrounded by a slender flat ring 
 everywhere distinct from its surface, and inclined to the ecliptic." 
 
 Huyghens was not content with merely demonstrating how 
 fully this assumption explained all the observed phenomena. He 
 submitted it to the further and most delicate test which can be 
 applied to any astronomical theory. He attempted by its aid to 
 make a prediction the fulfilment of which would necessarily give 
 his theory the stamp of truth. From his calculations he saw that 
 the planet would appear circular, about July or August in 1671. 
 This anticipation was practically verified, for the ring was seen to 
 vanish in May of that year. No doubt, with our modern calcula- 
 tions founded on long-continued and accurate observation, we are 
 
SATURN. 241 
 
 now enabled to make forecasts as to the appearance or the dis- 
 appearance of Saturn's ring- with far greater accuracy; but, re- 
 membering the early stage in the history of the planet at which 
 the prediction of Huyghens was made, we must regard its fulfil- 
 ment as quite sufficient, and as confirming in a satisfactory manner 
 the theory of Saturn and his ring. 
 
 The ring of Saturn having thus been thoroughly established 
 as an astronomical certainty, each generation of astronomers has 
 laboured, and successfully laboured, to find out more and more of 
 its marvellous features. In the frontispiece we have a view of the 
 planet as seen at the Harvard College Observatory, U.S.A., 
 between July 28th and October 20th, 1872. It has been drawn 
 by the skilful astronomer and artist Mr. L. Trouvelot and gives 
 a faithful and beautiful representation of this unique object. To 
 realise from the plate the stupendous size of the whole system, it 
 may be remembered that the volume of Saturn is 700 times that 
 of the earth. 
 
 The next great discovery in the Saturnian system after those of 
 Huyghens, showed that the ring surrounding the planet was marked 
 by a dark concentric line, which divided the ring into two parts 
 the outer being narrower than the inner. This line was first seen 
 by J. D. Cassini, when Saturn emerged from the rays of the sun in 
 1675. That this black line is not merely a black mark on the ring, 
 but that it is actually a separation, was rendered very probable by 
 the researches of Maraldi in 1715, followed many years later by 
 those of Sir William Herschel, who, with that thoroughness which 
 was a marked characteristic of the man, made a most minute and 
 scrupulous examination of Saturn. Night after night he followed 
 it for hours with his unrivalled instruments, and considerably 
 added to our knowledge of the planet and his system. He devoted 
 very particular attention to the examination of the line dividing 
 the ring. He saw that the colour of this line was not to be dis- 
 tinguished from the colour of the space intermediate between the 
 globe and the ring. He observed it for ten years on the northern 
 face of the ring, and during that time it continued to present the 
 same breadth and colour and sharpness of outline. He was then 
 
 
242 THE STORY OF THE HEAVENS. 
 
 fortunate enough to observe the southern side of the ring. There 
 again could the black line be seen, corresponding both in appear- 
 ance and in position with the dark line as seen on the northern 
 side. Hardly a doubt could remain as to the fact that Saturn was 
 girdled by two concentric rings equally thin, and the outer edge of 
 one closely approaching to the inner edge of the other. At the 
 same time it is right to add, that the only absolutely indisputable 
 proof of the division between the rings has not yet been yielded 
 by the telescope. The appearances noted by Herschel would be 
 consistent with the view that the black line was merely a part of 
 the ring extending through its thickness, and composed of materials 
 very much less capable of reflecting light than the rest of the ring. 
 It is still a matter of doubt how far it is ever possible actually to 
 see through the dark line. There is apparently only one satisfac- 
 tory method of accomplishing this. It would only occur under rare 
 circumstances, and it does not seem that the opportunity has as 
 yet arisen. Suppose that in the course of its motion through the 
 heavens the path of Saturn happened to cross directly between the 
 earth and a fixed star. The telescopic appearance of a star is merely 
 a point of light much smaller than the globe and rings of Saturn. 
 If the ring passed in front of the star and the black line on the 
 ring came over the star, we would, if the black line were really an 
 opening, see the star shining through the narrow aperture. Up to 
 the present, we believe, there has been no opportunity of submitting 
 the question of the duplicity of the ring to this crucial test. Let 
 us hope that as Saturn is now in a position well suited for examina- 
 tion, and as there are now so many telescopes in use adequate to 
 deal with the subject, there may, ere long, be observations made 
 which will decide the question. It can hardly be expected that a 
 very small star would be suitable. No doubt the smallness of the 
 star would render the observations more delicate and precise if the 
 star were visible ; but we must remember, that the star will be thrown 
 into contrast with the bright rings of Saturn on each margin, so 
 that unless the star were of considerable magnitude it would hardly 
 answer. It has, however, been recently observed that the globe of 
 the planet can be, .in some degree, discerned through the dark line; 
 
SATUEN. 243 
 
 this is practically a demonstration of the fact that the line is at all 
 events partly transparent. 
 
 The outer ring is also divided into two by a line much fainter 
 than that just described. It requires a good telescope and a good 
 night, combined with a good position of the planet, to render 
 this line a well marked object. It is most easily seen at the 
 extremities of the ring most remote from the planet. To the 
 present writer, who has examined the planet with the twelve 
 inch refractor of the South equatorial at Dunsink Observatory, 
 this outer line appears as broad as the well-known line; but it 
 is unquestionably fainter and has a more shaded appearance. It 
 certainly does not suggest the appearance of being actually an 
 opening in the ring. It rather seems as if the ring were at this 
 place thinner, and possessed less substance without being actually 
 divided. 
 
 On these points it may be expected that much additional infor- 
 mation will be acquired when next the ring places itself in such a 
 position that its plane, if produced, would pass between the earth and 
 the sun. Such occasions are but rare, and even when they do occur, 
 it may happen that the planet will not be well placed for observa- 
 tion. The next really good opportunity will not occur till 1907. 
 In this case the sunlight illuminates one side of the ring, while 
 it is the other side of the ring that is presented towards the earth. 
 Powerful telescopes are necessary to deal with the planet under such 
 circumstances ; but it may be reasonably hoped that the questions 
 relating to the division of the ring, as well as to many other 
 matters, will then receive some further elucidation. 
 
 Occasionally, other divisions of the ring, both inner and outer, 
 have been recorded. It may, at all events, be stated that no such 
 divisions can be regarded as permanent features. Yet their exist- 
 ence has been so frequently enunciated by skilful observers that it 
 is impossible to doubt that they have been sometimes seen. 
 
 It was about 200 years after Huyghens had first offered the 
 true theory of Saturn, that another very interesting and important 
 discovery was effected. It had, up to the year 1850, been always 
 supposed that the two rings, divided by the well-known black line, 
 
244 THE STORY OF THE HEAVENS. 
 
 comnrised the entire ring system surrounding the planet. In the 
 year just mentioned, Professor Bond, the distinguished astronomer 
 of Cambridge, Mass., startled the whole astronomical world by 
 the announcement of his discovery of a third ring surrounding 
 Saturn. As so often happens in such cases, the same object was 
 discovered independently by the English astronomer Dawes. This 
 third ring lay just inside the inner of the two well-known rings, 
 and extended to within about half the distance towards the body of 
 the planet. This new ring is therefore of considerable size, and 
 occupies a conspicuous position : how then came it to pass that it 
 eluded not only the telescopes of Galileo and Huyghens, but many 
 more perfect instruments subsequently constructed ? How, in 
 fact, did this third ring succeed in escaping the penetration of the 
 great Sir William Herschel, who devoted so much attention to 
 observing Saturn and his system ? The third ring seems to be 
 of a totally different character to the two others, in so far as 
 they present a comparatively substantial appearance. We shall, 
 indeed, presently show that they are in all probability not solid 
 not even liquid bodies but still when compared with the third 
 ring the older ones are of quite a substantial character. They 
 can receive and exhibit the deeply marked shadow of Saturn, and 
 they can throw a deep and black shadow upon Saturn themselves ; 
 but the third ring seems of a much more spiritual texture. It has 
 not the brilliancy of the others, it is rather of a dusky semi- 
 transparent appearance, and the expression " crape ring/' by which 
 it is often designated, is by no means inappropriate. It is the 
 faintness of this crape ring which led to its being so frequently 
 overlooked by the earlier observers of Saturn. 
 
 It has often been noticed that when an astronomical discovery 
 has been made with a good telescope, it afterwards becomes pos- 
 sible for the same object to be observed with instruments of 
 much inferior power. No doubt, when the observer knows what 
 to look for, he will often be able to see what would not other- 
 wise have attracted his attention. It may be regarded as an 
 illustration of this principle, that the crape ring of Saturn has 
 become an object familiar to those who are accustomed to work 
 
SATURN. 245 
 
 with good telescopes ; but it may, nevertheless, be doubted whether 
 the ease and distinctness with which the crape ring is now seen can 
 be entirely accounted for by this supposition. Indeed, it seems 
 hardly possible to resist the supposition that the crape ring has, 
 from some cause or other, gradually become more and more visible. 
 The supposed recent appearance of the crape ring is one of those 
 arguments now made use of, to prove that in all probability the 
 rings of Saturn are at this moment undergoing gradual trans- 
 formation ; but observations of Hadley would seem to show that the 
 crape ring was perhaps seen by him in 1720, or even previously by 
 J. J. Cassini. 
 
 The various features of the rings are well shown in the 
 beautiful drawing of Trouvelot. We here see the inner and the 
 outer ring, and the line of division between them. We see in the 
 outer ring the faint traces of the line by which it is divided, and 
 inside the inner ring we have a view of the curious and semi- 
 transparent crape ring. The black shadow of the planet is cast 
 upon the ring, thus proving that the ring, no less than the body of 
 the planet, shines only in virtue of the sunlight which falls upon it. 
 This shadow presents some anomalous features, and its curious 
 irregularity may be, to some extent, an optical illusion. The 
 drawing contains no trace of those other and finer lines which 
 are more or less problematical, but it is a faithful representation of 
 the planet, under good seeing conditions, and viewed in a telescope 
 of considerable power. 
 
 There can be no doubt that any attempt to depict the rings of 
 Saturn can only represent the salient features of that marvellous 
 system. We are situated at such a great distance that all objects 
 not of colossal dimensions are invisible. We have, indeed, only 
 an outline, which makes us wish to be able to fill in the details. 
 We long, for instance, to see the actual texture of the rings, and 
 to learn of what materials they are made ; we wish to comprehend 
 the strange and filmy crape ring, so unlike any other object known 
 to us in the heavens. There is no doubt that much may even yet be 
 learned under all the disadvantageous circumstances of our position; 
 there is still room for the labour of whole generations of astronomers 
 
246 THE STORY OF THE HEAVENS. 
 
 provided with splendid instruments. We want accurate drawing's 
 of Saturn under every conceivable aspect in which it may be 
 presented. We want incessantly repeated measurements, of the 
 most fastidious accuracy. These measures are to tell us the sizes 
 and the shapes of the ring's ; they are to measure with fidelity the 
 position of the dark lines and the boundaries of the rings. These 
 measures are to be protracted for generations and for centuries; 
 then and then only can terrestrial astronomers learn whether this 
 elaborate system has really the attributes of permanence ; or 
 whether it may be undergoing changes of marvellous rapidity, when 
 the gigantic nature of the objects involved is considered. 
 
 We have been accustomed to find that the law of universal 
 gravitation pervades every part of our system, and to look to 
 gravitation as the source of the explanation of many phenomena 
 otherwise inexplicable. We have good reasons for knowing that 
 in this marvellous Saturnian system, the law of gravitation is 
 paramount. There are satellites revolving around Saturn as 
 well as a ring; these satellites move, as other satellites do, in 
 conformity with the laws of Kepler; and from them we learn that 
 Saturn is really an attracting body, and, therefore, any theory as to 
 the nature of Saturn's ring must be formed subject to the con- 
 dition that it shall be attracted by the gigantic planet situated in 
 its interior. 
 
 To a hasty glance nothing might seem easier than to reconcile 
 the phenomena of the ring with the attraction of the planet. 
 We might suppose that the ring stands at rest symmetrically 
 around the planet. At its centre the planet pulls in the ring 
 equally on all sides, so that there is no tendency in it to move in 
 one way rather than another ; and, therefore, it will stay at rest. 
 This will not do. A ring composed of materials almost infinitely 
 rigid might possibly, under such circumstances, be for a moment 
 at rest ; but it could not remain permanently at rest any more 
 than can a needle balanced vertically on its point. In each case 
 the equilibrium is unstable. If the slightest cause of disturbance 
 arise, the equilibrium is destroyed, and the ring would inevitably 
 fall in upon the planet. Such causes of derangement are in- 
 
SATURN. 247 
 
 cessantly present, so that unstable equilibrium cannot be an appro- 
 priate explanation of the phenomenon. 
 
 Even if this difficulty could be removed, there is still another, 
 which would be quite insuperable if the ring be composed of any 
 materials with which we are acquainted. Let us ponder for a 
 moment on the matter, as it will lead up naturally to that ex- 
 planation of the rings of Saturn which is now most generally 
 accepted. 
 
 Imagine that you stood on the planet Saturn, near his equator ; 
 over your head stretches the ring which sinks down to the horizon 
 in the east and in the west. The half-ring above your horizon 
 would then resemble a mighty arch, whose span was about a 
 hundred thousand miles. Every particle of this arch is drawn 
 towards Saturn by gravitation, and if the arch continue to exist, 
 it must do so in pursuance of the ordinary mechanical laws 
 which regulate the railway arches with which we are familiar. 
 The subsistence of these arches depends upon the resistance of 
 the stones forming them to a crushing force. Each stone of an 
 arch is subjected to a vast pressure, but stone is a material capable 
 of resisting such pressure, and the arch remains. The wider the 
 span of the arch, the greater is the pressure to which each stone is 
 exposed. At length a span is reached which corresponds to a 
 pressure as great as the stones can safely bear, and accordingly 
 we thus find the limiting span over which a single arch of 
 masonry can be constructed. Apply these principles to the 
 stupendous arch formed by the ring of Saturn. It can be shown 
 that the pressure on the materials of the arch capable of spanning 
 an abyss of such awful magnitude would be something so enormous 
 that no materials we know of would be capable of bearing it. Were 
 the ring formed of the toughest steel that was ever made, the 
 pressure would be so great that the metal would be squeezed like a 
 liquid, and the mighty structure would collapse and fall down on 
 the surface of the planet. It is not credible that any materials 
 could exist capable of sustaining a stress so stupendous. The law 
 of gravitation accordingly bids us to search for a method by which 
 the intensity of this stress can be mitigated. 
 
248 THE STORY OF THE HEAVENS. 
 
 One method is at hand, and is obviously suggested by analogous 
 phenomena everywhere in our system. We have spoken of the 
 ring as if it were at rest; let us now suppose it to be animated 
 by a motion of rotation in its plane around Saturn as a centre. 
 Instantly we have a force developed directly antagonistic to the 
 gravitation of Saturn. This force is the so-called centrifugal force. 
 If, then, we imagine the ring to rotate, the centrifugal force at 
 all points acts in an opposite direction to the attractive force, and 
 hence the enormous stress on the ring can be abated, and one 
 difficulty can be overcome. 
 
 The theory of gravitation alone would thus tell us that the ring 
 must rotate, but we are fortunately not left merely to such specu- 
 lative inferences. That the ring does rotate, William Herschel 
 proved in 1789. In July of that year it happened that the edge of 
 the ring was directly turned towards the earth. In a small tele- 
 scope the thin edge of the ring ceases, under such circumstances, 
 to be visible ; but in Herschel's great reflecting telescope, twenty 
 feet long, the edge still remained as a broken line of light. He 
 was able to distinguish certain spots of light on this line, and he 
 perceived that they gradually moved; he saw that they travelled 
 from one end of the fine line to the other; he ascertained that 
 this was really due to the rotation of the ring ; and he determined 
 the period of rotation from these observations to be about 10 hours, 
 32 minutes, and 15 seconds. 
 
 Such is the period of rotation of the outer ring, and we can also 
 attribute to the inner ring a rotation which will partly relieve it 
 from the stress the arch would otherwise have to sustain. But we 
 cannot admit that the difficulty has been fully removed. Suppose 
 that the outer ring revolve at such a rate as shall be appropriate to 
 neutralise the gravitation on its outer edge, the centrifugal force 
 will be less at the interior of the ring, while the gravitation will be 
 greater ; and hence vast stresses will be set up in the interior parts 
 of the outer ring. Or suppose the ring to rotate at such a rate as 
 would be adequate to neutralise the gravitation at its inner margin ; 
 then the centrifugal force at the outer parts will largely exceed the 
 gravitation, and there will be a tendency to disruption of the ring 
 
SATURN. 249 
 
 outwards. To obviate this tendency we may suppose the outer parts 
 of each ring to rotate more slowly than the inner parts. This 
 naturally requires that the parts of the ring shall be mobile 
 relatively to one another, and thus we are conducted to the sug- 
 gestion that perhaps the rings are really composed of matter in a 
 fluid state. The suggestion is a plausible one, and superficially 
 it will, no doubt, account for the phenomena; each part of each 
 ring can then move with an appropriate velocity, and the rings 
 would thus exhibit a number of concentric circular currents with 
 different velocities. The mathematician can push this inquiry a 
 little farther, and he can study how this fluid would behave under 
 such circumstances. His symbols can pursue the subject into the 
 intricacies which cannot be described in general language. The 
 mathematician finds that waves would originate in the supposed 
 fluid, and that as these waves would lead to disruption of the 
 rings, the fluid theory must be abandoned. 
 
 But we can still make one or two more suppositions. What if it 
 be really true that the ring consist of an incredibly large number of 
 concentric rings, each animated precisely with the velocity which 
 would be suitable to the production of a centrifugal force just 
 adequate to neutralise the attraction ? No doubt this meets many 
 of the difficulties : it is also suggested by those observations which 
 have shown the presence of several dark lines on the ring. Here 
 again dynamical considerations must be invoked for the reply. Such 
 a system of solid rings revolving in the way that would be required 
 is not compatible with the laws of dynamics. We are, therefore, 
 compelled to make one last attempt, and still further to subdivide 
 the ring. It may seem rather startling to abandon entirely the sup- 
 position that the ring is in any sense a continuous body, but there 
 remains no alternative. Look at it how we will, we seem to be 
 conducted to the conclusion that the ring is really an enormous 
 shoal of extremely minute bodies ; each of these little bodies pur- 
 sues an orbit of its own around the planet, and is, in fact, 
 merely a satellite. These bodies are so numerous and so close 
 together that they seem to us to be continuous, and they may 
 be very minute perhaps not larger than the globules of water 
 
250 THE STORY OF THE HEAVENS. 
 
 found in an ordinary cloud over the surface of the earth, which, 
 even at a short distance, seems like a continuous body. 
 
 As Saturn's ring is itself unique, we cannot find elsewhere 
 any very pertinent illustration of a structure so remarkable as that 
 now claimed for the ring. Yet the solar system does show some 
 analogous phenomena. There is, for instance, one on a very grand 
 scale surrounding the sun himself. We allude to the multitude of 
 minor planets, all confined within a certain region of the system. 
 Imagine these planets to be vastly increased in number, and the 
 orbits of the irregular ones flattened down and otherwise adjusted, 
 and we should have a ring surrounding the sun, thus producing an 
 arrangement not dissimilar from that now attributed to Saturn. 
 On another and indeed a truly majestic scale, the heavens present 
 us with an imposing spectacle of an enormous myriad of bodies 
 forming a ring. We shall in a future chapter refer to that most 
 extensive object, the Milky Way, which is a vast zone of stars 
 girdling the entire heavens. No one supposes that the individual 
 points forming the Milky Way are small objects. It would be 
 more reasonable to suppose that they were splendid suns, perhaps 
 in many cases rivalling, or it may be excelling, our own sun 
 in brilliancy. But even admitting this, the objects forming 
 the Milky Way are extremely small when compared with the 
 dimensions of the vast zone in which they are placed; and to this 
 extent at least we may find in them an analogue on the most 
 stupendous scale to the phenomenon of Saturn's ring. 
 
 It is tempting to linger still longer over this beautiful system, 
 to speculate on the appearance which the ring would present to an 
 inhabitant of Saturn 3 to conjecture whether it is to be regarded as a 
 permanent feature of our system in the same way as we attribute 
 permanence to our moon or to the satellites of Jupiter. Looked 
 at from every point of view, the question is full of interest, and 
 it provides occupation abundant for the labours of every type of 
 astronomer. If he be furnished with a good telescope, then has he 
 ample duties to fulfil in the task of surveying, of sketching, and 
 of measuring. If he be one of those useful astronomers who 
 devote their energies not to actual telescopic work, but to forming 
 
SATURN. 251 
 
 calculations based on the observations of others, then the beautiful 
 system of Saturn provides copious material. He has to foretell 
 the different phases of the ring, to announce to astronomers when 
 each feature can be best seen, and at what hour each element 
 can be best determined. He has also to predict the times of 
 the movements of Saturn's satellites, and the other phenomena 
 of a system more elaborate than that of Jupiter; or, lastly, 
 if the astronomer be one of that class perhaps, from some 
 points of view, the highest class of all who employ the most 
 profound researches of the human intellect to unravel the dynamical 
 problems of astronomy, he, too, finds in Saturn problems which 
 task to the utmost, even if they do not utterly transcend, the 
 loftiest flights of analysis. He finds in Saturn's ring an object 
 so utterly unlike anything else, that new weapons of analysis have 
 to be forged for the encounter. He finds in the system so many 
 extraordinary features, and such delicacy of adjustment, that he is 
 constrained to admit that if he did not actually see Saturn's rings 
 before him, he would not have thought that such a system was 
 possible. The mathematician's labours on this wondrous system 
 are at present only in their infancy. Not alone are the researches 
 of so abstruse a character as to demand the highest mathematical 
 genius, but even yet the materials for the inquiry have not been 
 accumulated. In a discussion of this character, observation must 
 precede calculation. The scanty observations hitherto obtained, 
 however they may illustrate the beauty of the system, are still 
 utterly insufficient to form the basis of that great mathematical 
 theory of Saturn which must eventually be written. 
 
 But Saturn possesses an interest for a far more numerous class 
 of persons than those who are specially devoted to astronomy. It 
 is of interest, it must be of interest, to every cultivated person 
 who has the slightest love for nature. A lover of the picturesque 
 cannot behold Saturn in a telescope without feelings of the live- 
 liest emotion ; while, if his reading and reflection have previously 
 rendered him aware of the colossal magnitude of the object at which 
 he is looking, he will be constrained to admit that no more remark- 
 able spectacle is presented in the whole realm of natural scenery. 
 
250 THE 8TORT OF THE HEAVENS. 
 
 found in an ordinary cloud over the surface of the earth, which, 
 even at a short distance, seems like a continuous body. 
 
 As Saturn's ring is itself unique, we cannot find elsewhere 
 any very pertinent illustration of a structure so remarkable as that 
 now claimed for the ring. Yet the solar system does show some 
 analogous phenomena. There is, for instance, one on a very grand 
 scale surrounding the sun himself. We allude to the multitude of 
 minor planets, all confined within a certain region of the system. 
 Imagine these planets to be vastly increased in number, and the 
 orbits of the irregular ones flattened down and otherwise adjusted, 
 and we should have a ring surrounding the sun, thus producing an 
 arrangement not dissimilar from that now attributed to Saturn. 
 On another and indeed a truly majestic scale, the heavens present 
 us with an imposing spectacle of an enormous myriad of bodies 
 forming a ring. We shall in a future chapter refer to that most 
 extensive object, the Milky Way, which is a vast zone of stars 
 girdling the entire heavens. No one supposes that the individual 
 points forming the Milky Way are small objects. It would be 
 more reasonable to suppose that they were splendid suns, perhaps 
 in many cases rivalling, or it may be excelling, our own sun 
 in brilliancy. But even admitting this, the objects forming 
 the Milky Way are extremely small when compared with the 
 dimensions of the vast zone in which they are placed ; and to this 
 extent at least we may find in them an analogue on the most 
 stupendous scale to the phenomenon of Saturn's ring. 
 
 It is tempting to linger still longer over this beautiful system, 
 to speculate on the appearance which the ring would present to an 
 inhabitant of Saturn 3 to conjecture whether it is to be regarded as a 
 permanent feature of our system in the same way as we attribute 
 permanence to our moon or to the satellites of Jupiter. Looked 
 at from every point of view, the question is full of interest, and 
 it provides occupation abundant for the labours of every type of 
 astronomer. If he be furnished with a good telescope, then has he 
 ample duties to fulfil in the task of surveying, of sketching, and 
 of measuring. If he be one of those useful astronomers who 
 devote their energies not to actual telescopic work, but to forming 
 
SATURN. 251 
 
 calculations based on the observations of others, then the beautiful 
 system of Saturn provides copious material. He has to foretell 
 the different phases of the ring, to announce to astronomers when 
 each feature can be best seen, and at what hour each element 
 can be best determined. He has also to predict the times of 
 the movements of Saturn's satellites, and the other phenomena 
 of a system more elaborate than that of Jupiter; or, lastly, 
 if the astronomer be one of that class perhaps, from some 
 points of view, the highest class of all who employ the most 
 profound researches of the human intellect to unravel the dynamical 
 problems of astronomy, he, too, finds in Saturn problems which 
 task to the -utmost, even if they do not utterly transcend, the 
 loftiest flights of analysis. He finds in Saturn's ring an object 
 so utterly unlike anything else, that new weapons of analysis have 
 to be forged for the encounter. He finds in the system so many 
 extraordinary features, and such delicacy of adjustment, that he is 
 constrained to admit that if he did not actually see Saturn's rings 
 before him, he would not have thought that such a system was 
 possible. The mathematician's labours on this wondrous system 
 are at present only in their infancy. Not alone are the researches 
 of so abstruse a character as to demand the highest mathematical 
 genius, but even yet the materials for the inquiry have not been 
 accumulated. In a discussion of this character, observation must 
 precede calculation. The scanty observations hitherto obtained, 
 however they may illustrate the beauty of the system, are still 
 utterly insufficient to form the basis of that great mathematical 
 theory of Saturn which must eventually be written. 
 
 But Saturn possesses an interest for a far more numerous class 
 of persons than those who are specially devoted to astronomy. It 
 is of interest, it must be of interest, to every cultivated person 
 who has the slightest love for nature. A lover of the picturesque 
 cannot behold Saturn in a telescope without feelings of the live- 
 liest emotion ; while, if his reading and reflection have previously 
 rendered him aware of the colossal magnitude of the object at which 
 he is looking-, he will be constrained to admit that no more remark- 
 
 o" 
 
 able spectacle is presented in the whole realm of natural scenery. 
 
252 THE STORY OF THE HEAVENS. 
 
 We have pondered so long- over the fascinations of Saturn's 
 ring-, that we can only give a very brief account of that system of 
 satellites by which the planet is attended. We have already had 
 occasion to allude more than once to these bodies ; it only remains 
 now to enumerate a few further particulars. 
 
 It was on the 25th of March, 1655, that the first satellite of 
 Saturn was detected by Huyghens, to whose penetration we owe 
 the discovery of the true form of the ring-. On the evening of 
 the day referred to, Huyghens was examining Saturn with a tele- 
 scope constructed with his own hands, when he observed a small 
 starlike object near the planet. The next night he repeated his 
 observations, and it was found that the star was accompanying the 
 planet in its progress through the heavens. This showed that the 
 little object was really a satellite to Saturn, revolving around him 
 in a period of 15 days, 22 hours, 41 minutes. Such was the 
 commencement of that numerous series of discoveries of satellites 
 which accompany Saturn. One by one they were detected, so 
 that at the present time no fewer than eight are known to attend 
 the great planet through his wanderings. The subsequent dis- 
 coveries were, however, in no case made by Huyghens, for he 
 abandoned the search for any further satellites on grounds which 
 sound strange to modern ears. It appears that from some principle 
 of symmetry, Huyghens thought that it would accord with the 
 fitness of things that the number of satellites, or secondary planets, 
 should be equal in number to the primary planets themselves. 
 The primary planets, including the earth, numbered six; and 
 Huyghens' discovery now brought the total number of satellites 
 to be also six. The earth had one, Jupiter had four, Saturn had 
 one, and the system was complete. 
 
 Nature, however, knows no such arithmetical doctrines as those 
 which Huyghens attributed to her. Had he been less influenced 
 by such prejudices, he might, perhaps, have anticipated the 
 labours of Cassini, who, by discovering other satellites of Saturn, 
 demonstrated the absurdity of the doctrine of numerical equality 
 between planets and satellites. As further discoveries were made, 
 the number of satellites was at first raised above the number of 
 
 
SATURN. 253 
 
 planets; but in recent times, when the swarm of minor planets 
 came to be discovered, the number of planets speedily reached and 
 speedily passed the number of satellites. 
 
 It was in the year 1671, about sixteen years after the discovery 
 of the first satellite of Saturn, that a second was discovered by 
 Cassini. In the following year he discovered another ; and twelve 
 years later, in 1684, still two more; thus making a total of five 
 satellites to this planet. 
 
 The complexity of the Saturnian system had now no rival in 
 the heavens. Saturn had five satellites, and Jupiter had but four, 
 while at least one of the satellites of Saturn, named Titan, 
 was larger than any satellite of Jupiter. The discoveries of 
 Cassini had been made with telescopes of most portentous dimen- 
 sions. The length of the instrument, or rather the distance at 
 which the object-glass was placed, was one hundred feet or 
 more from the eye of the observer. It seemed hardly possible 
 to push telescopic research farther with instruments of this cum- 
 brous type. At length, however, the great reformation in the 
 construction of astronomical telescopes began to dawn. In the 
 hands of Herschel, it was found possible to construct reflecting 
 telescopes of manageable dimensions, vastly more powerful and 
 vastly more accurate than the long-focussed lenses of Cassini. A 
 great instrument of this kind forty feet long, just completed by 
 Herschel, was directed to Saturn on the 28th of August, 1789. 
 Never before had the wondrous planet been submitted to a scrutiny 
 so minute. Herschel was familiar with the labours of his pre- 
 decessors. He had often looked at Saturn and his five moons in 
 inferior telescopes; now again he saw the five moons and a star-like 
 object so near the plane of the ring that he conjectured this to be 
 a sixth satellite. A speedy method of testing this conjecture was 
 at hand. Saturn was then moving rapidly over the heavens. If 
 this new object were in truth a satellite, then it must be carried on 
 by Saturn. Herschel watched with anxiety to see whether this 
 would be the case. A short time sufficed to answer the question : 
 in two hours and a half the planet had moved to a distance quite 
 appreciable, and had carried with him not only the five satellites 
 
254 THE STORY OF THE HEAVENS. 
 
 already known, but also this sixth object. Had this been a star it 
 would have been left behind ; .it was not left behind, and hence 
 it, too, was a satellite. Thus, after the long lapse of a century, 
 the telescopic discovery of satellites to Saturn recommenced. 
 Herschel, as was his wont, observed this object with unre- 
 mitting ardour, and discovered that it was much nearer to Saturn 
 than any of the previously-known satellites. In accordance with 
 the general law, that the nearer the satellite the shorter the 
 period of revolution, Herschel found that this little moon com- 
 pleted a revolution in about 1 day, 8 hours, 53 minutes. The 
 same great telescope used with the same unrivalled skill, soon led 
 Herschel to a still more interesting discovery. An object so small 
 as only to appear like a very minute point in the great forty-foot 
 reflector was also detected by Herschel, and was by him proved to 
 be a satellite, so close to the planet that it completed a revolution 
 in the very brief period of 22 hours and 37 minutes. This is 
 an extremely delicate object, only to be seen in the best telescopes 
 in the brief intervals when it is not entirely screened from view by 
 the ring. 
 
 Again another long interval elapsed, and for almost fifty years 
 the Saturnian system was regarded as consisting of the series of 
 rings and of the seven satellites. The next discovery has a 
 singular historical interest. It was made simultaneously by two 
 observers Professor Bond, of Cambridge, Mass., and Mr. Lassell, 
 of Liverpool for on the 19th September, 1848, each of these astro- 
 nomer verified that a small point seen by each on previous nights 
 was really a satellite. This object is, however, at a considerable 
 distance from the planet, and requires 21 days, 7 hours, 28 minutes 
 for each revolution. 
 
 Such is at present the system of Saturn as we know it. It 
 seems certain that if there be any more satellites than the ample 
 number of eight already discovered, they must be objects much 
 more minute than those already detected. Accomplished observers, 
 provided with telescopes more powerful than that of Bond, and in 
 some cases more powerful than that of Lassell, have examined 
 and re-examined this object. They have made out many further 
 
SATURN. 255 
 
 details of the Saturnian system, but no further satellite has been 
 added. 
 
 A law has been observed by Professor Kirkwood, which con- 
 nects tog-ether the movements of four of the satellites of Saturn. 
 This law is fulfilled in such a manner as leads to the supposition 
 that it arises from the mutual attraction of the satellites. We 
 have already described a similar law relative to three of the 
 satellites of Jupiter. The problem relating to Saturn, involving as 
 it does no fewer than four satellites, is one of no ordinary com- 
 plexity. It involves the theory of Perturbations to a greater 
 degree than that to which mathematicians are accustomed in their 
 investigation of the more ordinary features of our system. To 
 express this law it is necessary to have recourse to the daily move- 
 ments of the satellites ; these are respectively 
 
 SATELLITE. DAILY MOVEMENT. 
 
 1 382-2. 
 
 II 262'74, 
 
 III 190-7. 
 
 IV 131-4. 
 
 The law states that if to five times the movement of the first 
 satellite we add that of the third and four times that of the fourth, 
 the whole will equal ten times the movement of the second satellite. 
 The calculation stands thus : 
 
 5 times I. equals 1911'0 
 
 III. 190'7 n. ... 262-74 
 
 4 times IV. 525 -6 10 
 
 2627'3 equals 2627 '4 nearly. 
 
 Nothing can be simpler than the verification of this law ; but 
 the task of showing the physical reason why it should be fulfilled 
 has not yet been accomplished. 
 
 Saturn was the most distant planet known to the ancients. It 
 revolves in an orbit far outside the other ancient planets, and until 
 the discovery of Uranus a century ago, the orbit of Saturn might 
 well be regarded as the frontier of the solar system. Saturn was 
 indeed an object worthy to occupy a position so distinguished. But 
 
256 THE STORY OF THE HEAVENS. 
 
 we now know that the mighty orbit of Saturn does not extend 
 to the frontiers of the solar system ; a splendid discovery, leading 
 to one still more splendid, has vastly extended the boundary, by 
 revealing two mighty planets, revolving in dim telescopic distance, 
 far outside the path of Saturn. These objects have not the beauty 
 of Saturn; they are much less brilliantly attended; they are, indeed, 
 in no sense effective telescopic pictures. Yet these outer planets 
 awaken an interest of a most special and remarkable kind. The 
 discovery of each is a classical event in the history of astronomy, 
 and the opinion has been maintained, and perhaps not without good 
 reason, that the discovery of Neptune, the more remote of the two, 
 is the greatest achievement in astronomy made since the time of 
 Newton. To the development of these discoveries our next chapters 
 must be devoted. 
 
CHAPTER XIY. 
 
 URANUS. 
 
 Contrast between Uranus and the other great Planets William Herschel His 
 Birth and Parentage Herschel' s Arrival in England His love of Learning 
 Commencement of his Astronomical Studies The Construction of Telescopes 
 Reflecting Telescopes Construction of Mirrors The Professor of Music be- 
 comes an Astronomer The Methodical Research The 13th March, 1781 The 
 Discovery of Uranus Delicacy of Observation Was the Object a Comet ? 
 The Significance of this Discovery The Fame of Herschel George III. and 
 the Bath Musician The King's Astronomer at Windsor Caroline Herschel 
 The Planet Uranus Numerical Data with reference thereto The Four Satel- 
 lites of Uranus Their Circular Orbits Early Observations of Uranus 
 Flamsteed's Observations Lemonnier saw Uranus Utility of their Measure- 
 mentsThe Elliptic Path The great Problem thus Suggested. 
 
 To the present writer it has always seemed that the history of 
 Uranus, and of the circumstances attending its discovery, form one 
 of the most pleasing and interesting episodes in the whole history 
 of science. We here occupy an entirely new position in the study 
 of the solar system. All the other great planets were familiarly 
 known to the ancients, however erroneous might be the ideas 
 entertained in connection with them. They were conspicuous 
 objects, and by their movements could hardly fail to attract the 
 attention of those whose pursuits led them to observe the stars. 
 But now we come to a great planet, whose very existence was utterly 
 unknown to the ancients ; and hence, in approaching the subject, 
 we have first to describe the actual discovery of this object, and 
 then to consider what we can learn as to its physical nature. 
 
 We have, in preceding pages, had occasion to mention the 
 revered name of William Herschel in connection with various 
 branches of astronomy ; but we have hitherto designedly postponed 
 any more explicit reference to this extraordinary man until we have 
 arrived at the present stage of our work. The story of Uranus, 
 B 
 
258 TEE STORY OF THE HEAVENS. 
 
 in its earlier stages at all events, is the story of the early career of 
 William Herschel. It would be alike impossible and undesirable 
 to attempt to separate them. 
 
 William Herschel, the illustrious astronomer, was born at 
 Hanover in 1738. His father was an accomplished man, pursuing, 
 in a somewhat humble manner, the calling of a Professor of Music. 
 He had a family of ten children, of whom William was the fourth; 
 and it may be noted that all the family of whom any record has 
 been preserved inherited their father's musical talents, and became 
 accomplished performers. Pleasing sketches have been given of 
 this interesting family, of the unusual aptitude of William, of the 
 long discussions on music and on philosophy, and of the little 
 sister Caroline, destined in later years for an illustrious career. 
 William soon learned all that his master could teach him in the 
 ordinary branches of knowledge, and by the age of fourteen he 
 was already a competent performer on the oboe and the viol. He 
 was engaged in the Court orchestra at Hanover, and was also a 
 member of the band of the Hanoverian Guards. Troublous times 
 were soon to break up HerscheFs family. The French invaded 
 Hanover, the Hanoverian Guards were overthrown in the battle of 
 Hastenbeck, and young William Herschel had some unpleasant 
 experience of actual warfare. He was not wounded, but after 
 passing the night in a ditch, he decided that he would make a 
 change in his profession. His method of doing so is one which 
 his biographers can scarcely be expected to defend ; for, to speak 
 plainly, he deserted, and succeeded in making his escape to England. 
 It is stated on unquestionable authority that on Herschel's first visit 
 to King George III. more than twenty years afterwards, his pardon 
 was handed to him by the King himself, written out in due form. 
 
 At the age of nineteen, the young musician began to seek 
 his fortunes by his profession in England. He met at first with 
 very considerable hardship, but industry and skill conquered all 
 difficulties, and by the time he was twenty-six years of age, he 
 was thoroughly settled in England, and doing well in his pro- 
 fession. In the year 1766, we find Herschel occupying a position 
 of some distinction in the musical world: he had become the 
 
URANUS. 259 
 
 organist of the Octagon Chapel at Bath, and his time was fully 
 employed in giving lessons to his numerous pupils, and with his 
 preparation for concerts and for oratorios. 
 
 Notwithstanding his busy professional life, Herschel still re- 
 tained that insatiable thirst for knowledge which he had when a 
 boy. Every moment he could snatch from his musical engage- 
 ments was eagerly devoted to study. In his desire to perfect his 
 knowledge of the more abstruse parts of the theory of music, he 
 had occasion to learn mathematics ; from mathematics the transition 
 to optics was a natural one ; and once he had commenced to study 
 optics, he was of course brought to a knowledge of the telescope, 
 and thence to astronomy itself. 
 
 His beginnings were made on a very modest scale. It was 
 through a. small and imperfect telescope that the great astronomer 
 obtained his first view of the celestial glories. No doubt he had 
 often before looked at the heavens on a clear night, and admired 
 the thousands of stars with which it was adorned ; but now, when 
 he was able to increase his powers of vision even to a slight extent, 
 he obtained a view which fascinated him. The stars he saw before, 
 he now saw far more distinctly ; but more than this, he found that 
 myriads of others previously invisible were now revealed to him. 
 Glorious, indeed, is this spectacle to any one who possesses a spark 
 of enthusiasm for natural beauty. To Herschel this view im- 
 mediately changed the whole current of his life. His success as a 
 Professor of Music, his oratorios and his pupils, were speedily to be 
 forgotten, and the rest of his life was to be devoted to the absorb- 
 ing pursuit of one of the noblest of the sciences. 
 
 Herschel could not remain contented with the small and 
 imperfect instrument which first interested him. Throughout his 
 career he determined to see everything for himself in the best 
 manner which his utmost powers could command. He at once 
 determined to have a better v instrument, and he wrote to a cele- 
 brated optician in London with a view of purchasing one. But 
 the price which the optician demanded seemed more than Herschel 
 thought he could or ought to give. Instantly his determination 
 was taken. A good telescope he must have, and as he could not 
 R 2 
 
262 THE STORY OF THE HEAVENS. 
 
 Herschel as a laborious musician, with considerable renown in his 
 profession, not only in Bath, but throughout the west of Eng- 
 land. His telescope, making was merely the occupation of his spare 
 moments, and was quite unknown to most of those who knew and 
 respected his musical attainments. It was in 1774 that Herschel 
 first enjoyed a view of the heavens through an instrument built 
 with his own hands. It was but a small one in comparison with 
 those which he afterwards fashioned, but at once he experienced 
 the advantage of being his own instrument maker. Night after 
 night he was able to make one improvement after another ; at one 
 time he was enlarging the size of his mirrors; at another he was 
 reconstructing the mounting, and trying to remedy defects in the 
 eye-pieces. With unwearying perseverance he aimed at the highest 
 excellence, and with each successive advance he found that he was 
 able to see more and more. His enthusiasm attracted to him a 
 few friends who were, like himself, ardently attached to science. 
 The mode in which he first made the acquaintance of Sir William 
 Watson, who afterwards became his warmest friend, was cha- 
 racteristic of both. Herschel was observing the mountains in the 
 moon, and as the hours passed on, he had occasion to bring his 
 telescope into the street in front of his house to enable him 
 to continue his work. Sir William Watson happened to pass 
 by, and was arrested by the somewhat unusual spectacle of an 
 astronomer in the public street, at the dead of night, using a 
 large and somewhat quaint-looking instrument. Having a love 
 for astronomy, he stopped, and when Herschel took his eye from 
 the telescope, asked, if he might be allowed to have a view of the 
 moon. The request was readily granted. Probably Herschel at 
 that time found but few in the gay city who cared for such 
 matters ; he was quickly drawn to Sir W. Watson, who at once 
 reciprocated the feeling, and thus began a friendship which bore 
 important fruit in HerscheVs subsequent career. 
 
 At length the year 1781 approached, which was to witness 
 his great achievement. Herschel had now some six or seven years' 
 practical experience in astronomy, and he had completed a tele- 
 scope of exquisite optical perfection, though greatly inferior in 
 
URANUS. 263 
 
 point of size to those which he afterwards erected. With this 
 telescope Herschel commenced a methodical piece of observation. 
 He formed the scheme of systematically examining all the stars 
 which were ahove a certain degree of brightness. It does not quite 
 appear what object Herschel proposed to himself when he under- 
 took this labour, but, in any case, he could hardly have anticipated 
 the extraordinary success with which the work was to be crowned. In 
 the course of this review, the telescope was directed to a star ; that star 
 was examined ; then another was brought into the field of view, and 
 it too was examined. Every star under such circumstances merely 
 shows itself as a point of light ; the point may be brilliant or not, 
 according as the star is bright or not ; the point will also, of course, 
 show the colour of the star, but it cannot show any recognisable size 
 or shape. The greater, in fact, the perfection of the telescope, the 
 smaller is the telescopic image of a star. 
 
 How many stars Herschel looked at in this review, we are not 
 told ; but at all events, on the ever-memorable night of the 13th 
 of March, 1781, he was pursuing his self-allotted task among the 
 stars in the constellation Gemini. Doubtless one star after another 
 was admitted to view, and was allowed to pass away. At length, 
 however, a star was placed in the field which differed from every 
 other star. It was not a mere point of light ; it had a minute, but 
 still a perfectly recognisable, disc. We say the disc was perfectly 
 recognisable, but we should be careful to add that it was so to the 
 delicate eye of Herschel alone. Other astronomers had often seen 
 this object before. It had actually been most deliberately and 
 carefully measured no fewer than seventeen times on former oc- 
 casions before the Bath musician, with his home-made telescope, 
 looked at it. Herschel perceived that this object was not what 
 previous astronomers had supposed it to be ; it was, indeed, not a 
 star at all. Even after the discovery was made, and when well- 
 trained observers with good instruments looked again under the 
 direction of Herschel at this object, one after another has borne 
 testimony to the extraordinary delicacy of Herschel's perception, 
 which enabled him almost at the first glance to discriminate between 
 this object and a star. 
 
264 THE 8TORT OF THE HEAVENS. 
 
 If not a star, what, then, could it be ? The first step to enable 
 this question to be answered was to observe the body for some 
 time. This Herschel did. He looked at it one night after another, 
 and soon he discovered another most fundamental difference between 
 this object and an ordinary star. The stars are, of course, charac- 
 terised by their fixity, but this object was not fixed; night after 
 night the place it occupied changed with respect to the stars. No 
 longer could there be any doubt that this body was a member of 
 the solar system, and that an interesting discovery had been made ; 
 many months however elapsed before Herschel knew the real merit 
 of his achievement. It does not seem at first to have occurred to 
 him that he had made the superb discovery of another mighty 
 planet revolving outside Saturn ; he thought at first that it could 
 only be a comet. No doubt this object looks very different from 
 the ordinary conception of a comet, which is decorated with a tail. 
 It was not, however, so entirely different from some forms of tele- 
 scopic comet as to make the suggestion of its being a body of this 
 kind unlikely ; and, in fact, the discovery was at first announced 
 to be that of a new comet. Time was necessary before the true 
 character of the object could be ascertained. It must be followed 
 for a considerable distance along its path, and measures of its posi- 
 tion at different epochs must be effected, before it is practicable for 
 the mathematician to calculate the path which the body pursues ; 
 once, however, attention was devoted to the subject, many astro- 
 nomers aided in making the necessary observations. These were 
 placed in the hands of mathematicians, and the result was pro- 
 claimed that this body was not a comet, but that, like all the 
 planets, it revolved in a nearly circular path around the sun, and 
 that the path lay millions of miles outside the path of Saturn, which 
 had so long been regarded as the boundary of the solar system. 
 
 It is hardly possible to over-estimate the significance of this 
 splendid discovery. The five planets had been known from all 
 antiquity ; they were all, at suitable seasons, brilliantly conspicuous 
 to the unaided eye. But Herschel showed that, far outside the 
 outermost of these planets, there was another splendid planet, larger 
 than Mercury or Mars, larger far larger than Venus and the 
 
UfaiNUS. 265 
 
 earth, and only surpassed in bulk by Jupiter and by Saturn. This 
 superb new planet was plunged into space to such a depth that, 
 notwithstanding its noble proportions, it seemed merely a tiny star, 
 being only on rare occasions within reach of the unaided eye. 
 Herschel showed that this great planet required a period of eighty- 
 seven years to complete its majestic path, and that the diameter of 
 that path was 3,600,000,000 miles. 
 
 Although the history of astronomy is the record of brilliant 
 discoveries of the discoveries of Copernicus, and of Kepler of the 
 telescopic achievements of Galileo, and the splendid theory of 
 Newton of the refined discovery of the aberration of light of 
 many other imperishable triumphs of intellect yet this achieve- 
 ment of the organist at the Octagon Chapel occupies a totally 
 different position from any other. There never before had been any 
 historic record of the discovery of one of the bodies of the particular 
 system to which the earth belongs. The older planets were no 
 doubt discovered by some one, but we can say little more about 
 these discoveries than we can about the discovery of the sun or of the 
 moon ; all are alike prehistoric. Here, then, was the first recorded 
 instance of the discovery of a planet which, like our earth, revolves 
 around the sun, and, like our earth, may conceivably be an in- 
 habited globe. So unique an achievement instantly arrested the 
 attention of the whole scientific world. The music-master at Bath, 
 hitherto unheard of as an astronomer, was speedily placed in the 
 very foremost rank of those entitled to the name. On all sides the 
 greatest interest was manifested about the unknown philosopher. 
 The name of Herschel, then unfamiliar to English ears, appeared 
 in every journal which pretended to the slightest importance, 
 and a curious list has been preserved of the number of blunders 
 which were made in spelling the name. The different scientific 
 societies hastened to convey their congratulations on an occasion 
 so memorable. 
 
 Tidings of the discovery made by the Hanoverian musician 
 reached the ears of George III., and he sent for Herschel to come 
 to the Court, that the King might learn what his achievement 
 actually was from the discoverer's own lips. Herschel brought with 
 
266 THE STORY OF THE HEAVENS. 
 
 him one of his telescopes, and he provided himself with a chart of 
 the solar system, with which to explain precisely wherein the 
 significance of the discovery lay. The King was greatly interested 
 in Herschel's narrative, and not less in Herschel himself. The 
 telescopes were erected at Windsor, and, under the astronomer's 
 guidance, the King was shown Saturn and other celebrated objects. 
 It is also told how the ladies of the Court the next day asked 
 Herschel to show them the objects which had so pleased the King. 
 The telescope was duly erected in a window of one of the Queen's 
 apartments, but when evening arrived the sky was found to be 
 overcast with clouds, and no stars could be seen. This was an 
 experience with which Herschel, like every other astronomer, is 
 unhappily only too familiar. But it is not every astronomer who 
 would have had the readiness of Herschel to escape gracefully from 
 the position. He showed to his lady pupils the construction of the 
 telescope ; he explained the mirror, and how he had fashioned it 
 and polished it ; and then, seeing the clouds were inexorable, he 
 proposed that, as he could not show them the real Saturn, permission 
 should be given to him to exhibit an artificial one. The permis- 
 sion granted, Herschel turned the telescope away from the sky, and 
 pointed it towards the wall of a distant garden. On looking into 
 the telescope there was Saturn, his globe and his system of rings, 
 so faithfully shown that, says Herschel, even a skilful astronomer 
 might have been deceived. The fact was that during the course of 
 the day Herschel saw that the sky would probably be overcast in 
 the evening, and he had provided for the emergency by cutting a 
 hole in a piece of cardboard, the shape of Saturn, which was then 
 placed against the distant garden wall, and illuminated by a lamp 
 at the back. 
 
 This visit to Windsor was productive of consequences mo- 
 mentous to Herschel, momentous to science. He had made so 
 favourable an impression, that the King proposed to create for him 
 the special appointment of King's Astronomer at Windsor. The 
 King was to provide the means for erecting the great telescopes, and 
 he allocated to Herschel a salary of 200 a year, the figures being 
 based, it must be admitted, on a somewhat moderate estimate of the 
 
UEANUS. 267 
 
 requirements of an astronomer's household. Herschel mentioned 
 these particulars to no one save to his constant and generous friend, 
 Sir W. Watson, who exclaimed, " Never bought monarch honour so 
 cheap." To all his other inquiring friends, Herschel merely said 
 that the King had provided for him. In accepting this post, 
 Herschel took no doubt a most serious step. He at once sacrificed 
 entirely his musical career, now, from many sources, a lucrative 
 one; but his determination was speedily taken. The splendid 
 earnest that he had already given of his devotion to astronomy was, 
 he knew, only the commencement of a series of most memorable 
 labours. He had indeed long been feeling that it was his bounden 
 duty to follow that path in life which his genius indicated. He 
 was no longer a young man. He had attained middle age, and the 
 years had become especially precious to one who knew that he had 
 still a mighty life-work to accomplish. He at one stroke freed 
 himself from all distractions ; his pupils and concerts, his whole 
 connection at Bath, were immediately renounced ; he accepted the 
 King's offer with alacrity, and after one or two changes settled 
 permanently at Slough, near Windsor. 
 
 With him went his sister Caroline, also a recipient of the King's 
 bounty, as assistant to her brother. It hardly comes within our 
 scope to narrate the wonderful career of this brother and sister. 
 It would lead us too far to attempt any description of the herculean 
 labours by which mighty telescopes were erected, and of the in- 
 domitable perseverance with which the observations were con- 
 ducted. How, from dusk to dawn, the great astronomer stood at 
 the telescope throughout the winter's night ; how he narrated the 
 wonders that he saw; how the faithful sister, sitting by, trans- 
 cribed those observations till the ink sometimes froze in her pen ; 
 how she devoted her days to making the necessary calculations ; 
 how for nearly forty years this wonderful brother and sister 
 worked together ; and how many and marvellous were the dis- 
 coveries which were made I All these matters cannot here be more 
 fully described. In the course of this work we shall still have 
 occasion to refer to Herschel's discoveries in particular branches of 
 astronomy, and so here we must forbear to pursue this most 
 
263 THE STORY OF THE HEAVENS. 
 
 attractive subject, merely recommending all who are interested in 
 the biography of a most remarkable woman to read the " Memoirs 
 of Caroline Herschel." * 
 
 It has, indeed, been well remarked that the most important 
 event in connection with the discovery of Uranus was the discovery 
 of Herschel's unrivalled powers of observation. Uranus must, 
 sooner or later, have been found. Had Herschel not lived, we 
 would still, in all probability, have known Uranus long ere this. 
 The really important point for science was that HerscheFs genius 
 should be given full scope, by setting him free from the engrossing 
 details of an ordinary professional calling. The discovery of 
 Uranus secured all this, and accordingly obtained for astronomy 
 all HerscheFs future labours. . , 
 
 Of Uranus itself as a planet we have but little to say. It is 
 plunged into space to such a remote distance that even the best of 
 our modern telescopes cannot make of it a striking picture. We 
 can see, as Herschel did, that it has a measurable disc, and from 
 measurements of that disc we conclude that the diameter of the 
 planet is about 31,700 miles. This is about four times as great as 
 the diameter of the earth, and we accordingly see that the volume 
 of Uranus must be about sixty-four times as great as that of the 
 earth. We also find that, like the other giant planets, Uranus seems 
 to be composed of materials much lighter, on the whole, than those 
 we find here ; so that though sixty-four times as large as the earth, 
 Uranus is only fifteen times as heavy. If we may trust to the 
 analogies of what we see everywhere else in our system, we can 
 feel but little doubt that Uranus must rotate about an axis. The 
 ordinary means of demonstrating this rotation can be hardly avail- 
 able in a body whose surface appears so small and so faint, that we 
 cannot conclude with any certainty as to the existence of markings 
 on that surface definite enough to be watched during rotation. The 
 period of rotation is accordingly unknown. 
 
 There is, however, one feature about Uranus which presents 
 many points of interest to those astronomers who are possessed of 
 telescopes of unusual size and perfection. Uranus is accompanied 
 * By Mrs. John Herschel. London, 1876. 
 
 
URANUS. 269 
 
 by a system of satellites, some of which are so faint as to require 
 the closest scrutiny for their detection. The discovery of these 
 satellites was one of the subsequent achievements of Herschel. 
 It is, however, remarkable that even his penetration and care 
 did not preserve him from errors with regard to these very 
 delicate objects. Some of the points which he thought to be 
 satellites must, it would now seem, have been merely stars enor- 
 mously more distant, which happened to lie in the field of view. 
 It has been finally ascertained that the known satellites of 
 Uranus are four in number, and their movements have been made 
 the subject of prolonged and interesting telescopic research. The 
 four satellites bear the names of Ariel, Umbriel, Titania, and 
 Oberon. Arranged in order of their distance from the central body, 
 Ariel, the nearest, accomplishes its journey in 2*52 days. Oberon, 
 the most distant satellite, completes its journey in 13*46 days. 
 
 The law of Kepler declares that the path of a satellite around 
 its primary, no less than of the primary around the sun, must be 
 an ellipse. It leaves us, however, boundless latitude in the choice 
 of the shape of the ellipse. The ellipse may be nearly a circle, it 
 may be absolutely a circle, or it may be something quite different 
 from a circle. The paths pursued by the planets are, generally 
 speaking, nearly circles ; but we meet with no exact circle among 
 planetary orbits. So far as we at present know, the closest 
 approach made to a perfectly circular movement is that by which 
 the satellites of Uranus revolve around their primary. We are not 
 prepared to say that these paths are absolutely circular. All that 
 can be said is that our telescopes fail to show any measurable 
 departure therefrom. It is also to be noted as an unparalleled 
 circumstance that the orbits of the satellites of Uranus all 
 lie in the same plane. This is not true of the orbits of the 
 planets around the sun, nor is it true of the orbits of any other 
 system of satellites around their primary. The most singular 
 circumstance attending the whole Uranian system is, however, 
 found in the position which this plane occupies. This is indeed 
 almost as great an anomaly in our system as are the rings of 
 Saturn themselves. We have already had occasion to notice 
 
270 THE STORY OF THE HEAVENS. 
 
 that the plane in which the earth revolves around the sun is 
 very nearly coincident with the planes in which all the other 
 great planets revolve. The same is true, to a large extent, of 
 the orbits of the minor planets ; though here, no doubt, we meet 
 with a few cases in which the plane of the orbit is inclined at no 
 inconsiderable angle to the plane in which the earth moves. The 
 plane in which the moon revolves also lies very close to this system 
 of planetary planes. So, too, do the orbits of the satellites of 
 Saturn and of Jupiter, while even the more recently discovered 
 satellites of Mars form no exception to the rule. The whole solar 
 system at least so far as the great planets are concerned would 
 require but comparatively little alteration if the orbits were to be 
 entirely flattened down into one plane. There is, however, one 
 notable exception to this all but universal rule. The satellites of 
 Uranus revolve in a plane which is far from coinciding with the 
 plane to which all other orbits approximate. In fact, the paths 
 of the satellites of Uranus lie in a plane nearly at right angles to 
 the orbit of Uranus. We are not in a position to give any satis- 
 factory explanation of this circumstance. It is, however, evident 
 that in the genesis of the Uranian system there must have been 
 some influence of a quite exceptional and local character. 
 
 Soon after the discovery of the planet Uranus, in 1781, 
 sufficient observations were accumulated to enable the orbit it. 
 follows to be determined. When the orbit was known, it was then 
 a mere matter of mathematical calculation to ascertain where the 
 planet was situated at any past time, and where it would be 
 situated at any future time. A very interesting inquiry was thus 
 suggested as to how far it might be possible to find any observations 
 of the planet made previously to its discovery by Herschel. Uranus 
 looks like a star of the sixth magnitude. Not many astronomers 
 were provided with telescopes of the perfection attained by 
 Herschel, and the personal delicacy of perception characteristic 
 of Herschel was a still more rare possession. It was, therefore, 
 to be expected that, if such previous observations existed, they 
 would merely record Uranus as a star visible, and indeed bright, 
 in a moderate telescope, but still not claiming any exceptional 
 
 
UEANUS. 271 
 
 attention over thousands of apparently similar stars. Many of 
 the early astronomers had devoted themselves to the useful and 
 laborious work of forming catalogues of stars. In the preparation 
 of a star catalogue,, the telescope was directed to the heavens, the 
 stars were observed, their places were carefully measured, the 
 brightness of the star was also estimated, and thus the catalogue 
 was gradually compiled in which each star had its place faithfully 
 recorded, so that at any future time it could be identified. The 
 stars were thus inventoried by hundreds and by thousands, at 
 various dates from the birth of accurate astronomy till the present 
 time. The suggestion was then made that, as Uranus looked so 
 like a star, and as it was quite bright enough to have engaged the 
 attention of astronomers possessed of even very moderate instru- 
 mental powers, there was a possibility that it had already been 
 observed, and thus actually lay recorded as a star in some of the 
 older catalogues. This was indeed a suggestion worthy of every 
 attention, and pregnant with the most important consequences in 
 connection with the immortal discovery to be discussed in our next 
 chapter. But how was such an examination of the catalogues to 
 be conducted ? Uranus is constantly moving about; does it not 
 seem that there is every element of uncertainty in such an in- 
 vestigation ? Let us consider a memorable example. 
 
 The great national observatory at Greenwich was founded in 
 1675, and the first Astronomer-Royal was the illustrious Flamsteed, 
 who in 1676 commenced that series of observations of the heavenly 
 bodies which has been continued to the present day with such 
 incalculable benefits to science. At first the instruments were of 
 the most primitive description, but in the course of some years 
 Flamsteed succeeded in procuring instruments adequate to the 
 production of a catalogue of stars, and he devoted himself with 
 extraordinary zeal to the undertaking. It is in this memorable 
 work, the " Historia Coelestis " of Flamsteed, that we have found 
 the first observation of Uranus. In the first place, it was known 
 that the orbit of Uranus, like the orbit of every other great planet, 
 was inclined at a very small angle to the ecliptic. It hence 
 follows that Uranus is at all times only to be met with along the 
 
272 THE STORY OF THE HEAVENS. 
 
 ecliptic, and it is possible to calculate where the planet was in 
 each year. It was thus seen that in the year 1690 the planet was 
 situated in that part of the ecliptic where Flamsteed was at the 
 same date making his observations. It was natural to search the 
 observations of Flamsteed, and see whether any of the so-called 
 stars could have been Uranus. An object was found in the 
 ( ' Historia Coelestis " which occupied a position identical with that 
 which Uranus must have filled on the same date. Could this be 
 Uranus ? A decisive test was at once available. The telescope was 
 directed to the spot in the heavens where Flamsteed saw a sixth- 
 magnitude star. If that were really a star, then would it still be 
 visible. The trial was made : no such star could be found, and 
 hence the presumption that this was really Uranus could hardly 
 be for a moment doubted. Speedily other confirmations flowed in. 
 It was shown that Uranus had been unconsciously observed 
 by Bradley and by Tobias Mayer, and it also became apparent 
 that Flamsteed had observed Uranus not only once, but that 
 he had actually measured its place four or five times between 
 the years 1690 and 1715. Yet Flamsteed was never conscious 
 of the discovery that lay so nearly in his grasp. He was of 
 course under the impression that all these observations related 
 to different stars. A still more remarkable case is found among 
 the observations of Lemonnier. After Uranus had been dis- 
 covered, Lemonnier turned to examine his former observations, 
 and among them he found that he had really observed Uranus 
 no less than three times, on each occasion, of course, recording 
 it as a distinct star. But Uranus had in reality a still 
 narrower escape from being detected by Lemonnier. Another 
 astronomer on going again over Lemonnier's work found no fewer 
 than nine additional observations of Uranus, and of these four 
 had been made on consecutive nights. How close, indeed, was 
 Lemonnier to the discovery which would have immortalised him ! 
 During the intervals between those four nights, the planet of course 
 moved,, and was of course taken for a different star each night. If 
 Lemonnier had only carefully looked over his own work ; if he 
 had perceived, as he might have done, how the star he observed 
 
URANUS. 273 
 
 yesterday was gone to-day, while the star visible to-day had moved 
 away by to-morrow, there is no doubt that Uranus would have 
 been discovered, and William Herschel would have been antici- 
 pated. Would Lemonnier have made as good use of his fame as 
 Herschel did ? This is a question hard, perhaps, to answer ; but 
 those who estimate Herschel as the present writer thinks he ought 
 to be estimated, will probably agree in thinking that it was most 
 fortunate for science that Lemonnier did not compare his obser- 
 vations.* 
 
 These early accidental observations of Uranus are not merely 
 to be regarded as matters of historical interest or curiosity. That 
 they are of the deepest importance with regard to the science itself 
 a few words will enable us to show. It is to be remembered that 
 Uranus requires no less than eighty-four years to accomplish his 
 mighty revolution around the sun. The planet has completed one 
 entire revolution, since its discovery, and up to the present time 
 (1885) has accomplished nearly one-third of another. For the 
 careful study of the nature of the orbit, it was desirable to have 
 as many observations as possible, and extending over the widest 
 possible interval. This was in a great measure secured by 
 the identification of the early observations of Uranus. An ap- 
 proximate knowledge of the orbit was quite capable of giving 
 the places of the planet with sufficient accuracy to identify it when 
 met with in the catalogues. But when by their aid the actual 
 observations have been discovered, they tell us precisely the place 
 of Uranus ; and hence, instead of our knowledge of the planet 
 being limited to but little more than one revolution, we have at 
 the present time information with regard to it extending over 
 considerably more than two revolutions. 
 
 From the observations of the planet the ellipse in which it 
 moves can be ascertained. We can compute this ellipse from 
 the observations made during the time since the discovery. We 
 can also compute the ellipse from the early observations made 
 
 * Arago says that " Lemonnier's records were the image of chaos." Bouvard 
 showed to Arago one of the observations of Uranus which was written on a paper 
 bag that in its time had contained hair-powder. 
 
274 THE STORY OF THE HEAVENS. 
 
 before the discovery. If Kepler's laws were rigorously verified, 
 then, of course, the ellipse performed in the present revolution must 
 differ in no respect from the ellipse performed in the preceding, 
 or indeed in any other revolution. We can test this point in an 
 interesting manner by comparing the ellipse derived from the 
 ancient observations with that deduced from the modern ones. 
 These ellipses closely resemble each other ; they are nearly the 
 same ; but it is most important to observe that they are not exactly 
 the same, even when allowance has been made for every known 
 source of disturbance in accordance with the principles explained in 
 the next chapter. The law of Kepler seems thus not absolutely 
 true in the case of Uranus. Here is, indeed, a matter demanding 
 our most earnest and careful attention. Have we not repeatedly 
 laid down the universality of the laws of Kepler in controlling the 
 planetary motions ? How then can we reconcile this law with the 
 irregularities proved beyond a doubt to exist in the motions of 
 Uranus ? 
 
 Let us look a little more closely into the matter. We know 
 that the laws of Kepler are a consequence of the laws of gravitation. 
 We know that it is in virtue of the sun's attraction that the planet 
 moves in an elliptic path around the sun, and we know that the 
 ellipse will be preserved without the minutest alteration if the sun 
 and the planet be left to their mutual attractions, and no other force 
 intervene to disturb them. The conclusion is irresistible. Uranus 
 does not move solely in consequence of the sun's attraction and 
 that of the planets of our system interior to Uranus ; there must 
 therefore be some further influence acting upon Uranus besides 
 those already known. To the development of this subject the 
 next chapter will be devoted. 
 
CHAPTER XV. 
 
 NEPTUNE. 
 
 Discovery of Neptune A Mathematical Achievement The Sun's Attraction All 
 Bodies Attract Jupiter and Saturn The Planetary Perturbations Three 
 Bodies Nature has Simplified the Problem Approximate Solution The 
 Sources of Success The Problem Stated for the Earth The Discoveries of 
 Lagrange The Eccentricity Necessity that all the Planets Kevolve in the 
 same Direction Lagrange's Discoveries have not the Dramatic Interest of the 
 more Recent Achievements The Irregularities of Uranus The Unknown 
 Planet must Revolve outside the Path of Uranus The Data for the Problem 
 Le Verrier and Adams both Investigate the Question Adams Indicates the 
 Place of the Planet How the Search was to be Conducted Le Verrier also 
 Solves the Problem The Telescopic Discovery of the Planet The Rival 
 Claims Early Observation of Neptune Difficulty of the Telescopic Study of 
 Neptune Numerical Details of the Orbit Is there any Outer Planet ? Con- 
 trast between Mercury and Neptune. 
 
 WE enter in this chapter into a discovery so extraordinary that the 
 whole annals of science may be searched in vain for a parallel. We 
 are not here concerned with technicalities of practical astronomy. 
 Neptune was first revealed to us by profound mathematical re- 
 search rather than by telescopic investigation. We must develop 
 the account of this striking- epoch in the history of science with the 
 fulness of detail which is commensurate with its importance ; and it 
 will accordingly be necessary, at the outset of our narrative, to make 
 an excursion into a difficult but most attractive region of astronomy, 
 to which we have as yet made little reference. 
 
 The supreme controlling power in the solar system is the 
 attraction of the sun. Each planet of the system experiences that 
 attraction, and, in virtue of it, the planet is constrained to move 
 around the sun in an elliptic path. The efficiency of the sun as an 
 attractive agent is directly proportional to its mass, and as its mass 
 is more than a thousand times as great as the mass of Jupiter, 
 which, itself, exceeds that of all the other planets collectively, the 
 B 2 
 
276 THE STORY OF THE HEAVENS. 
 
 attraction of the sun is necessarily the chief determining- force of 
 all the movements in our system. The law of gravitation, how- 
 ever, does not merely say that the sun attracts each planet. 
 Gravitation is a doctrine much more general, for it asserts that 
 every body in the universe attracts every other body. In obedience 
 to this law each planet must be attracted, not only by the sun, but 
 by innumerable bodies, and the movement of the planet must be 
 the joint effect of all the attractions. As to the influence of the 
 stars on our solar system, it may be at once set aside as in- 
 appreciable. The stars are no doubt enormous bodies, in many 
 cases possibly transcending 1 our sun in magnitude, but the law 
 of gravitation tells us that the intensity of the attraction de- 
 creases with the square of the distance. Most of the stars 
 are a million times as remote as the sun, and consequently 
 their attraction is so slender as to be absolutely inappreciable in 
 the discussion of this question. The only attractions necessary to 
 consider are those which arise from the action of one body of the 
 system upon another. Let us take, for instance, the two largest 
 planets of our system, Jupiter and Saturn. Each of these planets 
 moves mainly in consequence of the sun's attraction, but each 
 planet also attracts the other, and the consequence is that each 
 planet is slightly drawn away from the position it would other- 
 wise have occupied. In the language of astronomy we would 
 say that the path of Jupiter is perturbed by the attraction of 
 Saturn ; and, conversely, that the path of Saturn is perturbed by 
 the attraction of Jupiter. 
 
 For many years these irregularities of the planetary motions 
 presented problems which astronomers were not able to solve. 
 Gradually, however, one difficulty after another has been vanquished, 
 and though there are no doubt some small irregularities still out- 
 standing which have not been completely explained, yet all the 
 larger and more important phenomena of the kind are well 
 understood. The subject is one of the most difficult which the 
 astronomer has to encounter in the whole range of his science. 
 He has here to calculate what effect one planet is capable of pro- 
 ducing on another planet. Such calculations bristle with the most 
 
NEPTUNE. 277 
 
 formidable difficulties, which can only be overcome by consum- 
 mate skill in the loftiest branches of mathematics. Let us state 
 what the problem really is. 
 
 When two bodies move in virtue of their mutual attraction, 
 each of these bodies revolves in a path which admits of being 
 exactly calculated. Each path is, in fact, an ellipse, and the focus 
 of each ellipse is at the centre of gravity of the two bodies. In 
 the case of a sun and a planet, in which the mass of the sun 
 preponderates so enormously over the mass of the planet, the centre 
 of gravity of the two lies very near the centre of the sun ; the 
 actual path of the sun is in such case very small in comparison 
 with the path of the planet ; and hence we are justified in most 
 cases in regarding the sun as at rest, and the planet revolving 
 around it. All these matters admit of perfectly rigid calcula- 
 tion of a somewhat elementary character. But now let us add a 
 third body to the system. This third body attracts each of the 
 others, and is attracted by them. By this attraction, the third body 
 is displaced, and accordingly its influence on the others is modified ; 
 they in turn act upon it, and these actions and reactions introduce 
 endless complexity into the system. Such is the famous " problem 
 of three bodies," which has engaged the attention of almost every 
 great mathematician since the time of Newton. Stated in its 
 mathematical aspect, and without having its rigour abated by any 
 modifying circumstances, the problem is one that defies solution. 
 No one has yet been able to construct the analysis which shall be 
 powerful enough to cope with the mutual attractions of three bodies 
 moving freely in space. If the number of bodies be greater than 
 three, as is actually the case in the solar system, the problem 
 becomes still more hopeless. 
 
 Nature, however, has in this matter dealt kindly with us. She 
 has, it is true, proposed a problem which cannot be rigorously 
 solved ; but she has introduced into the problem, as proposed in the 
 solar system, certain special features which materially reduce the 
 difficulty. We are still unable to make what a mathematician would 
 describe as a rigorous solution of the question ; we cannot solve it 
 with the completeness of a sum in arithmetic ; but we can do what 
 
278 THE STOEY OF THE HEAVENS. 
 
 is nearly if not quite as useful. We can solve the problem approxi- 
 mately; we can find out what the effect of one planet on the other 
 is very nearly, and we can by additional labour reduce the limits of 
 uncertainty to as low a point as may be desired. We thus have 
 really a practical solution of the problem adequate for all the pur- 
 poses of science. It avails us little to know the place of a planet 
 with absolute mathematical accuracy. If we can calculate the 
 place with so close a degree of approximation to the true position 
 that no telescope could possibly disclose the difference, then 
 every practical end will have been attained. The reason why 
 in this case we are enabled to get round the difficulties which 
 we cannot surmount, lies in the exceptional character of the 
 problem of three bodies as exhibited in the solar system. In the 
 first place the sun is of such pre-eminent mass, that many 
 matters may be overlooked which would be of moment were he 
 rivalled in mass by any of the planets. Another great source 
 of success arises from the small inclinations of the planetary 
 orbits to each other ; while the fact that the orbits are nearly cir- 
 cular also greatly facilitates the work. The mathematicians who 
 reside in some of the other parts of the universe are not equally 
 favoured. Among the sidereal systems we find not a few cases 
 where the problem of three bodies, or even of more than three, 
 would have to be faced without any of the alleviating circum- 
 stances which our system presents. In such groups as the mar- 
 vellous quadruple star of Orion, we have four or more bodies 
 comparable in size, which must produce movements of the utmost 
 complexity. Even if terrestrial mathematicians had the hardihood 
 to face such problems, there is no likelihood of their being able to 
 do so for ages to come ; such researches must repose on accurate 
 observations as their foundation ; and the observations of these dis- 
 tant systems are at present utterly inadequate for the purpose. 
 
 The elliptic revolution of a planet around the sun, in conformity 
 with Kepler's law, would assure for that planet permanent con- 
 ditions of climate. Our earth, for instance, if guided solely by 
 Kepler's laws, would at each day of the year return exactly to the 
 same position which it had on the same day of last year. From age 
 
NEPTUNE. 279 
 
 to age the quantity of heat received by our earth would remain con- 
 stant if the sun himself remained constant, and our climate would 
 thus be preserved indefinitely. But since the existence of planetary 
 perturbation has become recognised, questions arise of the deepest 
 importance with reference to the possible effects which such pertur- 
 bations may have. We now see that the path of: the earth is not 
 absolutely fixed. That path is deranged by Venus and by Mars ; 
 it is deranged, it must be deranged, by every planet in our system. 
 It is true that in a year, or even in a century, the amount of de- 
 rangement produced is not very great ; the ellipse which represents 
 very nearly the path of our earth this year, does not differ much 
 from the ellipse which represented the movement of the earth one 
 hundred years ago. But the important question arises as to 
 whether the slight difference which does exist may not be con- 
 stantly increasing, and may not ultimately assume such propor- 
 tions as to profoundly modify our climates, or even to render 
 life utterly impossible. Indeed, if we look at the subject with- 
 out attentive calculation, nothing would seem more probable than 
 that such should be the fate of our system. Our earth revolves 
 in a path inside that of the mighty Jupiter. The earth is con- 
 stantly attracted by Jupiter, and when it overtakes Jupiter, and 
 comes between Jupiter and the sun, then the two planets are 
 comparatively close together, and the earth is pulled outwards 
 by Jupiter. It might, therefore, be supposed that the tendency 
 of Jupiter would be gradually to draw the earth away from the 
 sun, and thus to cause the earth to describe a path ever growing 
 wider and wider. It is, however, not possible to decide a dynamical 
 question by merely superficial reasoning of this character. The 
 question has to be brought before the tribunal of mathematical 
 analysis, where every element in the case is duly taken into account. 
 Such an inquiry is by no means a simple one. It worthily occupied 
 the splendid talents of Lagrange, and his discoveries in the theory 
 of planetary perturbation are some of the most remarkable contri- 
 butions ever made to theoretical astronomy. 
 
 We cannot here attempt to give even the slightest sketch of 
 the reasoning which Lagrange employed. It can only be expressed 
 
280 THE STORY OF THE HEAVENS. 
 
 by the formulae of the mathematician, and would then be hardly 
 intelligible without previous years of mathematical study. It 
 fortunately happens, however, that the results to which Lagrange 
 was conducted, and which have been abundantly confirmed by the 
 labours of other mathematicians, admit of being described in 
 simple language. 
 
 Let us suppose the case of the sun, and of two planets circulating 
 around him. These two planets are mutually disturbing each other, 
 but the amount of the disturbance is small in comparison with the 
 effect of the sun on each of them. Lagrange demonstrated that, 
 though the ellipse in which each planet moved was gradually 
 altered in some respects by the attraction of the other planet, 
 yet, there is one feature of the ellipse which the perturbation is 
 powerless to alter permanently : the longest axis of the ellipse that 
 is to say, a line equal to double the mean distance of the planet 
 from the sun must remain unchanged. This is really a discovery 
 as important as it was unexpected. It at once removes all fear as 
 to the effect which the perturbations can produce on the stability of 
 the system. It shows in particular, so far as the earth is concerned, 
 that, notwithstanding the attractions of Mars and of Venus, of 
 Jupiter and of Saturn, our earth will for ever continue to revolve 
 at the same mean distance from the sun, and thus the succession 
 of the seasons and the length of the year will, so far as this cause 
 at least is concerned, remain for ever unchanged. 
 
 13ut Lagrange went further into the inquiry. He saw that the 
 mean distance did not alter, but it remained to be seen whether 
 the eccentricity of the ellipse described by the earth could not be 
 affected by the perturbations. This is a matter of hardly less con- 
 sequence than that just referred to. It might be that, while the 
 earth still preserved the same average distance from the sun, yet 
 that the greatest and least distance might change enormously : the 
 earth might pass very close to the sun at one part of its orbit, and 
 then recede to a very great distance at the opposite part. So far 
 as the welfare of this earth and its inhabitants is concerned, this 
 is quite as important as the question of the mean distance; 
 too much heat in one half of the year would be indifferent 
 
NEPTUNE. 281 
 
 compensation for too little during the other half. Lagrange sub- 
 mitted this question also to his powerful analysis. Again he van- 
 quished the mathematical difficulties, and again he was able to give 
 assurance of the permanence of our system. It is true that he 
 was not this time able to say that the eccentricity of each path will 
 remain constant ; this is not the case. What he does assert, and 
 what he has abundantly proved, is that the eccentricity of each orbit 
 will always remain small. Here, then, we see that the shape of 
 the earth's orbit gradually swells and gradually contracts; the 
 length of the ellipse is invariable, but sometimes it approaches 
 more to a circle, and sometimes becomes more elliptical. These 
 changes are comprised within narrow limits ; so that, though they 
 may probably correspond with measurable climatic changes, yet 
 still the safety of the system is not imperilled, as it would be if the 
 eccentricity could increase indefinitely. Once again Lagrange 
 applied the resources of his calculus to study the effect which 
 perturbations can have on the inclination of the path in which the 
 planet moves. The result in this case was similar to that obtained 
 with respect to the eccentricities. If we commence with the 
 assumption that the mutual inclinations of the planets are small, 
 then Lagrange's formulae tell us that they must always remain 
 email. We are thus finally led to the conclusion that the planetary 
 perturbations are unable to threaten the stability of the solar 
 system. 
 
 We shall perhaps appreciate the more fully the importance of 
 these memorable researches, if we consider how easily matters 
 might have been otherwise. Let us suppose a system resembling 
 ours in every respect save one. Let that system have a sun, as 
 ours has ; a system of planets and of satellites like ours. Let the 
 masses of all the bodies in this hypothetical system be identical 
 with the masses in our system, and let even the distances and the 
 periodic times be the same in the two cases. Let all the planes of 
 the orbits be similarly placed; and yet this hypothetical system 
 might have the seeds of decay from which ours is exempt. There 
 is one point in the hypothetical system which we have not specified. 
 In our system all the planets revolve in the same direction around 
 
282 THE STORY OF THE HEAVENS. 
 
 the sun. Let us, in the hypothetical system, merely suppose this 
 law violated by reversing one planet on its path. That slight 
 change alone will turn the system into one doomed to destruction 
 by the planetary perturbations. Here, then, we find the explanation 
 of that remarkable feature in our system, the uniformity of the 
 directions in. which the planets revolve around the sun. Had that 
 not been the case, our system must, in all probability, have perished 
 ages ago, and we should not be here to discuss perturbations or any 
 other subject. 
 
 Great as was the success of the eminent French mathematician 
 who made these beautiful discoveries, it was left for this century 
 to witness the crowning triumph of mathematical analysis applied 
 to the law of gravitation. The work of Lagrange lacks the dramatic 
 interest of the discovery made by Le Verrier and Adams, which 
 gave still wider extent to the solar system by the discovery of the 
 planet Neptune revolving far outside Uranus. 
 
 We have already alluded to the difficulties which were expe- 
 rienced when it was sought to reconcile the early observations of 
 Uranus with those made since its discovery. We have shown 
 that the path in which Uranus revolved experienced change, and 
 that consequently Uranus was submitted to the action of some other 
 force besides the sun's attraction. 
 
 The question, then, arises as to the nature of these disturbing 
 forces. From what we have already learned as to the mutual dis- 
 turbing influence of one planet upon another, it seems natural to 
 inquire whether the irregularities of Uranus could not be accounted 
 for by the attraction of the other planets. Uranus revolves just 
 outside Saturn. The mass of Saturn is much larger than the mass 
 of Uranus. Could it not be that Saturn draws Uranus aside, 
 and thus causes the changes ? This is a question to be decided 
 by the mathematician. He can compute what Saturn is able to 
 do, and he finds, no doubt, that Saturn is capable of producing 
 some displacement of Uranus. In a similar manner Jupiter, with 
 his mighty mass, acts on Uranus, and produces some disturbance 
 which the mathematician calculates. When these calculations 
 had been made for all the known planets they were applied to 
 
NEPTUNE. 283 
 
 Uranus, and we might expect to find that they would fully account 
 for the observed irregularities of his path. This was, however, 
 not the case. After every known source of disturbance had been 
 carefully allowed for, Uranus was still shown to be influenced 
 by some further disturbance ; and hence the conclusion was 
 established that Uranus must be affected by some unknown 
 body. What could this unknown body be, and where must it 
 be situated ? Analogy was here the guide of those who speculated 
 on this matter. We know no cause of disturbance of a planet's 
 motion except it be the attraction of another planet. Could it 
 then be that Uranus was really attracted by some other planet at 
 that time utterly unknown ? This suggestion was made by many 
 astronomers, and it was easy to see some of the conditions which 
 the unknown body should fulfil. Its orbit must, in the first place, 
 lie outside the orbit of Uranus. This seemed obvious, because the 
 unknown planet must be a large and massive one to produce the 
 observed disturbances. If, therefore, it were nearer than Uranus, 
 it would be a conspicuous object, and must have been discovered 
 long ago. Other reasonings were also available to show, without 
 doubt, that if the disturbances of Uranus were caused by the attrac- 
 tion of a planet, that planet must revolve outside Uranus. The 
 general analogies of the planetary system might also be invoked in 
 support of the hypothesis that the path of the unknown planet, 
 though really elliptic, did not differ very widely from a circle, and 
 that the plane in which it moved must also be nearly coincident with 
 the plane of the earth's orbit. 
 
 The sole data available for the discovery of the planet were the 
 measured deviations of Uranus at the different points of its orbit. 
 We have, as it were, to fit the orbit of the unknown planet, as well 
 as the mass of that planet itself, in such a way as to account for 
 the various perturbations. Let us, for instance, assume a certain 
 distance for the hypothetical body, and try whether we can assign 
 an orbit and a mass for the planet, with that distance, which shall 
 account for the perturbations. The distance is perhaps too great. 
 We try again with a lesser distance. We can now represent the 
 observations with greater accuracy. A third attempt will give the 
 
284 THE STORY OF THE HEAVENS. 
 
 result closer still, until at length the distance of the unknown 
 planet is determined. In a similar way the weight of the body can 
 be also determined. We assume a weight, and calculate all the 
 perturbations. If the results seem greater than those obtained by 
 observations, then the assumed weight is too great. We amend the 
 assumption, and recompute with a lesser weight, and so on until 
 at length we obtain a mass for the planet which harmonises with 
 the results of actual measurement. The other elements of the 
 unknown orbit its eccentricity and the position of its axis are 
 all to be determined in a similar manner. At length it appeared 
 that the perturbations of Uranus could be completely explained if 
 the unknown planet had a certain definite mass, and moved in 
 an orbit which had a certain definite position, while it was also 
 manifest that no other orbit or greatly different mass would explain 
 the observed facts. 
 
 These very remarkable computations were undertaken quite 
 independently by two astronomers one in England and one in 
 France. Each of them attacked, and each of them succeeded in 
 solving, the great problem. The scientific men of England and the 
 scientific men of France joined issue on the question as to the 
 claims of their respective champions to the great discovery ; but 
 in the forty years which have elapsed since these memorable 
 researches the question has gradually become settled. It is the 
 impartial verdict of the scientific world outside England and 
 France, that the merits of this splendid triumph of science must be 
 divided equally between this present distinguished Professor J. C- 
 Adams, of Cambridge, and the late U. J. J. Le Verrier, the director 
 of the Paris Observatory. 
 
 Shortly after Mr. Adams had taken his degree at Cambridge, 
 in 1843, when he obtained the distinction of Senior Wrangler, he 
 turned his attention to the perturbations of Uranus, and, guided by 
 these perturbations alone, commenced his search for the unknown 
 planet. Long and arduous was the inquiry demanding an 
 enormous amount of arithmetical calculation, no less than con- 
 summate mathematical resource ; but gradually Mr. Adams over- 
 came the difficulties. Gradually, as the subject developed, he saw 
 
NEPTUNE. 285 
 
 how the perturbations of Uranus could be fully explained by the 
 existence of an exterior planet, and at length he had ascertained, 
 not alone the orbit of this exterior planet, but he was even able to 
 indicate the part of the heavens in which this planet was placed. 
 With his researches in this advanced condition, Mr. Adams, in 
 October of 1845, called on the Astronomer-Royal, Sir George Airy, 
 at Greenwich, and placed in his hands the computations which 
 indicated with marvellous accuracy the place of the unknown 
 planet. It thus appears that seven months before any one else had 
 solved this problem Mr. Adams had effected the solution, and had 
 actually located the planet in a position but little more than a 
 degree distant from the spot which it is now known to have oc- 
 cupied. All that was wanted to complete the achievement, and to 
 gain for Professor Adams and for English science the undivided 
 glory of this discovery, was a strict telescopic search of the heavens 
 in the neighbourhood indicated. 
 
 Why, it may be said, was not such an inquiry instituted at once ? 
 No doubt it would have been done at once, if the observatories forty 
 years ago had been furnished generally with those elaborate star- 
 chants which are now available. In the absence of such charts (and 
 it does not seem that any English astronomers then possessed them) 
 the search for the planet was a most laborious matter. It had 
 been suggested that it could be detected by its possessing a visible 
 disc ; but it must be remembered that even Uranus, so much closer 
 to us, had a disc so small that it was observed nearly a score of times 
 without particular notice, though it did not escape the eagle glance 
 of Herschel. There then remained only one available method of 
 finding Neptune. It was to construct a chart of the heavens in the 
 neighbourhood indicated, and then to compare this chart night 
 after night with the stars in the heavens. Before recommending 
 the commencement of a labour so onerous, the Astronomer-Royal 
 thought it right to submit Mr. Adams's researches to a crucial 
 preliminary test. Mr. Adams had shown how his theory rendered 
 exact account of the perturbations of Uranus in longitude. The 
 Astronomer-Royal asked Mr. Adams whether his theory would 
 give an equally clear explanation of the notable variations in the 
 
286 THE STORY OF THE HEAVENS. 
 
 distance of Uranus. There can be no doubt that his theory would 
 render a satisfactory account of these variations also; but, un- 
 fortunately, Mr. Adams seems not to have thought the matter of 
 sufficient importance to give the Astronomer- Royal any speedy 
 reply, and hence it happened that no less than nine months elapsed 
 between the time when Mr. Adams first communicated his results 
 to the Astronomer- Royal, and the time when the telescopic search 
 for the planet was systematically commenced. Up to this time also 
 no account of Mr. Adams's researches had been published. His 
 labours were known to but few besides the Astronomer-Royal and 
 Professor Challis of Cambridge, to whom the duty of making 
 the search was afterwards entrusted. 
 
 In the meantime the great French mathematician and astronomer 
 Le Verrier had had his attention specially invited by Arago to the 
 problem of the perturbations of Uranus. With the most exhaustive 
 analysis he investigated every possible known source of disturbance. 
 The influences of the older planets were estimated with the most 
 elaborate detail, but only to confirm the conclusion already arrived at 
 as to their inadequacy to account for the perturbations. Le Verrier 
 then, in complete ignorance of the labours of Adams, commenced 
 the search for the unknown planet by the aid of mathematical 
 investigation. On the 1st of June, 1846, Le Verrier's results 
 appeared; and then the Astronomer-Royal perceived that the 
 results obtained by him coincided practically with those of Adams, 
 insomuch that the places assigned to the unknown planet by the 
 two astronomers were not more than a degree apart ! This was, 
 indeed, a most remarkable result. Here was a planet unknown to 
 human sight, yet felt, as it were, by mathematical analysis with a 
 certainty so great that two astronomers, each in absolute ignorance 
 of the other's labours, actually concurred in locating the planet in 
 almost the same spot of the heavens. The existence of the planet 
 was thus raised nearly to a certainty, and it was incumbent on 
 practical astronomers at once to commence the search. In June, 
 1846, the Astronomer-Royal announced to the visitors at Greenwich 
 Observatory the close coincidence between the calculations of 
 Le Verrier and of Adams, and he urged the importance of the 
 
NEPTUNE. 287 
 
 proposal that a search should be at once commenced. Professor 
 Challis, having the command of the great Northumberland 
 equatorial telescope at Cambridge, was induced to undertake the 
 work, and on the 29th of July, 1846, he began his labours. 
 
 The plan of search adopted by Professor Challis was an onerous 
 one. He first took the theoretical place of the planet, as given by 
 Mr. Adams, and after allowing a margin for the necessary un- 
 certainties of a calculation so recondite, he marked out a certain 
 region of the heavens, near the ecliptic, in which it might be 
 anticipated that the unknown planet must be found. He then 
 determined to observe all the stars in this region and measure 
 their relative distances. When this work was once done it was to 
 be repeated a second time. His scheme even contemplated a third 
 complete set of measures of all the stars contained within this 
 limited region. There can be no doubt that this process would 
 determine the planet if it should be bright enough to come within 
 the limits of stellar magnitude which Professor Challis adopted. The 
 planet would be detected by its motion relatively to the stars, when 
 the three series of measures came to be compared together. The 
 scheme was organised so thoroughly that it must have led to the 
 discovery of the planet in fact it afterwards appeared that Pro- 
 fessor Challis actually did observe the planet more than once, and 
 the subsequent comparison of its places must infallibly have led to 
 its detection. 
 
 In the meantime Le Verrier was maturing his no less elaborate 
 investigations in the same direction. He felt confident of the 
 existence of the planet, and he even went so far as to predict 
 not only the place of the planet but its actual appearance. He 
 thought the planet would be large enough (though still of course 
 only a telescopic object) to be distinguished from the stars by the 
 possession of a disc. These definite predictions strengthened the 
 belief that we were on the verge of another great discovery in the 
 solar system, so much so that when Sir John Herschel addressed 
 the British Association on the 10th of September, 1846, he intro- 
 duced the following words : " The past year has given to us the 
 new planet Astrsea it has done more, it has given us the probable 
 
288 THE STORY OF THE HEAVENS. 
 
 prospect of another. We see it as Columbus saw America from 
 the shores of Spain. Its movements have been felt trembling 1 
 along the far-reaching line of our analysis, with a certainty hardly 
 inferior to ocular demonstration." 
 
 The time of the discovery was now rapidly approaching. On 
 the 18th of September, 1846, Le Verrier wrote to the astronomers of 
 the Berlin Observatory describing precisely the place of the planet 
 as indicated by his calculations, and asking their aid in making 
 its telescopic discovery. The request thus preferred was similar 
 to that made on behalf of Adams to Professor Challis. Both at 
 Berlin and at Cambridge the telescopic research was to be made in 
 the same region of the heavens. The Berlin astronomers were, 
 however, most fortunately possessed of an invaluable aid to the 
 research which was not, at the time, in the hands of Professor 
 Challis. We have mentioned how the search for a telescopic planet 
 can be enormously facilitated by the use of a carefully-executed 
 chart of the stars. In fact a mere comparison of the chart with the 
 sky is all that is necessary. It happened that the preparation of a 
 series of star charts had been undertaken by the Berlin Academy of 
 Sciences some years previously. On these charts the place of every 
 star, down even to the tenth magnitude, was faithfully engraved. 
 This work was one of very great utility, but its originators could 
 hardly have anticipated the brilliant discovery which would arise 
 from their herculean labours. It was only possible to accomplish 
 such a vast piece of surveying work by instalments, and accordingly 
 sheet by sheet, as the chart was completed, it issued from the press. 
 It happened that before the news of Le Verrier's labours reached 
 Berlin the chart of that part of the heavens had been engraved and 
 printed, although not actually published. 
 
 It was on the 23rd of September that Le Terrier's letter 
 reached the Berlin astronomers. The sky that night was clear, 
 and we can easily understand with what anxiety Dr. Galle 
 directed his telescope to the heavens. The instrument was 
 pointed in accordance with Le Verrier's instructions. The field 
 of view showed, as does every part of the heavens, a multitude of 
 stars. One of these was really the planet. The new chart was 
 
NEPTUNE. 289 
 
 unrolled, and, star by star, the heavens were compared with the 
 chart. As the process of identification went on, one object after 
 another was found in the heavens as engraved on the chart, and 
 was of course rejected. At length a star of the eighth magnitude 
 a brilliant object was brought into review. The map was 
 examined, but there was no star there. This object could not have 
 been in its present place when the map was formed. The object 
 was therefore a wanderer a planet. Yet it is necessary to be 
 excessively cautious in such a matter. Many possibilities had to 
 be guarded against. It was, for instance, possible that the object 
 was really a star which, by some mischance, eluded the careful eye 
 of the astronomer who constructed the map. It was even possible 
 that the star might be one of the large class of variables which 
 alternate in brightness, and it might have been conceivable that it 
 was too faint to be seen when the chart was made. Even if neither 
 of these explanations would answer, it was still necessary to show 
 that the object was now moving, and moving with that particular 
 velocity and in that particular direction which the theory of 
 Le Verrier indicated. The lapse of a single day was sufficient to 
 dissipate all doubts. The next night the object was again observed. 
 It had moved, and when its motion was measured it was found to 
 accord precisely with what Le Verrier had foretold. Indeed, as 
 if no circumstance in the confirmation should be wanting, it was 
 ascertained that the diameter of the planet, as measured by the 
 micrometers at Berlin, was practically coincident with that antici- 
 pated by Le Verrier. 
 
 The world speedily rang with the news of this splendid 
 achievement. Instantly the name of Le Verrier rose to a pinnacle 
 hardly surpassed by that of any astronomer of any age or country. 
 The whole circumstances of the discovery were most dramatic in 
 their character. We picture to ourselves the great astronomer buried 
 in profound meditation for many months ; his eyes are bent, not on 
 the stars, but on his calculations and his formulae. No telescope 
 is in his hand ; it is the human intellect which is the instrument he 
 alone uses. With patient labour, guided by the most consummate 
 mathematical artifice, he manipulates his columns of figures. He 
 T 
 
290 THE STORY OF THE HEAVENS. 
 
 attempts one solution after another. In each he learns some- 
 thing- to avoid; by each he obtains some light to guide him in his 
 future labours. At length he begins to see harmony in those 
 results where before there was but discord. Gradually the clouds 
 disperse, and he sees with a certainty little short of actual vision 
 the planet glittering in the far depths of space. He rises from his 
 desk and invokes the aid of a practical astronomer, and lo ! there 
 is the planet in the indicated spot. The annals of science present 
 no such achievement as this. It was the most triumphant proof 
 of the law of universal gravitation. The Newtonian theory had 
 indeed long ere this attained an impregnable position ; but, as if 
 to place its truth in the most dazzling and conspicuous light, this 
 discovery of Neptune was accomplished. 
 
 For a moment it seemed as if the French nation were to enjoy 
 the undivided honour of this splendid triumph; nor would it, indeed, 
 have been unfitting that the nation which gave birth to Lagrange 
 and to Laplace, and which developed the great Newtonian theory 
 by their immortal labours, should have obtained this distinction. 
 Up to the time of the telescopic discovery of the planet by Dr. 
 Galle at Berlin, no public announcement had been made of the 
 labours of Challis in searching for the planet, or even of the 
 theoretical researches of Adams on which those observations were 
 based. But in the midst of the paBans of triumph with which the 
 enthusiastic French nation hailed the discovery of Le Verrier, there 
 appeared a letter from Sir John Herschel in the Athenaum for 
 3rd October, 1846, in which he announced the researches made by 
 Adams, and claimed for him a participation in the glory of the 
 discovery. Subsequent inquiry has shown that this claim was a 
 just one, and it is now universally admitted by all independent 
 authorities. Yet it will easily be imagined that the French 
 savants, jealous of the fame of their countryman, could not at first 
 be brought to recognise a claim so put forward. They were asked 
 to divide the unparalleled honour between their own illustrious 
 countryman and a young foreigner of whom but few had ever 
 heard, and who had not even published a line of his work or had 
 any claim of his put' forward until after the whole work had been 
 
NEPTUNE 291 
 
 completely finished by Le Verrier. The demand made on behalf 
 of Adams was accordingly resented by the French nation; and a 
 somewhat embittered controversy arose on the matter, but point 
 by point the English astronomers succeeded in establishing 
 the claim of their countryman. It was true that Adams had 
 not published his researches to the world, but he had com- 
 municated them to the Astronomer-Royal, the official head of 
 astronomy in this country. They were also well known to Pro- 
 fessor Challis, the Professor of Astronomy at Cambridge. Then, 
 too, when the work of Adams was examined, it was found to be 
 quite as thorough and quite as successful as that of Le Verrier. It 
 was also found that the method of search adopted by Professor 
 Challis not only must have been eventually successful, but that it 
 actually was in a sense already successful. When the telescopic 
 discovery of the planet had been achieved, Challis turned naturally 
 to see whether he had observed the planet or not. It was on the 
 1st October that he heard of the success of Dr. Galle, and by that 
 time Challis ha,d accumulated observations in connection with this 
 research of no fewer than 3,150 stars. Among them he speedily 
 found that an object observed on the 12th of August was not in 
 the same place on the 30th of July. This was really the planet ; 
 and its discovery would thus have been assured when Challis 
 had had time to compare his measurements. In fact, if he had 
 only discussed his observations at once, there cannot be much 
 doubt that the entire glory of the discovery would have been 
 awarded to Adams. He would then have been first, no less in the 
 theoretical calculations than in the optical discovery of the planet. 
 It may also be remarked that in another way Challis very narrowly 
 missed making the telescopic discovery of Neptune. In his 
 paper, Le Verrier had pointed out the possibility of detecting 
 the planet by its disc. Challis made the attempt, and before the 
 intelligence of the actual discovery at Berlin had reached him, he 
 had made an examination of the region indicated by Le Verrier. 
 About 300 stars passed through the field of view, and among 
 them he selected one on account of its disc; it afterwards ap- 
 peared that this was indeed the planet. 
 T 2 
 
292 THE STORY OF THE HEAVENS. 
 
 Even if the researches of Le Verrier and of Adams had never 
 been undertaken it is certain that the distant Neptune must be 
 some time discovered ; yefc that discovery might have been made 
 in a manner which every true lover of science would now deplore. 
 We hear constantly of the discovery of minor planets,, yet no 
 one attaches to such achievements a fraction of the consequence 
 belonging to the discovery of Neptune. The danger to be feared 
 by delay was, that Neptune should have been discovered by simple 
 survey work,, just as Uranus was discovered, or just as the hosts 
 of minor planets are now found. In this case the science of 
 Theoretical Astronomy, the great science founded by Newton, and 
 raised to a marvellously interesting and beautiful system by the 
 labours of Lagrange and Laplace, would have been deprived of its 
 most brilliant illustration. 
 
 Neptune had, in fact, a very narrow escape on at least one 
 previous occasion of being discovered in a very simple way. This 
 was shown when sufficient observations had been obtained to enable 
 the path of the planet to be calculated. It was then possible to 
 trace back the movements of the planet among the stars and thus 
 to institute a search in the catalogues of earlier astronomers to see 
 whether they contained any record of Neptune. It was soon found 
 that the place of the planet on May 10th, 1795, must have coin- 
 cided with the place of a star recorded on that day in the " Histoire 
 Celeste"" of Lalande. It further appeared by actual examination 
 of the heavens that there was no star in the place indicated 
 by Lalande, so the fact that this was really an observation of 
 Neptune was placed quite beyond doubt. When reference was 
 made to the original observations of Lalande a matter of the very 
 greatest interest was brought to light. It was there found that he 
 had observed the same star (for so he regarded it) both on May 
 8th and on May 10th ; on each day he had determined its position, 
 and both are duly recorded. But when he came to prepare his 
 catalogue and found that the places on the two occasions were dif- 
 ferent, he discarded the earlier result, and merely printed the later. 
 Had Lalande but had the courage to believe implicitly in his own 
 observations, an immortal discovery lay in his grasp; had he man- 
 
NEPTUNE. 293 
 
 fully said, ff l was right on the 10th of May and I was right on the 
 8th of May; I made no mistake on either occasion, and the object 
 I saw on the 8th must have moved between that and the 10th/' 
 then he must without fail have found Neptune. But had he done 
 so, how great would have been the loss to science ! The discovery 
 of Neptune would then merely have been an accidental reward to 
 a laborious worker, instead of being one of the most glorious 
 achievements in the loftiest department of human reason. 
 
 With the conclusion of the brief sketch here given of the 
 discovery of Neptune, we nearly conclude all that can be narrated 
 of this planet. If we fail to see in Uranus any of those features 
 which make Mars or Venus, Jupiter or Saturn such attractive 
 telescopic objects, what can we expect to find in Neptune, which 
 is half as far again as Uranus ? With a good telescope and a suit- 
 able magnifying power we can see that Neptune has a disc, but no 
 features on that disc can be identified. We are consequently not 
 in a position to ascertain the period in which Neptune rotates 
 around its axis, though from the general analogy of the system 
 we must feel assured that it really does rotate. More successful 
 have been the attempts to measure the diameter of Neptune, which 
 is found to be nearly 35,000 miles, or more than four times the 
 diameter of the earth. It would also seem that, like Jupiter and 
 like Saturn, the planet must be enveloped with a vast cloud laden 
 atmosphere, for the density of the globe is only about one-fifth 
 that of the earth. This great globe revolves around the sun at a 
 mean distance of no less than 2,780 millions of miles, being about 
 thirty times as great as the mean distance from the earth to the 
 sun. The journey, though accomplished at a rate of more than 
 three miles a second, is yet so long that Neptune requires almost 
 155 years to complete one revolution. Since its discovery, some 
 forty years ago, Neptune has moved only through about one quarter 
 of its path, and even since the time it was first casually seen by 
 Lalande, in 1795, it has only had time to traverse half of its mighty 
 circuit. 
 
 Neptune, like our earth, is attended by a single satellite ; this 
 delicate object was discovered by Mr. Lassell with his colossal 
 
294 THE STORY OF THE HEAVENS. 
 
 reflecting telescope shortly after the planet itself was known. The 
 satellite performs its journey around Neptune in a period of but 
 little less than six days. By observing the motions of the satellite 
 we are enabled to determine the mass of the planet, and thus it 
 appears that Neptune is about one twenty-thousandth part of the 
 weight of the sun. 
 
 Our review of the planetary system closes when we have 
 arrived at Neptune ; whether any planets revolve around the sun 
 in orbits beyond Neptune it is impossible to say. All we can 
 assert is that no such planets have been seen, nor is there at 
 present any good ground for believing in their existence. The 
 negative evidence on the question is indeed very strong. We have 
 in our chapter on the minor planets entered into a full discussion 
 of the way in which these objects are discovered. It is by minute 
 and diligent comparison of the heavens with elaborate star charts 
 that these planets are found. But these inquiries would be 
 equally efficacious in searching for a trans- Neptunian planet ; in 
 fact, we could design no better method to hunt for a trans-Neptunian 
 planet than that which is at this moment in constant practice at 
 many observatories. The labours of those who search for small 
 planets have been abundantly rewarded by discoveries now counted 
 by hundreds. Yet it is a most noteworthy fact that all these 
 planets are limited to one region of the solar system. It is 
 approximately true to say that all their orbits are included between 
 those of Jupiter and of Mars. In one or two cases the orbits just 
 reach beyond the path of Jupiter on the one hand, or inside the path 
 of Mars on the other ; but never has a planet been found in this way 
 which goes out even as far as Saturn, and, of course, still less has 
 any trans-Neptunian object been found. It has sometimes been 
 conjectured that time may disclose perturbations in the orbit of 
 Neptune, and that these perturbations may lead to the discovery 
 of a planet still further, even though that planet be so remote 
 and so faint that it actually eludes telescopic research. At present, 
 however, and for generations to come, such an inquiry will hardly be 
 within the range of practical astronomy. Neptune has not yet done 
 more than travel through a quarter of its path since it was found 
 

 NEPTUNE. 295 
 
 by Le Verrier and Adams. Its movements since then have no doubt 
 been studied minutely, but it must at least describe one whole 
 revolution before it would be feasible to construct from any per- 
 turbations of its path the orbit of an unknown and still more 
 remote planet. 
 
 We have thus seen the planetary system to be bounded on one 
 side by Mercury and on the other by Neptune. The discovery of 
 Mercury was in itself a brilliant achievement of prehistoric times. 
 The early astronomer who, accomplished that feat, when devoid of 
 instrumental assistance and unsupported by accurate theoretical 
 knowledge, merits our hearty admiration for his untutored acute- 
 ness and penetration. On the other hand, the discovery of the 
 exterior boundary of the planetary system is worthy of special 
 attention from the fact that it was founded on profound theoretical 
 learning 1 , and verified by consummate instrumental and practical 
 skill. 
 
 Though we here close our account of the planets and their satel- 
 lites, we have still two chapters to add before we shall have com- 
 pleted what is to be said with regard to the solar system. A further 
 and notable class of bodies, neither planets nor satellites, own alle- 
 giance to the sun, and revolve around him in conformity to the 
 laws of universal gravitation. These bodies are the comets, and 
 their somewhat more humble associates, the shooting stars. We 
 find, in the study of these objects, many matters of interest, 
 to which we shall proceed in the ensuing chapters. 
 
CHAPTER XVI. 
 
 COMETS. 
 
 Comets Contrasted with Planets in Nature as well as in their Movements Coggia's 
 Comet Periodic Eeturns The Law of Gravitation Parabolic and Elliptic 
 .Orbits Theory in Advance of Observations Most Cometary Orbits are 
 sensibly Parabolic The Labours of Halley The Comet of 1682 Halley's 
 Memorable Prediction The Retardation Produced by Disturbance Successive 
 Returns of Halley's Comet Encke's Comet Effect of Perturbations Orbit of 
 Encke's Comet Attraction of Mercury and of Jupiter How the Identity of 
 the Comet is secured How to Weigh Mercury Distance from the Earth to 
 the Sun found by Encke's Comet The Disturbing Medium The Comets of 
 1843 and 18f 8 Passage of a Comet between the Earth and the Stars Comets 
 not composed of Gas of appreciable Density Can the Comet be Weighed ? 
 Evidence of the Small Mass of the Comet derived from the Theory of Perturba- 
 tion The Tail of the Comet Its Changes Views as to its Nature Carbon 
 present in Comets. 
 
 IN our previous chapters which treated of the sun and the moon, 
 the planets and their satellites, we found in all cases that the 
 celestial bodies with which we were concerned were nearly globular 
 in form, and many are undoubtedly solid bodies. All these objects 
 possess a density which, even if in some cases it be much less than 
 that of the earth, is still hundreds of times greater than the density 
 of merely gaseous substances. "We now, however, approach the con- 
 sideration of a class of objects of a totally different character. "We 
 have no longer to deal with globular objects possessing a consider- 
 able mass. Comets are of the most irregular shapes, they are in 
 large part, at all events, formed of materials in the utmost state of 
 tenuity, and their masses are so small that no means we possess have 
 enabled them to be measured. Not only are comets different in con- 
 stitution from planets or from the other more solid bodies of our 
 system, but the movements of comets are quite distinct from the 
 orderly return of the planets at their appointed seasons. The comets 
 appear sometimes with almost startling unexpectedness; they 
 
 
PLATE XH. 
 
 
 COQQIA'S COMET. 
 
 (AS SEEN ON JUNE IOTH AND JULY 8TH, 1874.) 
 

COMETS. 297 
 
 rapidly swell in size to an extent that in superstitious ages called 
 forth the utmost terror ; again, they disappear, often never again 
 to return. Modern science has, no doubt, removed a great deal 
 of the mystery which once invested the whole subject of comets. 
 Their movements are now to a large extent explained, and some 
 additions have been made to our knowledge of their nature, though 
 we must still confess that what we do know, bears but a very small 
 proportion to what remains unknown. 
 
 Let us first describe in general terms the nature of a comet, in 
 so far as its structure is disclosed by the aid of a powerful refracting 
 telescope. We represent in Plate XII. two interesting sketches 
 made at Harvard College of the great comet of 1874, distinguished 
 by the name of its discoverer Coggia. 
 
 We see here the head of the comet, containing as its brightest 
 spot what is called the nucleus, and in which the material of the 
 comet seems to be much denser than elsewhere. Surrounding the 
 nucleus we find certain definite layers of luminous material, from 
 which the tail seems to stream away. This view may be regarded 
 as that of a typical comet, but the varieties of structure presented 
 by different comets are almost innumerable. In some cases we 
 find the nucleus absent ; in other cases we find the tail absent. 
 The tail is, no doubt, a conspicuous feature in those great comets 
 which receive universal attention ; but in the small telescopic 
 comets, of which a few are generally found every year, the absence 
 of the tail is quite an ordinary character. Not only do comets 
 present great variety in appearance, but even the aspect of a 
 single comet undergoes great change. The comet will some- 
 times increase enormously in bulk; sometimes it will diminish, 
 sometimes it will have a large tail, or sometimes no tail at all. 
 Measurements of a comet's size are almost futile ; they may cease 
 to be true even during the few hours in which a comet is observed 
 in the course of a night. It is, in fact, impossible to identify a 
 comet by any description of its personal appearance. Yet the 
 question as to the personal identity of a comet is often of very great 
 consequence. We must provide means by which that identity can 
 be established, entirely apart from what the comet may be like. 
 
298 THE STORY OF THE HEAVENS. 
 
 It is now well known that several comets make periodic returns. 
 After being invisible for a certain number of years such a comet 
 comes again into view, and again retreats into space to perform 
 another revolution. The question then arises as to how are we to 
 recognise the comet when it comes back? The personal features 
 of its size or brightness, the presence or absence of a tail, large 
 or small, are fleeting characters of no value for such a purpose. 
 Fortunately, however, the law of elliptic motion established by 
 Kepler has suggested the means of defining the identity of a 
 comet with absolute precision. 
 
 After Newton had made his immortal discovery of the law of 
 gravitation, and after he had succeeded in demonstrating that the 
 elliptic paths of the planets around the sun were a necessary con- 
 sequence of that law, he was naturally tempted to apply the same 
 reasoning to explain the movements of comets. Here again he 
 met with marvellous success, and illustrated his theory by com- 
 pletely explaining the movements of the remarkable comet which 
 appeared in the year 1680. 
 
 There is a certain very beautiful curve known to geometricians 
 by the name of the parabola. Its form is shown in the adjoining 
 figure ; it is a curved line which bends in towards and around a 
 certain point known as the focus. This would not be the occasion 
 for any allusion to the geometrical properties of this curve ; these 
 are fully discussed in works on mathematics. It will here only be 
 necessary to point to the connection which exists between the 
 parabola and the ellipse. We have in a former chapter explained 
 the construction of the ellipse, and we have shown how it possesses 
 two foci. Let us suppose that a series of ellipses be drawn, 
 each of which has a greater distance between its foci than the 
 preceding one. Imagine the process carried on until at length the 
 distance between the foci became enormously great in comparison 
 with the distance from each focus to the curve, then each end of 
 this long ellipse will have practically the same form as a parabola. 
 "We may thus look on the parabola represented in Fig. 55 as being 
 one end of an ellipse whereof the other end is at an indefinitely 
 great distance. Newton showed that the law of gravitation would 
 
COMETS. 
 
 299 
 
 permit a body to move in ail ellipse of this very extreme type no 
 less than in one of the more ordinary proportions. If an object 
 revolve in a parabolic orbit about the sun at the focus, then the 
 object moves in gradually towards the sun, sweeps around the sun, 
 and then begins to retreat ; but there is one great and necessary dis- 
 tinction between parabolic and elliptic motion. In the latter case 
 
 \ 
 
 Jig. 55. The Parabolic Path of a Comet, 
 
 the body, after its retreat to a certain distance, will turn round and 
 again draw in towards the sun ; in fact, it will make periodic 
 returns, as was found to be the case with the planets. But in the 
 case of the true parabola the body can never return ; to do so it 
 would have to double the distant focus, and as that is infinitely 
 remote, it could not be reached except in the lapse of infinite time. 
 The characteristic feature of the movement in a parabola is thus 
 enunciated. The body draws in gradually towards the focus from 
 an indefinitely remote distance on one side, and after passing round 
 the focus gradually recedes to an indefinitely remote distance on the 
 
300 THE STORY OF THE HEAVENS. 
 
 other side, never again to return. When Newton had perceived 
 that parabolic motion of this type could arise from the law of 
 gravitation, it at once occurred to him that by its means the 
 movements of a comet might be explained. He knew that comets 
 must be attracted by the sun ; he saw that the usual course of a 
 comet was to appear suddenly, to sweep around the sun and then 
 retreat, never again to return. Was this really a case of parabolic 
 motion ? Fortunately, the materials for the trial of this important 
 theory were all ready to his hand. He was able to avail himself of 
 the measurements of the great comet of 1680, and of observations of 
 several other bodies of the same nature which had been accumulated 
 by the diligence of astronomers. With his usual sagacity he devised 
 a method by which, from the known facts, the path which the 
 comet pursues could be determined. He accordingly calculated the 
 path and he found that it was a parabola, and that the velocity of 
 the comet was governed by the law of describing equal areas in 
 equal times. Here was another convincing demonstration of the law 
 of universal gravitation. In this case, indeed, the theory may be 
 said to have been actually in advance of calculation. Kepler had 
 determined from observation alone that the paths of the planets 
 were ellipses, and Newton had shown how these ellipses were a 
 consequence of the law of gravitation. But in the case of the 
 comets their highly erratic orbits had never been reduced to 
 geometrical form until the theory of Newton showed him that they 
 were parabolic, and then he invoked the observations to verify the 
 anticipations of his theory. 
 
 The great majority of comets move in orbits which cannot be 
 sensibly discriminated from parabola?, and any comet whose orbit is 
 of this character can only be seen at a single apparition. The 
 theory of gravitation, though it admits the parabola as a possible 
 orbit for a comet, does not assert that the orbit must be necessarily 
 a parabola. We have pointed out that this curve is really only a 
 very extreme type of ellipse, and it would still be in perfect accord- 
 ance with the law of gravitation for a comet to pursue a path of 
 any elliptical form, provided only that the sun was placed at the 
 focus, and that the comet obeyed the rule of describing equal areas 
 
 
COMETS. 301 
 
 in equal times. If a comet move in an elliptic path, then it will 
 return to the sun again, and consequently we shall have regular 
 periodical visits fom the same object. 
 
 An interesting field of inquiry was here presented to the astro- 
 nomer. Nor was it long before the discovery of a periodic comet 
 was made which illustrated, in a striking manner, the truth 
 of the laws of universal gravitation. The name of the celebrated 
 astronomer Halley is, perhaps, best known from its associa- 
 tion with the great comet, whose periodicity was discovered 
 by his calculations. When Halley learned from the Newtonian 
 theory the possibility that a comet might move in an elliptic orbit, 
 he undertook a most laborious investigation; he collected from 
 various records of observed comets all the reliable particulars 
 that could be obtained, and thus he was enabled to learn, with 
 tolerable accuracy, the nature of the paths pursued by about 
 twenty-four large comets. One of these was the great comet of 
 1682, which Halley himself observed, and whose path he computed 
 in accordance with the principles of Newton. He then proceeded 
 to the inquiry whether this comet of 1682 could possibly have 
 visited our system at any previous period. To answer this question 
 he turned to the list of recorded comets which he had so carefully 
 elaborated, and he found that his comet very closely resembled, 
 both in appearance and in orbit, a comet observed in 1607, and 
 also another observed in 1531. Could these three bodies be 
 identical ? It was only necessary to suppose that a comet, instead 
 of revolving in a parabolic orbit, really revolved in an extremely 
 elongated ellipse, and that it completed each revolution in a period 
 of about seventy-five or seventy-six years. He submitted this 
 hypothesis to f:very test that he could devise ; he found that the 
 orbits, determined on each of the three occasions, were so nearly 
 identical that it would be beyond the wildest probability that the 
 coincidence should be accidental. Accordingly, he decided to 
 submit his theory to the most supreme test known to astronomy. 
 He ventured to make a prediction which posterity would have 
 the opportunity of verifying. If the period of the comet were 
 seventy-five or seventy- six years, as the former observations seemed 
 
302 THE STOKY OF THE HEAVENS. 
 
 to show, then Halley estimated that it, if unmolested, ought to 
 return in 1757 or 1758. There were, however, certain sources of 
 disturbance which Halley pointed out, and which would be quite 
 powerful enough to affect materially the time of the return. The 
 comet in its journey passes near the path of Jupiter, and ex- 
 periences great perturbations from that mighty planet. Halley 
 concluded that the return of the comet might be delayed till the 
 end of 1758 or the beginning of 1759. 
 
 This prediction was a memorable event in the history of 
 astronomy, inasmuch as it was the first attempt to foretell the 
 apparition of one of those mysterious bodies whose visits seemed 
 guided by no fixed law, and which were usually regarded as 
 omens of awful import. Halley felt the importance of his an- 
 nouncement. He knew that his earthly course would have run 
 long before the comet had completed its revolution; and, in 
 language almost touching, the great astronomer writes-: "Where- 
 fore if it should return according to our prediction about the year 
 1758, impartial posterity will not refuse to acknowledge that this 
 was first discovered by an Englishman." 
 
 As the time drew nigh when this great event was expected, it 
 awakened the liveliest interest among astronomers. The distin- 
 guished mathematician Clairaut undertook to compute anew, and by 
 the aid of improved methods, the effect which would be wrought on 
 the comet by the attraction of the planets. His analysis of the per- 
 turbations was sufficient to show that the object would be kept back 
 for 100 days by Saturn, and for 518 days by Jupiter. He there- 
 fore gave some additional exactness to the prediction of Halley, 
 and finally concluded that this comet would reach the point of 
 its path nearest to the sun about the middle of April, ] 759. The 
 sagacious astronomer (who, we must remember, lived long before 
 the discovery of Uranus and of Neptune) further adds that as this 
 comet retreats so far, it may possibly be subject to influences of 
 which we do not know, or to the disturbance even of some planet 
 too remote to be ever perceived. He, accordingly, qualifies his 
 prediction with the statement that, owing to these unknown 
 possibilities, his calculations may be a month wrong one way or 
 
COMETS. 303 
 
 the other. Clairaut made this memorable communication to the 
 Academy of Sciences on the 14th of November, 1758. The attention 
 of astronomers was immediately quickened to see whether the visitor, 
 who last appeared seventy-six years previously, was about to return. 
 Night after night the heavens were scanned. On Christmas Day in 
 1758 the comet was first detected, and it passed closest to the sun 
 about midnight on the 12th of March, just a month earlier than the 
 time announced by Clairaut, but still within the limits of error 
 which he had assigned as being possible. 
 
 The verification of this prediction was a further confirmation of 
 the theory of gravitation. Since then, Halley's comet has returned 
 once again, in 1835, under circumstances somewhat similar to those 
 just narrated. Further historical research has also succeeded in 
 identifying Halley' s comet with numerous memorable apparitions 
 of comets in former times. It has even been shown that a splendid 
 comet, which appeared eleven years before the commencement of 
 the Christian era, was merely that of Halley in one of its former 
 returns. Among the most celebrated visits of this object was that 
 of 1066, when the apparition attracted universal attention. A 
 picture of the comet on this occasion forms a quaint feature in 
 the Bayeux Tapestry. The next return of Halley's comet is ex- 
 pected about the year 1910. 
 
 There are now several comets known which revolve in elliptic 
 paths, and are, accordingly, entitled to be termed periodic. These 
 objects are chiefly telescopic, and are thus in strong contrast to the 
 splendid comet of Halley. Most of the other periodic comets have 
 also periods much shorter than that of Halley. Of these objects, 
 by far the most celebrated is that known as Encke's comet, which 
 merits our careful attention. 
 
 The object to which we refer has had a striking career, during 
 which it has suffered much at the hands of the law of gravitation. 
 We are not here concerned with the prosaic routine of a mere 
 planetary orbit. A planet is mainly subordinated to the compelling 
 sway of the sun's gravitation. It is also to some slight extent 
 affected by the attractions which it experiences from the other 
 planets. Mathematicians have long been accustomed to anticipate 
 
304 THE STORY OF THE HEAVENS. 
 
 the movements of the planets by actual calculation. They know 
 how the place of the planet is approximately decided by the sun's 
 attraction,, and they can discriminate the different adjustments 
 which that place is to receive, in consequence of the disturbances 
 produced by the other planets. The capabilities of the planets for 
 producing disturbance are greatly increased when the disturbed 
 body follows the eccentric path of a comet. It is frequently found 
 that the path of a comet comes very near to the track of a planet, 
 so that the comet may actually sweep by the very planet itself, 
 even if the two bodies do not actually run into collision. On such 
 an occasion the disturbing effect of the planet is enormously aug- 
 mented, and we therefore turn to the comets when we desire to 
 illustrate the great theory of planetary perturbations by some strik- 
 ing example. 
 
 Having decided to choose a comet, the next question is, 
 What comet? There cannot here be much room for hesitation. 
 Those splendid comets which appear so capriciously may be at once 
 excluded. They are visitors apparently coming for the first time, 
 and retreating without any distinct promise that mankind shall 
 ever see them again. A comet of this kind moves in a parabolic 
 path, sweeps once around the sun, and then retreats into the space 
 whence it came. We cannot completely study the effect of perturba- 
 tions on a comet until it has been watched during successive returns 
 to the sun. Our choice is thus limited to the comparatively small 
 class of objects known as periodic comets ; and, from a survey of 
 the entire group, we can select the most suitable to our purpose. 
 One comet, and only one, fulfils all the required conditions. 
 It is the celebrated object best known as Encke's comet, for, 
 though Encke was not the discoverer, yet it is to his calculations 
 that the comet owes its fame. This body is rendered more suitable 
 for our purpose by the memorable researches to which it has recently 
 given rise. 
 
 In the year 1818 a comet was discovered by the painstaking 
 astronomer Pons at Marseilles. We are not to suppose that this 
 comet was a splendid spectacle. It was a small telescopic object, 
 not unlike one of those dim nebulae which are scattered in thousands 
 
COMETS. 305 
 
 over the heavens. The comet is, however, readily distinguished 
 from a nebula by its rapid movement relatively to the stars, 
 while the nebula remains at rest. The position of this comet was 
 ascertained by its discoverer, as well as by other astronomers. 
 Encke found from the observations that the comet returned to the 
 sun once in every three years and a few months. This was a 
 startling- discovery. At that time no other comet of short period 
 had been detected, so that this new addition to the solar system 
 awakened the liveliest interest. The question was immediately 
 raised as to whether this comet, as it revolved so frequently, may 
 not have been observed during previous returns. The historical 
 records of the apparitions of comets are counted by hundreds, and 
 how among this host are we to select those objects which were 
 identical with the comet discovered by Pons ? 
 
 We may at once relinquish any hope of identification by drawings 
 of the comet itself, but fortunately, there is one feature of a comet 
 on which we can seize, and which no fluctuations of the actual 
 structure can modify or disguise. The path in which a comet 
 travels through space is independent of the bodily changes of the 
 comet. The shape of that path and its position depend entirely 
 upon those other bodies of the solar system which are specially 
 involved in the theory of Encke's comet. In Fig. 56 we show 
 the orbits of three of the planets. These orbits have been 
 chosen with such proportions as shall make the innermost repre- 
 sent the orbit of Mercury; the next is the orbit of the earth, 
 while the outermost is the orbit of Jupiter. Besides these 
 three, we also perceive a much more elliptical path, representing 
 the oibit of Encke's comet, projected down on the plane of the 
 earth's motion. The sun is situated at the focus of the ellipse 
 The comet is constrained to revolve in this curve by the attraction 
 of the sun, and it requires a little more than three years to accom- 
 plish a complete revolution. It passes close to the sun at peri- 
 helion, at a point inside the path of Mercury, while at its greatest 
 distance the path approaches to the path of Jupiter. This elliptic 
 path is mainly determined by the attraction of the sun. Whether 
 the comet weighed an ounce, a ton, a thousand tons, or a million 
 u 
 
306 
 
 THE STORY OF THE HEAVENS. 
 
 tons, whether it was a few miles, or many thousands of miles in 
 diameter, the orbit would still be the same. It is by the shape of 
 this ellipse, by its actual size, and by the position in which it lies 
 that we identify the comet. 
 
 Jupiter 
 
 Fig. 56. The Orbit of Encke's Comet. 
 
 Encke's comet is usually invisible, and even the most powerful 
 telescope in the world would not show a trace of it. After one of 
 its periodical visits, the comet withdraws until it passes to the outer- 
 most part of its path, then it will turn, and again approach the sun. 
 It would seem that it becomes invigorated by the sun's rays, and 
 commences to dilate under their genial influence. While moving in 
 this part of its path the comet lessens its distance from the earth. 
 It daily increases in splendour, until at length, partly by the intrinsic 
 

 COMETS. 307 
 
 increase in brightness, and partly by the decrease in distance from 
 the earth, it comes within the range of our telescopes. We can 
 generally anticipate when this will occur, and we can tell to what 
 point of the heavens the telescope is to be pointed which will discern 
 the comet at its next return to perihelion. The comet has eluded 
 the grasp of the artist, but it cannot elude the grasp of the 
 mathematician. The mathematician will tell when and where the 
 comet is to be found, but no one can say what it will be like. 
 
 Once recognised as a regular member of our system, Encke's 
 comet becomes a teacher of marvellous power. At one time its 
 periodical voyage brings it close to the sun and close to the path 
 of the planet Mercury. At another time the comet approaches 
 the orbit of Jupiter. Again, the wanderer comes within reach of 
 our telescopes, and we are able to glean many interesting facts 
 from its travels. These movements contain impressive evidence 
 as to the way in which the celestial phenomena are founded on 
 the law of gravitation. 
 
 Were all the other bodies of the system removed, then the path 
 of Encke's comet must be for ever performed in the same ellipse 
 and with absolute regularity. The chief interest for our present 
 purpose lies, however, not in the regularity of its path, but in the 
 irregularities introduced into that path by the presence of the other 
 bodies of the solar system. Let us, for instance, follow the progress 
 of the comet through its perihelion passage, in which the track lies 
 near that of the planet Mercury. It will usually happen that 
 Mercury is situated in a distant part of its path at the moment 
 the comet is passing, and the influence of the planet will then be 
 comparatively small. It may, however, sometimes happen that the 
 planet and the comet come close together. One of the most memor- 
 able instances of a close approach to Mercury took place on the 22nd 
 November, 1848. On that day the comet and the planet were only 
 separated by an interval of about one-thirtieth of the earth's distance 
 from the sun, i.e., about 3,000,000 miles. On two other occasions, 
 viz., 1835, August 23rd, and 1858, October 25th, the distance 
 between Encke's comet and Mercury has been less than 10,000,000 
 miles an amount of trifling import in comparison with the 
 U 2 
 
308 THE STORY OF THE HEAVENS. 
 
 dimensions of our system. Approaches so close as this are fraught 
 with serious consequences to the movements of the comet. Mer- 
 cury, though a small body, is still, as its name would suggest, 
 a massive body. It always attracts the comet, but the efficacy of 
 that attraction is enormously enhanced when the comet in its 
 wanderings comes near the planet. The effect of this attraction is 
 to force the comet to swerve from its path, and to impress certain 
 changes upon its velocity. As the comet recedes, the disturbing 
 influence of Mercury rapidly abates, and ere long becomes insensible. 
 But time cannot efface from the orbit of the comet the effect which 
 the disturbance of Mercury has actually accomplished. The dis- 
 turbed orbit is a different figure to the undisturbed ellipse which 
 the comet would have occupied had the influence of the sun alone 
 determined its shape. We are able to calculate the movements of 
 the comet as determined by the sun. We could also calculate the 
 effects arising from the disturbance produced by Mercury, provided 
 we know its mass. 
 
 Mercury, though one of the smallest of the planets, is perhaps 
 the most troublesome to the astronomer. It lies so close to the 
 sun, that it is seen but seldom in comparison with the other 
 great planets. Its orbit is very eccentric, and experiences dis- 
 turbances by the attraction of other bodies in a way not yet fully 
 understood. A special difficulty has also been found in the 
 attempt to place Mercury in the weighing scales. We can weigh 
 the whole earth, we can weigh the sun, the moon, and even 
 Jupiter and other planets, but Mercury presents difficulties of a 
 peculiar character. Le Verrier, however, succeeded in devising a 
 method of weighing Mercury. He demonstrated that our earth is 
 attracted by this planet, and he showed how the amount of attrac- 
 tion may be disclosed by observations of the sun, so that by exami- 
 nation of the observations, he made an approximate determination of 
 the mass of Mercury. Le Verrier's result indicated that the weight 
 of the planet was about the fifteenth part of the weight of the earth. 
 In other words, if our earth was placed in a mighty scale, and 
 fifteen globes, each equal to Mercury, were laid in the other, the 
 scales would be evenly balanced. It was necessary that this result 
 
COMETS. 309 
 
 should be received with great caution. It depended upon a very 
 delicate interpretation of somewhat precarious measurements. It 
 could only be regarded as a provisional value, to be discarded when 
 a better one should be obtained. 
 
 The approach of Encke's comet to Mercury, and the elaborate 
 investigations of Von Asten, by which the observations of the comet 
 have been discussed, have thrown much light on the subject. Von 
 Asten assumed, in the first place, that Mercury was really endowed 
 with the mass which Le Verrier had attributed to it ; and he was 
 able to calculate where the comet should be, assuming this deter- 
 mination to be correct. But the comet was not found in the place 
 indicated by the calculations ; it was fully an arc of one degree dis- 
 tant. The calculations must, therefore, have some erroneous element. 
 There was only one element which could be erroneous : it was the 
 mass of Mercury. Von Asten then tried a different value for the 
 mass of the planet. He repeated the calculations, and the calculated 
 place became more nearly coincident with the observed place. A 
 third attempt, and this time a final one, showed that by assuming 
 a suitable mass for Mercury the calculated places of the comet 
 can be reconciled with the observed places. Von Asten has thus 
 demonstrated that the weight of Mercury is much less than 
 Le Verrier had supposed. If our earth were divided into twenty- 
 five parts of equal mass, each one of these parts would weigh as 
 much as the planet. No different value of the mass of Mercury 
 can be made consistent with the information obtained from the 
 voyage of Encke's cornet. 
 
 Do we doubt the accuracy of this delicate weighing-machine ? 
 Look, then, at the orbit of Jupiter, to which the comet approaches 
 so nearly when it retreats from the sun. It will sometimes happen 
 that Jupiter and the comet are in close proximity, and then the 
 mighty planet seriously disturbs the pliable orbit of the comet. 
 The path of the latter bears indelible traces of the Jupiter pertur- 
 bations, as well as of the Mercury perturbations. It might seem 
 a hopeless task to discriminate between the influences of the two 
 planets, overshadowed as they both are by the supreme control 
 of the sun, but the contrivances of mathematical analysis are 
 
310 THE STORY OF THE HEAVENS. 
 
 adequate to deal with the problem. They point out how much is due 
 to Mercury, how much is due to Jupiter ; and so the wanderings of 
 Encke's comet can be made to disclose the mass of Jupiter as well 
 as that of Mercury. Here, then, we have a means of testing the 
 precision of our scales. The mass of Jupiter can be accurately 
 measured by his satellites, as we have already mentioned in a 
 previous chapter. As the satellites revolve round and round the 
 planet, they afford the means of measuring his weight by the 
 rapidity of their motion. They tell us that if the sun were placed 
 in one scale of the celestial balance, it would take 1048 bodies equal 
 to Jupiter in the other to weig'h him down. Hardly a trace of 
 uncertainty clings to this determination, and it is therefore of great 
 interest to test the theory of Encke's comet by seeing whether it 
 gives an accordant result. The comparison has been made by Von 
 Asten. Encke's comet tells us that the sun is 1050 times as heavy 
 as Jupiter ; so the results are practically identical, and the accuracy 
 of the indications of the comet are confirmed. 
 
 Encke's comet, by its long wanderings and by its frequent 
 visits, is, indeed, pregnant with information on various other points 
 relating to the solar system. A score of times that comet has 
 returned, and we can only render these returns consistent with 
 each other by making appropriate estimations of the various quan- 
 tities involved. It is most instructive to see how, in Von Asten's 
 hands, the most recondite information is wrung from the comet. 
 The distance of the earth from the sun is a very important matter 
 to Encke's comet. Our earth contributes its quota of disturbance 
 to the many afflictions of the suffering body. The capacity of our 
 earth to derange the comet depends in a measure upon the distance 
 from the earth to the sun. The investigation is of no little com- 
 plexity, but still it is easy to see that the earth's distance from the 
 sun must be among the numerous elements of the question. Unless 
 the right distance be chosen in the calculations, the observed place 
 will not correspond with the calculated place. This consideration 
 is sufficient for the mathematician. He can actually disentangle 
 the earth's distance from the other quantities involved. It is some- 
 what remarkable that Von Asten has found by his calculations a 
 
COMETS. 311 
 
 value of the sun's distance very appreciably less than that obtained 
 by other methods. Each of these results has good claims to 
 accuracy, and it will be a problem of interest for astronomers to 
 reconcile the discrepancy. 
 
 We have hitherto discussed the adventures of Encke's comet in 
 cases where they throw light on questions otherwise more or less 
 known to us. We now approach a celebrated problem, on which 
 Encke's comet is, if not our sole, at all events our principal 
 authority. Every 1,210 days that comet revolves completely around 
 its orbit, and returns again to the neighbourhood of the sun. The 
 return of the comet is, however, open to certain irregularities. We 
 have already explained how irregularities arise from Mercury and 
 from Jupiter. Additional irregularities also arise from the attrac- 
 tion of the earth and of the other remaining planets ; but all these 
 can be allowed for, and then we are entitled to expect, if the 
 law of gravitation be universally true, that the comet shall obey 
 the calculations of mathematics. Encke's comet has not justified 
 this anticipation ; at each revolution the period is getting steadily 
 shorter ! Each time the comet comes back to perihelion in two and 
 a half hours less than on the former occasion. Two and a half 
 hours is, no doubt, a small period of time in comparison with that 
 of an entire revolution ; but in the region of its path visible to us 
 the comet is moving so quickly that its motion in two and a half 
 hours is very considerable. This irregularity cannot be over- 
 looked, inasmuch as it has been confirmed by the returns during 
 about twenty revolutions. It has sometimes been thought that the 
 discrepancies might be attributed to some planetary perturbations 
 omitted or not fully accounted for. The masterly analysis of Von 
 As ten has, however, disposed of this explanation. He has minutely 
 studied all the observations down to 1875, but only to confirm the 
 reality of this regular retardation in the movements of Encke's 
 comet. 
 
 An explanation of these irregularities was suggested by Encke 
 long ago, and subsequent researches have tended to substantiate 
 his view. Let us briefly attempt to describe this memorable 
 hypothesis. When we say that a body will move in an elliptic path 
 
312 THE 8TOEY OF THE HEAVENS. 
 
 around the sun in virtue of gravitation, it is always assumed that 
 the hody has a free course through space. It is assumed that there 
 is no friction, no air, or other source of disturbance. But suppose 
 that this assumption should be incorrect ; suppose that there really 
 is some medium pervading- space which offers resistance to the 
 comet, in the same way as our air offers resistance to the passage of 
 a rifle bullet, what effect ought such a medium to produce? 
 
 Comet 
 
 Fig. 57. The Resisting Medium around the Sun. 
 
 The drawing in Fig. 57 represents the theory which Encke put 
 forward, and in support of which we have to add the more recent 
 researches of Von Asten. Even if the greater part of space be 
 utterly void, so that the path of the filmy and almost spiritual 
 comet is incapable of feeling resistance, yet in the neighbour- 
 hood of the sun it would seem that there must be some medium 
 of excessive tenuity capable of affecting the comet. It can be 
 demonstrated that the effect of a resisting medium such as we 
 have supposed is to diminish the size of the comet's path, and 
 thus to diminish the periodic time. The existence of the solar 
 corona and the strange phenomenon of the zodiacal light have 
 already shown us, on quite other grounds, the existence of a vast 
 extent of matter of some kind diffused around the sun. 
 
COMETS. 313 
 
 We have selected the comets of Halley and of Encke as 
 illustrations of the class of periodic comets, of which, indeed, 
 they are the most remarkable members. Of the much more 
 numerous class of non-periodic comets, examples in abundance may 
 be cited. We shall mention a few which have appeared during 
 the present century. There is first the splendid comet of 1843, 
 which appeared suddenly in February of that year, and was so 
 brilliant that it could be seen during full daylight. This comet 
 followed a path which could not be certainly distinguished from 
 a parabola, though there is no doubt that it might have been a 
 very elongated ellipse. It is impossible to decide a question of 
 this kind during the brief opportunities available for finding the 
 place of the comet. We can only see the object during a veiy 
 small arc of its orbit, and even then the comet is not a very well- 
 defined point which admits of being measured with the precision 
 attainable in observations of a star or a planet. This comet of 1843 
 is, however, especially remarkable for the rapidity with which it 
 moved, and for the close approach which it made to the sun. The 
 heat to which that comet was exposed during its passage around 
 the sun must have been enormously greater than the heat which 
 can be raised in our mightiest furnaces. If the materials had 
 been agate or cornelian, or the most infusible substances known 
 on the earth, they would have been fused and driven into vapour 
 by the appalling intensity of the sun's rays. 
 
 The great comet of 1858 was one of the most magnificent 
 comets of modern times. It was first observed on June 2nd of 
 that year by Donati, whose name the comet has subsequently 
 borne ; it was then merely a faint nebulous spot, and for about 
 three months it pursued its way across the heavens, without giving 
 any indications of the splendour which it was so soon to attain. 
 Even up to the end of August the comet had hardly become 
 visible to the unaided eye, and was only furnished with a very 
 small tail, but as it gradually drew nearer and nearer to the sun 
 in September, it soon became invested with splendour. A tail 
 of majestic proportions was quickly developed, and by the middle 
 of October, when the maximum brightness was attained, the tail 
 
314 THE STORY OF THE HEAVENS. 
 
 extended over an arc of forty degrees. The beauty and interest of 
 this comet were greatly enhanced by its position in the heavens 
 at a season when the nights were sufficiently dark. Some of the 
 more recent great comets have not been equally fortunate : they 
 have appeared in midsummer, when the nights are not dark, or 
 they have not been well placed in the heavens. 
 
 On the 22nd May, 1881, Mr. Tebbutt, of Windsor, in New 
 South Wales, discovered a comet which speedily developed into one 
 of the most interesting celestial objects seen by this generation. 
 About the 22nd of June the comet became visible from these 
 latitudes in the northern sky at midnight. Gradually it ascended 
 higher and higher till it passed around the pole. The nucleus of 
 the comet was as bright as a star of the first magnitude, and its 
 tail was about 20 long. On the 2nd of September it ceased to be 
 visible to the unaided eye, but it remained visible in telescopes 
 until the following February. This was the first comet which was 
 successfully photographed. To this comet also the spectroscope 
 was applied with especial success. We shall in a subsequent 
 chapter describe the use of this remarkable instrument ; here it will 
 be sufficient to observe that by its aid the existence of the element 
 carbon in this comet was demonstrated. 
 
 Another of the recent comets which received great and deserved 
 attention was that discovered early in September, 1882, in the 
 southern hemisphere. It increased so much in brilliancy that it 
 was seen in daylight by Mr. Common on the 17th September, 
 while on the 18th the astronomers at the Cape of Good Hope were 
 fortunate enough to have observed the comet actually approach up 
 to the sun's limb, where it ceased to be visible. On the same day 
 the spectrum of the comet was observed, and it was shown that the 
 elements sodium and iron were present therein. 
 
 As comets are much nearer to the earth than the stars, it will 
 occasionally happen that the comet must arrive at a position 
 directly between the earth and the star. There is a quite similar 
 phenomenon in the movement of the moon. A star is frequently 
 occulted in this way, and the observations of such phenomena 
 are very familiar to astronomers ; but when a comet passes in front 
 
COMETS. 315 
 
 of a star the circumstances are widely different. The star is indeed 
 seen nearly as well through the comet as it would be if the comet 
 were entirely out of the way. This has often been noticed. One 
 of the most celebrated observations of this kind was made by 
 the late Sir John Herschel on Biela's comet, which is one of the 
 periodic class, and will be alluded to in the next chapter. The illus- 
 trious astronomer on one occasion saw this object pass over a star 
 cluster. The cluster consisted of excessively minute stars they 
 could only be seen by a very powerful telescope, such as the one Sir 
 John was using. The very faintest haze or the merest trace of a 
 cloud would have sufficed completely to obliterate all the stars. 
 It was therefore with no little interest that the astronomer watched 
 the progress of Biela's comet. Gradually the wanderer encroached 
 on the group of stars, so that if it had any appreciable solidity the 
 numerous twinkling points would have been completely screened. 
 But what were the facts ? Down to the most minute star in that 
 cluster, down to the smallest point of light which the great tele- 
 scope could show, every object in the group was distinctly seen 
 to twinkle right through the mass of Biela's comet ! 
 
 This was a most important observation. We must recollect 
 that the screen interposed between the cluster and the telescope 
 was not merely a thin curtain ; it was a mass of cometary sub- 
 stance, very many thousands of miles in thickness. Contrast, 
 then, the almost inconceivable tenuity of a comet with the clouds 
 to which we are accustomed. A mass of cloud a few hundred 
 feet thick will hide not only the stars, but even the great sun 
 himself. The lightest haze that ever floated in a summer sky, 
 would do more to screen the stars from our view than would one 
 hundred thousand miles of such cometary material as was here 
 interposed. 
 
 The great comet of Donati passed over many stars which were 
 visible distinctly through its mass. Among these stars was a very 
 bright one the well-known Arcturus. The comet, fortunately, 
 happened to pass over Arcturus, and though nearly the densest part 
 of the comet was interposed between the earth and the star, yet 
 Arcturus twinkled on with undiminished lustre through the thick- 
 
316 THE STOET OF THE HEAVENS. 
 
 ness of this stupendous curtain. Recent observations have, how- 
 ever, shown that stars in some cases experience change in lustre 
 when the denser part of the comet passes over them. It is, indeed, 
 difficult to imagine that a star would remain visible if the nucleus 
 of a really large comet passed over it. It does not seem that an 
 opportunity of testing this supposition has yet arisen. 
 
 If the comet were made of transparent gaseous material such, 
 for instance, as our atmosphere the place of a star would be 
 deranged when the comet approached the star. The refractive 
 power of air is very considerable. When we look at the sunset, 
 we see the sun appearing to pass below the horizon ; yet the sun 
 has actually gone entirely below the horizon before it appears 
 to have commenced its descent. The refractive power of the 
 air is so great that it actually bends the luminous rays round 
 and shows the sun, though it is directly screened by the inter- 
 vening obstacles. If the comet were made of gaseous material, it 
 must act on the places of the stars by refraction, even if it does not 
 alter their brightness. The question has been carefully tested. A 
 comet has been observed to approach two stars ; one of those stars 
 was seen through the comet, while the other was seen directly. If 
 the body had any appreciable quantity of gas in its composition the 
 relative places of the two stars would be altered. This question 
 has been more than once submitted to the test of actual measure- 
 ment. It has sometimes been found that no appreciable change 
 of position could be detected, and that accordingly in such cases the 
 comet has no perceptible density. A series of careful measure- 
 ments made on the great comet of 1881 showed, however, that in 
 the neighbourhood of the nucleus the comet seemed to possess some 
 refractive power, though still vastly less than the refraction of our 
 atmosphere. 
 
 From these considerations it will probably be at once admitted 
 that the mass of a comet must be indeed a very small quantity 
 in comparison with its bulk. When, however, we attempt 
 actually to weigh the comet, all our efforts have proved abor- 
 tive. We have been able to weigh the mighty planets Jupiter 
 and Saturn ; we have even been able to weigh the vast sun himself; 
 
COMETS. 317 
 
 the law of gravitation has provided us with a stupendous weighing- 
 apparatus, which has been applied in all these cases with success, 
 but when we attempt to apply the same methods to the comets our 
 efforts are speedily seen to be illusory. No weighing machinery 
 known to the astronomer is delicate enough to determine the weight 
 of a comet. All that we can under any circumstances accomplish 
 is to weigh one heavenly body in comparison with another. 
 The comets seem to be almost imponderable when compared with 
 such robust masses as those of the earth, or any of the other great 
 planets, and so we learn nothing except that their weight is inap- 
 preciable. Of course it will be understood that when we say the 
 weight of a comet is inappreciable, we mean with regard to the 
 other bodies of our system. Perhaps no one now doubts that a great 
 comet must really weigh tons ; though whether those tons are to 
 be reckoned in tens, in hundreds, in thousands, or in millions, the 
 total seems quite insignificant when compared with a body like the 
 earth. 
 
 The small weight of comets is also brought before us in a very 
 remarkable way when we recall what has been said in the last 
 chapter on the important subject of the planetary perturbations. 
 We have there treated of the permanence of our system, and we 
 have shown that the permanence of the system depends upon 
 certain laws which the planetary motions must fulfil. The planets 
 move nearly in circles, their orbits are all nearly in the same plane, 
 and they all move in the same direction. The permanence of the 
 system would be imperilled if any one of these conditions was 
 not fulfilled. In that discussion we made no allusion to the 
 comets. Yet they are members of our system, and they far out- 
 number the planets. And comets laugh to scorn all those rules which 
 the planets so rigorously obey. Their orbits are never like circles ; 
 they are, indeed, more usually parabolic, and thus differ as widely 
 as possible from the circular path. Nor do the planes of the orbits 
 of the comets observe any particular locality ; they are inclined at 
 all sorts of angles, and the directions in which the comets move 
 seem to be mere matters of caprice. Every article of the planetary 
 convention is violated by the comets, but yet our system lasts ; it 
 
318 THE STORY OF THE HEAVENS. 
 
 has lasted for countless ages, and seems destined to last for ages to 
 come. The comets are attracted by the planets, and conversely, the 
 comets must attract the planets, and must perturb the orbits of the 
 planets to some extent ; but to what extent ? If the comets all 
 moved in orbits guided by the three general laws which charac- 
 terise planetary motion, then our argument would break down. 
 The planets might experience considerable derangements from 
 cometary attraction, and yet in the lapse of time those disturbances 
 would neutralise each other, and the permanence of the system would 
 be unaffected. But the case is very different when we deal with the 
 actual cometary orbits. If the comets could appreciably disturb 
 the planets, those disturbances would not neutralise ea"ch other, and 
 in the lapse of time the system would be wrecked by the con- 
 tinuous accumulation of irregularities. The facts show, however, 
 that the system has lived, and is living, notwithstanding the 
 comets, and hence we are forced to the conclusion that the cometary 
 masses are utterly insignificant in comparison with the great 
 planetary masses of our system. 
 
 These considerations exhibit the laws of universal gravitation and 
 their relations to the permanence of our system in a very striking 
 light. If we include the comets, we may say that the solar 
 system includes many thousands of bodies, in orbits of all sizes, 
 shapes, and positions, only agreeing in the fact that the sun is a 
 focus common to all. The great majority of these bodies are 
 imponderable, and their orbits are placed anyhow, so that, 
 although they may suffer much from the perturbations of the other 
 bodies, they can in no case inflict any appreciable disturbance. 
 There are, however, a few great planets capable of producing vast 
 disturbances ; and if their orbits were placed anyhow, chaos would 
 sooner or later be the result. By the mutual adaptations of their 
 orbits to a nearly circular form, to a nearly coincident plane, and to 
 a uniformity of direction, a permanent truce has been effected 
 among the great planets. They cannot now permanently dis- 
 organise each other, while the slight mass of the comets renders 
 them incompetent to do so. The stability of the great planets is 
 thus assured ; but it is to be observed that there is no guarantee of 
 
COMETS. 319 
 
 stability for comets. Their eccentric and irregular paths may 
 undergo the most enormous derangements ; indeed, the history of 
 astronomy contains many instances of the vicissitudes to which a 
 cometary career is exposed. 
 
 Great comets appear in the heavens under circumstances which 
 have but few features in common. There is no part of the heavens, 
 either in the northern hemisphere or the southern hemisphere, no 
 constellation or region, which is not liable to occasional visits from 
 these mysterious bodies. There is no season of the year, no hour 
 of the day or of the night, when comets may not be above the 
 horizon. In like manner, the size and the aspect of the comets 
 are of every character, from the dim spot just visible to an eye 
 fortified by a mighty telescope, up to a gigantic and brilliant object, 
 with a tail stretching across the heavens for a distance which is as 
 far as from the horizon to the zenith. So also the direction of 
 the tail of the comet seems at first to admit of every possible 
 position : it may stand straight up in the heavens, as if the comet 
 were about to plunge below the horizon; it may stream down 
 from the head of the comet, as if the body had been shot up from 
 below the horizon ; it may slope down to the right or rise to the 
 right, it may slope down to the left or rise to the left. Amid all 
 this variety and seeming caprice, can we discover any law common 
 to the different phenomena ? If we are to succeed in giving any 
 explanation of the tails of comets, we must first discover what 
 features there are if any there be which all these tails possess in 
 common ; and we shall find that there is a very remarkable law 
 which the tails of comets obey a law so true and satisfactory, that 
 if we are given the place of a comet in the heavens, it is possible at 
 once to point out in what direction the tail will lie. 
 
 A beautiful comet appears in summer in the northern sky. It 
 is near midnight ; we are gazing on the faintly luminous tail, which 
 stands up straight and points towards the zenith ; perhaps it may 
 be curved a little or possibly curved a good deal, but still, on the 
 whole, it is directed from the horizon to the zenith. We are 
 not here referring to any particular comet. Every comet, large or 
 small, that appears in the north must at "midnight have its tail 
 
320 THE STOEY OF THE HEAVENS. 
 
 pointed up in a nearly vertical direction. This fact, which has 
 been verified on numerous occasions, is a striking illustration 
 of the law of direction of comets' tails. Think for one moment 
 of the facts of the case. It is summer ; the twilight at the north 
 shows the position of the sun, and the tail of the comet points 
 directly away from the twilight and a way from the sun. Take 
 another case. It is evening; the sun has set, the stars have begun 
 to shine, and a long-tailed comet is seen. Let that comet be high 
 or low, north or south, east or west, its tail invariably points away 
 from that point in the west where the departing sunlight still lingers. 
 Again, a comet is watched in the early morning, and if the eye be 
 moved from the place where the first streak of dawn is appearing to 
 the head of the comet, then along that direction, streaming away 
 from the sun, is found the tail of the comet. This law is of still more 
 general application. At any season, at any hour of the night, the 
 tail of a comet must be directed away from the sun. The sun is, 
 of course, on some point of the hemisphere below our feet ; draw 
 a great circle from the sun, passing above the horizon to the head 
 of the comet, then the direction of the comet's tail will follow the 
 continuation of that circle. 
 
 From the earliest times this fact in the movement of comets 
 must have arrested the attention of those who pondered on the 
 movements of the heavenly bodies. It is a fact patent to ordinary 
 observation, it depends on no telescopic research, nor is it the 
 result of any elaborate mathematical discussion. It is the one fact 
 which gives some degree of consistency to the multitudinous 
 phenomena of comets, and it must be made the basis of our 
 inquiries into the structure of the tails. 
 
 In the adjoining figure, Fig. 58, we show a portion of the 
 parabolic orbit of a comet, and we also represent the position of the 
 tail of the comet at various points of its path. It would be, perhaps, 
 going too far to assert that throughout the whole vast journey of 
 the comet, its tail must always be directed from the sun. In the 
 first place, it must be recollected that we can only see the comet 
 during that small part of its journey when it is approaching to or 
 receding from the sun. It is also to be remembered that, while 
 
COMETS. 321 
 
 actually passing round the sun, the brilliancy of the comet is so 
 overpowered by the sun that the comet often becomes invisible, 
 just as the stars are invisible in daylight. But with these obvious 
 qualifications we can assert that the tail is always directed away 
 from the sun. 
 
 In a hasty consideration of the subject it might be thought that 
 as the comet was dashing along with an enormous velocity the tail 
 was merely streaming out behind, just as the shower of sparks 
 from a rocket are strewn along the path which it follows. This 
 
 Fig. 58. The Tail of a Comet directed from the Sun. 
 
 would be an entirely erroneous analogy ; the comet is moving not 
 through an atmosphere, but through open space, where there is no 
 medium sufficient to sweep the tail into the direction of motion. 
 Another very remarkable feature is the gradual growth of the tail 
 as the comet approaches the sun. While the body is still at a 
 great distance it has usually no perceptible tail, but as it draws in 
 the tail gradually developes, and in some cases reaches the most 
 stupendous dimensions. It is not to be supposed that this increase 
 is a mere optical consequence of the body coming nearer the earth. 
 It can be shown that the growth of the tail is enormously 
 greater than it would be possible to explain merely by the approach 
 of the comet. We are thus led to connect the formation of the 
 tail with the approach to the sun, and we are accordingly in the 
 
 ..- 
 
 v 
 
322 THE STORY OF THE HEAVENS. 
 
 presence of an enigma which has but little analogy among the other 
 bodies of our system. 
 
 That the comet as a whole is attracted by the sun, there can be 
 no doubt whatever. The fact that it moves in an ellipse or in a 
 parabola proves that the two bodies act and re-act on each other 
 in obedience to the law of universal gravitation. But while this 
 is true of the comet as a whole, it is no less certain that the tail 
 of the comet is repelled by the sun. It is impossible to speak with 
 certainty as to how this comes about, but the facts of the case 
 seem to point to an explanation of the following kind. 
 
 In the materials composing a comet we have some one or more 
 ingredients which give rise to the tail. As the comet draws near 
 the sun and experiences the invigorating effect of increased 
 heat, these ingredients become melted and driven off into vapour. 
 It would seem that though these substances in their solid state 
 are duly attracted to the sun, yet when driven into vapour of a 
 highly rarefied type the heat of the sun exerts on that vapour a 
 repellent power which entirely overcomes the attraction, and ac- 
 cordingly drives the vapour off in a direction pointing away from 
 the sun. We are thus to regard the tail as a stream of smoke 
 or vapour rapidly flying away, and constantly renewed from 
 the evaporation of fresh materials so long as the comet remains 
 sufficiently near to the sun. This explanation will give a plausible 
 reason for the direction of the tail, and, indeed, it seems impossible 
 to believe that the tail could be whirled around the sun with 
 sufficient rapidity if it were really a continuous object. 
 
 As to the nature of the repulsive force which we must postulate 
 for the formation of the tail, we are not yet in a position to speak 
 with absolute certainty. It is, however, impossible to omit some 
 account of the remarkable researches of Professor Bredichin, 
 which, when taken in combination with the suggestions of Pro- 
 fessor Osborne Reynolds and of others, afford at least a probable 
 explanation of the phenomena. Professor Bredichin has con- 
 ducted his labours in the strictly philosophical manner which 
 has already led to all the great discoveries in science. He has 
 carefully collaborated the measurements and drawings of the 
 
 
COMETS. 
 
 323 
 
 tails of various comets. He has obtained one result from this 
 preliminary part of his inquiry, which will possess a value 
 that cannot be affected even if the ulterior portion of his labours 
 should be found to require qualification. In the examination of 
 the tails of various comets, it is observed that the curvilinear 
 shapes of the outlines fall into one or other of three special types. 
 In the first type we have the straightest tails, which point almost 
 
 Fig. 59. Bredichin's Theory of Comets' Tails. 
 
 directly away from the sun in a straight line. In the next type 
 we have tails which, while still starting away from the sun, are 
 curved backwards from the direction towards which the comet 
 is moving. In the third type we find the tail curved in still 
 more towards the comet's path. It can be shown that the tails 
 of comets can almost invariably be identified with one or other 
 of these three groups ; and in cases where the comet exhibits two 
 tails, as has sometimes happened, then both of these tails will be 
 found to belong to one or other of the types. 
 
 The adjoining diagram (Fig. 59) is a sketch of an imaginary comet 
 
324 
 
 THE STORY OF THE HEAVENS. 
 
 furnished with tails of the three different types. The direction in 
 which the comet is moving is shown by the arrow-head on the 
 line passing through the nucleus. The straightest of the three tails, 
 marked as Type I., is most probably due to the element hydrogen ; 
 the tails of the second type are due to the presence of some of the 
 hydro-carbons in the body of the comet ; while the small tails of 
 the third type may be due to iron or to chlorine, or to some other 
 
 Fig. 60. Tails of the Comet of 1858. 
 
 element with a high atomic weight. It will of course be undei 
 stood that this is not the view of any actual comet. 
 
 A very beautiful illustration of this theory is afforded in tl 
 case of the celebrated comet of 1858 already referred to, of which 
 a drawing is shown in Fig. GO. "We see here, besides the great tail, 
 which is the characteristic feature of the comet, two other faint 
 streaks of light. These are really the edges of the hollow cone 
 which forms a tail of Type I., and when we look through 
 the central regions it will be easily understood that the light 
 is not sufficiently intense to be visible; at the edges, however, a 
 
COMETS. .325 
 
 good thickness of the cometary matter is looked through, and thus 
 we have the appearance shown in this figure. It would seem that 
 Donates comet possessed one tail due to hydrogen, and another due to 
 
 Fig. 61. Cheeeauxs Comet of 1744. 
 
 some of the compounds of carbon. The carbon compounds involved 
 seem to be of considerable variety, and there is, in consequence, 
 a disposition in the tails of the second type to a more indefinite 
 outline than in the hydrogen tails. Cases have been recorded in 
 which several tails have been seen simultaneously on the same 
 
326 THE STORY OF THE HEAVENS. 
 
 comet. The most celebrated of these is that which appeared in 
 the year 1744, and is now known as Cheseaux's comet. Professor 
 Bredichin has devoted special attention to the theory of this mar- 
 vellous object, and he lias shown with a high degree of probability 
 how the multiform tail could be accounted for on his theory. 
 
 It is possible to submit some of the questions involved to the test 
 of calculation, and it can be shown that the repulsive force adequate 
 to produce the very straigh test tail of Type I. need only be about twelve 
 times as large as the attraction of gravitation. Tails of the second type 
 could be produced by a repulsive force which was equal to gravita- 
 tion, while tails of the third type would only require a repulsive 
 force of one-quarter the power of gravitation. The chief repulsive 
 force known in nature is derived from electricity, and it has 
 naturally been surmised that the phenomena of comets' tails are 
 really due to the electric condition of the sun and of the comet. It 
 can be shown that when the cometary substances are driven into 
 vapour by the heat of the sun, the electric repulsion may equal 
 may even greatly exceed gravitation, and thus produce the phe- 
 nomena which are observed. It would be premature to assert that 
 the electric character of the comet's tail has been absolutely demon- 
 strated ; all that can be said is that, as it seems to account for the 
 observed facts, it would be undesirable to introduce some mere 
 hypothetical repulsive force. It must be remembered that on quite 
 other grounds it is known that the sun is the seat of certain electric 
 phenomena. 
 
 As the comet gradually recedes from the sun the heat abates, 
 the evaporation ceases, the repulsion has no longer any object on 
 which to exercise its power, and accordingly we find the tail of 
 the comet declines. If the comet be a periodic one, the same series 
 of changes may take place at its next return to perihelion. A new 
 tail is formed, which also gradually disappears as the comet regains 
 the depths of space. If we may employ the analogy of terrestrial 
 vapours to guide us in our reasoning, then it would seem that 
 as the comet retreats its tail would condense into myriads of small 
 particles. Over these small particles the law of gravitation would 
 resume its sway, no longer obscured by the superior efficiency of 
 
COMETS. 327 
 
 the repulsion produced by heat acting on the vapour. The mass 
 of the comet is, however, so extremely small that it would not be 
 able to recall these particles by the mere force of attraction. It 
 follows that, as the comet at each perihelion passage makes a tail, 
 it must on each occasion sacrifice a corresponding quantity of tail- 
 making material. Let us suppose that the comet was endowed in 
 the beginning with a certain capital of those particular materials 
 which are adapted for the production of tails. Each perihelion 
 passage witnesses the production of a tail, and the expenditure of a 
 corresponding proportion of the capital. It is obvious that this 
 operation cannot go on indefinitely. In the case of the great 
 majority of comets the visits to perihelion are so extremely rare 
 that the consequences of the extravagance are not very apparent ; 
 but in the case of those periodic comets which have short periods 
 and frequent returns, the consequences are precisely what might 
 have been anticipated : the tail-making capital has been gradually 
 squandered, and thus at length we have the frequent spectacle 
 of a comet without any tail at all. 
 
 It is not a little remarkable that some of the materials present 
 in a comet are identical with substances which are familiar on the 
 earth. The most notable instance is the element carbon. This is 
 especially interesting when we reflect on the significance of carbon 
 on the earth. We know that carbon is one of the most abundant 
 and the most important of the elements. We see it as the chief 
 constituent of all vegetable life ; we find it to be invariably present 
 in animal life and, indeed, in all organic substances. It is an 
 interesting fact that this element, of such transcendent importance 
 on the earth, should now have been proved to be present in these 
 wandering bodies. Although no one suggests that a comet could 
 be the abode of life, yet it is certainly a significant discovery 
 that the most important factor in the material substratum of life 
 has now been shown not to be limited to the surface of the earth, 
 but to be as widespread as comets themselves. 
 
 To say that carbon is a constituent of comets is, indeed, to 
 attribute to that element a presence throughout the length and 
 breadth, the depth and the height, of the material universe. There 
 
328 THE STORY OF THE HEAVENS. 
 
 is no quarter of the heavens from which comets may not appear. 
 They exist in such profusion that, as Kepler said, there may be 
 more comets in the sky than there are fishes in the ocean ; and as 
 to the distance to which comets recede in their wanderings, or from 
 which they may be drawn into our system, its calculation baffles 
 all astronomical measurement. We are in the habit of regarding 
 the orbits of most comets as sensibly of a parabolic shape ; and 
 assuming this to be the case, let us ask the law of gravitation to 
 describe the history of one of these wanderers. 
 
 Far away in the depths of space, at a distance so remote that 
 from it our earth and the other planets are all invisible, and from 
 which even our sun has been dwarfed to the magnitude of a mere 
 star, lies the future comet. Throughout the mighty abyss which 
 intervenes between the comet and the sun, the law of gravitation 
 reigns. Under its influence the comet begins to approach the 
 sun. The force is so enfeebled by the effect of distance that at 
 first the movements of the comet are very slow. Years roll on ; 
 the comet is still found to be approaching, and the motion, though 
 still extremely slow, has made some slight improvement. Centuries 
 roll on, and thousands of years roll on, and now the comet is seen 
 to be rapidly approaching. The distance decreases and the speed 
 increases. The attraction of the sun has now increased greatly, 
 owing to the diminished distance ; and the comet at length, with 
 an appalling velocity, amounting to hundreds of miles a second, 
 whirls around the sun, and in a few hours commences its retreat. 
 The mighty velocity of the body now tends to carry it away from 
 the sun, while the whole efficiency of the solar attraction is ex- 
 pended in the effort to recall it; but the comet if its orbit be 
 such as we are here considering will not be recalled. The effect 
 of the sun's attraction will speedily abate the velocity of the 
 motion. The comet retreats precisely as it approached. The 
 lapse of a year after it has left the sun finds the comet as far 
 as it was a year before it reached the sun. The same is true for 
 100 years for 1,000 years. More and more slowly will the comet 
 retreat as it gradually sinks into the depths of space. It never 
 can entirely escape from the sun's attraction. With ever-abating 
 
 
COMETS. 329 
 
 velocity it will recede further and further, but never again to 
 return to our system. 
 
 Such is a brief outline of the principal facts known with regard 
 to these most interesting, but most perplexing bodies. As to the 
 origin of comets, it is impossible to offer more than a conjecture. 
 It has been supposed that they may have been merely drawn into 
 our system from the depths of infinite space. It has also been 
 suggested that possibly comets may have originated in our system 
 that they may, in fact, have been merely fragments driven off 
 from the sun himself. We must be content with the mere 
 recital of what we know, rather than with attempting guesses about 
 matters beyond our reach. We have confined our remarks to the 
 humbler, but more certain process of indicating the part which 
 comets play in the solar system. We see that comets are obedient 
 to the great laws of gravitation, and afford the most striking 
 illustration of their truth. We have seen how modern science has 
 dissipated the fear and the superstition with which, in earlier ages, 
 the advent of a comet was regarded. We no longer regard a comet 
 as a sign of impending calamity ; we may rather look upon it as an 
 interesting and a beautiful visitor, which comes to please us and 
 to instruct us, but never to threaten or to destroy. 
 
CHAPTER XVII. 
 
 SHOOTING STARS. 
 
 Small Bodies of our System Their Numbers How they are Observed The 
 Shooting Star The Theory of Heat A great Shooting Star The November 
 Meteors Their Ancient History The Route followed by the Shoal Diagram 
 of the Shoal of Meteors How the Shoal becomes Spread out along its Path 
 Absorption, of Meteors by the Earth The Discovery of the Relation between 
 Meteors and Comets The remarkable Investigations concerning the November 
 Meteors Two Showers in Successive Years No Particles have ever been 
 Identified from the great Shooting Star Showers Meteoric Stones Chladnf s 
 Researches Early Cases of Stonefalls The Meteorite at Ensisheim Col- 
 lections of Meteorites The Rowton Siderite Relative frequency of Iron and 
 Stony Meteorites Fragmentary Character of Meteorites No Reason to connect 
 Meteorites with Comets Tschermak's Theory Effects of Gravitation on a 
 Missile ejected from a Volcano Can they have come from the Moon ? The 
 Claims of the Minor Planets to the Parentage of Meteorites Possible Terres- 
 trial Origin The Ovifak Iron. 
 
 IN the preceding chapters we have dealt with the gigantic bodies 
 which form the chief objects in what we know as the solar system. 
 We have studied mighty planets measuring thousands of miles in 
 diameter, and we have followed the movements of comets, whose 
 dimensions are to be told by millions of miles. Once, indeed, in a 
 previous chapter, we have made a descent to objects much lower in 
 the scale of magnitude, and we have examined that numerous class 
 of small bodies which we call the minor planets. It is now, however, 
 our duty to make a still further, and this time a very long step, 
 downwards in the scale of magnitude. Even the minor planets 
 must be regarded as colossal objects, when compared with those 
 little bodies whose presence is revealed to us in a most interesting, 
 and sometimes in a most striking manner. 
 
 These small bodies compensate in some degree for their minute 
 size, by the enormous profusion in which they exist. No attempt, 
 indeed, could be made to tell in figures the myriads in which 
 they swarm throughout space. They are probably of very varied 
 dimensions, some of them being many pounds or perhaps tons in 
 
SHOOTING STARS. 331 
 
 weight, while others seem to be not larger than pebbles, or even 
 than grains of sand. Yet, insignificant as these bodies may seem, 
 the great sun himself does not disdain to accept their control. 
 Each particle, whether it be as small as the mote in a sunbeam 
 or as mighty as the planet Jupiter, will perform its path around 
 the sun in conformity with the laws of Kepler. 
 
 Who does not know that very beautiful occurrence which we 
 call a shooting star, or which, in its more splendid forms, is some- 
 times called a meteor or fire-ball? It is to objects of this class 
 that we are now to to direct our attention. 
 
 A small body, perhaps as large as a paving-stone or larger, 
 more often perhaps not as large as a marble, is moving round the 
 sun. Just as a mighty planet revolves in an ellipse, so this small 
 object will move round and round in an ellipse, with the sun in the 
 focus. There are, at the present moment, inconceivable myriads 
 of such meteors moving in this manner. They are too small and 
 too distant for our telescopes, and we can never see them except 
 under extraordinary circumstances. 
 
 At the time we see the meteor it is usually moving with 
 enormous velocity, so that it often traverses a distance of more 
 than twenty miles in a second of time. Such a velocity is almost 
 impossible near the earth's surface : the resistance of the air would 
 prevent it. Aloft, in the emptiness of space, there is no air to resist 
 the meteor. It may have been moving round and round the sun 
 for thousands, perhaps for millions of years, without let or 
 hindrance ; but the supreme moment arrives, and the meteor 
 perishes in a streak of splendour. 
 
 In the course of its wanderings the body comes near the earth, 
 and within a few hundred miles of its surface of course begins to 
 encounter the upper surface of the atmosphere with which the 
 earth is enclosed. To a body moving with the appalling velocity 
 of a meteor, a plunge into the atmosphere is usually fatal. Even 
 though the upper layers of air are excessively attenuated, yet they 
 suddenly check the velocity, almost as a rifle bullet would be 
 checked when fired into water. As the meteor rushes through the 
 atmosphere the friction warms the surface of the meteor ; gradually 
 
332 THE STORY OF THE HEAVENS. 
 
 it becomes red-hot, becomes white-hot, and is usually driven off 
 into vapour with a brilliant light, while we on the earth, one or two 
 hundred miles below, exclaim : " Oh, look ! there is a shooting 
 star." 
 
 We have here on a very striking scale an experiment illus- 
 trating the mechanical theory of heat. It may seem incredible 
 that mere friction should be sufficient to generate heat enough to 
 produce so brilliant a display, but we must recollect two facts : first, 
 that the velocity of the meteor is, perhaps, one hundred times that of 
 a rifle bullet ; and second, that the efficiency of friction in develop- 
 ing heat is proportioned to the square of the velocity. The meteor 
 in passing through the air must therefore be heated by the friction 
 of the air abou^t ten thousand times as much as the rifle bullet is 
 heated. We do not make an exaggerated estimate in supposing 
 that by its rush through the air the rifle bullet becomes heated 
 ten degrees ; yet if this be admitted, we must grant that there 
 would be such, an enormous development of heat attending the 
 flight of the meteor that even a fraction of it would be sufficient 
 to drive the object into vapour. 
 
 Let us first consider the circumstances under which these ex- 
 ternal bodies are manifested to us, and for the sake of illus- 
 tration, we may take a remarkable fire-ball which occurred on 
 November 6th, 1869. This fire-ball was extensively seen from 
 different parts of England ; and by combining and comparing these 
 observations, we obtain accurate information as to the height of 
 the object and the velocity with which it travelled. 
 
 It appears that this meteor commenced to be visible at a 
 point ninety miles above Frome, in Somersetshire, and that it 
 disappeared at a point twenty-seven miles over the sea, near St. 
 Ives, in Cornwall, both the path, its height, and the principal 
 localities from which it was observed being shown in the map 
 (Fig. 62). The whole length of its course was about 170 miles, 
 which was performed in a period of five seconds, thus giving an 
 average velocity of thirty-four miles per second. A remarkable 
 feature in the appearance which this fire-ball presented was the long 
 persistent streak of luminous cloud, about fifty miles long and four 
 
SHOOTING STARS. 
 
 333 
 
 miles wide, which remained in sight for fully fifty minutes. We 
 have in this example an illustration of the chief features of the 
 
 Fig. 62. The Path of the Fireball of November 6th, 1869. 
 
 phenomena of a shooting- star presented on a very grand scale. 
 It is, however, to be observed that the persistent luminous streak 
 is not a universal, nor, indeed, a very common characteristic of a 
 ehooting star. 
 
 The small objects which occasionally flash across the field of 
 
334 THE STORY OF THE HEAVENS. 
 
 the telescope show us that there are innumerable telescopic shoot- 
 ing 1 stars, too small and too faint to be visible to the unaided eye. 
 These objects are all dissipated in the way we have described ; it 
 is, in fact, only at the moment, and during the process of their 
 dissolution, that we become aware of their existence. Small as 
 these missiles probably are, their velocity is so prodigious, that 
 they would render the earth uninhabitable were they permitted to 
 rain down unimpeded on its surface. We must, therefore, among 
 the other good qualities of our atmosphere, not forget that it con- 
 stitutes a kindly screen, which shields us from a tempest of missiles, 
 the velocity of which no artillery could equal. It is, in fact, 
 the very fury of these missiles which is the cause of their utter 
 destruction. Their anxiety to strike us is so great, that friction 
 dissolves them into harmless vapour. 
 
 Next to the splendid event of a mighty meteor such as that we 
 have just described, the most striking occurrence in connection with 
 shooting stars is what is known as a shower. These showers have, 
 within the last century, attracted a great deal of attention, and 
 they have abundantly rewarded the labour devoted to them by 
 affording some of the most interesting astronomical discoveries of 
 modern times. 
 
 The showers of shooting stars do not occur very frequently. 
 No doubt the quickened perception of those who especially attend 
 to meteors will detect a shower when others see only a few 
 straggling shooting stars ; but speaking generally, we may say that 
 the present generation can hardly have witnessed more than two or 
 three such occurrences. I have myself seen two great showers, one 
 of which, in November, 1866, has impressed itself on my memory as 
 a most glorious astronomical spectacle. 
 
 To commence the story of the November meteors it is necessary 
 to look back for nearly a thousand years. In the year 902 occurred 
 the death of a Moorish king, and in connection with this event 
 an old chronicler relates how "that night there were seen, as it 
 were lances, an infinite number of stars, which scattered themselves 
 like rain to right and left, and that year was called the Year of 
 the Stare/' 
 
SHOOTING STARS. 335 
 
 No one now believes that the heavens intended to commemorate 
 the death of the king by that display. The record is, however, 
 of considerable importance, for it indicates the year 902 as one in 
 which a great shower of shooting- stars occurred. It was with 
 the greatest interest that astronomers perceived that this was the 
 first recorded instance of the well-known periodical shower, one of 
 whose regular returns we saw in 1866. Further diligent literary 
 research has revealed here and there records of startling appear- 
 ances in the heavens, which fit in with the successive returns 
 of the November meteors. From the first instance, in 902, to 
 the present day there have been twenty-nine visits of the 
 shower ; and it is not unlikely that these twenty-nine showers 
 were all more or less seen in some parts of the earth. Some- 
 times they may have been witnessed by savages, who had neither 
 the inclination nor the means to place on record an apparition 
 which to them was a source of terror. Sometimes, however, 
 these showers were witnessed by civilised communities. Their 
 nature was not understood, but the records were made ; and 
 in some cases, at all events, these records have withstood the 
 corrosion of time, and have now been brought together to illustrate 
 this curious subject. We have altogether historical notices of twelve 
 of these showers, mainly collected by the industry of Professor 
 Newton, whose labours have contributed so much to the advance- 
 ment of our knowledge of shooting stars. 
 
 Let us imagine an enormous shoal of shooting stars moving in 
 space. Think of a shoal of herrings in the ocean, extending over 
 many square miles, and containing countless myriads of indi- 
 viduals ; or think of those enormous flocks of wild pigeons in 
 the United States of which Audubon has told us. The shoal of 
 shooting stars is perhaps much more numerous than the herrings 
 or the pigeons. The shooting stars are, however, not very close 
 together ; they are, on an average, probably some few miles apart. 
 The actual bulk of the shooting star shoal is therefore prodigious ; 
 and its dimensions are to be measured by hundreds of thousands of 
 miles. 
 
 The meteors cannot choose their own track, like the shoal of 
 
336 THE STORY OF THE HEAVENS. 
 
 herrings, for they are compelled to follow the route which is 
 prescribed to them by the sun. Each one pursues its own ellipse 
 in complete independence of its neighbour, and accomplishes its 
 mighty journey, thousands of millions of miles in length, every 
 thirty-three years. We cannot see the meteors in the greater part 
 of their orbit. There are countless myriads of the meteors at this 
 very moment coursing round their path. The great shoal is at present 
 (1885) far out from the sun, but we know they are there, though 
 we have not seen them. We know it precisely in the same way 
 as we know that there are herrings in the sea. We have not seen 
 the herrings at present in the sea : they are not often seen until 
 they are caught ; so it is with the meteors ; we never see them till 
 the earth catches them. Every thirty-three years the earth catches 
 these meteors just as successfully as the fisherman catches herrings, 
 and in very much the same way, for while the fisherman spreads his 
 net in which the fishes meet their doom, so the earth spreads out 
 the atmosphere wherein the meteors perish. We are told that 
 there is no fear of the herrings getting exhausted, for all the fisher- 
 men catch are as nothing compared to the boundless profusion 
 in which they exist. We may hold the same language true with 
 regard to the meteors. They exist in such myriads, that though the 
 earth swallows up millions each thirty-three years, yet plenty are 
 still left for future showers. The diagram (Fig. 63) will explain the 
 way in which the earth spreads her net. We there see the orbit in 
 which the earth moves around the sun, as well as the elliptic path 
 of the meteors, though it should be remarked that it is not con- 
 venient to draw the figure exactly to scale, so that the path of the 
 meteors is relatively much larger than here represented. Once a 
 year the earth accomplishes its revolution, and between the 12th 
 and the 14th of November, crosses the track in which the meteors 
 move. It will usually happen that the great shoal is not at this 
 point when the earth is passing. There are, however, some 
 stragglers all along the path, so that the earth usually obtains 
 a few of these every. year at this date. They dart into our atmos- 
 phere as shooting stars, and form what we usually speak of as the 
 November meteors. 
 
 :^rt^^ 
 
SHOOTING STARS. 337 
 
 It will, however, occasionally happen that when the earth is in 
 the act of crossing the track it encounters the dense body of 
 meteors. Through the mighty shoal the earth then plunges, 
 enveloped, of course, with the surrounding coat of air. Into this 
 net the meteors dash in countless myriads, never again to emerge. 
 In a few hours' time, the earth, moving at the rate of eighteen 
 miles a second, has crossed the track and emerges on the other 
 side, bearing with it the spoils of the encounter. The remaining 
 
 Meteors 
 
 Fig. 63. The Orbit of a Shoal of Meteors. 
 
 meteors of the shoal continue their journey without interruption ; 
 perhaps millions have been taken, but probably hundreds of 
 millions have been left. 
 
 Such was the occurrence which astonished the world on the 
 night between November 13th and 14th, 1866. We then plunged 
 into the middle of the shoal. The night was fine ; the moon was 
 absent. The meteors were distinguished not only by their enormous 
 multitude, but by their intrinsic magnificence. I shall never 
 forget that night. On the memorable evening I was engaged in 
 my usual duty at that time of observing nebulae with Lord 
 Bosse's great reflecting telescope. I was of course aware that a 
 shower of meteors had been predicted, but nothing that I had heard 
 
338 THE STORY OF THE HEAVENS. 
 
 prepared me for the splendid spectacle so soon to be -unfolded. It 
 was about ten o'clock at night when an exclamation from an 
 attendant by my side made me look up from the telescope, just in 
 time to see a fine meteor dash across the sky. It was presently 
 followed by another, and then again by others in twos and in 
 threes, which showed that the prediction of a great shower was 
 likely to be verified. At this time the Earl of Rosse (then Lord 
 Oxmantown) joined me at the telescope, and, after a brief interval, 
 we decided to cease our observations of the nebulae and ascend to the 
 top of the wall of the great telescope (Fig. 6, p. 17) from which a 
 clear view of the whole hemisphere of the heavens could be obtained. 
 There, for the next two or three hours, we witnessed a spectacle 
 which can never fade from my memory. The shooting stars 
 gradually increased in number until sometimes several were seen 
 at once. Sometimes they swept over our heads, sometimes to the 
 right, sometimes to the left, but they all diverged from the east. 
 As the night wore on the constellation of Leo ascended above the 
 horizon, and then the remarkable character of the shower was dis- 
 closed. All the tracks of the meteors radiated from Leo. (See Fig. 
 74, p. 383). Sometimes a meteor appeared to come almost directly 
 towards us, and then its path was so foreshortened that it had 
 hardly any appreciable length, and looked like an ordinary fixed 
 star swelling into brilliancy and then as rapidly vanishing. Occa- 
 sionally luminous trains would linger on for many minutes after the 
 meteor had flashed across, but the great majority of the trains in 
 this shower were evanescent. It would be impossible to say how 
 many thousands of meteors were seen, each one of which was bright 
 enough to have elicited a note of admiration on any ordinary night. 
 The adjoining figure (Fig. 64) shows the remarkable manner 
 in which the shooting stars of this, shower diverged from a point. 
 It is not to be supposed that all these objects were in view 
 at the same moment. The observer of a shower is provided with 
 a map of that part of the heavens in which the shooting stars 
 appear. He then fixes his attention on one particular shooting 
 star, and observes carefully its track with respect to the fixed stars 
 in its vicinity. He then draws a line upon his map in the direction 
 
SHOOTING STARS. 339 
 
 in which the shooting star moved. Repeating the same observation 
 for several other shooting stars belonging to the shower, his map 
 will hardly fail to show that the different tracks of almost all 
 the shooting stars tend from one point or region of the figure. 
 There are, it is true, a few erratic ones, but the majority observe 
 this law. It certainly looks, at first sight, as if all the shooting 
 stars did actually dart from this point ; but a little reflection will 
 
 Fig. 64. Radiant Point of Shooting Stars. 
 
 show that this is a case in which the real motion is different from 
 the apparent. If there actually were a point from which these 
 meteors diverged, then from different parts of the earth the point 
 would be seen in different positions with respect to the fixed stars ; 
 but this is not the case. The radiant, as this point is called, is 
 seen in the same part of the heavens from whatever station the 
 shower is visible. 
 
 We are, therefore, led to accept the simple explanation afforded 
 
 by the theory of perspective. Those who are acquainted with the 
 
 principles of this science know, that when a number of parallel 
 
 lines in an object have to be represented in a drawing they must 
 
 w 2 
 
340 THE STORY OF THE HEAVENS. 
 
 all be made to pass through the same point in the plane of the 
 picture, unless in the exceptional case where they are also lines 
 parallel to the picture. When we are looking at the shooting- 
 stars, what we really see are only the projections of their paths 
 upon the surface of the heavens. From the fact that all those 
 paths pass through the same point, we are to infer that the shoot- 
 ing stars belonging to the same shower are all moving in parallel 
 lines. 
 
 We are now able to ascertain the actual direction in which the 
 shooting stars are moving, because a line drawn from the eye of 
 the observer to the radiant point must be parallel to that direc- 
 tion. Of course, it is not intended to convey the idea that through- 
 out all space the shooting stars of one shower are moving in 
 parallel lines ; all we mean is, that during the short time in which 
 we see them the motion of each of the shooting stars is sensibly a 
 straight line, and that all these straight lines are parallel. 
 
 At present the great meteor shoal of the Leonids (for so this 
 shower is called) has accomplished about half its mighty journey. 
 In the year 1885 it attains its greatest distance from the sun, 
 and then commences to return. Each year the earth crosses 
 the orbit of the meteors, but the shoal is not met with, and no 
 shower of stars is perceived. Eveiy succeeding year will find the 
 meteors approaching the critical point, and the year 1899 will 
 bring the shoal to the earth's track. In that year, therefore, 
 another brilliant meteoric shower may be looked for. The shoal 
 of meteors is of such enormous length that it takes more than a 
 year for the mighty procession to pass through the critical portion 
 of its orbit which lies across the track of the earth. We thus see 
 that the meteors cannot escape the earth. It may be that when the 
 shoal begins to reach this neighbourhood the earth will have just 
 left this part of its path, and a year will have elapsed before the 
 earth gets round again. Those meteors that have the good fortune 
 to be in the front of the shoal will thus escape the net, but some 
 of those behind will not be so fortunate, and the earth will again 
 devour an incredible host. It has sometimes happened that casts 
 into the shoal have been obtained in two consecutive years. 
 
SHOOTING STARS. 341 
 
 If the earth happened to pass through the front part in one year, 
 then the shoal is so long that the earth will have moved right 
 round its orbit of 600,000,000 miles, and will again dash through 
 the critical spot before the entire number have passed. History con- 
 tains records of cases when, in two consecutive Novembers, brilliant 
 showers of Leonids have been seen. 
 
 As the earth consumes such myriads of Leonids each thirty-three 
 years, it follows that the total number must be decreasing. The 
 splendour of the showers in future ages will, no doubt, be affected 
 by this circumstance. They cannot be always as bright as they 
 have been. It is also of interest to notice that the shape of the 
 shoal is gradually changing. Each meteor of the shoal moves in 
 its own ellipse around the sun, and is quite independent of the rest 
 of these bodies. Each one has thus a special period of revolution 
 which depends upon the length of the ellipse in which it happens 
 to revolve. Two meteors will move around the sun in the same time 
 if the lengths of their ellipses are exactly equal, but not otherwise. 
 The lengths of these ellipses are many hundreds of millions of 
 miles, and it is impossible that they can be all absolutely equal. 
 In this may be detected the origin of a gradual change in the 
 character of the shower. Suppose two meteors A and B be such 
 that A travels completely round in thirty-three years, while B 
 takes thirty -four years. If the two start together, then when A 
 has finished the first round B will be a year behind; the next 
 time B will be two years behind, and so on. The case is exactly 
 parallel to that of a number of boys who start for a long race, 
 in which they have to run several times round the course before 
 the distance has been accomplished. At first they all start in a 
 cluster, and perhaps for the first round or two they may remain in 
 comparative proximity ; gradually, however, the faster runners get 
 ahead and the slower ones lag behind, so the cluster becomes 
 elongated. As the race continues, the cluster becomes dispersed 
 around the entire course, and perhaps the first boy will even 
 overtake the last. Such seems the destiny of the November 
 meteors in future ages. The cluster will in time to come be 
 spread out around the whole of this mighty track, and no longer 
 
342 THE STORY OF THE HEAVENS. 
 
 will a superb display have to be recorded every thirty-three 
 years. 
 
 It was in connection with the shower of November meteors in 
 186(5 that a very interesting and beautiful discovery in mathe- 
 matical astronomy was made by Professor Adams. We have seen 
 that the Leonids must move in an elliptic path, and that they 
 return every thirty-three years, but the telescope cannot follow 
 them during their wanderings. All that we know by observation 
 is the date of their occurrence, the point of the heavens from 
 which they radiate, and the great return every thirty-three years. 
 Putting these various facts together, it is possible, not exactly to 
 determine the ellipse in which the meteors move the facts do 
 not go so far they only tell us that the ellipse must be one 
 of five possible orbits. These five possible orbits are firstly, 
 the immense ellipse in which we now know the meteorites 
 do revolve, and for which they require the whole thirty-three 
 years to complete a revolution ; secondly, a nearly circular orbit, 
 very little larger than the earth's path, which the meteors would 
 traverse in a few days more than a year ; another similar orbit, 
 in which the time would be a few days short of the year; and 
 two other small elliptical orbits lying inside the earth's orbit. It 
 was clearly demonstrated by Professor Newton that the observed 
 facts would be explained if the meteors moved in any one of 
 these paths, but that they could not be explained by any other 
 hypothesis. It remained to see which of these orbits was the 
 true one. Professor Newton himself made the suggestion of a 
 possible method of solving the problem. The test he proposed 
 was one of very great difficulty, and involved the most intricate 
 calculations. Fortunately, however, Professor Adams undertook 
 the inquiry, and by his successful labours the path of the Leonids 
 has been completely ascertained. 
 
 When the ancient records of the appearance of great Leonid 
 showers are examined, it is found that the date of their occurrence 
 undergoes a gradual and continuous change. It follows as a 
 necessary consequence that the point where the path of the meteors 
 crosses the earth's track is not fixed, but that at each successive 
 
SHOOTING STARS. '343 
 
 return the meteors pass at a point about half a degree further 
 on in the direction in which the earth is travelling 1 . It follows 
 that the orbit in which the meteors are revolving is under- 
 going change; the path they follow in one revolution varies 
 slightly from that pursued in the next. As, however, these 
 changes proceed in the same direction, they may gradually attain 
 considerable dimensions; and the amount of change which is 
 produced in the path of the meteors in the lapse of centuries 
 may be estimated by the two ellipses shown in Fig. 65. The 
 continuous line represents the orbit in A.D. 126; the dotted line 
 represents it at present. 
 
 This very definite change in the orbit is one that astronomers 
 attribute to what we have already spoken of as perturbation. It is 
 certain that the elliptic motion of these bodies is due to the sun, 
 and that if they were only acted on by the sun the ellipse would 
 remain absolutely unaltered. We see, then, in this gradual change 
 of the ellipse the influence of the attractions of the planets. It 
 was shown that if the meteors moved in the large orbit, this 
 shifting of the path must be due to the attraction of the planets 
 Jupiter, Saturn, Uranus, and the Earth ; while if the meteors 
 followed one of the smaller orbits, the planets that would be near 
 enough, and large enough to act sensibly on them, would be the 
 Earth, Venus, and Jupiter. Here, then, we see how the question 
 may be answered by calculation. It is difficult, but it is possible, 
 to calculate what the attraction of the planets would be capable of 
 producing for each of the five different suppositions as to the orbit. 
 This is what Adams has done. He found that if the meteors 
 moved in the great orbit, then the attraction of Jupiter would 
 account for two-thirds of the observed change, while the remaining 
 third was due to the influence of Saturn, supplemented by a small 
 addition on account of Uranus. In this way the calculation showed 
 that the large orbit was a possible one. Professor Adams also 
 computed the amount of displacement in the path that could be 
 produced if the meteors revolved in any of the four smaller ellipses. 
 This investigation was one of an extremely arduous character, but 
 the results amply compensated for the labour. It was shown that 
 
344 THE STORY OF THE HEAVENS. 
 
 in this case it would be impossible to obtain by the aid of pertur- 
 bations a displacement even one-half of that which was observed. 
 These four orbits were, therefore, impossible ones. Thus the 
 demonstration was complete as to the character of the path in 
 which the meteors revolve. 
 
 . The movements in each revolution are guided by Kepler's laws. 
 When at the part of its path most distant from the sun the velocity 
 of a meteor is at its lowest, being then but little more than a mile 
 a second; as it draws in the speed gradually increases, until 
 when the meteor crosses the earth's track its velocity is no less 
 than twenty-six miles a second. The earth is moving very nearly 
 in the opposite direction at the rate of eighteen miles a second, 
 so that if the meteor happen to strike the earth's atmosphere, 
 it does so with an enormous velocity of nearly forty-four miles a 
 second. If the meteor escape the earth, then it resumes its outward 
 journey with gradually declining velocity, and by the time it has 
 again completed its mighty journey a period of thirty-three years 
 years and a quarter will have elapsed. 
 
 The innumerable meteors which form the Leonids are arranged 
 in an enormous stream, of a breadth very small in comparison with 
 its length. If we represent the orbit by an ellipse whose length 
 is seven feet, then the meteor stream will be represented by a 
 thread of the finest sewing-silk, about a foot and a half or two feet 
 long, creeping along the orbit.* The size of this stream may be 
 estimated from the consideration that even its width cannot be less 
 than 100,000 miles. Its length may be estimated from the circum- 
 stance that, although it is moving about twenty- six miles a second, 
 yet the stream takes about two years to pass the point where 
 its orbit crosses the earth's track. On the memorable occasion of 
 the night between the 13th and 14th of November, 1866, the earth 
 plunged into this stream near its head, and did not emerge on the 
 other side until five hours after. During that time it happened 
 that the hemisphere of the earth which was in front contained the 
 
 * This illustration, as well as the figure of the path of the meteors, has been 
 derived from Mr. G. J. Stoney's interesting lecture on "The Story of the November 
 Meteors," at the Royal Institution in 1879. 
 
SHOOTING STARS. 
 
 345 
 
 continents of Europe, Asia, and Africa, and consequently it was in 
 the Old World that the great shower was seen. On that day 
 twelvemonth, when the earth had regained the same spot, the shoal 
 had not entirely passed, and the earth made another plunge. This 
 time the American continent was in the van, and consequently it 
 
 Fig. 65. The History of the Leonids. 
 
 was there that the shower of 1867 was seen. Even in the following 
 year the great shoal had not entirely passed, and a few stragglers 
 along the route are still occasionally encountered at each annual 
 transit of the earth across the great meteoric highway. 
 
 The diagram is also designed to indicate a remarkable specula- 
 tion which was put forward on the high authority of Le Verrier, 
 with the view of explaining how the shoal came to be introduced 
 
846 THE STORY OF THE HEAVENS. 
 
 into the solar system. The orbit in which the meteors revolve 
 does not intersect the paths of Jupiter,, Saturn, or Mars, but it 
 does intersect the orbit of Uranus. It must, therefore, sometimes 
 happen that Uranus will be passing through this point of its path 
 just as the shoal has arrived there. Le Verrier has demonstrated 
 that such an event took place in the year A.D. 126, but that it has 
 not happened since. We thus seem to have a clue to a very won- 
 derful past history : 
 
 " All would be explained if we may suppose that before the year 
 126, the meteors had been moving beyond the solar system ; and that in 
 that year they chanced to cross the path of the planet Uranus, travelling 
 along some such path as that represented in the diagram. Had it not 
 been for the planet, they would have kept on the course marked out with 
 a dotted line, and after having passed the sun, would have withdrawn 
 on the other side into the depths of space, to the same measureless 
 distance from which they had originally come. But their stumbling on 
 the planet changed their whole destiny. Even so great a planet would 
 not sensibly affect them until they got within a distance, which would 
 look very short indeed upon our diagram. But they seem to have 
 almost grazed his surface, and while they were very close to such a 
 planet, he would be able to drag them quite out of their former course. 
 This the planet Uranus seems to have done ; and when, pursuing his 
 own course, he again got too far Off to influence them sensibly, they 
 found themselves moving slowly backwards and slowly inwards ; and 
 accordingly began the new orbit round the sun, which corresponds to 
 the situation into which they had been brought, and the direction and 
 moderate speed of their new motion. 
 
 " They seem to have passed Uranus while they were still a small, 
 compact cluster. Nevertheless, those members of the group which 
 happened to be next the planet as they swept past, would be attracted 
 with somewhat more force than the rest ; the farthest members of the 
 group with the least. The result of this must inevitably have been 
 that when the group were soon after abandoned to themselves, they 
 did not find themselves so closely compacted as before, nor moving with 
 an absolutely identical motion, but with motions which differed, although 
 perhaps very little, from one another. These are conditions which 
 would have started them in those slightly differing orbits round the 
 sun, which, as we have seen, would cause them, as time wore on, to be 
 drawn out into the long stream in which we now, after seventeen cen- 
 turies, find them. 
 
SHOOTING STAES. 347 
 
 " What is here certain is, that there was a definite time when the 
 meteors entered upon the path they are now pursuing that this time 
 was the end of February or beginning of March in the year 126 is still 
 a matter of probability only. It is, however, highly probable, because 
 it explains all the phenomena at present known ; but astronomers are 
 not yet in a position to assert that it is ascertained, since one link in 
 the complete chain of proof is wanting. We who live now should be 
 in possession of this link if our ancestors had made sufficiently full 
 observations ; and our posterity will have it when they compare the 
 observations they can make with those which we are now carefully 
 placing on record for their use. They will then know whether the 
 rate at which the stream is lengthening out is such as to indicate that 
 A.D. 126 was the year in which this process began. If so, Le Verrier's 
 hypothesis will be fully proved."* 
 
 The determination of the true elliptic path of the Leonid? 
 led Professor Schiaparelli to a discovery of rather an unexpected 
 character, which revealed in a manner almost startling, a myste- 
 rious connection between the shooting star showers and periodic 
 comets. 
 
 In the year 1866 a comet appeared. This comet was observed, 
 and from these observations the path in which the comet moved 
 could be calculated. What was the astonishment of astronomers, 
 when it became known that the path of this comet was also the 
 path followed by the Leonids ! To emphasise this coincidence, it is 
 desirable to think separately of the various points which it implies. 
 Take, for instance, the plane in which the comet moves : that plane 
 may have any position, subject only to the condition that it shall 
 pass through the centre of the sun. So, too, the orbit of the 
 meteors may lie in any one of the myriad planes passing through 
 the sun's centre. That these two planes should absolutely coincide 
 is a matter which would be almosfc infinitely improbable if it were 
 merely a question of chance. The coincidence does not rest even 
 here ; not only do the planes coincide, but the directions of the axes 
 of the orbits in these planes are also identical : this, again, would be 
 infinitely unlikely were the question one merely of chance. But we 
 
 * G-. J. Stoney : " The Story of the November Meteors." 
 
348 THE STORY OF THE HEAVENS. 
 
 have not yet exhausted the subject, for it further appears that the 
 shortest distance from the sun to the orbit is also in each case 
 identical. We have here a concentration of evidence which proves 
 to demonstration that there must be some profound physical con- 
 nection between comets and swarms of meteors. The observed 
 facts would be accounted for if we suppose the comet to be pre- 
 ceded or followed in its path by a meteor stream ; but this, of course, 
 implies some very intimate connection between the comet and the 
 meteors, though at the present time it is not possible to state the 
 real nature of that connection. 
 
 The remarkable association between comets and meteors has 
 been noticed in the case of several other meteoric streams besides 
 the Leonids. We may, for instance, cite one of the showers which 
 is most constant in its annual appearance, and is called the Perseids, 
 from the circumstance that the radiant point lies in the constella- 
 tion of Perseus. This shower is known also as that of the 
 August meteors, which fall from the 9th to the llth of that 
 month in each year. Their orbit has been carefully determined, 
 and it has been found to be also the path of a comet. There are 
 several other showers of which the same statement may be made. 
 We shall instance one in particular which has attracted great and 
 deserved attention. 
 
 A copious meteoric shower took place on the night of the 
 27th November, 1872. On this occasion the shooting stars 
 diverged from a radiant point in the constellation of Andromeda. 
 I estimated at the time that there were at least three hundred 
 meteors per hour. It is true that these shooting stars were not 
 bright, nor were their trains very long. As a spectacle, it was, in 
 my judgment, unquestionably inferior to the magnificent display of 
 1866. But though, from a picturesque point of view, the shower 
 of 1872 is inferior to that of 1866, yet it is difficult to say which of 
 the two showers is of greater scientific importance. 
 
 It surely is a remarkable coincidence that the earth should 
 encounter the Andromedes (for so this shower is called) at the very 
 moment when it is crossing the track of Biela's comet. We have 
 observed the direction from which the Andromedes come when they 
 
ff. ^TTSPJ 
 
 \&^/ 
 
 SHOOTING STAES. 349 
 
 plunge into the atmosphere; we can ascertain also the direction 
 in which Biela's comet is moving when it passes the earth's track, 
 and we find that the direction in which the comet moves and the 
 direction in which the meteors move are identical. We have there- 
 fore learned that the orbit" in which the Andromedes move and the 
 orbit in which Biela's comet moves are coincident. This is, in 
 itself, a strong and almost overwhelming presumption that the 
 comet and the shooting stars are connected ; but this is not all. 
 This comet was observed in 1772, and again in 1805-6, before its 
 periodic return in seven years was discovered. It was again dis- 
 covered by Biela in 1826, and was observed in 1832. In 1846 the 
 astronomical world was startled to find that there were now two 
 comets in place of one, and the two fragments were again perceived 
 at the return in 1852. No trace of Biela's comet was seen in 
 1859, nor in 1865-66, when its return was also due. It there- 
 fore happens that at the end of 1872 the time had arrived for 
 the return of Biela's comet, and thus the occurrence of the great 
 shower of the Andromedes took place about the time when Biela's 
 comet was actually due. The inference is irresistible that the 
 shooting stars, if not actually a part of the comet itself, are at all 
 events most intimately connected therewith. This shower will 
 also be memorable by reason of the telegram sent from Professor 
 Klinkerfues to Mr. Pogson at Madras. The telegram ran as fol- 
 lows: "Biela touched earth last night. Search near Theta 
 Centauri." Pogson did search and did find a comet, though 
 whether this was really the comet of Biela is still a matter which 
 some astronomers doubt. 
 
 We have only mentioned three of the periodic showers ; they 
 are, indeed, the only three which produce a display worthy of 
 note, from a picturesque point of view. There are, however, 
 many others well known to those who have studied the subject ; 
 and the annual reports of the British Association are replete with 
 information sedulously collected from every available source on the 
 entire subject of luminous meteors. 
 
 It is a noticeable circumstance that the great meteoric showers 
 seem never yet to have succeeded in projecting a missile which 
 
350 THE STORY OF THE HEAVENS. 
 
 has reached the earth's surface. Out of the myriads of Leonids, 
 of Perseids, or of Andromedes, not one particle has ever been 
 seized and identified. Those bodies which do fall from the sky 
 to the earth, and which we call meteorites, never come from 
 the great showers, so far as we know. They seem, indeed, 
 to be phenomena of quite a different character to the periodic 
 meteors. 
 
 It is somewhat curious that the belief in the celestial origin of 
 meteorites is of modern growth. In ancient times there were, no 
 doubt, rumours of wonderful stones which had fallen down from 
 the heavens on the earth, but these reports seem to have obtained 
 but little credit. They were a century ago regarded as perfectly 
 fabulous, though there was abundant testimony on the subject. 
 Eye-witnesses averred that they had seen the stones fall. The 
 stones themselves were unlike other objects in the neighbourhood, 
 and cases were even authenticated where men had been killed by 
 blows from these celestial visitors. 
 
 The objects were, however, generally seen by ignorant and 
 illiterate persons. The true parts of the record were so mixed up 
 with imaginary additions, that science refused to adopt the belief 
 that such objects really fell from the sky. Even at the present 
 day it is often extremely difficult to obtain accurate testimony as 
 to the circumstances attending the fall of a meteorite. For 
 instance, the fall of a meteorite was observed by a Hindoo in the 
 jungle. The stone was there, its meteoric character was un- 
 doubted, and the witness was duly examined as to the details of 
 the occurrence ; but he was so frightened by the noise and by the 
 danger he had narrowly escaped, that he could tell little or nothing. 
 The only fact of which he felt certain was that the meteorite 
 had hunted him for some time through the jungle before it fell to 
 the earth ! 
 
 In the year 1794 Chladni published an account of the remark- 
 able mass of iron which the traveller Pallas had discovered in Siberia. 
 It was then for the first time that the important step was taken of 
 recognising that this object and others similar to it were really 
 of celestial origin. But even Cbladni's reputation and the argu- 
 

 SHOOTING STARS. 351 
 
 ments he brought forward failed to procure universal assent. 
 Shortly afterwards, in 1795, a stone of fifty-six pounds was ex- 
 hibited in London, which several witnesses declared they had seen 
 fall at Wold Cottage, in Yorkshire ; and it was subsequently depo- 
 sited in the collection at the British Museum. The evidence then 
 began to pour in from other quarters ; portions of stone from Italy 
 and from Benares were found to be identical with the Yorkshire 
 stone. The incredulity of those who had doubted the celestial origin 
 of these objects began to give way. A careful memoir on the 
 Benares stone, by Howard, was published in the "Philosophical 
 Transactions" for 1802, while, as if to complete the demonstration, 
 a great shower of falling stones took place in the following year 
 at I/Aigle, in Normandy. The French Academy deputed the 
 physicist Biot to make a detailed examination on the spot of all 
 the circumstances attending this memorable shower. His inquiry 
 completely removed every trace of doubt, and the meteoric stones 
 have accordingly been transferred from the dominions of geology 
 to those of astronomy. It may be noted as a coincidence that the 
 recognition of the celestial origin of meteorites was simultaneous 
 with that of the discovery of the first of the series of minor planets. 
 In each case our knowledge of the solar system has been extended 
 by the addition of numerous minute bodies, which, notwithstand- 
 ing their insignificant dimensions, are pregnant with information. 
 Once the reality of these stone-falls has been admitted, we can 
 turn with great interest to the ancient records, and restore to them 
 the credit which many of them undoubtedly merit, and which was 
 for so many centuries withheld. Perhaps the earliest of all the 
 records which can be said to have much pretension to historical 
 accuracy, is that of the shower of stones which Livy describes as 
 having fallen, about the year 654 B.C., on the Alban Mount, near 
 Rome. Among the more modern instances, we may mention one 
 which was authenticated in a very emphatic manner. It occurred 
 in the year 1492 at Ensisheim, in Alsace. The Emperor Maximilian 
 ordered a minute narrative of the circumstances to be drawn up 
 and deposited with the stone in the church. The stone was 
 suspended in the church for three centuries, until in the French 
 
352 THE STORY OF THE HEAVENS. 
 
 Revolution it was carried off to Colmar, and pieces were broken from 
 it, one of which is now in the British Museum. Fortunately, this 
 interesting object has been restored to its ancient position in the 
 church at Ensisheim, where it remains an attraction to sight-seers 
 to this day. The account is as follows : " In the year of the 
 Lord 1492, on the Wednesday before St. Martin's Day, November 
 7th, a singular miracle occurred, for between eleven o'clock and noon 
 there was a loud clap of thunder and a prolonged confused noise, 
 which was heard at a great distance, and a stone fell from the air 
 in the jurisdiction of Ensisheim which weighed 260 pounds, and the 
 confused noise was at other places much louder than here. Then a 
 child saw it strike on ploughed ground in the upper field towards 
 the Rhine and the 111, near the district of Gisgang, which was sown 
 with wheat, and it did no harm, except that it made a hole there ; 
 and then they conveyed it from the spot, and many pieces were 
 broken from it, which the Land Vogt forbade. They therefore 
 caused it to be placed in the church, with the intention of suspending 
 it as a miracle, and there came here many people to see this stone, 
 so there were many remarkable conversations about this stone; 
 the learned said they knew not what it was, for it was beyond the 
 ordinary course of nature that such a large stone should smite from 
 the height of the air, but that it was really a miracle from God, 
 for before that time never was anything heard like it, nor seen, nor 
 written. When they found that stone, it had entered into the earth 
 to half the depth of a man's stature, which everybody explained to 
 be the will of God that it should be found, and the noise of it was 
 heard at Lucerne, at Villingen, and at many other places, so loud that 
 the people thought that houses had been overturned; and as the 
 King Maximilian was here, the Monday after St. Catherine's Day 
 of the same year, his Royal Excellency ordered the stone which had 
 fallen to be brought to the castle, and after having conversed a 
 long time about it with the noblemen, he said that the people of 
 Ensisheim should take it and order it to be hung up in the church, 
 and not to allow anybody to take anything from it. His Excellency, 
 however, took two pieces of it, of which he kept one, and sent the 
 other to Duke Sigismund of Austria, and they spoke a great deal 
 
SHOOTING STARS. 353 
 
 about the stone, which they suspended in the choir, where it still is, 
 and a great many people came to see it." 
 
 Once we have recognised the celestial origin of the meteorites, 
 they claim our closest attention. They afford the only direct 
 method we possess of obtaining a knowledge of the materials of 
 bodies exterior to our planet. We can take a meteorite in our 
 hands, we can analyse it, and find the materials of which it is 
 composed. We shall not attempt to enter into any very detailed 
 account of the structure of meteorites ; it is rather a matter for the 
 consideration of chemists and mineralogists than for astronomers. 
 A few of the more obvious features will be all that we require. 
 They will serve as a preliminary to the discussion of the probable 
 origin of these bodies. 
 
 In the British Museum we may examine a superb collection 
 of meteorites. They have been brought together from all parts 
 of the earth, and vary in size from bodies not much larger than 
 a pin's head up to vast masses weighing many hundredweights. 
 There are also many models of celebrated meteorites, of which the 
 originals are dispersed through various other museums. 
 
 Many of these objects have nothing very remarkable in their 
 external appearance. If they were met with on the sea beach, 
 they would be passed by without more notice than would be given 
 to any other stone. Yet, what a history such a stone might tell 
 us if we could only manage to obtain it ! It fell ; it was seen to 
 fall from the sky ; but what was its course anterior to that move- 
 ment? Where was it 100 years ago, 1,000 years ago? Through 
 what regions of space has it wandered ? Why did it never fall 
 before ? Why has it actually now fallen ? Such are some of the 
 questions which crowd upon us as we ponder over these most 
 interesting bodies. Some of these objects are composed of very 
 characteristic materials ; take, for example, one of the more recent 
 meteorites, known as the Rowton siderite. This body differs 
 very much from the more ordinary kind of stony meteorite. It is 
 an object which even a casual passer-by would hardly pass without 
 notice. Its great weight would also attract attention, while if it 
 be scratched or rubbed with a file, it would be found that it was 
 X 
 
354 THE STORY OF THE HEAVENS. 
 
 not in any sense a stone, but that it was a mass of nearly pure 
 iron. We know the circumstances under which that piece o 
 iron fell to the earth. It was on the 20th of April, 1876, about 
 3.40 p.m., that a strange rumbling 1 noise, followed by a start- 
 ling explosion, was heard over an area of several miles in extent 
 among the villages in Shropshire, eight or ten miles north of 
 the Wrekin. About an hour after this occurrence, a farmer 
 noticed the ground in one of his grass-fields to have been dis- 
 turbed, and he probed the hole which the meteorite had made, 
 and found it, still warm, about eighteen inches below the sur- 
 face. Some men working at no great distance had actually 
 heard the noise of its descent, but without being able to indicate 
 the exact locality. This remarkable object weighs 7f Ibs. It is 
 an irregular angular mass of iron, though all its edges seem to 
 have been rounded by fusion in its transit through the air, and 
 it is covered with a thick black pellicle of the magnetic oxide of 
 iron, except at the point where it first struck the ground. It was 
 first exhibited publicly at a bazaar in Wolverhampton, and by the 
 Duke of Cleveland, on whose property it fell, it was presented to 
 the British Museum, where, as the Rowton siderite, it attracts the 
 attention of every one who is interested in these wonderful bodies. 
 This siderite is specially interesting on account of its distinctly 
 metallic character. Falls of the siderites (as they are called) are not 
 so common as those of the stony meteorites ; in fact, there are only 
 a few known instances of meteoric irons having been actually seen to 
 fall, while the falls of stony meteorites are to be counted in scores 
 or in hundreds. The inference is that the iron meteorites are much 
 less frequent than the stony ones. This is, however, not the im- 
 pression that the visitor to the British Museum would be likely to 
 receive. In that extensive collection the meteoric irons are by far 
 the most striking objects. The explanation is not difficult. Those 
 gigantic masses of iron are unquestionably meteoric : no one 
 doubts that this is the case. Yet the vast majority of them have 
 never been seen to fall ; they have simply been found, under cir- 
 cumstances which point unmistakably to their meteoric nature. 
 Suppose, for instance, that a traveller out on one of the plains 
 
 
SHOOTING STARS. 355 
 
 of Siberia or of Central America finds a mass of metallic iron 
 lying on the surface of the ground, what explanation can be 
 rendered of such an occurrence? No one has brought the iron 
 there, and there is no iron within hundreds of miles. Man never 
 fashioned that object, and the iron is found to be alloyed with 
 nickel in a manner that is always observed in known meteorites, 
 and is generally regarded as a sure indication of a meteoric origin. 
 Observe also, that as iron perishes by corrosion in our atmosphere, 
 that great mass of iron cannot have lain there for indefinite ages ; 
 it must have been placed there at some finite time. There is only 
 one source of such an object conceivable; it must have fallen 
 from the sky. On those boundless plains the stony meteorites 
 also have, doubtless, fallen in hundreds and in thousands, but 
 the stony ones crumble away, and in any case would not arrest 
 the attention of the traveller as the iron meteorites do. Hence it 
 follows, that although the stony meteorites seem to fall much more 
 frequently, yet, unless they are actually observed in the moment 
 of descent, they are much more liable to be overlooked than the 
 meteoric irons. Hence it is, that the great meteorites which are 
 the characteristic feature of the British Museum collection are 
 mainly the meteoric irons. 
 
 We have said that a loud noise accompanied the descent of the 
 Rowton siderite, and the Emperor Maximilian has recorded the 
 loud explosion which took place when the meteorite fell at En- 
 sisheim. In this we have a characteristic feature of the phe- 
 nomenon. Nearly all the descents of meteorites that have been 
 observed, seem to have been ushered in by a loud detonating 
 explosion. We do not, however, assert that this is a quite invariable 
 character; and it is also the case that meteors often detonate 
 without throwing down any stones which are observed. The 
 violence associated with the phenomenon is forcibly illustrated by 
 the Butsura meteorite. This object fell in India about twenty 
 years ago. A loud explosion was heard, several fragments of stone 
 were collected at distances miles apart ; and when brought to- 
 gether, they were found to fit, so as to enable the primitive form of 
 the meteorite to be re- constructed. A few of the pieces are wanting 
 x 2 
 
356 THE STORY OF THE HEAVENS. 
 
 (they were, no doubt, lost by falling unobserved into localities from 
 which they could not be recovered), but we have obtained pieces 
 quite numerous enough to j ermit us to form a very good idea of 
 the irregular shape of the object before the explosion occurred 
 which shattered it into fragments. This is one of the ordinary 
 stony meteorites, and is thus contrasted with the Rowton siderite 
 which we have just been considering. There are also other types 
 of meteorites. The Breitenbach iron, as it is called, is a good 
 representation of a class of meteorites which are intermediate 
 
 Fig. 66. Section of the Chaco Meteorite. 
 
 between the meteoric irons and the stones. It consists of a coarse 
 cellular structure of iron, the cavities being filled with mineral 
 substances. In the British Museum will be seen sections of inter- 
 mediate forms in which this structure is exhibited. 
 
 Look first at the most obvious characteristic of these meteorites. 
 We do not now allude to their chemical composition, but to their 
 external appearance. What is the most remarkable feature in the 
 shape of these objects ? surely it is that they are fragments. They 
 are evidently pieces that are broken from some larger object. This is 
 apparent by merely looking at their form ; it is still more manifest 
 when we examine their mechanical structure. It is often found that 
 meteorites are themselves composed of smaller fragments, united 
 together in the manner of certain well-known volcanic rocks; 
 others are, again, composed of very small particles, analogous to 
 what are known as volcanic tufas. The structure of the meteorites 
 
SHOOTING STARS. 357 
 
 may be illustrated by a section of an aerolite found on the Sierra of 
 Chaco, weighing about 301bs. (Fig. 66). 
 
 The section here represented shows the composite structure of 
 this object, which belongs to the class of stony meteorites. Its 
 shape shows that it was really a fragment with angular edges and 
 corners. No doubt it may have been much more considerable when 
 it first dashed into the atmosphere. The angular edges now seen 
 on the exterior may be due to an explosion which then occurred ; 
 but this will not account for the structure of the interior. We there 
 see irregular pieces of the most varied form and material conglo- 
 merated into a single mass. If we would seek for any analogous 
 objects on the earth, we must look to some of the volcanic rocks, 
 where we have multitudes of irregular angular fragments adhering 
 together by a cement in which they are imbedded. The evidence 
 presented by this meteorite is conclusive as to one circumstance, 
 with regard to the origin of these objects. They must have come 
 as fragments, from some body of considerable, if not of vast, dimen- 
 sions. In this meteorite there are numerous small grains of iron 
 mingled with mineral substances. The iron in many meteorites 
 has, indeed, characters resembling those produced by the actual 
 blasting of iron by dynamite. Thus, a large meteoric iron from 
 Brazil has been found to have been actually shivered into frag- 
 ments at some time anterior to its fall on the earth. These 
 fragments have been cemented together again by irregular veins of 
 mineral substances. 
 
 For an aerolite of a very different type we may refer to the 
 carbonaceous meteorite of Orgueil, which fell in France on the 14th 
 May, 1864. On the occasion of its descent a splendid meteor was 
 seen, rivalling the full moon in size. The actual diameter of this 
 globe of fire must have been some hundreds of yards. There were 
 actually found nearly a hundred fragments of the body scattered 
 about over a tract of country fifteen miles long. This object is 
 of particular interest, inasmuch as it belongs to a rare group of 
 aerolites, from which metallic iron is absent. It contains many of 
 the same minerals which are met with in other meteorites, but 
 in these objects they are associated with carbon, and with sub- 
 
358 THE STORY OF THE HEAVENS. 
 
 stances of a white or yellowish crystallisable material, soluble in 
 ether, and resembling 1 some of the hydro-carbons. Such a sub- 
 stance, if found on the earth, would probably be deemed a product 
 resulting from animal or vegetable life ! 
 
 We have pointed out how a body moving with great velocity 
 and impinging upon the air may become red-hot and white-hot, 
 or even driven off into vapour. How, then, does it happen that 
 meteorites escape this fiery ordeal, and fall down to the earth, no 
 doubt with a great velocity, but still, with a velocity very much 
 less than that which would have sufficed to drive them off into 
 vapour ? Had that K/owton siderite, for instance, struck our 
 atmosphere with a velocity of twenty miles a second, it seems 
 unquestionable that it would have been entirely dissipated into 
 vapour, though, no doubt, the particles would ultimately coalesce 
 together, and slowly descend to the earth in microscopic beads of 
 iron. How has the meteorite escaped this fate ? It must be 
 remembered that our earth is also moving with a velocity of about 
 eighteen miles per second, and that the velocity with which the 
 meteorite plunges into the air is to be obtained by compounding 
 the velocity of the meteorite with the velocity of the earth. If the 
 meteorite come into direct collision with the earth, the velocity of 
 the collision will be extremely great; but it may happen that 
 though the actual velocities of the two bodies are both enormously 
 great, yet the relative velocity may be comparatively small. This 
 is, at all events, one conceivable explanation of the arrival of a 
 meteorite on the surface o the earth. , . , 
 
 We have shown in the earlier parts of this chapter that the 
 well-known star showers are all intimately connected with comets. 
 In fact, each star shower revolves in the path pursued by a comet, 
 and the shooting star particles have, in all probability, been them- 
 selves derived from the comet. Showers of shooting stars and 
 comets have, therefore, an intimate connection, but there is no 
 ground for supposing that meteorites have any connection with 
 cornets the facts, indeed, all seem to me to point in the opposite 
 direction. It has already been remarked that meteorites have never 
 been known to fall from the great star showers. No particle of a 
 
SHOOTING STAE8. 359 
 
 meteorite was ever dropped from the countless host of the Leonids 
 or of the Perseids ; the Ly raids never dropped a meteorite, nor did 
 the Quarantids, the Geminids, or the many other showers with 
 which every astronomer is familiar. There is no reason to connect 
 meteorites with these showers, and there is, accordingly, no reason 
 to connect meteorites with comets. Indeed, the appearance of a 
 comet and the history of its movements and its changes seem 
 entirely at variance with the supposition that it is composed of 
 materials resembling those in meteorites. 
 
 With reference to the origin of meteorites it is difficult to 
 speak with any great degree of confidence. Every theory of 
 meteorites is in itself improbable, so it seems that the only course 
 open to us is to choose that view of their origin which seems 
 least improbable. It appears to me that this condition is best 
 fulfilled in the theory entertained by the Austrian mineralogist, 
 Tschermak. He has made a study of the meteorites in the 
 rich collection at Vienna, and he has come to the conclusion 
 that the e< meteorites have had a volcanic source on some celestial 
 body/' Let us attempt to pursue this hypothesis and discuss the 
 problem, which may be thus stated : Assuming that meteorites 
 have been ejected from, volcanoes, on what body or bodies in the 
 universe must these volcanoes be situated ? This is really a 
 question for astronomers and mathematicians. Once the minera- 
 logists assure us that these bodies are volcanic, the question 
 becomes one of calculation and of the balance of probabilities. 
 
 The first step in the inquiry is to realise distinctly the 
 dynamical conditions of the problem. Conceive a volcano to be 
 located on a planet. The volcano is supposed to be in a state of 
 eruption, and in one of its mighty throes projects a missile aloft : 
 this missile will ascend, it will turn round, and fall down again. 
 Such is the case at present in the eruptions of terrestrial volcanoes. 
 Cotopaxi has been known to hurl prodigious stones to a vast height, 
 but these stones assuredly return to earth. The gravitation of the 
 earth has gradually overcome the velocity produced by the explosion, 
 and down the body falls. But let us suppose that the eruption is 
 still more violent, and that the stones are projected from the planet 
 
360 THE STORY OF THE HEAVENS. 
 
 to a still greater height above its surface. Suppose, for instance, 
 that the stone should be shot up to a height equal to the planet's 
 radius, the attraction of gravitation will then be reduced to one-fourth 
 of what it was at the surface, and hence the planet will find greater 
 difficulty in pulling back the stone the higher it ascends. Not 
 only is the distance through which the stone has to be pulled back 
 increased as the height increases, but the efficiency of gravitation is 
 weakened, so that in a two-fold way the difficulty of recalling the 
 stone is increased. Once again, let us suppose the stone projected 
 with still greater speed, it will rise up and up; the gravitation 
 is still tending to pull the stone back, but as it ascends the 
 gravitation is lessened; when the stone has attained a height of 
 ten times the radius of the planet, the attraction is reduced to one 
 hundredth part of its original intensity, and thus it may happen 
 that if the velocity be sufficiently great, the stone will rise higher 
 and higher, and gravitation will never be able to recall it. There 
 is thus a certain critical velocity appropriate to each planet, and 
 depending on its mass. If the missile be projected upwards with 
 a velocity equal to this, then it will ascend never to return. We 
 all recollect Jules Verne's voyage to the moon, in which he described 
 the Columbiad, an imaginary cannon, capable of shooting out a 
 projectile with a velocity of six miles a second. This is the critical 
 velocity for the earth. If we could imagine the air removed, then 
 a cannon of six-mile power would project a body upwards which 
 would not fall down. 
 
 The great difficulty about Tschermak's view of the volcanic 
 origin of the meteorites, lies in the tremendous initial velocity 
 which is required. The Columbiad is a myth, and we know no 
 agent, natural or artificial, at the present time on the earth, 
 adequate to the production of a velocity so appalling. The 
 thunders of Cotopaxi are said to be heard one hundred and fifty 
 miles away, but in its mightiest throes, it discharges no missiles 
 at the present time with a velocity of six miles a second. We are 
 therefore led to inquire whether any of the other celestial bodies 
 are entitled to the parentage of the meteorites. We cannot see 
 volcanoes on any other body except the moon ; all the other bodies 
 
SHOOTING STARS. 361 
 
 are too remote for an inspection so minute. Does it seem likely 
 that volcanoes on the moon can ever launch forth missiles which 
 fall upon the earth ? 
 
 This was a belief sustained by eminent authority. The mass of 
 the moon is very small as compared with that of the earth. It 
 would take over eighty moons rolled into one, to equal the mass 
 of the earth. A body could therefore be projected from the surface 
 of the moon, so as to leave the moon altogether, with much less 
 velocity than is required to project a body entirely away from the 
 surface of the earth. It would not be true to assert that the 
 velocity of projection varies directly as the masses. The correct 
 law is/ that it varies directly as the square root of the mass, and 
 inversely as the square root of the radius. It is hence easy to see 
 that the velocity required to project a missile from the moon is 
 only about one-sixth of that which would be required to project a 
 missile from the earth. If the moon had on its surface volcanoes 
 of one mile power, it is quite conceivable that these might be 
 the source of meteorites. We have seen how the surface of the 
 moon shows -traces of intense volcanic activity. There are vast 
 craters, which must have been once the seat of mighty eruptions, so 
 that the whole surface of the moon can only be compared with the 
 most intensely volcanic regions on the earth. A missile thus 
 projected from the moon could undoubtedly fall on the earth, and 
 it is not impossible that some of the meteorites may really have 
 come from this source. There is, however, one great difficulty about 
 the volcanoes on the moon which must be borne in mind. Suppose 
 an object were so projected, it would, under the attraction of the 
 earth, in accordance with Kepler's laws, move around the earth as 
 a focus. If we set aside the disturbances produced by all other 
 bodies, as well as the disturbance produced by the moon itself, we 
 see that the meteorite if it once misses the earth can never fall 
 thereon. It would be necessary that the shortest distance of the 
 earth's centre from the orbit of the projectile shall be less than 
 the radius of the earth, so that if a lunar meteorite is to fall 
 on the earth, it must do so the first time it goes round. The 
 journey of a meteorite from the moon to the earth is only a matter 
 
362 THE STOItY OF THE HEAVENS. 
 
 of days, and therefore, as meteorites are still falling-, it would 
 follow that they must still be constantly ejected from the moon. 
 The volcanoes on the moon are, however, not now active; ob- 
 servers have long studied its surface, and they find no reliable 
 traces of volcanic activity at the present day. It is utterly out 
 of the question, whatever the moon may once have been able 
 to do, that at the present date she should still continue to 
 launch forth meteorites. It is just possible that a meteorite 
 expelled from the moon in remote antiquity, when its volcanoes 
 were active, may, under the influence of the disturbances previously 
 excepted, have its orbit so altered, that at length it comes within 
 the reach of the atmosphere and falls to the earth, but under no 
 other circumstances could the moon send us a meteorite at present. 
 We are therefore compelled to look elsewhere in our search for 
 volcanoes fulfilling the conditions of the problem. 
 
 Let us now direct our attention to the planets, and examine the 
 circumstances under which volcanoes located thereon could eject 
 a meteorite which should ultimately tumble on the earth. We 
 cannot see the planets well enough to tell whether they have or 
 ever had any volcanoes ; but the almost universal presence of heat 
 in the large celestial masses seems to leave us in little doubt that 
 some form of volcanic action may be found in the planets. We 
 may at once dismiss the giant planets, such as Jupiter or Saturn : 
 their appearance is very unlike a volcanic surface; while their 
 great mass would render it necessary to suppose that the meteorites 
 were expelled with terrific velocity if they are to succeed in escaping 
 from the gravitation of the planet. Applying- the rule already 
 given, a volcano on Jupiter would have to be five or six times 
 as powerful as the volcano on the earth. To avoid this difficulty, 
 we naturally turn to the smaller planets of the system ; take, for 
 instance, one of that innumerable host of minor planets, and let us 
 inquire how far this body is likely to have ejected a missile which 
 should fall upon the earth. Some of these planets are only a few 
 miles in diameter. There are bodies in the solar system so small 
 that a very moderate velocity would be sufficient to project a 
 missile away from them altogether. We have, indeed, already 
 
SHOOTING STARS. 363 
 
 illustrated this point in discussing the minor planets. In a cricket 
 match, on a planet twenty miles in diameter, a successful hit 
 might actually drive the ball away never to return ; the field would 
 wait in vain for its descent, the ball would have started off in a 
 conic section around the sun in its focus, and would never return 
 to the planet from which it had been driven. It has been supposed 
 that a volcano placed on one of the minor planets might be quite 
 powerful enough to drive off the meteorites so as to wander through 
 space until the chapter of accidents brought them into collision 
 with the earth. There is but little difficulty in supposing that 
 there should be such volcanoes, and that they should be sufficiently 
 powerful to drive bodies from the surface of the planet; but we 
 must remember that the bodies are to fall down on the earth, and 
 certain dynamical considerations here come in which merit our close 
 attention. To concentrate our ideas, we shall consider one of the 
 minor planets in particular, and for this purpose let us take Ceres, 
 which is a small object revolving in an orbit exterior to Mars. If 
 a meteorite is to fall upon the earth, it must pass through the 
 narrow ring some 8,000 miles wide, which marks the earth's path; 
 it will not do for the missile to pass through the ecliptic on the 
 inside or on the outside of the ring, it must be actually through 
 this narrow strip, and then if the earth happens to be there at the 
 same moment the meteorite will fall. The first condition to be 
 secured is, therefore, that the path of the meteorite shall traverse 
 this narrow ring. This is to be effected by projection from 
 some point in the orbit of Ceres. The missile on leaving Ceres 
 will ascend in a conic section around the sun in the focus, and will 
 then descend so as to cross the ecliptic again, at a point within the 
 narrow strip referred to. Ceres is itself moving in its orbit with 
 a high velocity of about eleven miles a second, so that the actual 
 velocity of the projectile will be the resultant obtained by com- 
 pounding the volcanic velocity with the orbital velocity. It can 
 be shown on purely dynamical grounds that a meteorite projected 
 from a point in the orbit of Ceres could not intersect the earth's 
 orbit unless the velocity perpendicular to the radius at the instant 
 of projection were about eight miles a second. But the velocity 
 
364 THE STORY OF THE HEAVENS. 
 
 of the planet is about eleven miles a second in this direction. It 
 would therefore be necessary for the volcanic velocity to abate the 
 velocity perpendicular to the radius by at least three miles a second. 
 This is a significant result; the volcanic energy sufficient to re- 
 move the projectile from Ceres may be small enough, but if that 
 projectile is ever to cross the earth's track, the dynamical require- 
 ments of the case show that we must have a volcano on Ceres at 
 the very least of three-mile power. We have thus gained but little 
 by the suggestion of a minor planet, for we have not found that a 
 moderate volcanic power would be adequate. But there is another 
 difficulty in the case of Ceres, inasmuch as the ring on the 
 ecliptic is very narrow in comparison with the other dimensions 
 of the problem. Ceres is a long way off, and it would require 
 very great accuracy in volcanic practice on Ceres to project a 
 missile so that it should just traverse this ring and fall neither 
 inside nor outside. There must be a great many misses for every 
 hit. We have attempted to make the calculation by the aid of the 
 theory of probabilities, but it is a question not very easy to state 
 definitely. If the total velocity with which the projectile leaves 
 the orbit of Ceres be less than eight miles a second, then the 
 projectile will fall short of the earth's track. On the other hand, 
 if the total initial velocity exceed sixteen miles a second the orbit in 
 which the projectile passes will be hyperbolic, and though it may 
 cross the earth's track once, it will never do so again. Taking 
 a mean between these extreme velocities, we may investigate the 
 following problem. Suppose that a projectile is discharged in a 
 random direction from Ceres with the total initial velocity of twelve 
 miles a second (resulting, of course, from the composition of the 
 velocity of projection with the actual orbital velocity of Ceres), 
 what is the probability that this projectile shall cross the earth's 
 track? Solved by the theory of probabilities, we find that the chances 
 against this occurrence are about 50,000 to 1, so that out of every 
 50,000 projectiles hurled from a point in the orbit of Ceres only a 
 single one can be expected to cross the track of the earth. It is thus 
 evident that there are two objections to Ceres (and the same may 
 be said of the other minor planets) as a possible source of the 
 
SHOOTING STARS. 365 
 
 meteorites. Firstly, that notwithstanding the small mass of the 
 planet a very powerful volcano would still be required ; and secondly, 
 that we are obliged to assume that for every meteorite which could 
 ever fall on the earth at least 50,000 must have been ejected. It is 
 thus plain that if the meteorites have really been driven from some 
 planet of the solar system, large or small, the volcano must, from 
 one cause or another, be a very powerful one. As we must have a 
 very powerful volcano in any case, we are led to inquire which 
 planet possesses on other grounds the greatest probability in its 
 favour. 
 
 We have already spoken of the volcanoes on the earth, and we 
 admit of course that at the present time they are utterly devoid 
 of the necessary energy ; but were the terrestrial volcanoes always 
 so feeble as they are at present? Grounds are not wanting for 
 the belief that in the very early days of geological time the 
 volcanic energy on the earth was much greater than at present. 
 We admit fully the difficulties of the view that the meteorites 
 have really come from the earth ; but they must have some origin, 
 and it is reasonable to indicate the source which seems to have 
 most probability in its favour. Grant for a moment that in the 
 primaeval days of volcanic activity there were some mighty throes 
 which hurled forth missiles with the adequate velocity : these missiles 
 would ascend, they would pass from the gravitation of the earth, 
 they would be seized by the gravitation of the sun, and they 
 would be compelled to revolve around the sun for ever after. No 
 doubt the resistance of the air would be a very great difficulty, but 
 this resistance would be greatly lessened were the crater at a very 
 high elevation above the sea level. Some of these objects might 
 perhaps revolve in hyperbolic orbits, and retreat never to return, 
 while others would be driven into elliptic paths. Round the sun these 
 objects would revolve for ages, but at each revolution and here is 
 the important point they would traverse the point from which they 
 were originally launched. In other words, every object so pro- 
 jected from the earth, would at each revolution cross the track of 
 the earth. We have in this fact an enormous probability in favour 
 of the earth as contrasted with Ceres. Only one Ceres-ejected 
 
366 THE STORY OF THE HEAVENS. 
 
 meteorite out of every 50,000 could possibly cross the earth's track, 
 while every earth-projected meteorite would necessarily do so. 
 
 If this view be true, then there must be countless hosts of mete- 
 orites traversing- space in elliptic orbits around the sun. All these 
 orbits have one feature in common : they all intersect the track 
 of the earth. It will sometimes happen that the earth is found 
 at this point at the moment the meteorite is crossing- ; when this 
 is the case the long travels of the meteorite are at an end, and 
 it tumbles back on the earth from which it parted so many 
 ages ago. 
 
 It is well to emphasise the contrast between the lunar theory 
 of meteorites (which we think improbable), and the terrestrial 
 theory (which appears to be probable). For the lunar theory 
 it would, as we have seen, be necessary that some of the lunar 
 volcanoes should be still active. In the terrestrial theory it is 
 only necessary to suppose that the volcanoes on the earth once 
 possessed sufficient explosive energy. No one supposes that the 
 volcanoes at present on the ^arth eject now the fragments which 
 are to form future meteorites; but it seems probable that the 
 earth may be now slowly gathering back, in these quiet times, 
 the fragments she r ejected in an early stage of her history. As- 
 suming, therefore, with Tschermak that the meteorites have had 
 a volcanic origin on some considerable celestial body, we are led 
 to agree with those who think that most probably that body is 
 the earth. 
 
 We cannot forbear to mention one or two circumstances which 
 seem to corroborate the view that the meteorites are really of 
 ancient terrestrial origin. The most characteristic constituent of 
 meteorites is the alloy of iron and nickel, which is almost univer- 
 sally present. Sometimes, as in the Rowton siderite, the whole 
 object consists of little else, sometimes this alloy is in grains 
 distributed through the mass. When Nordenskjold discovered 
 in Greenland a mass of native iron containing nickel, this was 
 at once regarded as a celestial visitor. It was called the 
 Ovifak meteorite, and large pieces of the iron were conveyed 
 to our museums. There is, for instance, in the British Museum 
 
SHOOTING STARS. 367 
 
 a most interesting exhibit of the Ovifak substance. Close exami- 
 nation shows that this so-called meteorite lies in a bed of basalt 
 which has been vomited from the interior of the earth. Those 
 who believe in the meteoric origin of the Ovifak iron are con- 
 strained to admit that shortly after the eruption of the basalt, 
 and while it was still soft, this stupendous iron meteorite of 
 gigantic mass and bulk happened to fall into this particular 
 soft bed. The view is, however, steadily gaining ground that 
 this great iron mass was no celestial visitor at all, but that it 
 simply came forth from the interior of the earth with the basalt 
 itself. The beautiful specimens in the British Museum show how 
 the iron graduates into the basalt in such a way as to make it 
 highly probable that the source of the iron is really to be sought 
 in the earth and not external thereto. Should further research 
 establish this, as now seems probable, a most important step will 
 have been taken in proving the terrestrial origin of meteorites. 
 If the Ovifak iron be really associated with the basalt, we have 
 a proof that the iron-nickel alloy is indeed a terrestrial substance, 
 found deep in the interior of the earth, and associated with volcanic 
 phenomena. This being so, it will be no longer difficult to account 
 for the iron in undoubted meteorites. When the vast volcanoes were 
 in activity they ejected masses of this iron -alloy, which, having 
 circulated round the sun for ages, have at last come back again. 
 As if to confirm this view, Professor Andrews discovered par- 
 ticles of native iron in the basalt of the Giant's Causeway, while 
 the probability that large masses of iron are there associated with 
 the basaltic formation was proved by the researches on magnetism 
 of the late Provost Lloyd. 
 
 Besides the more solid meteorites which seem to be terrestrial, 
 there can be no doubt that the debris of the ordinary shooting stars 
 must rain down upon the earth in gentle showers of celestial dust. 
 The evidence on this point is overwhelming. The snow in the 
 Arctic regions has often been found stained with traces of dust 
 which contains particles of iron. Similar particles have been found 
 in the towers of cathedrals and under many other circumstances. 
 There can be hardly a doubt that some of the motes in the sun- 
 
363 THE STORY OF THE HEAVENS. 
 
 beam,, and many of the particles which good housekeepers abhor as 
 dust, are really of a cosmical origin. In the famous cruise of the 
 Challenger the dredges brought up from the depths of the Atlantic 
 no " wedges of gold, great anchors, heaps of pearl/'' but among 
 the mud which they raised are to be found numerous magnetic 
 particles which there is every reason to believe fell from the 
 sky, and thence subsided to the depths of the ocean. Sand 
 from the deserts of Africa, when examined under the microscope, 
 yields traces of minute iron particles which bear the marks of 
 having experienced a high temperature. 
 
 The earth draws in this cosmic dust continuously, but the earth 
 now never parts with a particle of its mass. The consequence is 
 inevitable; the mass of the earth must be growing, and though 
 the change may be a small one, yet to those who have studied 
 Darwin's treatise on " Earth-worms/' or to those who are ac- 
 quainted with the modern theory of evolution, it will be manifest 
 that stupendous results can be achieved by slight causes which 
 tend in one direction. It is quite probable that an appreciable 
 part of the solid substance of the earth may thus have been derived 
 from meteoric matter, which in perennial showers descends upon 
 its surface. 
 
CHAPTER XVIII. 
 
 THE STARRY HEAVENS. 
 
 Whence the Importance of the Solar System? Home View in Space Other 
 Stellar Systems The Sun a Star Stars are Self-Luminous We see the Points 
 of Light, but nothing else The Constellations The Great Bear and the 
 Pointers The Pole Star Cassiopeia Andromeda, Pegasus, and Persetis The 
 Pleiades: Auriga, Capella, Aldebaran Taurus, Orion, Sirius; Castor and 
 Pollux The Lion Bootes, Corona, and Hercules Virgo and Spica Vega 
 and Lyra The Swan. 
 
 IN the previous chapters of this work we have considered the 
 sun and the system of planets and other bodies which revolve 
 in obedience to his potent sway. We have found in the survey 
 of this system much to impress us, and much which is cal- 
 culated to awaken our conceptions as to the stupendous scale on 
 which the heavens are constructed. It is, however, desirable that 
 we should not disguise from our thoughts the circumstances which 
 lead us to attribute to the solar system a position of such im- 
 portance in the scheme of the universe. It is the fact of our 
 residence on a planet belonging to the solar system, which gives 
 the solar system its great importance in our eyes. The solar 
 system is our abode in the universe. The other planets, Mercury, 
 Venus, Mars, Jupiter, and Saturn are our neighbours; the moon 
 is our inseparable attendant; while the sun is the lamp which 
 gives us light, and the fire which gives us warmth. It thus 
 happens that in our eyes the solar system has the interest and 
 the familiarity of home. We are surrounded by it; its welfare 
 is our welfare. We live our lives without perhaps bestowing many 
 thoughts as to whether there may not be other systems besides 
 ours, or as to whether our solar system may not be far surpassed in 
 size and in splendour by many, perhaps we might say by countless, 
 other systems in the universe. 
 
 Y 
 
870 THE STORY OF THE HEAVENS. 
 
 When we become aware of the existence of such systems, a 
 multitude of questions arise. We long to know their details ; we 
 long to know the sizes of those great suns, and the dimensions of 
 the planets which circulate around them ; we long to see the con- 
 figuration of their planets, to learn what their surface is like, and, 
 it may be, to speculate on the possibility of their being inhabited. 
 We long to compare or to contrast such systems with our own 
 to ask whether those planets have satellites, as ours have j whether 
 those suns show the remarkable features which characterise our 
 sun; whether they are attended like our own sun by innumerable 
 comets. Have they showers of shooting stars, as we have? Such 
 are a few of the questions which occur to us when we pass from 
 a review of the solar system to the study of the other systems in 
 space. 
 
 These questions can be asked, but can they be answered? 
 In great part they cannot. These other systems are plunged 
 into space to a distance so appalling, that all those details which 
 we are so anxious to learn become utterly lost by distance. 
 Of the planets surrounding these systems we can see nothing, 
 though here and there their presence can be, as it were, felt, 
 and thus their existence is made probable. Of these systems, all 
 that we see are what we call the stars those gems of light 
 that stud the midnight sky. These stars are the suns of the 
 distant systems, but their distance is such that they have entirely 
 lost the glory which we attach to the word sun. Yet that they 
 really are suns can be easily shown. We shall merely allude to the 
 subject now, and we shall discuss it more fully hereafter. Suppose 
 that a traveller, endowed with a miraculous power of voyaging 
 through space, were to start from the solar system and soar away 
 into the stellar regions. As he receded he would find that the 
 earth and the other planets dwindled down and gradually became 
 invisible. Still further on he would find that the sun began to lose 
 its splendour, and even its pre-eminence, among the celestial host. 
 On and on the traveller wings his way, until, by the time he has 
 travelled over a distance comparable with that which intervenes 
 between the sun and the stars, he would find that our sun had 
 
THE STARRY HEAVENS. 371 
 
 dwindled down to a star not so bright as many of those that 
 twinkle in our skies every night. We are thus led to realise 
 that our solar system is a little island group, situated at the 
 most appalling distances from the stars. The solar system is 
 isolated from its neighbours just as a rock a few yards square 
 in the middle of the Atlantic would be isolated from the coasts 
 of Europe and of America. 
 
 It is very essential to observe that the stars we see are bodies 
 which shine by their own light. Aldebaran shines with a light 
 not very unlike that which comes from Mars. The planets are, 
 indeed, not unfrequently mistaken for stars, and stars are often 
 mistaken for planets ; yet no language can be too strong to 
 emphasise the vast difference between bodies belonging to these two 
 classes. If Aldebaran looks like Mars, the resemblance is quite a 
 fortuitous one. Aldebaran is probably thousands of times as large 
 as Mars, and it is certainly hundreds of thousands of times as far 
 off. But this is by no means all the difference ; it is not even the 
 chief difference. Aldebaran is a sun. It has its own light, and is 
 a body intensely heated and glowing, just like our sun ; but Mars 
 has no light of its own. Mars is as dark as an ordinary stone, and 
 is only visible from the fact that the rays of light from our sun 
 shine upon it. 
 
 We are thus led to perceive, that although the points of light 
 which we see in stellar space are one and all suns on their own 
 account, yet we do not see we need not expect to see any of the 
 dark bodies in their neighbourhood which are illuminated by their 
 light. We can hardly avoid acquiring by this thought a greatly 
 enlarged conception of the importance and the extent of the 
 stellar regions. As a traveller at night looks down on the distant 
 city from the height which he has attained, he sees in that 
 city, not the houses or the monuments, not even the churches or 
 the greatest buildings, all he sees are the bright points of light 
 scattered here and there in the gloom. Some, perhaps, may 
 be the lights which serve to guide the wayfarer through the 
 streets ; some may be the lights proceeding from the houses where 
 rejoicing and feasting are going on; some will be the light? 
 T 2 
 
372 THE STORY OF THE HEAVENS. 
 
 where patient watchers minister to the wants of the sick and the 
 dying; but of the purport of these lights, of what these lights 
 illuminate, the distant traveller sees nothing and knows nothing 
 he sees the lights, but he can see nothing else. So it is when we 
 look on the starry host we see the bright points oE light, but we 
 see nothing else ; of all the dark objects illuminated by those lights 
 we see absolutely nothing. We cannot resist the conjecture that 
 this unseen universe is of great interest and complexity, though 
 we are unable to see anything more than the system of lights by 
 which it is illuminated. 
 
 But though our acquaintance with the sidereal universe is so 
 narrowly limited that only objects of colossal dimensions can be 
 seen in our greatest telescopes, yet the field within our reach is 
 replete with interest. Let us in this chapter make a general 
 survey of the sidereal heavens, and we shall subsequently develop 
 the subject in its different aspects. 
 
 The student of astronomy should make himself acquainted 
 with the principal constellations in the heavens. This is a most 
 pleasing acquirement, and should form a part of the educa- 
 tion of every child in the kingdom. We shall commence our 
 discussion of the sidereal system with a brief account of the 
 principal constellations visible in the northern hemisphere, and we 
 accompany our description with such outline maps of the stars as 
 will enable the beginner to identify the chief features of the starry 
 heavens. 
 
 We have in an earlier chapter directed the attention of the 
 student to the remarkable constellation of stars which is known 
 to astronomers as Ursa Major, or the Great Bear. It is the most 
 conspicuous group in the northern skies, and in northern latitudes 
 it never sets. At eleven p.m. in the month of April the Great Bear 
 is directly overhead ; at the same hour in the month of September 
 it is low down in the north ; in July it will be seen in the west ; at 
 Christmas it is in the east. From the remotest antiquity this 
 group of stars has attracted attention. The stars in the Great Bear 
 were comprised in the great catalogue of stars, made two thousand 
 years ago, which has been handed down to us. From the positions 
 
THE STARRY HEAVENS. 373 
 
 of the stars given in this catalogue it is possible to reconstruct the 
 Great Bear as it appeared two thousand years ago. This has been 
 done, and it appears that the seven principal stars have not changed 
 in this lapse of time to any large extent, so that the configura- 
 
 Fig. 67. The Great Bear and Pole Star. 
 
 tion of the Great Bear remains practically the same now as it 
 was then. The beginner must first obtain an acquaintance with 
 this group of seven stars, and then his further progress iir this 
 branch of astronomy will be greatly facilitated. The Great Bear 
 is, indeed, a splendid constellation, and its only rival is to be 
 found in Orion, which contains more brilliant stars, though it does 
 not occupy so large a region in the heavens. 
 
374 THE STORY OF THE HEAVENS. 
 
 In the first place, we observe how the Great Bear enables the 
 Pole Star, which is the most important object in the northern 
 heavens, to be readily found. The Pole Star is very conveniently 
 indicated by the direction of the two stars, {3 and a, of the Great 
 Bear, which are, accordingly, generally known as the "pointers/' 
 This use of the Great Bear is shown on the diagram in Fig. 67, in 
 which the line /3 a, produced onwards and slightly curved, will 
 
 Fig. 68. The Great Bear and Cassiopeia. 
 
 conduct to the Pole Star. There is no likelihood of making any 
 mistake in this star, as it is the only bright one in the neighbour- 
 hood. Once it has been seen, it will be most readily identified on 
 future occasions, and the observer will not fail to notice how 
 constant is the position which it preserves in the heavens. The 
 other stars either rise or set, or, like the Great Bear, they dip down 
 low in the north without actually setting, but the Pole Star 
 exhibits no changes of this magnitude. In summer or winter, by 
 night or by day, the Pole Star is ever found in the same place 
 at least, so far as ordinary observation is concerned. No doubt, 
 when we use the accurate instruments of the observatory the 
 notion of the fixity of the Pole Star is dissipated ; we then see that 
 
THE STARRY HEAVENS. 375 
 
 it has a slow motion, and that it describes a circle every twenty- 
 four hours around the true pole of the heavens, which is not 
 coincident with the Pole Star, though closely adjacent thereto. 
 This distance is at present somewhat less than a degree and a half, 
 and it is gradually lessening, until, in the year A.D. 2095, the 
 distance will be under half a degree. 
 
 The Pole Star itself belongs to another not inconspicuous group 
 of stars, known as the Little Bear. The two principal stars of this 
 group, next in brightness to the Pole Star, are sometimes called 
 the " Guards." The Great Bear and the Little Bear, with their 
 cynosure the Pole Star, form a group in the northern sky not 
 paralleled, either in beauty or in utility, by anything in the 
 southern heavens. At the South Pole there is no conspicuous 
 star to indicate its position approximately a circumstance dis- 
 advantageous to astronomers and to navigators in the southern 
 hemisphere. 
 
 It will now be easy to add a third constellation to the two 
 already acquired. On the opposite side of the Pole Star to the 
 Great Bear, and at about the same distance, lies a very pleasing 
 group of five bright stars, forming a W. These are the more 
 conspicuous members of the constellation Cassiopeia, which con- 
 tains altogether about fifty-five bright stars. When the Great 
 Bear is low down in the north, then Cassiopeia, on the contrary, is 
 high overhead ; when the Great Bear is high overhead, then Cas- 
 siopeia is to be looked for low down in the north. The configura- 
 tion of the leading stars is so striking, that once the eye has 
 recognised them, future identification will be very easy the more 
 so when it is borne in mind that the Pole Star lies midway 
 between Cassiopeia and the Great Bear (Fig. 68). These im- 
 portant constellations will serve as guides to all the rest. We 
 shall accordingly proceed step by step to show how the learner 
 may distinguish the various other groups visible from the British 
 Islands or similar northern latitudes. 
 
 The next constellation to be recognised is the imposing group which 
 contains the Great Square of Pegasus. This is not, like Ursa Major, 
 or like Cassiopeia, said to be te circumpolar/' The Great Square of 
 
376 
 
 THE STORY OF THE HEAVENS. 
 
 Pegasus sets and rises daily. It cannot be seen conveniently during 
 the spring and the summer, but in autumn and in winter the four 
 stars which mark the corners of the square can be very easily per- 
 ceived. There are certain small stars within the square ; perhaps 
 about thirty can be counted by an unaided eye of ordinary power in 
 these latitudes. In the south of Europe, where purer and brighter 
 skies are to be found, the number of small stars appears to be 
 
 ig. 69. The Great Square of Pegasus. 
 
 greatly increased. An acute observer at Athens has counted 102 
 in the same region. 
 
 The Great Square of Pegasus can be readily identified by 
 imagining a line from the Pole Star over the end of Cassiopeia. 
 This line produced about as far again will conduct the eye to the 
 centre of the Great Square of Pegasus (Fig. 69) . 
 
 The line through ft and a in Pegasus produced 45 to the south 
 points out the important star Fomalhaut in the mouth of the 
 southern fish. To the right of this line, nearly half-way down, is 
 the rather vague constellation of Aquarius, where a small equi- 
 lateral triangle with a star in the centre may be noticed. 
 
 The Square of Pegasus is one illustration of the absurd way in 
 
THE STARRY HEAVENS. 377 
 
 which the boundaries of the constellations are defined. There can 
 be no more completely associated group than the four stars of this 
 square, and all four ought surely to belong to the same constellation. 
 Three of the corners marked a, /3, 7 do belong to Pegasus, but 
 the fourth corner also marked a is sometimes placed in a dif- 
 ferent constellation, known as Andromeda, whereof it is, indeed, 
 the brightest member. The remaining part of the constellation 
 Andromeda or, at all events, its brightest stars can now be 
 readily found. They are marked j3 and 7, and are readily identified 
 by producing one side of the Square of Pegasus in a curved direction. 
 We have now a remarkable group of seven stars, which it is easy to 
 identify and easy to remember, although formed out of parts belong- 
 ing to three different constellations. They are respectively a, /3, 
 and 7 from Pegasus, a, /?, and 7 from Andromeda, and a from 
 Perseus. The three form a sort of handle, as it were, extending 
 to a great length on one side of the square, and are a group both 
 striking in appearance, and very useful in the further identification 
 of the celestial objects. /3 Andromedse with two smaller stars form 
 the girdle of the unfortunate heroine. 
 
 a Persei lies between two other stars (7 and $) of the same 
 constellation. The three form a curve, and if we prolong that curve, 
 we are conducted to one of the gems of the northern heavens the 
 beautiful reddish star Capella, in the constellation of Auriga (Fig. 
 70). Close to Capella are three small stars forming an isosceles 
 triangle, these are the Hoedi or Kids. Capella and Vega are the 
 two most brilliant stars in the northern heavens; and though 
 Vega, with its whiter ray, is generally considered the more lustrous 
 of the two, yet the opposite opinion has been entertained. Dif- 
 ferent eyes will frequently form various estimates of the relative 
 brilliancy of stars which approach each other in brightness. The 
 difficulty of making a satisfactory comparison between Vega and 
 Capella is greatly increased by the wide distance in the heavens 
 by which they are separated, as well as by a slight difference in 
 colour, for Vega is distinctly whiter than Capella. This contrast 
 between the colour of stars is often another source of uncertainty 
 in the attempt to compare their relative brilliancy ; so that when 
 
378 ^ THE STORY OF THE HEAVENS. 
 
 actual measurements are to be effected by instrumental means, it 
 is necessary to compare the two stars alternately with some object 
 of intermediate colour. 
 
 On the opposite side of the pole to Capella, but not quite so 
 far away, will be found four stars in a quadrilateral. They form 
 
 v: Glorious 
 Star Cluster 
 
 Capella 
 
 1st. Mag m *" 7 Algol the 
 
 '"'-. ,' WonderfulVr 
 
 The Kids 
 
 The Pleiades 
 
 [Fig 1 . 70. Perseus and its neighbouring Stars. 
 
 the head of the dragon, the rest of whose form coils right round 
 the pole. 
 
 If we continue the curve formed by the three stars 7, a, and 8 
 in Perseus, and if we bend round this curve gracefully into one of 
 an opposite flexion, in the manner shown in Fig. 70, we are first 
 conducted to two other principal stars in Perseus, marked e and f. 
 The region of Perseus is one of the richest in the heavens. We have 
 here a most splendid portion of the Milky Way, and the field of 
 
THE STARRY HEAVENS. 379 
 
 the telescope is crowded with stars beyond number. Even a small 
 telescope or an opera-glass directed to this teeming 1 constellation 
 cannot fail to delight the observer,, and convey to him a profound 
 impression of the extent and majesty of the sidereal heavens. We 
 shall return later on to a brief enumeration of some of the remark- 
 able telescopic objects in Perseus. Pursuing in the same figure 
 the line e and f, we are conducted to the remarkable little group 
 known as the Pleiades. We have here a suggested resemblance to 
 the Great Bear on a much smaller scale ; indeed, in many ways 
 the designation of Little Bear would be applied with greater 
 
 Fig. 71. The Pleiades. 
 
 propriety to the Pleiades than to the constellation which is actually 
 so called. 
 
 The Pleiades form a group so universally known and so 
 easily identified, that it hardly seems necessary to give any further 
 specific instructions for their . discovery. It may, however, be 
 observed that in these latitudes they cannot be seen during the 
 summer. Let us suppose that the search is made at about 11 p.m. 
 at night : on the 1st of January the Pleiades will be found high 
 up in the sky in the south-west; on the 1st of March, at the same 
 hour, they will be seen to be setting in the west. On the 1st of 
 May they are not visible; on the 1st of July they are not visible; 
 on the 1st of September they will be seen low down in the east. 
 On the 1st of November they will be high in the heavens in the 
 
380 THE STORY OF THE HEAVENS. 
 
 south-east. On the ensuing 1st of January the Pleiades will be 
 in the same position as they were on the same date in the previous 
 year, and so on throughout the cycle. It need, perhaps, hardly be 
 explained here that these changes are not really due to movements 
 of the constellations ; they are due, of course, to the apparent 
 annual motion of the sun among the stars. 
 
 The Pleiades are shown in the figure on the previous page, a 
 group of ten, being about the number visible with the unaided eye 
 to those who are gifted with very acute vision. The lowest teles- 
 copic power will increase the number of stars' up to thirty or 
 forty (Galileo saw more than forty with his first telescope), while 
 with telescopes of greater power the number is largely increased ; 
 indeed, no fewer than 625 have been counted with the aid of a 
 powerful telescope. The group is, however, rather too large and 
 coarse for an effective telescopic object, except with a large field 
 and low power. In an opera-glass it is a very pleasing spectacle. 
 
 If we draw a ray from the Pole Star to Capella, a point already 
 determined, and produce this ray on sufficiently far, as shown in 
 the adjoining figure, we come to the glorious constellation of our 
 winter sky, the splendid group of Orion. The brilliancy of the stars 
 in Orion, the conspicuous belt, and the superb telescopic objects 
 which it contains, alike render this group remarkable, and place it 
 perhaps at the head of the constellations. The leading star in 
 Orion is known either as a Orionis, or as Betelgeuze, by which 
 name it is designated in the figure. It lies above the three stars, 
 S, e, and f, which form the belt. Betelgeuze is a star of the first 
 magnitude, and so also is Rigel, on the opposite side of the belt. 
 Orion thus enjoys the distinction of containing two stars of the 
 first magnitude in its group, while the five other stars here shown 
 are of the second magnitude. 
 
 The neighbourhood of Orion contains some of the most im- 
 portant stars. If we carry on the line of the belt upwards to the 
 right, we are conducted to another star of the first magnitude, 
 Aldebaran, which strongly resembles Betelgeuze in its ruddy colour. 
 Aldebaran is the brightest star in the constellation of Taurus. It 
 is this constellation which contains the Pleiades already referred 
 
THE STARRY HEAVENS. 
 
 381 
 
 POLE STAR 
 
 ^Capella 
 
 Aldebarai 
 ' 1st. Mag 
 
 The Belt of ,< 
 Orion j' fc 
 
 Rigel 
 1st. Mag 
 
 Fig. 72. Orion, Sirius, and the neighbouring Stars. 
 
 to, and another more scattered group known as the Hyades, 
 which will be easily discovered near Aldebaran. 
 
 The line of the belt of Orion continued downwards to the left 
 
382 THE STORY OF THE HEAVENS. 
 
 conducts the eye to the gem of the sky, the splendid star Sirius. 
 This is an object which is beyond all controversy the most brilliant 
 star in the heavens. It has, indeed, been necessary to create a 
 special order of magnitude wherein to place Sirius by himself ; all 
 the other first magnitude stars, such as Vega and Capella, Betel- 
 gueze and Aldebaran, coming a long way behind. Sirius, with a 
 few other stars of much less lustre, form the constellation of 
 Canis Major. 
 
 It is useful for the learner to note the large configuration, of 
 an irregular lozenge shape, of which the four corners are the first 
 
 'HE GREAT BEAI 
 
 THE TWINS 
 Castor* A Beautiful Double Star 
 
 THE LITTLE DOG 
 
 Fig. 73. Castor and Pollux. 
 
 magnitude stars, Aldebaran, Betelgeuze, Sirius, and Rigel (Fig. 72). 
 The belt of Orion is placed symmetrically in the centre of the 
 group, and the whole figure is so striking that once perceived it 
 is not likely to be forgotten. 
 
 One-third of the way from the Square of Pegasus to Aldebaran 
 a bright star of the second magnitude is the chief star in the 
 Ram ; with two others it forms a curve, at the other end of which 
 will be found 7 of the same constellation, which was the first 
 double star ever noticed. 
 
 We can again invoke the aid of the Great Bear to point out the 
 stars in the constellation of Gemini (Fig. 73). If the diagonal join- 
 ing the stars 8 and ft of the body of the Bear be produced in the 
 opposite direction to the tail, it will lead to Castor and Pollux, two 
 remarkable stars of the second magnitude. This same line carried 
 
THE STARRY HEAVENS. 383 
 
 a little further on passes near the star Procyon, of the first magni- 
 tude, which is the only conspicuous object in the constellation of 
 the Little Dog. 
 
 The pointers in the Great Bear marked a, will also serve to 
 indicate the constellation of the Lion. If we produce the line 
 joining them in the opposite direction from that used in finding the 
 
 Fig. 74. The Great Bear and the Lion. 
 
 Pole, we are brought into the body of the Lion. This group will 
 be recognised by the star of the first magnitude called Regulus. 
 It is one of a series of stars forming an object somewhat resembling 
 a sickle ; three of the group are of the second magnitude. The 
 Sickle is specially famous in astronomy as containing the radiant 
 point from which the periodic shooting star shower known as the 
 Leonids diverges, while Regulus lies alongside the sun's highway 
 through the stars, at a point which he passes on the 21st of 
 August every year. 
 
384 
 
 THE STORY OF THE HEAVENS. 
 
 Between Gemini and Leo lies the inconspicuous constellation 
 of the Crab; the most striking object in it is the misty patch 
 called Praesepe or the Bee-Hive, which the smallest opera-glass 
 will resolve into its component stars. 
 
 The tail of the Great Bear, when prolonged with a continuation 
 of the curve which it possesses, leads to a brilliant star of the first 
 magnitude known as Arcturus, the principal star in the constella- 
 
 Coldyred 
 Doubly Star 
 
 HE CROWN 
 
 
 THE GREAT BEAR 
 
 Arcturus < 
 1st. Mag 
 
 Fig. 75. Bootes and the Crown. 
 
 tion of Bootes (Fig. 75). A few other stars, marked 6, 7, S, 
 and e in the same constellation, are also shown in the figure. It 
 seems an open question whether among the stars visible in these 
 latitudes Arcturus is not to be placed next to Sirius in point of 
 brightness, the other two candidates for the place being Vega and 
 Capella. Two stars in the southern hemisphere invisible in these 
 latitudes termed a Centauri and Canopus, are both much brighter 
 than Arcturus, Vega, or Capella. 
 
 In the immediate neighbourhood of Bootes is a striking semi- 
 circular group known as the Crown or Corona Borealis. It will bo 
 
THE STARRY HEAVENS. 385 
 
 readily found from its position as indicated in the figure, or it may 
 be identified by following the curved line indicated by & 8, e and f 
 in the Great Bear. 
 
 THE GREAT BEAR 
 
 '(lst. -Mag) 
 
 THE GREAT LION 
 
 THJE VjffolN 
 
 pica 
 (1st. Mag) 
 
 Fig. 76. Virgo and the neighbouring Constellations. 
 
 The constellation of Virgo is principally characterised by the 
 first magnitude star called Spica, or a Virginis. This may 
 be found from the Great Bear; for if the line joining the 
 two stars a and 7 in that constellation he prolonged with a 
 slight curve, it will conduct the eye to Spica. We may here 
 notice another of those large configurations which are of great 
 assistance in the study of the stars. There is a fine equilateral 
 z 
 
386 THE STORY OF THE HEAVENS 
 
 triangle, whereof Arcturus and Spica form two of the corners, while 
 the third is indicated by Denebola, the bright star near the tail of 
 the Lion (Fig. 76). 
 
 In the summer evenings when the Crown is overhead, a line 
 from the Pole Star through its fainter edge continued nearly to the 
 
 ? the Double Doub 
 Vega 
 ^-Jst. Mag 
 
 HE GREAT BEAR 
 
 ARCTURUS*' 
 
 Fig. 77. The Constellation of Lyra. 
 
 southern horizon, encounters the brilliant red star Cor Scorpion is, or 
 the Scorpion's Heart (Antares), which was the first star mentioned 
 as having been seen with the telescope in the daytime. 
 
 The first magnitude star, Vega, in the constellation of the Lyre, 
 can be, perhaps, most readily found at the corner of a bold triangle, 
 of which the Pole Star and Arcturus form the base (Fig. 77). The 
 brilliant whiteness of Vega cannot fail to arrest the attention, while 
 the small group of neighbouring stars which form the Lyre pro- 
 duces one of the best defined constellations. 
 
THE STARRY HEAVENS. 387 
 
 Near Vega is another important constellation, known as the 
 Swan or Cygnus. The brightest star will be identified as the 
 vertex of a right-angled triangle, of which the line from Vega to 
 the Pole Star is the base, as shown in Fig. 78. There are in 
 Cygnus five principal stars, which form a constellation of rather 
 remarkable form. 
 
 POLE STAR 
 
 *Vega 
 
 ni* 1st. Mag. 
 
 A Most Beautiful 
 Coloured Double Star 
 
 78. Vega, the Swan, and the Eagie. 
 
 The last constellation which we shall here describe is that of 
 Aquila or the Eagle, which contains a star of the first magnitude, 
 often known as Altair ; this group can be readily found by a line 
 from Vega over ft Cygni, which passes near the line of three 
 stars, forming the characteristic part of the Eagle. 
 
 We have taken the opportunity to indicate in these sketches of 
 the constellations the positions of some other remarkable telescopic 
 objects, the description of which we must postpone to the follow- 
 ing chapters. 
 
CHAPTER XIX. 
 
 THE DISTANT SUNS. 
 
 Comparison between the Sun and the Stars Sirius Contrasted with the Sun- 
 Stars can be Weighed, but not Measured The Companion of Sirius Deter- 
 mination of the Weights of Sirius and his Companion Dark Stars Variable 
 Stars Enormous Number of Stars. 
 
 WIDE indeed is the contrast between the splendour of the noon-day 
 sun and the feeble twinkling of even the brightest of the stars. 
 This contrast, so forcible to our ordinary observation, can be sub- 
 mitted to the test of actual measurement. Let us take the most 
 brilliant star, Sirius. We can determine by experiment the number 
 of times by which the light of Sirius is surpassed by the light of the 
 sun. It is true we cannot make that comparison directly. In the 
 bright daylight Sirius cannot even be seen, much less can his light 
 be measured. But we can take the full moon as an intermediate 
 step between the glory of the sun and the feeble twinkle of Sirius. 
 It has been found that the light from the sun is about 600,000 
 times as great as the light from the full moon. Sirius and the 
 full moon can be compared together, and by suitable photometers 
 the quantity of light from the two bodies can be measured. It can 
 be shown that the light of about 33,000 stars, equal to Sirius, 
 would produce an illumination equal to that of the moon. We 
 have thus the necessary figures for comparing the brilliancy of the 
 sun with the brilliancy of Sirius, and the result is indeed signi- 
 ficant. The light from the sun is about twenty thousand million 
 times as great as the light from Sirius. 
 
 But do these figures truly represent the relative importance of 
 the sun and Sirius ? Recollect where we are placed. Our earth is 
 
THE DISTANT SUNS. 389 
 
 near the sun ; it is very far from Sirius. Our earth is not properly 
 placed for an impartial comparison between the splendour of the 
 sun and the splendour of Sirius. To make such a comparison, the 
 earth should be placed midway between the two bodies, so that we 
 could look at Sirius on one side and the sun on the other, under 
 precisely similar circumstances. Into such a position our earth 
 never has come, and never will come. How, then, is the real 
 comparison to be accomplished ? In this, as in many other cases, 
 it is possible to determine by calculation what it is impossible to 
 ascertain by experiment. It has been found, by observation, that 
 Sirius is about one million times as far from us as the sun. If we 
 take the distance of Sirius from the earth, and sub-divide it into 
 one million equal parts, each of these parts would be long enough 
 to span that great distance of 92,700,000 miles from the earth 
 to the sun. If, therefore, the earth were to be placed half-way 
 between the sun and Sirius, the earth would be five hundred 
 thousand times as far away from the sun as it is at present. What 
 would be the effect of this change upon the light received from 
 the sun ? Take the light of a candle at one foot as unity, then at 
 two feet the light is reduced to one-fourth, and at three feet to 
 one-ninth, and so on. The light received from a luminous source 
 varies " inversely as the square of the distance." Transplanted to a 
 distance 500,000 times as great as our present distance from the 
 sun, to what a vast extent must his radiance be diminished ! At 
 present our sun is as bright as 20,000,000,000 stars each equal 
 to Sirius ; but viewed from this central point, the light is reduced 
 to an almost incredible extent. Our sun would hare totally lost 
 his pre-eminence, and would only send to us one-twelfth part of 
 the light which we now receive from Sirius. On the other hand, 
 by shifting our station from the vicinity of the sun to a point half- 
 way to Sirius, the light of Sirius will become intensified in the ratio 
 of four to one. Assuming, then, that the earth was placed in the 
 correct position for testing the comparative splendour of Sirius and 
 the sun, we should find, first, that Sirius was increased four-fold, 
 and that the sun was so enormously reduced in intensity as to look 
 only one-twelfth of the original brightness of Sirius. The con- 
 
390 THE STORY OF THE HEAVENS. 
 
 sequence is irresistible, and fatal to the pre-eminence of the sun. We 
 may state it thus : Sirius sheds actually forty-eight times as much 
 light around it as does our sun ; we are not exactly entitled to 
 say that Sirius is forty-eight times as big as the sun, but we 
 can say that Sirius is forty-eight times as brilliant or as splendid. 
 In making this calculation we have taken a lower determination of 
 the brightness of Sirius relatively to the sun than some other care- 
 ful observations would have warranted. It will thus be seen that 
 if there be any uncertainty in our result it must only be as to 
 whether Sirius is not really more than forty-eight times as bright 
 as the sun. 
 
 Is this surpassing glory of Sirius merely due to excessive 
 brilliancy, or is Sirius really a body which vastly surpasses our sun, 
 both in intrinsic importance as well as in radiance ? The question 
 can only be fully answered by measuring the circumference of 
 Sirius, and then placing him in the weighing scales. At first 
 sight it might seem more difficult to achieve the latter than the 
 former. How is it possible that Sirius, at a distance of a hundred 
 billions of miles, should be actually weighed ? Yet we can weigh 
 Sirius, and we cannot measure him. We have no idea what the 
 actual bulk of Sirius may be, but as we find that he is certainly 
 heavier than the sun, we may conclude that he is probably larger 
 than the sun, though we can have no certain knowledge on this 
 point. 
 
 It is certainly a very remarkable fact, that out of the thousands 
 of stars with which the heavens are adorned, no single star has 
 yet been found which certainly shows an appreciable disc in the 
 telescope. We are aware that some skilful observers have thought 
 that certain small stars do show discs ; but we may lay this aside, 
 and appeal only to the ordinary fact that our best telescopes turned 
 on the brightest stars show merely glittering points of light, so 
 hopelessly small" as to elude our most delicate micrometers. The 
 ideal astronomical telescope is, indeed, one which will show Sirius 
 or any other bright star as nearly identical as possible with 
 Euclid's definition of a point, being that which has no parts and 
 no magnitude. It will throw some light on this question to 
 
THE DISTANT SUNS. 391 
 
 consider the telescopic appearance which the sun would present 
 if viewed from Sirius. This is a question which we can 
 answer. We know the diameter of the sun, and we can cal- 
 culate how large our sun would look if viewed from the stand- 
 point of Sirius. The answer is indeed significant. The size of 
 the sun from this distance would correspond to the size of a half- 
 penny 1,600 miles away. It is hopeless to expect that a 
 quantity so minute as this could be detected by any telescope. 
 When we have a telescope sufficiently powerful to show animals 
 on the moon, or to show buildings on the planet Mars, then 
 we may hope that we shall see the discs of the stars. But this 
 is utterly beyond our reach. The weakness is not merely in 
 the telescope; it is inherent in every circumstance of the pro- 
 blem. The stability of our earth is not sufficient to afford a 
 secure foundation for a telescope of such capacities, while even 
 the purest skies would form media far too turbid. We ought, 
 therefore, to feel no surprise at not being able to detect any 
 disc of Sirius ; were his diameter ten times that of the sun, and 
 his bulk, therefore, one thousand times as great, the task would 
 be equally hopeless. 
 
 Abandoning, therefore, all hope of measuring Sirius, let us 
 attempt to weigh him. Here we can be successful, as we shall 
 proceed to demonstrate. The story is, indeed, one of no little 
 interest in the history of astronomical discovery. 
 
 The splendid pre-eminence of Sirius had caused it to be 
 observed with minute care from the earliest times in the history 
 of astronomy. Each generation of astronomers devoted time and 
 labour to determine the exact place of the brightest star in the 
 heavens. A vast mass of observations as to the place of Sirius 
 among the stars had thus been accumulated, and it was found that, 
 like many other stars, Sirius had what astronomers call proper 
 motion. Comparing the place of Sirius now in the heavens with 
 the place of Sirius one hundred years ago, there is a difference 
 of about two minutes (131") in its situation. This is a small 
 quantity : it is so small that the unaided eye could not see it. 
 Were we suddenly to be transplanted back one century ago, we 
 
392 THE STORY OF THE HEAVENS. 
 
 should still see Sirius in its well-known place to the left of 
 Orion. Careful alignment by the eye would hardly detect that 
 Sirius was moving, in two, or even in three or in four centuries. 
 But the accuracy of the meridian circle grasps these minute 
 quantities, and gives to them their true significance. To the 
 eye of the astronomer, Sirius, instead of creeping along with a 
 movement which centuries will not show, is pursuing its majestic 
 course with a velocity appropriate to his dimensions. It is easy to 
 calculate that Sirius must be sweeping along at the rate of a thou- 
 sand miles a minute. 
 
 It is of the utmost importance to observe that though the 
 velocity of Sirius is about 1,000 miles a minute, yet it is sometimes 
 a little more and sometimes a little less than its mean value. To 
 the astronomer this fact is pregnant with significance. Were 
 Sirius one isolated star, attended only by planets of comparative 
 insignificance, there could be no irregularity in its motion. If it 
 were once started with a velocity of 1,000 miles a minute, then it 
 must preserve that velocity. Neither the lapse of centuries nor 
 the mighty length of the journey can alter it. The path of Sirius 
 would be inflexible in its direction ; and it would be traversed with 
 unalterable velocity. 
 
 The fact, then, that Sirius was not moving uniformly was of such 
 interest that it arrested the attention of Bessel when he discovered 
 the irregularities in 1844. Believing, as Bessel did, that there 
 must be some adequate cause for these disturbances, it was hardly 
 possible to doubt what the cause must be. When motion is 
 disturbed there must be force in action, and the only force that we 
 recognise in such cases is that known as gravitation. But gravity 
 can only act from one body to another body : so that when we seek 
 for the derangement of Sirius by gravitation, we are obliged to 
 suppose that there must be some mighty and massive body near 
 Sirius. The question was taken up again by Peters and by 
 Auwers, who were able to discover, from the irregularities of 
 Sirius, the nature of the path of the disturbing body. They were 
 able to show that this disturbing body must revolve around Sirius 
 in a period of about fifty years ; and although they could not tell 
 
THE DISTANT SUNS. 393 
 
 the actual distance of the unknown body from Sirius, yet they were 
 able to point out the direction in which it must lie. 
 
 In many respects this was a problem analogous to the ever 
 memorable discovery of the planet Neptune. In each case the 
 unknown body had been manifested by the perturbations it pro- 
 duced, and in each case the unknown object was discovered by the 
 calculations of the mathematician, before it was revealed to the 
 penetration of the astronomer. Nearly twenty years had elapsed 
 after Bessel had predicted the disturber of Sirius, before the tele- 
 scopic discovery which confirmed it was made. The circumstances 
 under which that discovery was made are not, indeed, so dramatic 
 as those which attended the discovery of Neptune, but yet they 
 have an interest of their own. In February, 1862, Messrs. Alvan 
 Clark and Sons, the celebrated telescope makers of Boston, were 
 testing a superb 18-inch object-glass, just completed for the 
 Chicago Observatory.* Turning the instrument on Sirius, for the 
 purpose of trying it, the practised eye of the younger Clark soon, 
 detected something unusual, and he exclaimed, " Why, father, the 
 star has a companion ! " The father looked, and there was a faint 
 companion due east from the bright star, and distant about ten 
 seconds. This was exactly the predicted direction of the com- 
 panion of Sirius, and yet the observers knew nothing of the pre- 
 diction. As the news of this discovery spread, many great telescopes 
 were pointed on Sirius ; and it was found that when observers knew 
 exactly where to look for the object, many instruments would show 
 it. The new companion star to Sirius lay in the true direction, 
 and it was now watched with the keenest interest, to see whether 
 it was moving in the way which calculation had assigned to the 
 body whose existence had been foretold. Four years of observation 
 showed that this was the case, so that hardly any doubt could re- 
 main. The correspondence between the observed motions and the 
 predicted motions has not since proved quite exact ; for the com- 
 panion appears to have moved about half a degree per annum more 
 rapidly than had been calculated. This difference, though larger 
 
 * This account is taken from Newcomb's " Popular Astronomy," p. 439. 
 
39i THE STORY OF THE HEAVENS. 
 
 than was expected, may be partly due to the inevitable errors of 
 the difficult observations from which the movements of the theo- 
 retical companion were computed. 
 
 Recent researches have exhibited the movements of Sirius in a 
 very unexpected way. Mr. Huggins demonstrated with his spec- 
 troscope that in 1868 Sirius was receding from the sun, while he has 
 since made the discovery, which has been confirmed at Greenwich, 
 that since 1881 Sirius has been approaching the sun ! 
 
 The discovery of the attendant of Sirius and the measures 
 which have been made thereon, give us an answer to the ques- 
 tion " What is the weight of Sirius ? " Let us attempt to 
 illustrate this subject. It must, no doubt, be admitted that the 
 numerical estimates we employ are to be received with a certain 
 degree of caution. The companion of Sirius is a difficult object 
 to observe, and the measurements require great delicacy. We are, 
 therefore, hardly as yet in a position to speak with accuracy as to 
 the periodic time in which the companion completes its revolution. 
 We shall, however, using the best observations available, take 
 this time to be forty-nine years. We also know the distance from 
 Sirius to his companion, and we may take it to be about thirty- 
 seven times the distance from the earth to the sun. It is useful, in 
 the first place, to compare the revolution of the companion around 
 Sirius with the revolution of the outermost planet, Neptune, 
 around the sun. Taking the earth's distance as unity, the radius 
 of the orbit of Neptune is about thirty, and Neptune takes 165 
 years to accomplish a complete revolution. We have no planet in 
 the solar system at a distance of thirty-seven, but from Kepler's 
 third law it is very easy to calculate that if there were such a 
 planet its periodic time must be about 225 years. We have now 
 the necessaiy materials for making the comparison between the 
 mass of Sirius and the mass of the sun. A body revolving 
 around Sirius at a certain distance completes its journey in 
 49 years. A body revolving around the sun at the same 
 distance completes its journey in 225 years. The quicker the 
 body is moving the greater must be the centrifugal force, and 
 consequently the greater must be the attractive power of the 
 
THE DISTANT SUNS. 395 
 
 central body. It can be easily shown from the principles 
 of dynamics that the attractive power varies inversely as the 
 square of the periodic time. Hence, then, the attractive power 
 of Sirius must bear to the attractive power of the sun, the pro- 
 portion which the square of 225 has to the square of 49. As 
 the distances are in each case supposed to be equal, the attractive 
 powers will be proportional to the masses, and hence we conclude 
 that the mass of Sirius, together with that of his companion, is to 
 
 ^fcfcifcWM^ """^^^^ *" "^ ^*"*"****""*** 
 
 the mass of the sun in the ratio of 20 to 1. We had already 
 learned that Sirius was much brighter than the sun ; now we have 
 learned that it is also much more massive. 
 
 Before we leave the consideration of Sirius, there is one addi- 
 tional point of considerable interest which it is necessary to con- 
 sider. It is remarkable to observe the contrast between the brilliancy 
 of Sirius and his companion. Sirius is a star far transcend- 
 ing all other stars of the first magnitude, while his companion 
 is extremely faint. Even if it were completely withdrawn from 
 the dazzling proximity of Sirins, the companion would be only 
 a small star of the eighth or ninth magnitude, far below the 
 limits of visibility of the unaided eye. To put the matter in 
 numerical language, Sirius is 5,000 times as bright as its com- 
 panion, but only about twice as heavy ! Here is a very great 
 contrast ; and this point will appear even more forcible if we 
 contrast the companion of Sirius with our sun. The companion 
 is seven times as heavy as our sun ; seven suns equal to ours in 
 one pan of the scales, would only just turn with the companion 
 in the other pan ; but in spite of its inferior bulk, our sun is much 
 more powerful as a light-giver. One hundred of the companions 
 to Sirius would not give as much light as our sun] This is a 
 result of very considerable significance. It teaches us that besides 
 the great bodies in the universe which attract attention by their 
 brilliancy, there are also other bodies of stupendous mass which 
 have but little brilliancy, probably some of them none at all. This 
 suggests a greatly enhanced conception of the majestic scale of 
 the universe. It almost invites us to the belief that the universe 
 which we behold, bears but a small ratio to the far larger part 
 
396 THE 1STORY OF THE HEAVENS. 
 
 which is invisible in the sombre shades of night. In the wide 
 extent of the material universe we have here or there a star or a 
 mass of gaseous matter sufficiently heated to become luminous, and 
 thus to become visible from the earth ; but our observation of these 
 luminous points can tell us little of the remaining contents of 
 the universe. 
 
 For the purposes of practical astronomy it has been found con- 
 venient to divide the stars into groups, according to their relative 
 degrees of brightness. In this way we denote the brightness of 
 the star by a certain number which is called the magnitude of 
 the star, and the lower the number which expresses the magni- 
 tude, the brighter is the star. Of the stars of the first magnitude, 
 which include all the brightest stars in the heavens, there are about 
 twenty. Among this number Sirius is included, though if the 
 classification were to be carried out with logical precision, a dis- 
 tinct class of exceptional brilliancy would have to be created for 
 the reception of Sirius alone. 
 
 The stars of the second magnitude are those in which there is 
 one distinct step downwards from the brilliancy of the first magni- 
 tude. The brighter stars in the constellation of the Great Bear 
 may be taken as examples. In the entire heavens we have about 
 65 stars of the second magnitude. Immediately below the second 
 magnitude we have the stars of the third magnitude, to the number 
 of 190. Next comes the fourth, 425 ; the fifth, 1,100 ; and so on 
 down to the sixth, 3,200, which completes the stars visible to the 
 unaided eye. In stars of telescopic magnitude we have the seventh, 
 to the number of about 13,000; while the eighth has 40,000, and 
 the ninth 142,000. 
 
 It will thus be seen that the number of stars increases when we 
 approach the lower magnitudes, and when we come to the magni- 
 tudes still lower than the ninth the numbers speedily swell from 
 thousands to millions. The minutest stars visible in powerful 
 telescopes are usually stated to be of the fourteenth or fifteenth 
 magnitude, while in the very greatest instruments magnitudes two 
 or three steps lower can be observed. 
 
 The number of stars visible without a telescope in England 
 
THE DISTANT SUNS. 397 
 
 may be estimated at about 3,000. Argelander has given to the 
 world a well-known catalogue of the stars in the northern hemi- 
 sphere, accompanied by a series of charts on which these stars are 
 depicted. All the stars of the first nine magnitudes are included, 
 as well as a very large number of stars lying between the ninth 
 and the tenth magnitudes. The total number of these stars is 
 324,188, and yet they are all within reach of a telescope of three 
 inches in aperture ! 
 
 Amid the hosts of stars, a considerable number specially 
 attract our attention by the peculiar changes they undergo in 
 brilliancy. They are known as variable stars; some of them 
 run through a cycle of changes in a day or two, some take many 
 months. Some appear once, and never appear again. Some are 
 conspicuous to the unaided eye, some are faint telescopic objects. 
 Though the number of variable stars is very large, yet, com- 
 pared with the ordinary fixed stars, they must be regarded as 
 very infrequent. 
 
 The most celebrated of all the variable stars is that known as 
 Algol, whose position in the constellation of Perseus is shown in 
 Fig. 70. This star is very conveniently placed for observation, 
 being visible every night in the northern hemisphere, and its 
 wondrous and regular changes can be observed without any tele- 
 scopic aid. Every one who desires to become acquainted with tke 
 great truths of astronomy should be able to recognise this star, 
 and should have also followed it during one of its periods of 
 change. Algol is usually a star of the second magnitude, but 
 in a period between two or three days, or, more accurately, in an 
 interval of 2 days 20 hours 48 minutes and 55 seconds, its 
 brilliancy goes through a most remarkable cycle of variations. 
 The series commences with a gradual decline of the star's bright- 
 ness, which in the course of three or four hours falls from the 
 second magnitude down to the fourth. At this lowest stage of 
 brightness Algol remains for about twenty minutes, and then 
 begins to increase, until in three or four hours it regains the 
 second magnitude, at which it continues for about 2 days 13 hours, 
 when the same series commences anew. It seems that the period 
 
398 THE STORY OF THE HEAVENS. 
 
 required by Algol to go through its changes is itself subject to a 
 slow but certain variation. 
 
 The claim of our sun to be admitted as a star having been 
 conceded, the question arises as to whether he shall be en- 
 titled to the distinction of being admitted to the select class of 
 variable stars, or whether he shall not take rank among the far 
 more numerous class which dispense their beams with uniformity. 
 I do not think we can have much hesitation in answering the 
 question. It is obvious that the light from our sun is for all 
 practical purposes absolutely constant. No doubt the spots and 
 the other objects 011 the sun are variable, but it would be merely 
 fantastic to speak of the sun as a variable star. It is no doubt 
 variable to the close observation of terrestrial beings who are 
 placed in its immediate vicinity ; it is not variable in the sense in 
 which we speak of Algol, or any of the other variable stars which 
 diversify the brilliancy of the sky. 
 
 That the sun is no more than a star, and the stars are no less 
 than suns, is the cardinal doctrine of astronomy. The imposing 
 magnificence of this truth is only realised when we attempt to 
 estimate the countless myriads of stars. This is a problem on 
 which our calculations are necessarily vain. Let us, therefore, 
 invoke the aid of the poet to attempt to express the innumerable, 
 and conclude this chapter with the following lines of Mr. Ailing- 
 ham : 
 
 " But number every grain of sand, 
 Wherever salt wave touches land ; 
 Number in single drops the sea ; 
 Number the leaves on every tree, 
 Number earth's living creatures, all 
 That run, that fly, that swim, that crawl. 
 Of sands, drops, leaves, and lives, the count 
 Add up into one vast amount, 
 And then for every separate one 
 Of all those, let a flaming SUN 
 Whirl in the boundless skies, with each 
 Its massy planets, to outreach 
 All sight, all thought : for all we see, 
 Encircled with infinity, 
 Is but an island." 
 
CHAPTER XX. 
 
 DOUBLE STARS. 
 
 Interesting Stellar Objects What is a Double Star ? Stars Optically Double The 
 great Discovery of tho Binary Stars made by Herschel The Binary Stars 
 describe Elliptic Paths Why is this so important ? The Law of Gravitation 
 Special Double Stars Castor Hizar The Pole Star The Coloured Double 
 Stars ft Cygni, 7 Andromedae. 
 
 THE sidereal heavens contain few more interesting objects for the 
 telescope than can be found in the numerous class of double stars. 
 They are to be counted now in thousands ; indeed, many thousands 
 can be found in the catalogues devoted to this special branch of astro- 
 nomy. Many of these objects are, no doubt, small and comparatively 
 uninteresting, but some of them are among the most conspicuous 
 stars in the heavens. We shall in this brief account select for 
 special discussion and illustration a few of the more remarkable 
 double stars. We shall particularly notice some of .those that can 
 be readily observed with a small telescope, and we have indicated 
 on the sketches of the constellations in a previous chapter how the 
 positions of these objects in the heavens can be ascertained. 
 
 In 1678, about 100 years before Herschel's observations com- 
 menced, it had been shown by Cassini that certain stars, which to 
 the unaided eye appeared single points of light, really consisted of 
 two or more stars, so close together that the telescope was required 
 for their separation. The number of these objects was gradually 
 increased by fresh discoveries, until in 1781 (the same year in 
 which Herschel discovered Uranus) a list, containing eighty double 
 stars, was published by the astronomer Bode. These interesting 
 objects claimed the attention of Herschel during his memorable 
 researches. The list of known doubles rapidly swelled. HerschePs 
 discoveries are to be enumerated by hundreds, while he also com- 
 menced systematic measurements of the distance by which the stars 
 
400 THE STORY OF THE HEAVENS. 
 
 were separated, and the direction in which the line joining them 
 pointed. It was these measurements which ultimately led to one 
 of the most important and instructive of all HerschePs discoveries. 
 When, in the course of years, his observations were repeated, 
 Herschel found that in some cases the relative position of the stars 
 had changed. He was thus led to the discovery that in many of 
 the double stars the components are so related that they revolve 
 around each other. Mark the importance of this result. We must 
 remember that the stars are suns, comparable, it may be, with our 
 sun in magnitude ; so that here we have the pleasing spectacle of a 
 pair of suns in mutual revolution. But this is not the chief point 
 of interest in this great discovery. There is nothing very surprising 
 in the fact that movements should be observed, for in all pro- 
 bability every body in the universe is in motion. It is the par- 
 ticular character of the movement which is specially interesting 
 and instructive. 
 
 It had been in the first instance supposed that the proximity of 
 the two stars forming a double was really only accidental. It was 
 thought that amid the vast host of stars in the heavens, it not 
 unfrequently happened that one star was so nearly behind another 
 that when the two were viewed in a telescope they produced the 
 effect of a double star. No doubt many of the so-called double 
 stars are produced in this way. Herschel's discovery shows that 
 this explanation will not always answer, but that in many cases 
 we really have two stars close together, and physically connected. 
 
 When the measurements of the distances and the positions 
 of double stars made during many years by many astronomers 
 had been accumulated, they were handed over to the mathema- 
 tician. There is one peculiarity about these observations : they 
 have not they cannot have the accuracy which can be obtained 
 in the determination of the places of the planets, or in meridian 
 work generally. This latter work is so accurate that almost any 
 superstructure of mathematical reasoning can be reared on its 
 foundation. But the case is very different when we come to the 
 binary stars. If the distance between the pair of stars forming a 
 binary be four seconds, the orbit we have to scrutinize is only as 
 
DOUBLE STARS. 401 
 
 large as the apparent size of a penny-piece at the distance of one 
 mile. It would require very careful measurement to make out the 
 form of a penny a mile off, even with good telescopes. If the 
 penny were tilted a little, it would appear, not circular, but oval ; 
 and it would be possible, by measuring this oval, to determine how 
 much the penny was tilted. All this requires skilful work the 
 errors, viewed intrinsically, may not be great, but viewed with 
 reference to the whole size of the quantities under consideration, 
 they are very appreciable. We therefore find the errors of obser- 
 vation far more prominent in observations of this class, than is 
 generally the case when the mathematician assumes the task of 
 discussing the labours of the observer. 
 
 The interpretation of Herschel's discovery was not accom- 
 plished by himself ; the light of mathematics was turned on his 
 observations of the binary stars by Savary, and afterwards by 
 other mathematicians. Under their searching inquiry the errors 
 of the observations were disclosed, and they were purified from 
 the grosser part of their inaccuracy. Mathematicians could then 
 apply to their corrected materials the methods of inquiry with 
 which they were familiar; they could deduce with fair precision the 
 actual shape of the orbit of the binary stars, and the position of 
 the plane in which that orbit is contained. The result is not a 
 little remarkable. It has been proved that the motion of each of 
 the stars is performed in an ellipse which contains the centre of 
 gravity of the two stars in its focus. This has been actually shown 
 to be true in many binary stars ; it is believed to be true in all. 
 But why is this so important ? Is not motion in an ellipse common 
 enough ? does not the earth revolve in an ellipse around the sun ? 
 and do not the planets also revolve in ellipses ? 
 
 It is this very fact that elliptic motion is so common in the 
 planets of the solar system which renders its discovery in binary 
 stars of such importance. From what does the elliptic motion 
 in the solar system arise? Is it not due to the law of at- 
 traction, discovered by Newton, which states that every mass 
 attracts every other mass with a force which varies inversely as 
 the square of the distance? That law of attraction had been 
 
 A A 
 
402 THE STORY OF THE HEAVENS. 
 
 found to pervade the whole solar system, and it explained the 
 movements of the bodies of our system with marvellous fidelity. 
 But the solar system, consisting of the sun, and the planets, with 
 their satellites, the comets, and a host of smaller bodies, formed 
 merely a little island group in the universe. In the economy of 
 this tiny cosmical island the law of gravitation reigns supreme ; 
 before Herschel's discovery we never could have known whether 
 that law was not merely a piece of local legislation, specially 
 contrived for the exigencies of our particular system. This dis- 
 covery gave us the knowledge which we could have gained from 
 no other source. From the binary stars came a whisper across 
 the vast abyss of space. That whisper told us that the law 
 of gravitation was not peculiar to the solar system. It told us 
 the law extended to the distant shores of the abyss in which our 
 island is situated. It gives us grounds for believing that the law 
 of gravitation is obeyed throughout the length, the breadth, the 
 depth, and the height of the entire visible universe. 
 
 One of the finest binary stars is that known as Castor, the 
 brighter of the two principal stars in the constellation of Gemini. 
 The position of Castor on the heavens is indicated in Fig. 73, page 
 382. Viewed by the unaided eye, Castor resembles a single star ; 
 but with a moderately good telescope it is found that what seems 
 to be one star is really two separate stars, one of which is of the 
 third magnitude, while the other is somewhat less. The angular 
 distance of these two stars in the heavens is not so great as 
 the angle subtended by a line an inch long viewed at a distance 
 of half-a-mile. Castor is one of the double stars in which the 
 components have been observed to possess a motion of revolution. 
 The movement is, however, extremely slow, and the lapse of cen- 
 turies will be required before a revolution is completely effected. 
 
 A very beautiful double star can be readily identified in the 
 constellation of Ursa Major (see Fig. 75, page 384). It is known 
 as Mizar, and is the middle star (f ) of the three which form the tail. 
 In the close neighbourhood of Mizar is the small star Alcor, which 
 can be readily seen with the unaided eye ; but when we speak of 
 Mizar as a double star, it is not to be understood that Alcor is one 
 
DOUBLE STARS. 403 
 
 of the components of the double. In the magnifying power of the 
 telescope Alcor is seen to be transferred a long way from Mizar, 
 while Mizar itself is split up into two suns close together. These 
 components are of the second and the fourth magnitudes respectively, 
 and as the apparent distance is nearly three times as great as in 
 Castor, they are observed with the greatest facility even in a small 
 telescope. This is, indeed, the best double star in the heavens for 
 the beginner to commence his observations upon. We cannot, 
 however, assert that Mizar is a binary star, inasmuch as observa- 
 tions have not yet established the existence of a motion of revolu- 
 tion. Still less are we able to say whether Alcor is also a member 
 of the same group, or whether it may not merely be a star which 
 happens to fall nearly in the line of vision. 
 
 Another object of considerable interest is the remarkable double- 
 double star found in the constellation Lyra, and usually known as 
 e Lyra3 (see Fig. 77, page 386). In this case the unaided eye 
 suffices to show a pair of stars so close that they can only just be 
 distinguished. When a telescope is applied to the object, the stars 
 are, of course, at once widely opened, and then the interesting ob- 
 servation is made that each of these is itself composed of two stars 
 extremely close together. The entire object thus consists of four 
 stars in two fine binary pairs, and in each of these pairs revolution 
 is taking place. It is difficult to resist the conclusion that the 
 two pairs are connected together, and each revolving, in some 
 period of stupendous duration, around the common centre of gravity 
 of the two. 
 
 The Pole Star is also an interesting double, but it differs widely 
 from the objects already mentioned, and must be regarded as a 
 representative of that class of doubles in which the two components 
 are of very unequal brilliancy. The Pole Star itself is between the 
 second and third magnitude; its companion is, however, only of 
 the ninth magnitude. This object requires a better telescope than 
 is sufficient for the others we have mentioned ; in fact, it is some- 
 times regarded as a suitable " test " for the performance of a small 
 instrument of from two to three inches in diameter. 
 
 A very pleasing class of double stars are those in which we 
 
 A A 2 
 
404 THE STORY OF THE HEAVENS. 
 
 have the remarkable phenomena of colours,, differing in a striking 
 degree from the colours of ordinary stars. Among the latter 
 we find in the great majority of cases, no very characteristic 
 hue ; some are, however, more or less tinged with red, some are 
 decidedly ruddy, and some are intensely red. Stars of a bluish or 
 greenish colour are much more rare,* and when a star of this 
 character does occur, it is almost invariably as one of a pair which 
 form a double. The other star of the double is sometimes of the 
 same hue, but more usually it is yellow or ruddy. 
 
 One of the loveliest of these objects, and one of those which 
 are fortunately within reach of telescopes of very moderate preten- 
 sions, is that found in the constellation of the Swan, and known as 
 ft Cygni (Fig. 78, p. 387). This exquisite object is composed of 
 two stars. The larger, about the third magnitude, is of a golden- 
 yellow, or topaz, colour ; the smaller, of the fifth magnitude, is of a 
 light blue. These colours are nearly complementary, but still there 
 can be no doubt that the effect is not merely one of contrast. It is 
 indeed the fact that these two stars are both tinged with the hues 
 we have stated, as can be shown by hiding each in succession behind 
 a bar placed in the field of view. It has also been confirmed in 
 a very striking manner by spectroscopic investigation ; for Mr. 
 Huggins has shown that the blue star has experienced a special 
 absorption of the red rays, while the more ruddy light of the other 
 star has arisen from the absorption of the blue rays. The contrast 
 of the colours in this object can often be very effectively seen by 
 putting the eye-piece out of focus. The discs thus produced show 
 the contrast of colours better than when the object exhibits merely 
 two stellar points. 
 
 Another rather more difficult coloured double star is y Andro- 
 medaB (see Fig. 69, page 376). The larger star of the third magni- 
 tude is a deep yellow colour; the other, of the fifth magnitude, 
 has a greenish hue. The interest of this object has been greatly 
 
 * Perhaps if we could view the stars without the intervention of the atmos- 
 phere, blue stars would he more common. The absorption of the atmosphere 
 specially affects the greenish and bluish colours. Professor Langley gives us good 
 reason for believing that the sun itself would be blue if it were not for the effect of 
 the air. 
 
DOUBLE STARS. 405 
 
 increased by the discovery that the blue star, the smaller of the 
 two, is itself composed of two minute stars, so close together that 
 their separation is quite beyond the reach of ordinary telescopes ; 
 instruments of considerable size are required to separate them. 
 7 Andromedse is, therefore, to be described as a triple star rather 
 than as a double. There are several instances of such complex 
 systems. One of the most interesting triples is found in the constel- 
 lation of the Crab, where it is known as f Cancri. It will be readily 
 identified at one-sixth of the distance from Prsesepe to Betelgueze. 
 The brightest star is of the fifth magnitude, while the star near it 
 is between the fifth and the sixth. They have been watched through 
 the course of one complete revolution in a period a little more than 
 sixty years. Their comparatively distant companion of intermediate 
 brightness has only accomplished about one-twelfth of a circuit in 
 that interval. The recent researches of M. Seeliger have added 
 to the complexity of this system by demonstrating with a high 
 degree of probability the existence of a dark attendant circulating 
 around the third star in about eighteen years. 
 
 Such are a few of these double and multiple stars. Their 
 numbers are being daily augmented ; indeed one observer Mr 
 Burnham, of Chicago has within the last ten or twelve years 
 added by his own researches no fewer than 1,000 new doubles to 
 the list of those previously known. 
 
 The interest in this class of objects must necessarily be increased 
 when we reflect that, small as these stars appear to be in our tele- 
 scopes, they are in reality suns of great size and splendour, in many 
 cases rivalling our own sun, or, perhaps, even surpassing him. 
 Whether these suns have planets attending upon them we cannot 
 tell; the light reflected from the planet would be utterly inade- 
 quate to the penetration of the vast extent of space which separates 
 us from the stars. If there be planets surrounding these objects, 
 then, instead of a single sun, such planets will be illuminated by 
 two, or, perhaps, even more suns. What wondrous effects of light 
 and shade must be the result ! Sometimes both suns will be above 
 the horizon together, sometimes only one sun, and sometimes 
 both will be absent. Especially remarkable would be the condi- 
 
406 THE STORY OF THE HEAVENS. 
 
 tion of a planet whose suns were of the coloured type. To-day 
 we have a red sun illuminating the heavens, to-morrow it would 
 be a blue sun, and, perhaps, the day after both the red sun and the 
 blue sun will be in the firmament together. What endless variety 
 of scenery such a thought suggests ! There are, however, grave 
 dynamical reasons for doubting whether the conditions under which 
 such a planet would exist could be made compatible with life in 
 any degree resembling the life with which we are familiar. The 
 problem of the movement of a planet under the influence of two 
 suns is one of the most difficult that has ever been proposed to 
 mathematicians, and it is, indeed, impossible in the present state 
 of analysis to solve with accuracy all the questions which it 
 implies. It seems not at all unlikely that the disturbances of 
 the planet's orbit would be so great, that it would be exposed 
 to vicissitudes of light and of temperature far transcending those 
 experienced by a planet moving, like the earth, under the supreme 
 control of a single sun. 
 
CHAPTER XXI. 
 
 THE DISTANCES OF THE STARS. 
 
 Sounding-line for Space The Labours of W. Herschel His Reasonings Illustrated 
 by Vega Suppose this Star to recede 10 times, 100 times, or 1,000 times 
 Some Stars 1,000 times as far as others Herschel's Method incomplete The 
 Labours of Bessel Meaning of Annual Parallax Minuteness of the Parallactic 
 Ellipse Illustrated The case of 61 Cygni Different Comparison Stars 
 used Difficulty owing to Refraction How to be avoided The Proper Motion 
 of the Star Bessel's Preparations The Heliometer Struve's Investigations 
 Can they be Reconciled? Researches at Dunsink Conclusion obtained 
 Accuracy which such Observations admit Examined How the Results 
 are Discussed The Proper Motion of 61 Cygni The Permanence of 
 the Sidereal Heavens Changes in the Constellation of the Great Bear 
 since the time of Ptolemy The Star Groombridge 1,830 Large Proper 
 Motion Its Parallax Velocity of 200 Miles a Second -The New Star in 
 Cygnus Its History No Appreciable Parallax A Mighty Outburst of Light 
 The Movement of the Solar System through Space Herschel's Discovery 
 Journey towards Hercules Probabilities Conclusion. 
 
 WE have long- known the dimensions of the solar system with 
 more or less accuracy. Our knowledge includes the distances of 
 the planets and the comets from the sun, as well as their move- 
 ments. We have also considerable knowledge of the diameters 
 and the masses of many of the different bodies which belong to 
 the solar system. We have long known, in fact, many details of 
 the isolated group nestled together under the protection of the 
 sun. The problem for consideration in the present chapter involves 
 a still grander survey than is required for measures of our solar 
 system. We propose to carry the sounding-line across the vast abyss 
 which separates the group of bodies closely associated about our sun, 
 from the other stars which are scattered through the realms of space. 
 For centuries the great problem of star distance has engaged the 
 attention of those who have studied the heavens. It would be im- 
 possible to attempt here even an outline of the various researches 
 which have been made on the subject. In the limited survey which 
 
408 THE STORY OF THE HEAVENS. 
 
 we can make, we must glance first at the remarkable speculative 
 efforts which have been directed to the problem, and then we shall 
 refer to those labours which have introduced the problem into the 
 region of accurate astronomy. 
 
 It was Herschel who first attempted, in a scientific manner,, 
 to give any accurate conception of the extent and magnitude of 
 the sidereal system. There was in particular one form of specula- 
 tion which Herschel employed with great success, for it possesses 
 very strong claims on our imagination. It would, indeed, have 
 been idle, or worse than idle, to indulge in speculation whenever 
 the means of certainly ascertaining the truth were available. This 
 Herschel never did. But when a majestic problem was incessantly 
 being proposed which he could find no means of certainly solving, 
 what course was he to adopt ? He was entitled to attempt its solu- 
 tion by analogy and by probability, in default of more trustworthy 
 guides. 
 
 We now know the distances of a few of the stars, perhaps of 
 twenty or thirty, with more or less accuracy ; but of the distances 
 of the great majority we are still ignorant, while of the thou- 
 sands of nebula? we have not yet found the distance of even a 
 single one. Our knowledge of the distances of the sidereal objects 
 is therefore scanty in the extreme, but the little that we do know 
 is vastly more than Herschel knew. It was not until many years 
 after Herschel's death that the great problem of finding the 
 distance of a star was first solved by Bessel. 
 
 During Herschel's celebrated sweeps of the sidereal heavens, 
 hundreds of thousands of objects perhaps we might say millions 
 passed before his view. We do not now refer to the members of 
 the solar system, the comets, or the planets and their satellites : we 
 are speaking solely of the sidereal heavens, whose denizens are 
 vastly more numerous, including the stars, the clusters, and the 
 nebulre. All this countless host passed before the review of 
 Herschel, yet there was not a single one of all these objects of 
 which he knew the distance. 
 
 To so enthusiastic an astronomer as Herschel the questions 
 " How far ? " and " How big ? " were ever suggested during his 
 

 TILE DISTANCES OF THE STARS. 409 
 
 nightly watches. He first exhausted all the means he could think 
 of to obtain an answer to these questions, and. having failed, he 
 then but not until then sought to ascertain by reasoning, an 
 answer to those problems which his observations had failed to 
 determine. Herschel's reasoning on this point is most plausible ; 
 and we now make use of it to supplement and to extend the positive 
 knowledge of the distances of the stars which has been acquired from 
 observation. It is by this combination that we seek to obtain an 
 adequate conception of the scale on which the vast fabric of the 
 universe is constructed. It would be useless to attempt to enter 
 into any detail on this point; let it suffice to give a single 
 illustration of the kind of reasoning which Herschel employed in 
 his speculations. 
 
 Take, for instance, the beautiful star Vega, or a Lyra?, the 
 brightest star in the constellation of Lyra, and, with but few 
 exceptions, the brightest star in the whole sky. To this star let us 
 apply the arguments of Herschel. We shall proceed to sound the 
 depths of the universe to which the telescope can penetrate, and we 
 shall use the distance of Vega as the unit of our measurement. 
 Herschel has shown how we can estimate the distances of the 
 faintest stars, not only of those which are visible to the unaided 
 eye, but even of the very faintest and most remote stars which can 
 be shown in the most powerful telescope. 
 
 Imagine Vega to be moved away from the earth; then, as it 
 receded further and further from our view its brilliancy would 
 gradually decline. Speed on the journey, nor let a halt be made 
 until Vega has moved to a distance which is ten times its present 
 distance. The brilliancy has now decreased so much that Vega 
 is no longer one of the brightest stars in the heavens. At ten 
 times its present distance from the earth it would have sunk in 
 brilliancy to a star of the sixth magnitude, and would only be just 
 visible to the unaided eye. 
 
 Let Vega again resume its journey. It will now become 
 invisible to the unaided eye, but the telescope can still follow the 
 star as its distance rapidly increases. Let us not make another 
 halt until the distance has again increased tenfold, and has become 
 
410 THE STORY OF THE HEAVENS. 
 
 100 units. Yet even at this vast distance Vega can still be seen with 
 a telescope of moderate pretensions. Once again let the star start 
 on its voyage. Let it travel tenfold the distance which it had 
 hitherto attained ; and now, for the last time, let the wanderer stop 
 when it has plunged into space to a distance 1,000 times as great 
 as it was at first. It has long passed from the ken of the ordinary 
 telescope, but it would still remain within view of the colossal 
 instruments of which there are now so many in the world. 
 Herschel himself said that his telescopes could pursue the star 
 until it had retreated 900 times as far as it was at first. Telescopes 
 have been much improved since his time, so that we shall not 
 exaggerate if we take 1,000 as the present limit of visibility. In 
 all this there is no assumption whatever ; we shall have to make 
 an assumption presently, but we have not made it yet. There can 
 be no doubt that if Vega were moved to ten times its distance it 
 would still be faintly visible to the unaided eye. There can be no 
 doubt that if Vega were moved 100 times as far, it would still be 
 visible in small telescopes; while if it were 1,000 times as far, it 
 would even then not have passed beyond the scrutiny of the grand 
 instruments now in use. 
 
 In coming to the application of these principles, we here for the 
 first time make an assumption. Herschel at this point adopted a 
 sort of statistical reasoning, in which he deals with the stars, 
 not in units, or even tens, but in thousands and millions. No one 
 will suppose for a moment that the stars are all the same size : in 
 fact, we know perfectly well that there is a vast difference in the 
 actual sizes of some of the stars ; but when we deal with the stars 
 in their thousands, we may assume some principles which are, at 
 ail events, intelligible. We must, however, apprise the reader that 
 these results are to be received with caution. They have not been 
 demonstrated by observation, they cannot be demonstrated by cal- 
 culation. They are plausible, but nothing more. 
 
 In the absence of all information to the contrary, Herschel 
 thought it would be reasonable to assume that the average sizes of 
 the stars distributed far and wide through space were the same as 
 the average sizes of the stars with which we are familiar. He 
 
THE DISTANCES OF THE STABS. 411 
 
 saw clearly that those stars which are brightest would seem the 
 faintest of all stars if they were moved ],000 times as far, but 
 there is no reason to suppose that the faint stars which we do see 
 are intrinsically smaller than the bright ones around us. All would 
 be explained perfectly if we suppose that the average size of the 
 stars in different regions of space is the same, and if we made the 
 further supposition that the faintest stars are, as a general rule, 
 about 1,000 times as far off as the brightest. 
 
 Such was the principle of HerscheFs method of probing the 
 heavens ; but it was incomplete. He saw that the relative distances 
 of the faint stars and the bright stars must be as we have described, 
 but what was the absolute value of those distances ? This was the 
 problem which baffled Herschel all his life. He never succeeded in 
 finding the actual distance of one of the stars from the earth. He 
 tried it in the early part of his career; it was, in fact, while he 
 was so engaged that he discovered Uranus. He tried it sub- 
 sequently in a different manner, and again he was unsuccessful in 
 his immediate object, though the observations led to the splendid 
 discovery of the binary stars. The problem has been solved since 
 HerscheFs time, and we have been able to understand why he was 
 frustrated. The distances are so great that the observations required 
 are of the utmost delicacy. HerscheFs great instruments, though of 
 vast space-penetrating power, were wanting in the nice refinements 
 of measurement necessary for this purpose. But now that the dis- 
 tances of some of the stars have been ascertained, we can make use 
 of HerscheFs reasoning to determine the distances ef the others 
 with some degree of plausibility. 
 
 No attempt to solve the problem of the absolute distances of 
 the stars was successful until many years after HerscheFs labours 
 were closed. Fresh generations of astronomers, armed with fresh 
 appliances, have for many years pursued the object with un- 
 remitting diligence, but for a long time the effort seemed hope- 
 less. The distances of the stars were so great that they 
 could not be ascertained until the utmost refinements of mecha- 
 nical skill and the most elaborate methods of mathematical 
 calculation were brought to converge on the difficulty. At last 
 
412 THE STORY OF THE HEAVENS. 
 
 it was found that the problem was beginning to yield. A few 
 stars have been induced to disclose the secret of their distance. 
 We are able to give some answer to the question How far are 
 the stars ? though it must be confessed that our reply up to the 
 present moment is both hesitating and imperfect. Even the little 
 knowledge which has been gained, possesses interest and im- 
 portance. As often happens in similar cases, the discovery of 
 the distance of a star was made independently about the same 
 time by two or three astronomers. The name of Bessel stands 
 out conspicuously in this memorable chapter of astronomy. Bessel 
 proved (1840) that the distance of the star known as 61 Cygni 
 was a measurable quantity. His demonstration possessed such 
 unanswerable logic that universal assent could not be withheld. 
 Almost simultaneously with the classical labours of Bessel, we have 
 Struve's measurement of the distance of Vega, and Henderson's 
 determination of the distance of the southern star a Centauri. Great 
 interest was excited in the astronomical world by these discoveries, 
 and the Royal Astronomical Society awarded its gold medal to 
 Bessel. It appropriately devolved on Sir John Herschel to deliver 
 the address on the occasion of the presentation of the medal : that 
 address is a most eloquent tribute to the labours of the three 
 astronomers. We cannot resist quoting the few lines in which Sir 
 John said : 
 
 f< Gentlemen of the Royal Astronomical Society, I con- 
 gratulate you and myself that we have lived to see the great and 
 hitherto impassable barrier to our excursion into the sidereal 
 universe, that barrier against which we have chafed so long and so 
 vainly cestuantes angusto Umite mnndi almost simultaneously 
 overleaped at three different points. It is the greatest and most 
 glorious triumph which practical astronomy has ever witnessed. 
 Perhaps I ought not to speak so strongly ; perhaps I should hold 
 some reserve in favour of the bare possibility that it may be all 
 an illusion, and that future researches, as they have repeatedly 
 before, so may now fail to substantiate this noble result. But I 
 confess myself unequal to such prudence under such excitement. 
 Let us rather accept the joyful omens of the time, and trust that. 
 
THE DISTANCES OF THE STARS. 413 
 
 as the barrier has begun to yield, it will speedily be effectually 
 prostrated." 
 
 Before proceeding further, it will be convenient to explain 
 briefly how it is possible for the distance of a star to be measured. 
 The problem has to be approached in a very different manner to 
 that of the sun's distance, which we have already discussed in 
 these pages. The observations for the determination of stellar 
 parallax are founded on the familiar truth that the earth revolves 
 around the sun. We may for our present purpose assume that 
 
 Jan. 
 
 July 
 
 Fig. 79. The ParaUactic Ellipse. 
 
 the earth revolves in a circular path. The centre of that path is 
 at the centre of the sun, and the radius of the path is 92,700,000 
 miles. Owing to our position on the earth, we observe the stars 
 from a point of view which is constantly changing. In summer 
 the earth is 185,400,000 miles distant from the position which 
 it had in winter. It follows that the apparent positions of 
 the stars, as projected on the background of the sky, must present 
 corresponding changes. We do not now mean that the actual 
 positions of the stars are really displaced. The changes are only 
 apparent, and while oblivious of our own motion, which produces 
 the displacements, we attribute the changes to the stars. 
 
 On the diagram in Fig. 79 is an ellipse with certain months 
 viz., January, April, July, October marked upon its circum- 
 
414 THE STORY OF THE HEAVENS. 
 
 ference. This ellipse may be regarded as a miniature picture of 
 the earth's orbit around the sun. In January the earth is at the 
 spot so marked ; in April it has moved a quarter of the whole 
 journey ; and so on round the whole circle, returning to its original 
 position in the course of one year. When we look from the position 
 of the earth in January, we see the star A projected against the 
 point of the sky marked 1. Three months later, the observer 
 with his telescope is carried round to April ; but he now sees the 
 star projected to the position marked 2. Thus, as the observer 
 moves around the whole orbit in the annual revolution of the 
 earth, so the star appears to move round in an ellipse on the 
 background of the sky. In the technical language of astronomers, 
 we speak of this as the parallactic ellipse, and it is by measuring 
 the major axis of this ellipse that we determine the distance of the 
 star from the sun. 
 
 The figure shows another star, B, more distant from the earth 
 and the solar system generally than the star previously considered. 
 This star also describes an elliptic path. We cannot, however, 
 fail to notice that the parallactic ellipse belonging to B is much 
 smaller than that of A. The difference in the sizes of the ellipses 
 arises from the different distances of the stars from the earth. 
 The nearer the star is to the earth the greater is the ellipse, so 
 that the nearest star in the heavens will describe the largest 
 ellipse, while the most distant star will describe the smallest 
 ellipse. We thus see that the distance of the star is inversely pro- 
 portional to the size of the ellipse, and if we measure the angular 
 value of the major axis of the ellipse, then, by a manipulation which 
 every mathematician understands, the distance of the star can be 
 expressed as a multiple of a radius of the earth's orbit. Assuming 
 that radius to be 92,700,000 miles, the distance of the star is 
 obtained by simple arithmetic. The difficulty in the process 
 arises from the fact, that these ellipses are so small that our micro- 
 meters often fail to detect them. Take, for example, the Great 
 Bear. Each of the stars in the Great Bear describes an ellipse in 
 the course of a year. It would, however, be quite an exaggeration 
 to suppose that the actual shape and position of the Great Bear is 
 

 THE DISTANCES OF THE STARS. 415 
 
 appreciably distorted by parallax. Indeed, the dimensions of the 
 ellipses have an almost immeasurably small ratio to the dimensions 
 of the constellation. 
 
 How shall we adequately describe the extreme minuteness of 
 the parallactic ellipses in the case of even the nearest stars ? In the 
 technical language of astronomers, we may state that the longest 
 diameter of the ellipse can never subtend an angle of more than 
 two seconds. In a somewhat more popular manner, we would 
 say that one thousand times the major axis of the very largest 
 parallactic ellipse would not greatly exceed the diameter of the 
 full moon. For a still more simple illustration, let us endeavour 
 to think of a penny-piece placed at a distance of two miles. If 
 looked at edgeways it will be linear, if tilted a little it would be 
 elliptic ; but the ellipse would, even at that distance, be greater 
 than the greatest parallactic ellipse of any star in the sky. Sup- 
 pose a sphere described around an observer with a radius of two 
 miles. If a penny-piece were placed on this sphere, in front of 
 each of the stars, every parallactic ellipse would be totally 
 concealed. 
 
 The star in the Swan known as 61 Cygni is not remark- 
 able either for its size or for its brightness. It is barely visible 
 to the unaided eye, and there are some thousands of stars which 
 are apparently larger and brighter. It is, however, a very 
 interesting example of that remarkable class of objects known as 
 double stars. It consists of two nearly equal stars close together, 
 and evidently connected by a bond of mutual attraction. The 
 attention of astronomers is also specially directed towards the star 
 by its large proper motion. In virtue of that proper motion, the 
 two components are carried together over the sky at the rate of 
 nve seconds annually. A proper motion of this magnitude is 
 extremely rare, yet we do not say it is unparalleled, for there are 
 some few stars which have a proper motion even more rapid; 
 but the remarkable duplex character of 61 Cygni, combined with 
 the large proper motion, render it an unique object, at all events, 
 in the northern hemisphere. 
 
 When Bessel proposed to undertake the great research with 
 
416 THE STORY OF THE HEAVENS. 
 
 which his name will be for ever connected, he determined to devote 
 one, or two, or three years to the continuous observations of one 
 star, with the view of measuring carefully its paral lactic ellipse. 
 How was he to select the object on which so much labour was to be 
 expended? It was all important to choose a star which should 
 prove sufficiently near to reward his efforts by exhibiting a mea- 
 surable parallax. Yet he could have but little more than surmise 
 
 BESSEL SOUTH 
 
 -' 5 BESSEL 
 
 1838 **" / 
 
 1853* : /' 
 
 1 1878* 
 
 NORTH BRUNNOW 
 
 Fig. 80. 61 Cygni and the Comparison Stars. 
 
 and analogy as a guide. It occurred to him that the exceptional 
 features of 61 Cygni afforded the necessary presumption, and he 
 determined to apply the process of observation to this star. . He 
 devoted the greater part of three years to the work, and succeeded 
 in discovering its distance from the earth. 
 
 Since the date of Sir John HerschePs address, 61 Cygni has 
 received the devoted and scarcely remitted attention of astronomers. 
 In fact, we might say that each succeeding generation undertakes 
 a new discussion of the distance of this star, with the view of con- 
 firming or of criticising the original discovery of Bessel. The 
 diagram here given (Fig. 80) is intended to illustrate the recent 
 history of 61 Cygni. 
 
THE DISTANCES OF THE STARS. 417 
 
 When Bessel engaged in his labours, the pair of stars form- 
 ing the double were at the point indicated on the diagram 
 by the date 1838. The next epoch occurred fifteen years later, 
 when the younger Struve undertook his researches, and the pair 
 of stars had by that time moved to the position marked 1853. 
 Finally, when the same object was more recently observed at 
 Dunsink Observatory, the pair had made still another advance to 
 the position indicated by the date 1878. Thus, in forty years this 
 double star had moved over an arc of the heavens upwards 
 of three minutes in length. The actual path is, indeed, more 
 complicated than a simple rectilinear movement. The two stars 
 which form the double have a certain relative velocity, in con- 
 sequence of their mutual attraction. It will not, however, be 
 necessary to take this into account, as the displacement thus 
 arising in the lapse of a single year is far too minute to produce 
 any inconvenient effect on the parallactic ellipse. 
 
 There is an ever present enemy with which the parallax seeker 
 has to contend. That enemy is the atmosphere, which, even when 
 free from clouds and fogs, is a source of uncertainty and of error. 
 On the clearest and finest night, in the most superb climate, the 
 refraction of the atmosphere distorts each star from its true place. 
 The direction in which the telescope must be pointed to see the 
 star is not the true direction; it is not the direction in which the 
 telescope would be pointed were the atmosphere absent. The 
 grosser part of this distortion is, no doubt,* amenable to calcu- 
 lation ; it can be allowed for, and the true place of the star can 
 be approximately ascertained. The amount of refraction under 
 specified circumstances, as to the elevation of the object above 
 the horizon, the temperature of the air and the height of the 
 barometer, admits of being calculated with an accuracy sufficient for 
 many purposes in astronomy ; but the calculated result is still only 
 an approximation to the truth. There are minute irregularities in 
 the refraction which render its amount so variable that we cannot 
 entirely eliminate its effects, and the corrected places of the stars 
 may, from this cause alone, be occasionally erroneous to the extent 
 of a second, or even more. For the present purpose errors of this 
 B B 
 
418 THE STORY OF THE HEAVENS. 
 
 magnitude are fatal. The parallactic ellipse which we want to 
 measure is, perhaps, itself not a second in diameter. If, therefore, 
 our observations are wrong by a large fraction of a second, the 
 errors would bear such a large proportion to the total quantity that 
 success would be hopeless. 
 
 There is, however, one way, and probably only one way, of 
 evading the difficulty. It is founded on the fact which we have 
 endeavoured to illustrate by the two stars shown in Fig. 79. A 
 star which is very distant has a small parallactic ellipse ; a star 
 which is near has a comparatively large ellipse. If, therefore, two 
 stars can be seen in the same field of view of the telescope, and if 
 one is much more remote than the other, the sizes of the parallactic 
 ellipses will be very different. On the diagram, Fig. 80, four stars 
 are indicated besides 61 Cygni; two of these bear the name of 
 Bessel, one bears the name of Struve, and the fourth has the name of 
 Briinnow. These stars are small telescopic objects : they are merely 
 units of the vast host with which the whole heavens teem. They 
 are much inferior in brightness to 61 Cygni; they have not its 
 remarkable duplicity, they have not its remarkable proper motion. 
 They are more remote from the earth than 61 Cygni, which we 
 are to suppose standing out boldly in the foreground, while the 
 others are the distant objects in the picture. The parallactic ellipse 
 described by 61 Cygni is therefore larger than that described 
 by these other stars. Bessel proposed to measure the apparent 
 angular distance from 61 Cygni to the stars marked with his 
 name at every available opportunity. This distance must be 
 constantly changing by the effect of parallax. If the reader 
 have been imbued with geometrical instinct, he will understand 
 how from these measures the form and the position of the parallactic 
 ellipse can be certainly ascertained. 
 
 It may be urged that the refraction must surely derange these 
 measures also. No doubt each of the stars is greatly displaced by 
 refraction, but fortunately, they are both displaced by almost the 
 same amount. The distance between the two stars is thus nearly 
 the same as it would be if refraction were entirely absent, and the 
 minute outstanding difference can be accurately allowed for. By 
 
THE DISTANCES OF THE STARS. 419 
 
 this happy contrivance for eliminating- the effect of refraction, the 
 research for annual parallax is brought within the grasp of practical 
 astronomy. The measures of the angular distance, though thus 
 purged from refraction, are, however, still complicated by the proper 
 motion of 61 Cygni. The movement of the star in its parallactic 
 ellipse becomes amalgamated with the rectilinear movement due to 
 proper motion. This circumstance will not embarrass the expe- 
 rienced computer. He is able to disentangle the composite move- 
 ment, and determine with accuracy the elliptic and the rectilinear 
 components. 
 
 Bessel commenced the preparations for his great work in a 
 somewhat startling- manner. He ordered the opticians to cut the 
 object-glass of his telescope in half. One-half was rigidly fixed in 
 the tube in its proper position, the other half was mounted on 
 a slide, so that its movement was rectilinear, and at right angles 
 to the axis of the telescope. An instrument so treated constitutes 
 a heliometer. When directed towards a star, each half of the 
 object-glass forms a distinct image of the star. If the halves 
 be in their ordinary position the two images of the star are 
 coincident, and the telescope performs like one of the usual type. 
 As, however, the movable part of the object-glass changes its 
 position the two images separate, and the distance between the 
 images is equal to the distance through which the half object-glass 
 has been moved. The movement is effected by a screw of delicate 
 workmanship, so that the distance admits of being determined with 
 accuracy. 
 
 The application of the heliometer to the present problem will 
 be easily understood. The telescope is directed to 61 Cygni, 
 so that the comparison star shall be also in the field, and then 
 as the two halves of the object-glass separate so do the two 
 images of the comparison star. Gradually the screw is turned 
 until one of these two images is brought exactly to the central point 
 between the two components of 61 Cygni. The distance through 
 which the half object-glass has been displaced is then to be read 
 off, and it will represent the apparent distance from the centre of 
 61 Cygni to the comparison star. For three years Bessel devoted 
 B B 2 
 
420 . THE STORY OF THE HEAVENS. 
 
 himself to these measurements ; and the result of his observations 
 was the discovery and the measurement of the parallactic ellipse. 
 It was, indeed, but a small object ; it only looked as large as 
 a penny-piece six or seven miles off. But as he found the same 
 ellipse from each of the two comparison stars independently, and 
 as he had exhausted every refinement which consummate skill could 
 suggest, his labours elicited the glowing language of Herschel 
 which has been already cited. 
 
 Fifteen years later (1853) the celebrated Russian astronomer, 
 Otto Struve, undertook the labour of a new determination of the 
 distance of 61 Cygni. He determined to vary the method of 
 observation employed by Bessel in every possible detail. Bessel had 
 measured the distance of the central point between the components 
 of 61 Cygni from his comparison stars. Struve chose a single com- 
 ponent of 61 Cygni and a comparison star marked on the figure by 
 his name, which is different from either of those used by Bessel. 
 Struve also rejected the divided object-glass, and used instead the 
 instrument previously described in this work and known as the 
 parallel wire micrometer. With this he measured the distance and 
 the position angle of the comparison star from the upper com- 
 ponent of 61 Cygni for a period of one year. Struve's labours 
 were also rewarded with success, and he was enabled to obtain 
 the parallactic ellipse from his measurements of the distance, while 
 he obtained an independent determination from his measures of the 
 position angle. The two investigations led to identical results. 
 
 Struve and Bessel had thus each shown that the parallactic 
 ellipse and therefore the distance of 61 Cygni was a measurable 
 quantity, but there was a substantial difference between their 
 results. Struve' s determination indicated that 61 Cygni was much 
 nearer to us than BessePs work would have led us to suppose. 
 Bessers distance is, indeed, half as much again as Struve's. Bessel 
 concluded that the distance was about sixty billions of miles ; Struve 
 thought it could not be more than forty billions of miles. There 
 was thus a discrepancy between the two astronomers of twenty 
 billions of miles. Is a discrepancy of twenty billions of miles 
 so very serious? No doubt, viewed by our terrestrial standards, 
 
THE DISTANCES OF THE STARS. 421 
 
 the amount is enormous. It would take about 300,000 years 
 even to count twenty billions. But the discrepancy is not so 
 very significant when it has occurred in these colossal magni- 
 tudes, the stellar distances. We do not know the parallax of 
 even a dozen of stars well enough to specify their distance 
 accurately to a distance of within twenty billions of miles. A 
 parallax-seeker would feel contented and satisfied with his work 
 if he could be sure that his error was well within this limit. 
 
 The case of 61 Cygni is, however, exceptional. It is one of our 
 nearest neighbours in the heavens. We can never find its distance 
 accurately to one or two billions of miles ; but still we have a con- 
 sciousness that an uncertainty amounting to twenty billions is too 
 large. We shall presently show that we believe Struve was right, yet 
 it does not necessarily follow that Bessel was wrong. The apparent 
 paradox can be easily explained. It would not be easily explained 
 if Struve had used the same comparison star as Bessel had done ; 
 but Struve's comparison star was different from either of BesseFs, 
 and this is probably the cause of the discrepancy. It will be recol- 
 lected that the whole essence of the process consists of the compari- 
 son of the small ellipse made by the distant star, with the larger 
 ellipse made by the nearer star. If the two stars were at the same 
 distance, the process would be wholly inapplicable. In such a case, 
 no matter how near the stars were to the earth, no parallax could 
 be detected. For the method to be completely successful, the com- 
 parison star should be at least eight times as far as the principal 
 star. Bearing this in mind, it is quite possible to reconcile the 
 measures of Bessel with those of Struve. We need only assume 
 that BessePs comparison stars are about three times as far as 
 6L Cygni, while Struve's comparison star is at least eight or ten 
 times as far. We may add that, as the comparison stars used by 
 Bessel are brighter than that of Struve, there really is a presumption 
 that the latter is the most distant of the three. 
 
 We have here a characteristic feature of this method of de- 
 termining parallax. Even if all the observations and the reductions 
 of a parallax series were mathematically correct, we could not with 
 strict propriety describe the final result as the parallax of one star. 
 
422 THE STORY OF THE HEAVENS. 
 
 It is only the difference between the parallax of the star and that 
 of the comparison star. We can therefore only assert that the 
 parallax sought cannot be less than the quantity determined. 
 Viewed in this manner, the discrepancy between Struve and Bessel 
 entirely vanishes. Bessel asserted that the distance of 61 Cygni 
 could not be more than sixty billions of miles. Struve did not 
 contradict this nay, he certainly confirmed it when he showed 
 that the distance could not be more than forty billions. 
 
 A quarter of a century has elapsed since Struve made his 
 observations. Those observations have not been challenged ; they 
 have been, on the whole, confirmed by other investigations. In a 
 critical review of the whole subject several years ago, Auwers had 
 shown that Struve's determination is more worthy of confidence than 
 is that of any other astronomer. Yet, notwithstanding this authori- 
 tative dictum, the question has been recently opened afresh. Dr. 
 Briinnow, the recent Astronomer- Royal of Ireland, had successfully 
 measured the parallax of several stars ; the instrument he used being 
 the South Equatorial at Dunsink of twelve inches aperture, which 
 we have described in a previous chapter. He commenced a series of 
 observations on the parallax of 61 Cj^gni which were continued and 
 completed by his successor. Briinnow chose a fourth comparison 
 star (marked on the diagram), different from any of those which 
 had been used by the earlier observers. The method of observing 
 which Briinnow employed was quite different to that of Struve, 
 though the filar micrometer was used in both cases. Briinnow 
 sought to determine the parallactic ellipse by measuring the 
 difference in declination between 61 Cygni and the comparison 
 star. In the course of a year it is found that the difference in 
 declination undergoes a periodic change, and from that change the 
 parallactic ellipse can be computed. In the first series of obser- 
 vations we measured the difference of declination between the 
 preceding star of 61 Cygni and the comparison star; in the second 
 series we took the other component of 61 Cygni and the same 
 comparison star. We had thus two completely independent 
 determinations of the parallax resulting from two years' work. 
 The first of these makes the distance forty billions of miles, and 
 
THE DISTANCES OF THE STARS. 
 
 423 
 
 the second makes it almost exactly the same. There can be no 
 doubt that this work supports Struve's determination in correction 
 of Bessel's, and therefore we may perhaps sum up the present 
 state of our knowledge of this question by saying- that the distance 
 of 61 Cygni is much nearer to the forty billions of miles which 
 Struve found, than to the sixty billions which Bessel found.* 
 
 APR./ JUNE AUG. OCT. 
 
 Fig. 81. Parallax in Declination of 61 Cygni. 
 
 It is desirable to give the reader the means of forming his 
 own opinion as to the quality of the evidence which is available 
 in such researches. The diagram in Fig. 81 here shown has 
 been constructed with this object. It is intended to illustrate 
 the second series of observations of difference of declination 
 which we have made at Dunsink. Each of the dots repre- 
 sents one night's observations. The height of the dot is the 
 observed difference of declination between 61 (B) Cygni and 
 
 * The distance of 61 Cygni has again been recently investigated by Prof. Asaph 
 Hall, of Washington, who has obtained a result practically coincident with that 
 found at Dunsink. 
 
424 THE STORY OF THE HEAVENS. 
 
 the comparison star. The distance along the horizontal line 
 or the abscissa, as a mathematician would call it represents 
 the date. These observations are grouped more or less regularly 
 in the vicinity of a certain curve. That curve expresses where 
 the observations should have been, had they been absolutely 
 perfect. The distances between the dots and the curve may be 
 regarded as the errors which have been committed in making the 
 observations. 
 
 Perhaps it will be thought that in many cases these errors 
 appear to have attained very undesirable dimensions. Let us, 
 therefore, hasten to say that it Avas precisely for the purpose 
 of setting forth these errors that this diagram has been shown ; 
 we have to exhibit the weakness of the case no less than 
 its strength. The errors of the observations are not, however, 
 intrinsically so great as might at first sight be imagined. To 
 perceive this, it is only necessary to interpret the scale on which 
 this diagram has been drawn, by comparison with familiar stan- 
 dards. The distance from the very top of the curve to the hori- 
 zontal line denotes an angle of only four- tenths of a second. This 
 is about the apparent diameter of a penny-piece at a distance of 
 ten miles ! We can now appraise the true magnitude of the errors 
 which have been made. It will be noticed that no one of the dots is 
 distant from the curve by much more than half of the height of 
 the curve. It thus appears that the greatest error in the whole 
 series of observations amounts to but two or three tenths of a 
 second. This is equivalent to our having pointed the telescope to 
 the upper edge of a penny-piece fifteen or twenty miles off, instead 
 of to the lower edge. This is not a great blunder. A rifle team 
 whose errors in pointing were more than a hundred times as great, 
 might still easily win every prize at Wimbledon. The parallaetic 
 ellipse is, however, so small, that the errors, minute as they are, 
 bear a large proportion to the total quantity under consideration. 
 This it is which constitutes the weakness of parallax observations, 
 but notwithstanding these difficulties the results are seemingly 
 entitled to confidence. It is in the very nature of such work 
 that by increasing the number of observations the errors tend 
 
THE DISTANCES OF THE STARS. 425 
 
 to counteract each other. To ensure the neutralization of errors, 
 the star is observed on as many nights as possible, and a 
 considerable number of observations are usually secured on each 
 night. As the number of observations is increased, so does the 
 error of the final result diminish. 
 
 The observations being all completed, the astronomer finds 
 before him a vast mass of, perhaps, fifteen thousand figures. The 
 parallax, of which he is in search, can be sufficiently expressed by 
 two digits indeed, almost by a single one. With the collection of 
 these observations the functions of the observer are at an end. It 
 now becomes the duty of the mathematician to elaborate from this 
 mass of detail the one or two figures which embody the final result. 
 He is not only able to elicit the most probable value of the 
 parallax, but he can appraise the value of his result, and express 
 numerically the degree of confidence to which it is entitled ; in 
 mathematical language, he determines the probable error of the 
 parallax, as well as its most probable value. 
 
 We have entered into the history of 61 Cygni with some detail, 
 because it is the star whose distance has been most studied. We 
 do not say that 61 Cygni is the nearest of all the stars ; it would, 
 indeed, be very rash to assert that any particular star was the 
 nearest of all the countless millions in the heavenly host. We 
 certainly know one star which seems nearer than 61 Cygni; it lies 
 in one of the southern constellations, and its name is a Centauri. 
 This star is, indeed, of memorable interest in the history of the 
 subject. Its parallax was first determined at the Cape of Good 
 Hope by Henderson; subsequent researches have confirmed his 
 observations, and the elaborate investigations of Mr. Gill have 
 proved that the parallax of this star is about three-quarters of a 
 second, so that it is only two-thirds of the distance of 61 Cygni. 
 By what sagacious intuition was Henderson guided to a com- 
 paratively near star in the southern hemisphere, and Bessel to 
 one in the northern ? 
 
 61 Cygni arrested our attention, in the first instance, by the 
 circumstance that it had the large proper motion of five seconds 
 annually. We have also ascertained that the annual parallax is 
 
426 THE STOEY OF THE HEAVENS. 
 
 about half a second. The combination of these two statements 
 leads to a result of considerable interest. It teaches us that 
 61 Cygni must each year traverse a distance of not less than ten 
 times the radius of the earth's orbit. Translating this into 
 ordinary figures, we learn that this star must travel nine hundred 
 and twenty million miles per annum. ' It must move between two 
 and three million miles each day, but this can only be accomplished 
 by maintaining the prodigious velocity of thirty miles per second. 
 There seems to be no escape from this conclusion. The facts 
 which we have described, and which are now sufficiently well 
 established, are inconsistent with the supposition that the velocity 
 of 61 Cygni is less than thirty miles per second; the velocity may 
 be greater, but less it cannot be. 
 
 For forty years, for one hundred years, we know that 61 Cygni 
 has been moving in the same direction and with the same velocitv. 
 Prior to the existence of the telescope we have no observation to 
 guide us ; we cannot, therefore, be absolutely certain as to the 
 earlier history of this star, yet it is only reasonable to suppose that 
 61 Cygni has been moving from remote antiquity with a velocity 
 comparable with that it has at present. If disturbing influences 
 were entirely absent, there could be no trace of doubt about the 
 matter. Some disturbing influence, however, there must be; the 
 only question is whether that disturbing influence is sufficient 
 to seriously modify the assumption we have made. A powerful 
 disturbing influence might greatly alter the velocity of the star ; 
 it might deflect the star from its rectilinear course ; it might even 
 force the star to move around a closed orbit. We do not, how- 
 ever, believe that any disturbing influence of this magnitude need 
 be contemplated, and there can be no reasonable doubt that 61 
 Cygni moves at present in a path very nearly straight, and with 
 a velocity very nearly uniform. 
 
 As the distance of 61 Cygni from the sun is forty billions of 
 miles, and as its velocity is thirty miles a second, it is easy to 
 find how long the star would take to accomplish a journey 
 equal to its distance from the sun. The time required will be 
 about 40,000 years. In the last 400,000 years, 61 Cygni will have 
 
THE DISTANCES OF THE STARS. 427 
 
 moved over a distance ten times as great as its present distance 
 from the sun, whatever be the direction of motion. This star 
 must therefore have been about ten times as far from the earth 
 400,000 years ago as it is at present. Though this epoch is in- 
 credibly more remote than any historical record, it is perhaps not 
 incomparable with the duration of the human race ; while compared 
 with the vast lapse of geological time, such periods seem trivial and 
 insignificant. Geologists have long ago repudiated mere thousands 
 of years; they now claim millions, and many millions of years, 
 for the performance of geological phenomena. If the earth has 
 existed for the millions of years which geologists assert, it becomes 
 reasonable for astronomers to speculate on the phenomena which 
 have transpired in the heavens in the lapse of similar ages. By 
 the aid of our knowledge of star distances, combined with an 
 assumed velocity of thirty miles per second, we can make the 
 attempt to peer back into the remote past, and show how great 
 are the changes which our universe seems to have undergone. 
 
 In a million years 61 Cygni will, apparently, have moved through 
 a distance which is twenty-five times as great as its present 
 distance from the sun. Whatever be the direction in which 61 
 Cygni is moving whether it be towards the earth or from the 
 earth, to the right or to the left, it must have been about twenty- 
 five times as far off a million years ago as it is at present ; but 
 even at its present distance 61 Cygni is a small star; were it ten 
 times as far it could only be seen with a good telescope ; were it 
 twenty-five times as far it could barely be a visible point in our 
 greatest telescopes. 
 
 The conclusions arrived at with regard to 61 Cygni may 
 be applied with varying degrees of emphasis to other stars. 
 We are thus led to the conclusion that many of the stars with 
 which the heavens are strewn are apparently in slow motion. 
 But this motion though apparently slow may really be very rapid. 
 When standing on the sea-shore, and looking at a steamer on the 
 distant horizon, we can hardly notice that the steamer is moving. 
 It is true that by looking again in a few minutes we can detect 
 a change in the place ; but the motion of the steamer seems slow. 
 
428 THE STORY OF THE HEAVENS. 
 
 Yet if we were near the steamer we would find that it was rushing 
 along 1 at the rate of many miles an hour. It is the distance which 
 causes the illusion. So it is with the stars : they seem to move 
 slowly because they are very distant, but were we near them, we 
 could see that in the majority of cases their motions are enormously 
 rapid, and probably a thousand times as fast as the swiftest steamer 
 that ever ploughed the ocean. 
 
 It thus appears that the permanence of the sidereal heavens, and 
 the fixity of the constellations in their relative positions, are only 
 ephemeral. When we rise to the contemplation of such vast periods 
 of time as the researches of geology disclose, the durability of the 
 constellations vanishes ! In the lapse of those stupendous ages stars 
 and constellations gradually dissolve from view, to be replaced by 
 others of no greater permanence. 
 
 In order to convey some idea of the magnitudes of the quantities 
 with which we have to deal when we are discussing the proper 
 motion of the stars, let us study the effects of proper motion on 
 the principal stars of the Great Bear. Night after night we see 
 these stars in the same relative positions, and the general out- 
 line of the constellation has remained unaltered for centuries. 
 Homer spoke of it thousands of years ago as the bear which feared 
 to dip into the sea, alluding to the fact that in these latitudes 
 this constellation never sets. 
 
 But it is very interesting to inquire whether some changes may 
 not have occurred in the stars of the Great Bear since the earliest 
 times. Fortunately, we have the means of answering the question 
 with a considerable degree of accuracy. Nearly two thousand years 
 ago Ptolemy constructed a catalogue, in which he recorded the 
 position of a large number of fixed stars. Ptolemy had no tele- 
 scope, but he had constructed certain instruments for measuring 
 the positions of the stars in the heavens. No doubt we should 
 now consider these contrivances excessively rude and inadequate, 
 and vastly inferior to those used every day by modern astronomers. 
 Yet the antiquity of Ptolemy 's observations confers on them a 
 positive value, which more than compensates for their inaccuracy. 
 Among the stars which Ptolemy observed were those of the Great 
 
THE DISTANCES OF THE STARS. 429 
 
 Bear; and from his observations we are able to construct the map 
 of the Great Bear as it appeared to him, and compare it with 
 the present appearance of the same constellation. The comparison 
 shows that the shape of the Great Bear has not changed much for 
 the last 2,000 years. There is, however, some difference between 
 the past and present places of the stars. A part of the differences 
 can be explained by the inevitable errors of the crude methods of 
 measuring used by Ptolemy ; but making every allowance for these 
 errors, there can still be no doubt that the positions of the stars 
 have perceptibly changed in 2,000 years. 
 
 We next proceed to the history of a very remarkable star, 
 which has been the subject of much parallax research and of much 
 ingenious speculation. The star will be found in the Great Bear, 
 but is only of the sixth or the seventh magnitude, and therefore 
 invisible to the unaided eye. It is one of that myriad host of stars 
 still remaining unchristened, so that astronomers refer to it either 
 by its place on the heavens, or by citing Groombridge's well known 
 star catalogue, in which the star is No. 1,830. Yet, though small 
 and inconspicuous, this object is in one respect the most remarkable 
 star known among the thousands in the northern hemisphere. It 
 is distinguished by the exceptionally large amount of its proper 
 motion. In virtue of this proper motion, Groombridge 1,830 moves 
 over no less than seven seconds of arc per annum. It should also be 
 remembered that our telescopes only give us that component of the 
 proper motion which is projected on the surface of the heavens. 
 The resultant proper motion must of course be generally greater 
 than this single component. By Mr. Huggins' spectroscopic 
 method, to be described in a future chapter, it might be possible to 
 measure the velocity of the star along the line of sight. If this 
 could be ascertained, then by the composition of velocities we 
 could determine the numerical value of the true proper motion. In 
 the absence of such measures, we can only assert that the proper 
 motion of this star may be greater, but cannot be less, than seven 
 seconds per annum. We have calculated the most probable value 
 of the proper motion from the theory of probabilities, and have 
 found that it is just as likely to exceed nine seconds per annum as 
 
430 THE STORY OF THE HEAVENS. 
 
 to fall short thereof ; we shall, however, only assume the velocity 
 to be seven seconds, as that will be amply sufficient for our 
 purpose. 
 
 We first translate the conception of the proper motion into 
 other forms in which it may be more readily intelligible. Imagine 
 a star placed at the Pole, and moving with the velocity of seven 
 seconds per annum across the heavens to the Belt of Orion. This 
 is a distance of nearly ninety degrees, and the entire journey would 
 occupy about forty-six thousand years. Even in a few months the 
 movement would be large enough to be readily detected by tele- 
 scopic observation, while in a period of two hundred and fifty years 
 this star would traverse a distance on the surface of the heavens 
 equal to the apparent diameter of the full moon. 
 
 When we speak of a large proper motion, it is to be understood 
 that we refer for comparison to the ordinary proper motions of 
 ordinary stars. Compared with the proper motions of objects 
 belonging to the solar system, the proper motions of the stars are 
 extremely small. The slowest and most remote planet, Neptune, 
 moves over eight hundred times as much arc per annum as 
 the most rapidly moving star. We must be prepared to find that 
 although Groombridge 1,830 only changes its place at the rate of 
 seven seconds annually, yet, that as its distance is enormously great, 
 the intrinsic velocity of the star may attain portentous dimensions. 
 
 Several attempts have been made to measure the distance of 
 Groombridge 1,830. Astronomers were specially attracted to the 
 research by the exceptional proper motion, which seemed to suggest 
 that a measurable distance might be expected. Strange to say, 
 however, this anticipation has not been realised ; the star is not 
 among the sun's nearer neighbours. Perhaps the most successful 
 determinations are those obtained by Struve and by Briinnow. 
 The latter has, indeed, produced a memoir upon the parallax of 
 Groombridge 1,830, which is a classical work in this branch of 
 astronomical literature. Both these observers have shown that 
 this star is at least four or five times as far from the earth 
 as the star 61 Cygni, which engaged our attention in the 
 earlier part of this chapter. By the combination of the 
 
THE DISTANCES OF THE STARS. 431 
 
 measured distance and the known proper motion, we arrive at 
 a result which is very startling-. An observer situated on the 
 star and looking at the solar system, would find the radius 
 of the earth's orbit to subtend an angle of one-tenth of a 
 second ; we, on the other hand, see that the star moves over seven 
 seconds in a year. From this it follows that the annual journey of 
 Groombridge 1,830 must be no less than seventy times the radius 
 of the earth's orbit ; in other words, the velocity of this star is so 
 enormous that it would travel from the earth to the sun in about 
 five days. It does not seem possible that there can be any 
 substantial error in this conclusion ; indeed, the known facts would 
 be quite consistent with the supposition that the star travelled 
 over a distance equal to one hundred radii of the earth's orbit per 
 annum. "We have taken the lowest estimate that can be recon- 
 ciled with the observations, in stating the velocity to be seventy 
 radii per annum. A very simple calculation will transform this 
 result into more familiar language. 
 
 We thus obtain, finally, the extraordinary result that the actual 
 velocity of this star is not less than two hundred miles per second ! 
 In ten minutes Groombridge 1,830 has travelled 120,000 miles. 
 In one minute this wonderful star would perform the journey from 
 London to Pekin. If our earth moved equally fast, the journey 
 around the sun would be accomplished in about a month, instead of 
 the year which is now required. We do not assert that so great a 
 velocity is absolutely unparalleled in the universe. In the full 
 flush of its perihelion swoop, a comet, as it passes close to the sun, 
 may attain this velocity, or may even exceed it. This is, however, 
 a mere explosive outburst of vigour on the part of the comet ; the 
 great velocity is soon abated, a reaction sets in, and in the remote 
 parts of its orbit the motion is very slow. The velocity of 
 Groombridge 1,830 is no mere spasmodic effort ; with a stately 
 uniformity, worthy of the dignity of a majestic sun, it sweeps 
 along, alike inflexible in the direction of its motion and in the 
 velocity with which its journey is performed. 
 
 But it not uufrequently happens that a parallax research proves 
 abortive. The labour is accomplished, the observations are reduced 
 
432 THE STORY OF THE HEAVENS. 
 
 and discussed, and yet no value of the parallax can be obtained. 
 The distance of the star is so vast that our base line, although 
 it .is nearly two hundred millions of miles long, is too short to 
 bear any appreciable ratio to the distance of the star. Even from 
 such failures, however, information may often be drawn. 
 
 Let me illustrate this by an account derived from our expe- 
 rience at Dunsink. On the 24th November, 1876, a well- 
 known astronomer Dr. Schmidt, of Athens noticed a new 
 bright star of the third magnitude in the constellation Cygnus, 
 in a place where he was sure that no corresponding star had 
 existed four days previously. The star which thus suddenly 
 burst into splendour seems never to have been observed before. 
 The charts and catalogues were searched, and they showed no star 
 in that position; so that if Nova Cygni were really only an 
 outbreak of brilliancy in a small star, that star must have been 
 so small as to have escaped the attention of all the survey- 
 ing astronomers who had previously examined that region. The 
 suddenly acquired splendour of Nova Cygni was not of long 
 duration. In a single week it had ceased to be a conspicuous 
 object; in a fortnight it was no longer visible to the unaided eye; 
 and at the last it dwindled down to a minute telescopic point. 
 
 This is, in any case, a very remarkable career. We do not say 
 that it is unparalleled, as there have been other similar cases 
 recorded; but it may be regarded as certain that there was 
 never a new star which was honoured by so much attention 
 as was shown to Nova Cygni. From the outbreak of splendour 
 to its decline, this wonderful star has been relentlessly pursued by 
 equatorials, meridian circles, micrometers, and the paraphernalia 
 of the modern observatory. By the spectroscope especially many 
 interesting facts have been ascertained. It has been shown that 
 the constitution of this star is totally different from that of the 
 ordinary stars. The spectroscope proves that the brilliant splendour 
 of Nova Cygni was due to the presence of glowing and incan- 
 descent gases, while the same subtle instrument showed. that when 
 the star was declining to invisibility, the last rays which reached 
 the earth were emitted not from a solid but from some gaseous 
 
THE DISTANCES OF THE STARS. 433 
 
 body. It is impossible to resist the conclusion, that the ephemeral 
 splendours of the star were connected with some prodigious out- 
 break of luminous vapour, on the cause of which it is hard, to 
 resist hazarding a speculation. Most astronomers are of opinion 
 that in all probability the sudden outburst of splendour was due 
 to a collision between two bodies, which dashed together with a 
 high relative velocity. It is known that such a collision would 
 be an adequate cause of brilliant incandescence. If, for example, 
 two bodies, each equal to the earth, and moving, as the earth 
 does, with a velocity of eighteen miles a second, were to collide, 
 a prodigious flash of light would be the consequence, while the 
 heat developed would be adequate to dissipate the entire mass of 
 both into glowing vapour. 
 
 On the 20th of November Nova Cygni was invisible. Whether 
 it first burst forth on the 21st, 22nd, or 23rd, no one can tell; but 
 on the 24th it was discovered. Its brilliancy even then seemed 
 to be waning ; so, presumably, it was brightest at some moment 
 between the 20th and 24th of November. The outbreak must 
 thus have been comparatively sudden, and we know of no cause 
 which would account for such a phenomenon more simply than a 
 gigantic collision. The decline in the brilliancy was much more 
 tardy than its growth, and more than a fortnight passed before the 
 star relapsed into insignificance two or three days for the rise, two 
 or three weeks for the fall. Yet even two or three weeks was a short 
 time in which to extinguish so mighty a conflagration. It is com- 
 paratively easy to suggest an explanation of the sudden outbreak ; 
 it is not equally easy to understand how it can have been subdued 
 in a few weeks. A good-sized iron casting in one of our foundries 
 takes nearly as much time to cool as sufficed to abate the celestial 
 fires in Nova Cygni ! , 
 
 On this ground it seemed not unreasonable to suppose that 
 perhaps Nova Cygni was not really a very extensive confla- 
 gration. But, if such were the case, the star must have been 
 comparatively near to the earth, since it presented so brilliant a 
 spectacle and attracted so much attention. It therefore appeared 
 a plausible object for a parallax research ; and consequently a series 
 c c 
 
434 THE STORY OF THE HEAVENS. 
 
 of observations were recently made at Dunsink. We were at the 
 time too much engaged with other work to devote very much 
 labour to a research which might, after all, only prove illusory. We 
 simply made a sufficient number of micrometric measurements to 
 test whether a large parallax existed. It has been already pointed 
 out how each star appears to describe a minute parallactic ellipse, in 
 consequence of the annual motion of the earth and by measurement 
 of this ellipse the parallax and therefore the distance of the 
 star can be determined. Under ordinary circumstances, when the 
 parallax of a star is being investigated, it is necessary to measure 
 the position of the star in its ellipse on many different occasions, 
 distributed over a period of at least an entire year. The method 
 we adopted was much less laborious. It was sufficiently accurate 
 to test whether or not Nova Cygni had a large parallax, though it 
 might not have been delicate enough to disclose a small parallax. 
 At a certain date, which can be readily computed, the star is at one 
 end of the parallactic ellipse, and six months later the star is at the 
 other end. By choosing suitable times in the year for our observa- 
 tions, we can measure the star in those two positions when it is most 
 deranged by parallax. It was by observations of this kind that we 
 sought to detect the parallax of Nova Cygni. Its distance from a 
 neighbouring star was carefully measured by the micrometer at the 
 two seasons when, if parallax existed, those distances should show 
 their greatest discrepancy ; but no certain difference between these 
 distances could be detected. The observations, therefore, failed to 
 reveal the existence of a parallactic ellipse or, in other words, 
 the distance of Nova Cygni was too great to be measured by 
 observations of this kind. 
 
 I feel certain that if Nova Cygni had been one of the nearest 
 stars these observations would not have been abortive. We are 
 therefore entitled to believe that Nova Cygni must be at least 
 20,000,000,000,000 miles from the solar system; and the sugges- 
 tion that the brilliant outburst was of small dimensions must be at 
 once abandoned. The intrinsic brightness of Nova Cygni, when at 
 its best, cannot have been greatly if at all inferior to the brilliancy 
 of our sun himself. If the sun were withdrawn from us to the 
 
THE DISTANCES OF THE STARS. 435 
 
 distance of Nova Cygni, it would seemingly have dwindled down 
 to an object not more brilliant than the variable star. How the 
 lustre of such a stupendous object declined so rapidly, remains, 
 therefore, a mystery not easy to explain. 
 
 Have we not said that the outbreak of brilliancy in this star 
 occurred between the 20th and the 24th of November, 1876? It 
 would be more correct to say that the tidings of that outbreak 
 reached our system at the time referred to. The real outbreak must 
 have taken place years previously three years, at all events, and 
 possibly much more. At the time that the star excited such com- 
 motion in the astronomical world it had really relapsed again into 
 insignificance, for the rays of light which start from the star take 
 years before they traverse the mighty distance to the solar system. 
 
 In connection with the subject of the present chapter we have 
 to consider a great problem which was proposed by Sir William 
 Herschel. He saw that the stars were animated by proper motion ; 
 he saw also that the sun is a star, one of the countless host of 
 heaven, and he was therefore led to propound the stupendous ques- 
 tion as to whether the sun, like the other stars which are its peers, 
 was also in motion. 
 
 Consider all that this great question involves. The sun has 
 around it a retinue of planets and their attendant satellites, the 
 comets, and a host of smaller bodies. The question is, whether all 
 this superb system is revolving around the sun at rest in the 
 middle, or whether the whole system sun, planets, and all is not 
 moving on bodily through space. 
 
 Herschel was the first to solve this noble problem ; he discovered 
 that our sun and all the splendid retinue by which it is attended are 
 moving bodily in space. He not only discovered this, but he ascer- 
 tained the direction in which the system was moving, as well as the 
 velocity with which that movement was performed. He has shown 
 that the sun and his system is now rapidly hastening towards a 
 certain point of the constellation Hercules. The velocity with 
 which the motion is performed corresponds to the magnitude of 
 the system : quicker than the swiftest rifle-bullet that was ever 
 fired, the sun, bearing with it the earth and all the other planets, 
 c o 2 
 
436 THE STORY OF THE HEAVENS. 
 
 is now sweeping onwards. We on the earth participate in that 
 motion. Every half-hour we are about ten thousand miles 
 nearer to the constellation of Hercules than we would have been 
 if the solar system was not animated by this motion. As we 
 are proceeding at this stupendous rate towards Hercules, it might 
 at first be supposed that we ought soon to get there ; but the dis- 
 tances of the stars in Hercules are enormously great, as are those 
 of the stars elsewhere, and we may be certain that the sun and his 
 system must travel at the present rate for far more than a million 
 years before we have crossed the abyss between our present position 
 and the frontiers of Hercules. 
 
 It remains to explain the method of reasoning which Herschel 
 adopted, by which he was able to make this great discovery. It 
 may sound strange to hear that the detection of the motion of the 
 sun was not made by looking at the sun himself j all the obser- 
 vations of the sun with all the telescopes in the world would never 
 tell us of that motion, for the simple reason that the earth, from 
 whence our observations must be made, participates in it. A 
 passenger in the cabin of a ship usually becomes aware that the 
 ship is moving by the roughness of the sea; but if the sea be 
 perfectly calm, then, though the tables and chairs in the cabin 
 are moving as rapidly as the ship, yet we do not see them moving, 
 because we are also travelling with the ship. If we could not go 
 out of the cabin, nor look out through the windows, we would 
 never know whether the ship was moving or at rest ; nor could we 
 have any idea as to the direction in which the ship was going, or as 
 to the velocity with which that motion was performed. 
 
 The sun, with his attendant host of planets and satellites, may 
 be likened to the ship. The planets may revolve around the sun 
 just as the passengers may move about in the cabin, but as the 
 passengers, by looking at objects on board, can never tell where 
 the ship is going to, so we, by merely looking at the sun, or the 
 other planets or members of the solar system, can never tell 
 whether our system as a whole is in motion. 
 
 The conditions of a perfectly uniform movement along a 
 perfectly calm sea are not often fulfilled on the waters with 
 
THE DISTANCES OF THE STARS. 437 
 
 which we are acquainted, but the course of the sun and his 
 system is untroubled by any disturbance, so that the majestic 
 progress is conducted with the most absolute uniformity. We feel 
 neither the rolling nor the pitching, neither the jolts nor the rattle 
 of machinery, which apprise us of motion in the ship ; and as all the 
 planets are travelling- with us, we can get no information from them 
 as to the common motion by which the whole system is animated. 
 
 The passengers are, however, at once apprised of the ship's 
 motion when they go on deck, and when they look at the sea sur- 
 rounding them. Let us suppose that their voyage is nearly accom- 
 plished, that the distant land appears in sight, and, as evening 
 approaches, the harbour is discerned into which the ship is to 
 enter. Let us suppose that the harbour has, as is often the case, 
 a narrow entrance, and that its mouth is indicated by a light- 
 house on each side. When the harbour is still a long way 
 off, near the horizon, the two lights are seen close together, and 
 now that the evening has closed in, and the night has become quite 
 dark, these two lights are all that is visible. While the ship is 
 still some miles from its destination the two lights look close 
 together, but as the distance decreases the two lights seem to open 
 out ; gradually the ship gets nearer while the lights are still open- 
 ing, till finally, when the ship enters the harbour, instead of the 
 two lights being directly in front, as at the commencement, one 
 of the lights is passed by on the right hand, while the other is simi- 
 larly found on the left. If, then, we are to discover the motion of 
 the solar system, we must, like the passenger, look at objects 
 unconnected with our system, and learn our own motion by their 
 apparent movements. But are there any objects in the heavens 
 unconnected with our system ? If all the stars were like the earth, 
 merely the appendages of our sun, then we never could discover 
 whether we were at rest or whether we were in motion : our 
 system might be in a condition of absolute rest> or it might be 
 hurrying on with an inconceivably great velocity, for anything we 
 could tell to the contrary. But the stars do not belong to the 
 system of our sun ; they are, rather, suns themselves, and do not 
 recognise the sway of our sun, as this earth is obliged to do. The 
 
438 THE 8TOET OF THE HEAVENS. 
 
 stars will, therefore, act as the external objects by which we can 
 test whether our system is voyaging through space. 
 
 With the stars as our beacons, what ought we to expect if our 
 system be really in motion ? Remember that when the ship was 
 approaching the harbour the lights gradually opened out to the 
 right and left. But the astronomer has also lights by which he 
 can observe the navigation of that vast craft, our solar system, 
 and these lights will indicate the path along which he is borne. If 
 our solar system be in motion, we should expect to find that the 
 stars were gradually spreading away from that point in the heavens 
 towards which our motion tends. This is precisely what we do 
 find. The stars in the constellations are gradually spreading away 
 from a central point in the constellation of Hercules, and hence 
 we infer that it is towards Hercules that the motion of the solar 
 system is directed. 
 
 There is one great difficulty in the discussion of this question. 
 Have we not had occasion to observe that the stars themselves are 
 in actual motion ? It seems certain that all the stars, including 
 the sun himself as a star, have each an individual motion of 
 their own. The motions of the stars as we see them are partly 
 apparent as well as partly real ; they partly arise from the actual 
 motion of each star and partly from the motion of the sun, in which 
 we partake, and which produces an apparent motion of the star. 
 How are these to be discriminated ? Our telescopes and our obser- 
 vations can never effect this decomposition directly. To accomplish 
 the analysis, Herschel resorted to certain geometrical methods. 
 His materials at that time were but scanty, but in his hands they 
 proved adequate, and he boldly announced his discovery of the 
 movement of the solar system. 
 
 So majestic an announcement demanded the severest test which 
 the most refined astronomical resources could suggest. There is 
 a certain powerful and subtle method which astronomers use in the 
 effort to interpret nature. Bishop Butler has said that probability 
 is the guide of life. If we wish to see this maxim applied with 
 logical perfection, we must visit the study of a mathematician when 
 he is discussing a problem like that now before us. The proper 
 
THE DISTANCES OF THE STARS. 439 
 
 motion of a star has to be decomposed into two parts, one real and 
 the other apparent. When several stars are taken, we may conceive 
 an infinite number of ways into which the movements of each star 
 can be so decomposed. Each one of these conceivable divisions 
 will have a certain element of probability in its favour. It is the 
 business of the mathematician to determine the amount of that 
 probability. The case, then, is as follows : Among all the various 
 systems one must be true. We cannot lay our finger for certain on 
 the true one, but we can take that which has the highest degree of 
 probability in its favour, and thus follow the precept of Butler to 
 which we have already referred. A mathematician would describe 
 his process by calling it the method of least squares. We do 
 not say that Herschel employed these refinements. Indeed, Sir 
 William Herschel was not a mathematician in the extended sense 
 of the word, and he employed methods which would not be suffi- 
 ciently capacious to deal with the ample materials now accumulated. 
 Since Herschel's discovery, one hundred years ago, many a mathe- 
 matician using observations of thousands of stars has attacked the 
 same problem. The stars in the northern hemisphere have been 
 discussed with far greater completeness ; the stars in the southern 
 hemisphere, which Herschel knew not, have been made to bear 
 their testimony. The stars which move quickly have been interro- 
 gated, and so have the stars which hardly move at all. Mathe- 
 maticians have exhausted every refinement which the theory of 
 probabilities can afford, but only to establish conclusively the truth 
 of that splendid theory which seems merely to have been one of 
 the flashes of Herschel's genius. 
 
 We thus learn that our whole system, comprising the sun in 
 the centre, with the planets which circulate around the sun, the 
 comets, and the incredible host of minute bodies, are all together 
 bound on a stupendous voyage through the realms of space. The 
 progress of our sun is marked by the dignified solemnity worthy 
 of so majestic a body. The sun requires almost two days to move 
 through a length equal to his own diameter. Every two days the 
 solar system accomplishes a stage of about a million miles in its 
 journey towards the constellation of Hercules. 
 
CHAPTER XXII. 
 
 THE SPECTROSCOPE. 
 
 A New Department of Science The Materials of the Heavenly Bodies Meaning 
 of Elementary Bodies Chemical Analysis and Spectroscopic Analysis The 
 Composite Nature of Light "Whence Colours ? The Rainbow The Prism 
 Passage of Light through a Prism Identification of Metals by the Rays they 
 emit when Incandescent The great Discovery of the Identity of the D-Lines 
 with Sodium The Dark Lines in the Solar Spectrum Interpreted Metals 
 present in the Sun Examination of Light from the Moon or the Planets The 
 Prominences surrounding the Sun Photographs of Spectra Measurement of 
 the Motion of the Stars along the Line of Sight. 
 
 THE revelations of the spectroscope form an important chapter in 
 modern science. By its aid a mighty stride has been taken in our 
 attempt to comprehend that elaborate system of suns and other 
 bodies which adorn the skies. 
 
 The subject is widely dissimilar to those branches of astronomy 
 which had been previously cultivated. The discoveries of the early 
 astronomers had taught us how the earth revolves around the sun. 
 We have been told how the distances of the different bodies were 
 to be measured, and how their sizes were found. More wonder- 
 ful still, the older astronomy narrated how we could weigh one 
 body in the heavens against another. We knew that the sub- 
 stance of which the sun was made glowed with intense heat ; but 
 we could not tell, we had no means of telling, what was the nature 
 of that substance. To this the old astronomy could render no answer 
 whatever; and, indeed, half a century ago it would have been 
 thought incredible that such a question could be answered. If this 
 be the case with regard to the sun, what are we to say of the other 
 and still less known bodies of the heavens ? What is the physical 
 nature of the other planets ? What are the substances present in 
 the stars, and in those dim nebulae which lie on the confines of 
 
THE SPECTROSCOPE. 44] 
 
 the visible universe ? It is the triumph of modern astronomy to 
 have afforded an answer to these questions : not, indeed, with all 
 the fulness that might be desired, and that, perhaps, may yet be 
 expected ; but still they have been answered in some degree, and 
 the results are of great interest. 
 
 Let us first enunciate the problem in a definite shape ; and here 
 we must call in the aid of chemistry to enable us to obtain 
 distinct views on the subject. What are we to understand by the 
 substances of which the sun is composed ? What do we even mean 
 by the substances of which the earth is composed? At a first 
 glance we would say that our globe is composed of rocks and clay, 
 of air and water, but the chemist will strive to render our ideas 
 more accurate. He shows us how rocks are composed of certain 
 other substances, which he calls elements, and the elements are 
 substances which he cannot decompose into anything else. He 
 will also tell us how the air is composed of two elementary 
 substances, oxygen and nitrogen, mingled together; and how water 
 is composed of oxygen and hydrogen in a state of combination. 
 The chemist pursues his analysis through every solid, liquid, and gas 
 on the globe; he decomposes animal or vegetable substances into 
 their elements, and the outcome of his labours is to demonstrate 
 that every particle of matter en our globe, or in the atmosphere 
 surrounding it, is composed of one or more elementary bodies, and 
 that the whole number of such elementary bodies is about sixty 
 or rather more. The elements may be solids, like iron, or gold, 
 or carbon; or they may be gases, like oxygen, or hydrogen, or 
 nitrogen. Many of the elements are extremely rare. About 
 twenty of the more ordinary ones are all that need concern us 
 at present. 
 
 We are now enabled to state a special problem of modern 
 astronomy. Are these elements of which the earth is composed 
 peculiar to the earth, or are they found on the other bodies which 
 teem throughout space? Take, for example, the most abundant 
 of all metals, iron, which exists in rich profusion near the surface 
 of the earth, and which exists perhaps in no less abundance in 
 the interior of the earth. Is iron a product limited to this, our 
 
442 THE STORY OF THE HEAVENS. 
 
 globe? or does it enter into the composition of other bodies in 
 the universe ? This is one of the questions which modern science 
 has answered. How that answer has been given it is the object 
 of this chapter to unfold. 
 
 Quite a new phase of astronomy is here opened up. Great 
 telescopes revealed faint objects, and showed the features clearly 
 which small telescopes only showed dimly, but all the telescopes 
 in the world would not answer the question as to whether iron 
 was found in the sun. A totally distinct branch of inquiry was 
 necessary, which is known as spectrum analysis. We could not 
 expect to actually see iron in the sun. The sun is itself a mighty 
 glowing globe, infinitely hotter than a Bessemer converter or a 
 Siemens furnace ; if iron is in the sun, that iron is not only white- 
 hot and molten, but actually driven off into vapour ; but vapour of 
 iron is not distinguishable. How would you recognise it ? How 
 would you know it if commingled with the vapour of many other 
 metals or other substances? It is, in truth, a delicate piece of 
 analysis to discriminate iron in the glowing atmosphere of the sun. 
 But the spectroscope is adequate to the task, and it renders its 
 analysis with an evidence that is absolutely convincing. 
 
 Wide indeed is the gulf which separates this branch of analysis 
 from those to which chemists were formerly restricted. To analyse 
 a body in the old fashion, the chemist must have a sample of 
 that body, and then by his re-agents and his test tubes, he could 
 determine what the body contained. But how is the chemist to 
 procure a sample of the sun? He can never procure a sample 
 of any body external to the earth. No doubt, in the case where 
 a body, which we call a meteorite, falls to the earth, we certainly 
 can submit to chemical analysis an actual fragment derived from 
 external space. It is a matter of significance to observe that 
 when a chemist analyses a meteorite, he finds in it no element 
 with which he is not already familiar ; nay, rather, he is struck 
 by the remarkable fact that while nearly all meteorites contain 
 iron, some are almost entirely composed of that widely distributed 
 metal. We cannot, however, for the reasons set forth in Chapter 
 XVII., draw any reliable conclusions from meteorites as to the 
 
TEE SPECTROSCOPE. 443 
 
 constituents of bodies exterior to the earth. In our attempts to 
 analyse the sun, the moon, the planets, or the stars, we are there- 
 fore deprived of the ordinary resources of chemistry. To deal with 
 the problem a new branch of science has been created. 
 
 What we receive from the sun is warmth and light. The 
 intensely heated mass of the sun radiates forth its beams in all 
 directions with boundless prodigality. Each beam we feel to be 
 warm, and we see to be brilliantly white, but a more subtle 
 analysis than mere feeling or mere vision is required. Each 
 sunbeam really bears indelible marks of its origin. These marks 
 are not visible until a special process has been applied, but then 
 the sunbeam can be made to tell its story, and it will disclose to 
 us the nature of the sun's constitution. 
 
 We speak of the sun's light as colourless, just as we speak of 
 water as tasteless, but both of these expressions relate rather to 
 our own feelings than to anything really characteristic of water 
 or of sunlight. We regard the sunlight as colourless because it 
 forms, as it were, the background on which all other colours are 
 depicted. The fact is, that white is so far from being colour- 
 less, that it contains every hue known to us blended together 
 in certain proportions. The sun's light is really extremely com- 
 posite; Nature herself tells us this if we will but give her 
 the slightest attention. Whence come the beautiful hues with 
 which we are all familiar? Look at the lovely tints of a 
 garden ; the red of the rose is not in the rose itself. All the rose 
 does is to grasp the sunbeams which fall upon it, extract from 
 these beams the red which is in them, and radiate that red light 
 to our eyes. Were there not red rays commingled with the other 
 rays in the sunbeam, there could be no red rose to be seen by sun- 
 light. This point is, in fact, well known in ordinary conversation ; 
 a lady will often say that a dress which looks very well in the day- 
 light does not answer in the evening. The reason is that the dress 
 is intended to show certain colours which exist in the sunlight ; 
 but these colours do not exist to the same degree in gaslight, and 
 consequently the dress has a different hue. The fault is not in the 
 dress, the fault lies in the gas ; and when the electric light is used, 
 
444 THE STORY OF THE HEAVENS. 
 
 it sends forth beams more nearly resembling those from the sun, 
 and the colours appear again with all their intended beauty. 
 
 The most glorious natural indication of the nature of the sun- 
 light is seen in the rainbow. Here the sunbeams are refracted and 
 reflected from the tiny globes of water falling as rain-drops from 
 the clouds ; these convey to us the sunlight, and in doing so de- 
 compose the white beams into the seven primary colours red, 
 orange, yellow, green, blue, indigo, and violet. 
 
 The bow set in the cloud is typical of that great department 
 of modern science which we discuss in this chapter. The globes 
 of water decompose the solar beams ; and we f ollow the course 
 suggested by the rainbow, and analyse the sunlight into its 
 
 Fig. 82. The Prism. 
 
 constituents. "We are enabled to do this with scientific accuracy 
 when we employ that admirable invention, the spectroscope. The 
 beams of white sunlight really consist of innumerable beams of 
 every hue all commingled together. Every shade of red, of yellow, 
 of blue, and of green can be found in a sunbeam. The magician's 
 wand, with which we can strike the sunbeam and instantly sort 
 out into perfect order the tangled skein, is the simple instrument 
 known as the glass prism. We have represented a prism in its 
 simplest form in the adjoining figure (Fig. 82). It is a piece of 
 pure and homogeneous glass, ground and polished to the shape of a 
 wedge. When a ray of light from the sun or from any source falls 
 upon the prism, it passes through the transparent glass and emerges 
 on the other side ; a remarkable change is, however, impressed on 
 the ray by the influence of the glass. It is bent by refraction 
 from the path it originally pursued, and it is compelled to follow 
 a, different path. If, however, the prism bent all rays of light 
 
THE SPECTROSCOPE. 445 
 
 equally, then it would be of no service in the analysis o light ; 
 but it fortunately happens that the prism acts with varying 
 efficiency on the rays of different hues. A red ray is not refracted 
 so much as a yellow ray; a yellow ray is not refracted so 
 much as a blue one. It consequently happens that when the 
 composite beam of sunlight in which all the different rays are 
 blended, passes through the prism, they emerge in the manner 
 shown in the annexed figure. Here then we have the source of 
 the analysing power of the prism: it bends the different hues 
 unequally, and consequently the beam of composite sunlight, after 
 passing through the prism, no longer shows mere white light, but 
 
 Fig. 83. Dispersion of Light by the Prism. 
 
 is expanded into a coloured band of light, with hues like the rain- 
 bow, passing from deep red at one end through every intermediate 
 grade to the violet. 
 
 We have in the prism the means of decomposing the light 
 from the sun, or the light from any other source, into its com- 
 ponent parts. The examination of the quality of the light when 
 analysed will often enable us to learn something of the constitution 
 of the body from which this light has emanated. Indeed, in some 
 simple cases the mere colour of a light will be sufficient to indicate 
 the source from which the light has come. There is, for instance, 
 a splendid red light sometimes seen in displays of fireworks, due 
 to the metal strontium. The eye can at once identify the element 
 by the mere colour of the flame. There is also a very characteristic 
 yellow light produced by the flame of common salt burned with 
 
446 THE STORY OF THE HEAVEN'S. 
 
 spirits of wine. Sodium is the important constituent of salt, so 
 here we can recognise another substance merely by the colour it 
 emits when burning. We may also mention a third substance, 
 magnesium, which burns with a brilliant white light, eminently 
 characteristic of the metal. 
 
 The three metals strontium, sodium, and magnesium are thus 
 to be identified by the colours they produce when incandescent. 
 In this simple observation lies the germ of the modern method of 
 research known as spectrum analysis. We examine with the prism 
 the colours of the sun and the colours of the stars, and from this 
 examination we can learn something of the materials which enter 
 into their composition. When we study the sunlight through the 
 prism, it is found that the coloured band or the spectrum, as it is 
 called does not extend continuously from one end to the other, 
 but is shaded over by a multitude of dark lines, a few only of 
 which are shown in the adjoining plate. These lines are a per- 
 manent feature in the spectrum. They are as characteristic of the 
 sunlight as the prismatic colours themselves, and full of interest 
 and of information with regard to the sun. These lines are, indeed, 
 the characters in which the history and the nature of the sun are 
 written. Viewed through an instrument of adequate power, dark 
 lines are to be found crossing the solar spectrum in hundreds and 
 in thousands. They are of every variety of strength and faintness j 
 their distribution seems guided by no simple law. At some parts 
 of the spectrum there are but few lines ; in other regions they are 
 crowded so closely together that it is difficult to separate them. They 
 are in some places so exquisitely fine and delicate, that they 
 never fail to excite the admiration of every one who looks at this 
 interesting spectacle in a good instrument. 
 
 There can be no better method of expounding the rather difficult 
 subject of spectrum analysis than by actually following the steps 
 of the original discovery which first gave a clear demonstration 
 of the significance of the lines. Let us concentrate our attention 
 specially upon that line of the solar spectrum marked D. This line, 
 when seen in the spectroscope, is found to consist of two lines, very 
 delicately separated by a minute interval, one of these lines being 
 
THE SPECTROSCOPE. 447 
 
 slightly darker than the other. Suppose that while the attention 
 is concentrated on these lines, the flame of an ordinary spirit-lamp 
 coloured by common salt be held in front of the instrument, so that 
 the ray of direct and brilliant solar light passes through the flame 
 before entering the instrument. The observer sees at once the two 
 lines known as D flash out with a greatly increased blackness and 
 vividness, while there is no other perceptible effect on the spectrum. 
 A few trials show that this intensifying of the D lines is due to the 
 vapour of sodium, arising from the salt burning in the lamp through 
 which the sunlight has passed. 
 
 It is quite impossible that this marvellous connection between 
 sodium and the D lines of the spectrum can be a merely casual 
 matter. Even if there were only a single line concerned, it would 
 be in the highest degree unlikely that the coincidence should arise 
 by accident; but when we find the sodium affecting both of the 
 two close lines which form D, our conviction that there must be 
 some profound connection between these lines and sodium rises to 
 absolute certainty. Suppose, now, that the sunlight be cut off, 
 and that all other light be excluded save that emanating from the 
 glowing vapour of sodium in the spirit flame. We shall then find, 
 on looking through the spectroscope, that we have no longer all 
 the colours of the rainbow; the light from the sodium is now 
 concentrated into two bright yellow lines, filling precisely the 
 position which the dark D lines occupied in the solar spectrum, 
 and the darkness of which the sodium flame seemed to intensify. 
 We must here remove an apparent paradox. How is it, that though 
 the sodium flame produces two bright lines when viewed in the 
 absence of other light, yet it actually appears to intensify the 
 two dark lines in the sun's spectrum ? The explanation of this 
 conducts us at once to the cardinal doctrine of spectrum analysis. 
 The so-called dark lines in the solar spectrum are only dark by 
 contrast with the brilliant illumination of the rest of the spectrum. 
 A good deal of solar light really lies in the dark lines, though not 
 enough to be seen when the eye is dazzled by the brilliancy around. 
 When the flame of the spirit-lamp charged with sodium intervenes, 
 it sends out a certain amount of light, which is entirely localised 
 
448 THE STORY OF THE HEAVENS. 
 
 in these two lines. So far, it would seem that the influence of the 
 sodium flame ought to be manifested in diminishing- the darkness 
 of the lines and rendering them less conspicuous. As a matter 
 of fact, they are far more conspicuous with the sodium flame than 
 without it. This arises from the fact that the sodium flame pos- 
 sesses the remarkable property of cutting off the sunlight which 
 was on its way to those particular lines; and thus, though the 
 sodium contributes some light to the lines on the one hand, yet on 
 the other it intercepts a far greater quantity of the light that 
 would otherwise have illuminated those lines, and hence they 
 became darker with the sodium flame than without it. 
 
 Here, then, we are conducted to a remarkable principle, which 
 has led to the interpretation of the dark lines in the spectrum of 
 the sun. We find that sodium vapour, when heated, gives out 
 light of a very particular type, which, viewed through the prism, is 
 concentrated in two lines. But the sodium vapour possesses also 
 this property, that light from the sun can pass through it with- 
 out any perceptible absorption, save and except of those particular 
 rays which are of the same refrangibilities as the two lines in 
 question. In other words, we say that if the heated vapour of a 
 substance gives a spectrum of bright lines, corresponding to lights 
 of various refrangibilities, this same vapour will act as an opaque 
 screen to lights of those special refrangibilities, while remaining 
 transparent to light of every other kind. This is a matter of some 
 little complexity, but it is of such momentous importance in 
 the theory of spectrum analysis that we cannot avoid giving a 
 further example. Let us take the element iron, which in a very 
 striking degree illustrates the law in question. In the solar 
 spectrum some hundreds of the dark lines are known to correspond 
 with the spectrum of iron. This correspondence is exhibited in a 
 vivid manner when, by suitable contrivance, the light of an. 
 electric spark from poles of iron is examined in the spectroscope 
 simultaneously with the solar spectrum. It can be shown that 
 hundreds of the lines in the sun are identical in position with the 
 lines in the spectrum of iron. But the spectrum of iron, as here 
 described., consists of bright lines, while those with which it is 
 
THE SPECTROSCOPE. 449 
 
 compared in the sun consist of dark lines. These dark lines can 
 be completely understood if we suppose that the vapour arising- 
 from intensely heated iron be present in the atmosphere which 
 surrounds the luminous strata on the sun. This vapour would 
 then, by the law in question, stop precisely the same rays as it 
 emits when incandescent, and hence we learn the important fact 
 that iron, no less than sodium, must, in one form or another, be a 
 constituent of the sun. 
 
 Such is, in brief outline, the celebrated discovery of modern 
 times which has given an interpretation to the dark lines of the 
 solar spectrum. A large number of terrestrial substances have 
 been examined, in conjunction with the solar spectrum, and thus 
 it has been established that several of the elements known on 
 the earth are present in the sun. These elements are as follows : 
 hydrogen, sodium, barium, calcium, magnesium, aluminium, iron, 
 manganese, chromium, cobalt, nickel, zinc, copper, titanium, 
 cadmium, strontium, cerium, uranium, lead, potassium. 
 
 It would, indeed, lead us far beyond the prescribed limits of 
 this volume if we were to attempt to give any detailed account 
 of the beautiful researches in the application of spectrum analysis 
 to astronomy. It is, however, impossible that we could entirely 
 pass over discoveries of such interest ; and, therefore, we shall give 
 a brief though it must be a very brief account of some of the 
 principal points. 
 
 When we apply the spectroscope to the moon or to the planets, 
 we recognise in the light they send us the features characteristic 
 of the solar light. The reflected sunlight, even from the moon, is 
 not bright enough to show all the lines that can be seen when the 
 sun is examined directly. Venus, when examined with care, will 
 probably show more of the dark lines in the solar spectrum than 
 the moon. In the case of the planets, also, we often find certain 
 additional lines, or bands of lines, added to the spectrum. These 
 lines have arisen from the further special absorption of sunlight 
 which takes place in the planet's atmosphere. It should also be 
 mentioned that some of the lines in the solar spectrum are to 
 be referred to the absorption in our own atmosphere. This fact 
 D D 
 
450 THE STORY OF THE HEAVENS. 
 
 is clearlv brought out when the sun is observed near sunset, 
 for then the rays have a vast depth of air to traverse, and, 
 accordingly, we find the atmospheric bands to be defined with 
 increased vividness. 
 
 We have hitherto spoken of the light from the sun, without 
 attempting to distinguish the light from the different parts of 
 the sun. The spectroscope can, however, be applied to each part 
 of the sun separately, and thus it has been possible to establish 
 remarkable differences between the spectra of the several parts. 
 The most interesting discovery made in connection with this sub- 
 ject was that accomplished almost simultaneously by Janssen and 
 Lockyer, which has given a wide extension to the possibilities of 
 solar observation. In our chapter on the sun we have dwelt on 
 the very remarkable objects known as the solar prominences, which 
 become visible on the rare occasions of a total eclipse of the sun. 
 The remarkable discovery just referred to disclosed a method by 
 which the solar prominences could be observed at any time with the 
 aid of the spectroscope. 
 
 The prominences are composed of glowing gas, principally 
 hydrogen ; and the light, instead of being spread all over the 
 spectrum, like ordinary solar light, is mainly concentrated into the 
 few lines which constitute the spectrum of hydrogen. When the 
 spectroscope is directed upon the edge of the sun the rays of 
 ordinary diffused sunlight are distributed over the whole length 
 of the spectrum, and are, of course, correspondingly weakened. The 
 light from the prominences, being all concentrated into the lines, is, 
 therefore, relatively increased in intensity to such an extent that 
 they become visible. 
 
 The first line of Plate XIII. represents the solar spectrum, on 
 which a few only of the principal lines are introduced for com- 
 parison. It is, however, to be understood that the solar spectrum 
 really exhibits thousands of lines when viewed with a spectroscope of 
 adequate power. Among the varied snectra of the stars different 
 types have been recognised, the characteristic features of which are 
 also depicted. The first type of stars are those which are intensely 
 white, of which we may take Sirius and Vega as examples. In the 
 
CO f 
 
 0) 
 
 li 
 
 0) 01 
 
THE SPECTROSCOPE. 
 
 spectra of these stars there are comparatively few lines. The lines 
 of hydrogen are however very strong, but the metallic lines of 
 sodium and magnesium are barely perceptible. The presence of 
 hydrogen in the atmosphere of these stars has been confirmed in 
 a very remarkable manner by the photographs of the spectra of 
 these stars which have been obtained by Dr. Huggins. In these 
 photographs, lines can be seen in that part of the spectrum in- 
 visible to the eye, and these lines also have been found to coincide 
 with lines in the photographic spectrum of hydrogen. 
 
 The second type of stars exhibit spectra closely resembling that 
 of the sun ; a good illustration of this type is found in the star 
 Capella. Aldebaran has also a spectrum belonging to the second 
 type, though verging on the third. The chief hydrogen lines are still 
 conspicuous, but many metallic lines are coming strongly out. 
 
 The spectrum of a Orionis, or Betelgeuze, exhibits a typical 
 spectrum of the third class, which contains many metallic lines and 
 shaded bands; but the most characteristic feature of this type 
 is the absence of the hydrogen lines. 
 
 Another most remarkable achievement in spectrum analysis 
 has been accomplished by Dr. Huggins in his very beautiful 
 discovery of the spectroscopic method of measuring the motion of 
 stars along the line of sight. The place of a line of hydrogen, for 
 instance, will be slightly moved to one side or the other, according 
 as the star be approaching the earth or receding from the earth. 
 The amount of displacement is very small; but in the case of 
 several stars large enough to enable an approximation to be made 
 to the velocity with which the star is moving towards or moving 
 from the earth. 
 
 The theory of this method is beautifully verified by observations 
 on the sun. As the eastern edge of the sun is approaching and the 
 western is receding, there is a corresponding difference in the 
 spectra of the two edges, and the observed amount gives a velocity 
 of rotation practically coincident with that otherwise known. 
 
 r> D 2 
 
CHAPTER XXIII. 
 
 STAR CLUSTERS AND NEBULJ3. 
 
 Interesting Sidereal Objects Stars not Scattered uniformly Star Clusters Their 
 Varieties The Cluster in Perseus The Globular Cluster in Hercules The 
 Milky "Way A Cluster of Minute Stars Nebulae distinct from Clouds 
 Number of known Nebulae The Constellation of Orion The Position of the 
 Great Nebula The Wonderful Star Orionis The Drawing of the Great 
 Nebula in Lord Rosse's Telescope Photographs of this Wonderful Object 
 The Importance of Accurate Drawings The Great Survey Photographs of the 
 Heavens Magnitude of the Nebula The Question as to the Nature of a 
 Nebula Is it composed of Stars or of Gas ? How Gas can he made to Glow 
 Spectroscopic Examination of the Nebula The Great Nebula in Andromeda 
 Its Examination by the Spectroscope The Annular Nebula in Lyra 
 Resemblance to Vortex Rings Planetary Nebulae Drawings of Several 
 Remarkable Nebulse Distance of Nebulae Conclusion. 
 
 WE have already mentioned Saturn as one of the most glorious 
 telescopic spectacles in the heavens. Setting aside the obvious 
 claims of the sun and of the moon, there are, perhaps, two other 
 objects visible from these latitudes which rival Saturn in the 
 splendour and the interest of their telescopic picture. One of 
 these objects is the star cluster in Hercules; the other is the 
 great nebula in Orion. We may take these objects as typical of 
 the two great classes of bodies to be discussed in this chapter, 
 under the head of Star Clusters and Nebulse. 
 
 The stars that, to the number of several millions, bespangle the 
 sky, are not scattered uniformly. We see that while some regions 
 are comparatively bare of stars, others contain stars in profusion. 
 Sometimes we have a small group, like the Pleiades ; sometimes we 
 have a stupendous region of the heavens strewn over with stars, as 
 in the Milky Way. Such objects are called star clusters. We find 
 every variety in the clusters ; sometimes the stars are remarkable 
 for their brilliancy, sometimes for their enormous numbers, and 
 sometimes for the remarkable form in which they are grouped. 
 
STAR CLUSTERS AND NEBULM 453 
 
 Sometimes a star cluster is adorned with brilliantly-coloured stars ; 
 sometimes the stars are so close together that their separate rays 
 cannot be disentangled ; sometimes the stars are so minute or so 
 distant that the cluster is barely distinguishable from the nebula. 
 
 Of the clusters remarkable at once both for richness and 
 brilliancy of the individual stars, we may mention the cluster in 
 the Sword-handle of Perseus. The position of this object is 
 marked on Fig. 70, page 378. To the unaided eye a dull spot is 
 visible, which in the telescope expands into two clusters separated 
 by a short distance. In each of them we have innumerable 
 stars, crowded together so as to fill the field of view of the tele- 
 scope. The splendour of this object is appreciated when we reflect 
 that each one of these stars is itself a brilliant sun, perhaps 
 rivalling our own sun in lustre. There are, however, regions in 
 the heavens near the Southern Cross, of course invisible from 
 northern latitudes, in which parts of the Milky Way present a richer 
 appearance even than the cluster in Perseus. 
 
 The most striking type of star cluster is well exhibited in 
 the constellation of Hercules. In this case we have a group of 
 minute stars apparently in a roughly globular form. Fig. 84 
 represents this object as seen in Lord Rosse's great telescope, and 
 it shows three radiating streaks, in which the stars seem less 
 numerous than elsewhere. It is estimated that this cluster must 
 contain from 1,000 to 2,000 stars, all concentrated into an ex- 
 tremely small part of the heavens. Viewed in a very small tele- 
 scope, this object resembles a nebula. The position of the cluster in 
 Hercules is shown in a diagram previously given (Fig. 75, page 384) . 
 We have already referred to this glorious aggregation of stars as 
 one of the three especially interesting objects in the heavens. 
 
 The Milky Way forms a girdle which, with more or less regularity, 
 sweeps completely around the heavens ; and when viewed with the 
 telescope, is seen to consist of myriads of minute stars. In some 
 places the stars are much more numerous than elsewhere. All these 
 stars are incomparably more distant than the sun, which they entirely 
 surround, so it is evident that our sun, and of course the system 
 which attends him, lies actually inside the Milky Way. It seems 
 
454 THE STORY OF THE HEAVENS. 
 
 tempting to pursue the thought here suggested, and to reflect that 
 the whole Milky Way may, after all, be merely a star cluster, 
 comparable in size with some of the other star clusters which we 
 see, and that viewed from a remote point in space, the Milky Way 
 would seem to be but one of the countless clusters of stars con- 
 taining our sun as an indistinguishable unit. 
 
 Fig. 84. The Globular Cluster in Hercules. 
 
 When we direct a good telescope to the heavens, we shall 
 occasionally meet with one of the remarkable celestial objects which 
 are known as nebulae. They are faint cloudy spots, or stains of 
 light on the black background of the sky. They are nearly all 
 quite invisible to the naked eye. These celestial objects must not 
 for a moment be confounded with clouds, in the ordinary meaning 
 of the word. The latter exist only suspended in our atmosphere, 
 while nebulae are immersed in the depths of space. Clouds shine 
 
STAR CLUSTERS AND NEBULJZ. 455 
 
 only by the light of the sun, which they reflect to us ; nebulae 
 shine with no borrowed light : they are self-luminous. Clouds 
 change from hour to hour ; nebulae do not change even from year 
 to year. Clouds are far smaller than the earth ; while the smallest 
 nebula known to us is incomparably greater than the sun. Clouds 
 are within a few miles of the earth ; nebulae are not within many 
 millions of miles. 
 
 The systematic study of the nebulae may be said to have com- 
 menced with the colossal labours of William Herschel at Slough. 
 The scheme which Herschel proposed for this task was indeed a 
 comprehensive one. He determined to make a survey of the entire 
 heavens with a powerful telescope, and to note all the features 
 of interest which he could detect. But though we can thus 
 summarily describe the undertaking, yet a little reflection will show 
 the gigantic amount of labour which it entailed. Considering how 
 rapidly we can sweep our eyes over the heavens, it might seem a 
 very easy matter to turn a telescope to one spot after another, until 
 the whole sky has been reviewed. The two cases are, however, as 
 different as possible ; a glance of the eyes takes in an enormous 
 region of the heavens, while the field of a large telescope only 
 includes a very small region. This is a point which very often 
 surprises those who for the first time look through a large telescope. 
 People sometimes expect to see the whole northern hemisphere, and 
 perhaps the signs of the zodiac, all at once. It is even unreasonable to 
 ask to be shown the Great Bear in a large telescope ; the telescope 
 can be pointed to special parts of the constellation ; but if we want 
 a comprehensive view of the whole, we must take an opera-glass, 
 or something of that description, not a great and powerful instru- 
 ment. A large telescope will hardly show even so much as the 
 whole of the moon at once. When we look through the eye-piece, 
 we find the entire field filled with the brilliant body of the moon, 
 and it will be necessary to move the telescope a little up and 
 down, and a little to the right and to the left, in order to bring 
 the whole surface of the moon under review. The moon only 
 occupies a very small portion of the sky ; but small as that portion 
 is, the field of view in a large telescope is not so great. Suppose 
 
456 THE STORY OF THE HEAVENS. 
 
 the whole surface of the heavens to be covered over with moons 
 quite close together, the apparent size of each being the same as 
 that of our moon, the heavens would then form a mosaic of about 
 200,000 pieces. Assuming that a great telescope can show about 
 half of the moon at once, the whole surface of the heavens would 
 form about 400,000 fields of view. In such an instrument a com- 
 plete survey of the heavens must therefore involve no less than 
 400,000 examinations of what the telescope can reveal in one view. 
 This estimate includes the southern heavens ; if we confine our 
 thoughts to that portion of the heavens which is visible from these 
 latitudes, we may perhaps say that about a quarter of a million fields 
 of view have to be carefully examined. The scheme, then, which 
 Herschel sketched out for his labours at Slough involved bringing 
 a quarter of a million fields under review ; it will readily be believed 
 that considerable organisation was necessary to enable so vast an 
 undertaking to be accomplished. It was necessary to provide that 
 none of the fields should be allowed to escape, and that none should 
 be observed oftener than was necessary. 
 
 But there is another way ia which we can obtain an adequate 
 conception of the enormous labour which Herschel contemplated, 
 and which he lived in great part to complete. Instead of counting 
 the number of fields of view which he would have to examine, let 
 us attempt to form an estimate of the number of objects that would 
 be likely to come under his notice. We need hardly say that such 
 an estimate must be only an approximate one, but for the purpose 
 of conveying to the reader some idea of the extent of Hersehel's 
 labours, it will be quite sufficient. There are several different 
 classes of objects in the heavens, but the objects which are most 
 numerous and most characteristic are the fixed stars. The con- 
 stellations, with which every one is familiar, are formed of fixed 
 stars of every conceivable brightness, from the splendour of Sirius 
 down to the merest point of light that can be discerned in the most 
 powerful telescope. Stars can be found of every tint, from the red 
 at one end of the spectrum to the blue at the other, and they are 
 scattered in boundless profusion over the whole extent of the 
 heavens. 
 
STAR CLUSTERS AND NEBULM 457 
 
 Suppose that a great telescope like that which Herschel used in 
 his researches be employed for the purpose of counting the stars, 
 each one of the 250,000 fields of view would have to be regularly- 
 inventoried. In almost every one of those fields of view some stars 
 would be seen ; in many fields there would be a large number of 
 stars; while in others the stars would be found in countless 
 multitudes. 
 
 Immediately after Herschel and his sister had settled at 
 Slough he commenced his review of the northern heavens in a 
 systematic manner. For observations of this kind it is essential 
 that the sky be free from cloud, while even the light of the moon 
 is sufficient to obliterate the fainter and more interesting objects. 
 It was in the long and fine winter nights, when the stars were 
 shining brilliantly and the pale path of the Milky Way extended 
 across the heavens, that the labour was to be done. The great 
 telescope being directed to the heavens, the ordinary diurnal 
 motion by which the sun and stars appear to rise and set carries 
 the stars across the field of view in a majestic panorama. The 
 stars enter slowly into the field of view, slowly move across it, and 
 slowly leave it, to be again replaced by others. Thus the observer, 
 by merely remaining passive at the eye-piece, sees one field after 
 another pass before him, and is enabled to examine their contents. 
 It follows, that even without moving the telescope a long narrow strip 
 of the heavens is brought under review, and by moving the telescope 
 slightly up and down the width of this strip can be suitably in- 
 creased. On another night the telescope is brought into a different 
 position, and another strip of the sky is examined ; so that in the 
 course of time the whole heavens can be carefully scrutinised. 
 
 Herschel stands at the eye-piece to watch the glorious pro- 
 cession of celestial objects. Close by, his sister Caroline sits at her 
 desk, pen in hand, to take down the observations as they fall from 
 her brother's lips. In front of her is a chronometer from which 
 she can note the time, and a contrivance which indicates the 
 altitude of the telescope, so that she can record the exact posi- 
 tion of the object in connection with the description which her 
 brother dictated. Such was the splendid scheme which this brother 
 
458 THE STORY OF THE HEAVENS. 
 
 and sister had arranged as the object of their life-long devotion. 
 The discoveries which Herschel was destined to make were to be 
 reckoned not by tens or by hundreds, but by thousands. The 
 records of these discoveries are to be found in the "Philosophical 
 Transactions of the Royal Society/' and they are among the richest 
 treasures of those volumes. It was left to Sir John Herschel, the 
 
 Fig. 85. Tne Constellation of Orion, showing the position of the Great Nebula. 
 
 only son of Sir William, to complete his father's labour by extend- 
 ing the survey to the southern heavens. He undertook with this 
 object a journey to the Cape of Good Hope, and sojourned there 
 for the years necessary to complete the great work. 
 
 As the result of the gigantic labours thus inaugurated, there are 
 now three or four thousand nebulae known to us, and with every 
 improvement of the telescope fresh additions are made to the list. 
 They are scattered over both hemispheres, and some are to be found 
 in every constellation. They differ from one another as four 
 thousand pebbles selected at random on a sea-beach might differ 
 
STAR CLUSTERS AND NEBULA. 459 
 
 namely, in form, size, c6lour, and material but yet have, like the 
 pebbles, a certain generic resemblance to each other. To describe 
 this class of bodies in any detail would altogether exceed the limits 
 of this chapter ; we shall merely select a few of the nebula?, 
 choosing naturally those of the most remarkable character, and 
 also those which are representatives of the different groups into 
 which nebulae may be divided. 
 
 We have already alluded to the great nebula in the constella- 
 tion of Orion as being one of the most interesting objects in the 
 heavens. It is alike remarkable whether we look at its size or its 
 brilliancy, the care with which it has been studied, or the success 
 which has attended the efforts to learn something of its cha- 
 racter. To find this object, we refer to Fig. 85 for the sketch of 
 the chief stars in this constellation, where the letter A indicates 
 the middle one of the three stars which form the sword-handle 
 of Orion. Above the handle are the three stars which form the 
 well-known belt so conspicuous in the wintry sky. The star A, when 
 viewed attentively with the unaided eye, presents a somewhat misty 
 appearance. In the year 1618 Cysat directed a telescope to this 
 star, and saw surrounding it a curious luminous haze, which was 
 the great nebula. Ever since his time this object has been 
 diligently studied by many astronomers, so that innumerable 
 observations have been made of it, and even whole volumes have 
 been written which treat of nothing else. Any ordinary telescope 
 will show the object to some extent, but the more powerful the 
 telescope the more are the curious details revealed. 
 
 In the first place, the star A (6 Orionis) is in itself the most 
 striking multiple star in the whole heavens. It consists really of 
 six stars, represented in the next diagram (Fig. 86) ; these points 
 are so close together that their commingled rays cannot be dis- 
 tinguished without a telescope. Four of them are, however, 
 easily seen in quite small instruments, but the two smaller stars 
 require telescopes of considerable power. And yet these stars are 
 suns, comparable, it may be, with our sun in magnitude. 
 
 It is not a little remarkable that this unrivalled group of six 
 suns should be surrounded by the renowned nebula; the nebula 
 
460 THE STORY OF THE HEAVENS. 
 
 or the multiple star would, either of them alone, be of exceptional 
 interest, and here we have a combination of the two. It seems 
 impossible t) resist drawing the conclusion that the multiple star 
 really lies in the nebula, and not merely along the same line of 
 vision. It would, indeed, seem to be at variance with all probability 
 to suppose that the presentation of these two exceptional objects in 
 the same field of view was merely accidental. If the multiple star 
 be really in the nebula, as seems most likely, then this object 
 affords evidence that in one case at all events the distance of a 
 nebula is a quantity of the same magnitude as the distance of a 
 
 'v' // 
 
 *-.X 
 
 Fig. 86. The Multiple Star (0 Orionis) in the Great Nebula of Orion. 
 
 star. This is unhappily almost the entire extent of our knowledge 
 of the distances of the nebulae from the earth. 
 
 The great nebula of Orion surrounds the multiple star, and 
 extends out to a vast distance into the neighbouring space. 
 The dotted circle drawn around the star marked A in figure 85 
 represents approximately the extent of the nebula, as seen in a 
 moderately good telescope, The nebula is of a faint bluish 
 colour, impossible to represent in a drawing. Its brightness is 
 much greater in some places than in others; the central parts 
 are, generally speaking, the most brilliant, and the luminosity 
 gradually fades away as the edge of the nebula is approached. 
 In fact, we can hardly say that the nebula has any definite 
 boundary, for with each increase of telescopic power faint new 
 branches can be seen. There seems to be an empty space in the 
 nebula immediately surrounding the multiple star, but it is not 
 
STAR CLUSTERS AND NEBULA. 461 
 
 unlikely that this is merely an illusion, produced by the contrast of 
 the brilliant light of the stars. At all events, the spectroscopic 
 examination of the nebula seems to show that the nebulous matter 
 is continuous over the stars. 
 
 The plate of the great nebula in Orion which is here shown 
 represents, in a reduced form, the elaborate drawing of this object 
 which has been made by the Earl of Rosse with the great reflecting 
 telescope at Parsonstown.* A telescopic view of the nebula shows 
 a large number of stars, two hundred or more, scattered over its 
 surface. It is not necessary to suppose that these stars are all 
 immersed in the substance of the nebula as the multiple star 
 appears to be ; they may be either in front of it, or, less probably, 
 behind it, so as to be projected on the same part of the sky. In the 
 production of this drawing, the first task was to construct an accurate 
 chart of these stars, to serve as a skeleton on which the nebula 
 could be delineated. The execution of the elaborate drawing 
 taxed the powers of the great telescope for a period of some years. 
 It must, however, be remembered that it is only during a part of 
 the year that the nebula can be seen. Very fine nights are neces- 
 sary for observing these objects. There must be no clouds, and the 
 moon must be absent, for its bright light would quite extinguish 
 the fainter portions of a nebula. The Milky Way should be seen 
 clearly if the night is to be employed in drawing such objects. In 
 fact, taking the weather alone into consideration, there are in the 
 British Islands only about one hundred hours in the whole year which 
 are really suited for the very best astronomical observations. When 
 it is further remembered that the great telescope at Parsonstown is 
 a meridian instrument, with only a limited range of lateral motion, 
 it is plain that the nebula in Orion will only be within its reach 
 for an hour, or rather less, each evening. Unless, then, the sky be 
 clear at that particular hour, the whole night is lost so far as this 
 drawing is concerned, and hence it cannot be a matter of surprise 
 that much time has been expended in the completion of a work 
 requiring so much careful comparison and examination. There is, 
 indeed, considerable difficulty in seeing the fainter portions of this 
 * T am indebted for this drawing to the kindness of Messrs. De la Hue. 
 
462 THE STORY OF THE HEAVENS. 
 
 nebula. The long projecting arms, which seem so plain in the 
 drawing, can only be seen by the most careful scrutiny with a 
 practised eye. A little will be seen one night : this will be 
 verified on a subsequent evening, and the faint object will be 
 traced a little further. But this faintness is only to be understood 
 as applying to the outlying portions. The central part of the 
 nebula is bright enough to be seen in any telescope, and in the 
 great instrument presents a scene of surpassing glory. 
 
 A considerable number of drawings of this unique object have 
 been made by other astronomers. Among these, we must mention 
 that executed by Professor Bond, in Cambridge, Mass., which 
 possesses a degree of faithfulness in detail that every student of 
 this object is bound to acknowledge. Of late years also successful 
 attempts have been made to photograph the great nebula. The late 
 Professor Draper was fortunate enough to obtain some admirable 
 photographs. In England, too, Mr. Common has taken most ex- 
 cellent photographs of the nebula, which have deservedly gained 
 for their author the high honour of the Koyal Astronomical 
 Society's gold medal. 
 
 It may be asfced, What is the object of devoting such elaborate 
 care to the examination and drawing of this single object ? The 
 answer to this question is really two-fold. As a geographer would 
 survey a new country for the sake of ascertaining the details of its 
 mountains, its plains, and its rivers ; as he would sketch the most 
 important features in its scenery, and fill his note-books with 
 measurements and with descriptions ; so does the astronomer con- 
 sider a detailed survey of the heavens to be one of the numerous 
 duties which it is incumbent on him to discharge. The contents of 
 the heavens are to be carefully measured, they are to be accurately 
 described, they are to be artistically drawn; and the result 
 ultimately aimed at is a complete and exhaustive delineation of the 
 magnitude, weight, position, and peculiar features of every comet or 
 planet, every star and every nebula, which the utmost powers of our 
 greatest instruments can disclose to us. We can hardly hope that 
 this majestic scheme will be ever fully realised. The task is too 
 great for human labour to accomplish completely, but important 
 
STAR CLUSTERS AND NEBULA. 463 
 
 contributions to the undertaking are made every year. In pur- 
 suance of this object, the lives of some astronomers are devoted to 
 the preparation of catalogues in which the positions and magnitudes 
 of the fixed stars are recorded. Others are engaged in the delinea- 
 tion of charts of portions of the sky, in which the minutest stars 
 are depicted.* Others, again, expend their energies in making 
 drawings of the features visible on the moon, or on drawings and 
 measures of the planets, or in the measurement of double stars, or 
 in tracing the faint outlines of the nebulae. These are the results 
 of special surveys of different provinces in the vast kingdom. Now, 
 of all the objects in the heavens beyond the confines of our solar 
 system, one of the most splendid and the most remarkable is 
 that now before us ; and hence, in the great celestial survey it is 
 of paramount importance that we should possess faithful draw- 
 ings and measurements of every detail that can be seized of the 
 great nebula in Orion. 
 
 The laborious work of surveying is the first and the simplest 
 duty of the astronomer. Far more difficult, and perhaps far more 
 interesting problems are to follow. One of these problems is to 
 determine the movements of the heavenly bodies, and another the 
 most difficult of all is to determine their nature. The drawings of 
 the great nebula in Orion are designed to aid in the solution of both 
 
 * The tedious process of forming star charts by measurements or estimations of 
 each separate star, is apparently to be superseded by the more comprehensive 
 methods of photography. It has been found that very minute telescopic stars can 
 be made to produce a sensible picture when an exposure of suitable length has been 
 given. Stars of the 14th and 15th magnitudes are thus shown, and it is even 
 possible for stars to be recorded on the photographic plate, which are unable to 
 excite vision in the most powerful telescope. A plate taken by Mr. J. Roberts con- 
 tained an area of the sky about two degrees square. On comparison of this photo- 
 graph with the well-known map of Argelander, Father Perry states that he found 
 not only all the fifty-eight stars of the map, but at least thirty-two new stars, all 
 probably below the 9th or 10th magnitude. At the Paris Observatory an exposure 
 of an hour gave a negative of about five square degrees of the heavens, on which 
 2,790 stars from the 5th to the 14th magnitude were depicted, while traces of stars 
 of the 15th magnitude were visible. It is hoped, therefore, that in some six or 
 eight years a complete survey of the heavens on this splendid scale may be 
 accomplished. Many important results may be anticipated from this research : for 
 instance, a minor planet is at once revealed by its motion, which gives a streak 
 instead of a point. 
 
464 THE STORY OF THE HEAVENS. 
 
 these great problems ; to show whether the nebula has any appre- 
 ciable movements, and to elucidate its actual structure and physical 
 condition. It is a most interesting- question to decide, whether this 
 great nebula is really in a process of change. Looked at from 
 night to night, it does not alter ; even from year to year it still 
 seems the same. But is this permanence real ? Do not those arms 
 of light or the bright central regions in any degree wax or wane 
 become absorbed or extended, dissipated or dispersed like the 
 ordinary clouds whose evanescence is proverbial? This is a question 
 of the very deepest interest ; and how is it to be answered ? The 
 life of an astronomer is not long enough, even if his memory were 
 good enough, to pronounce finally upon this question. But by 
 making an elaborate drawing with the utmost care and precision, 
 by measuring the relative positions of the stars in the nebula with 
 the last degree of exactness, and by careful comparison ascertaining 
 that each minute part of the drawing represents the corresponding 
 part of the nebula as closely as it is possible for art to represent 
 nature ; then those drawings can at a future time be compared with 
 the nebula, and the question as to its permanence can thus be in- 
 vestigated. This has already been done, and the labour expended 
 upon these drawings has been amply rewarded. The entire subject 
 has been critically examined, and it has been demonstrated that 
 in parts of the mighty nebula certain changes are in progress, 
 though probably much more time must elapse before the full 
 import of these changes can be understood. 
 
 The magnitude of this gigantic object can perhaps never become 
 fully known to us. In fact, the light fades away so gradually that 
 we cannot tell how large a portion of sky it really occupies. What 
 can be seen certainly occupies an area of the sky two or three 
 times as great as that covered by the full moon. But what shall 
 we say of the real magnitude of this object ? As we do not know 
 its distance, we cannot, of course, attempt any estimate of its bulk, 
 though it is possible to enunciate a minor limit to its dimensions. 
 The magnitude of our earth, vast as it is, will not serve as a means 
 of comparison; we must have a far larger unit of measurement 
 for a bulk so stupendous. The earth sweeps around the sun in a 
 
STAR CLUSTERS AND NEBULM 465 
 
 mighty path, whose diameter is not less than 1 85,40 0,000 miles. 
 Let us imagine a sphere so mighty that this circle would just 
 form a girdle around its equator, and let this gigantic globe be the 
 measure wherewith to enunciate the bulk of the vast nebula of 
 Orion. It can be demonstrated that a million of these mighty 
 globes rolled into one would not equal the great nebula in bulk ; 
 though how much greater than this the nebula may really be we 
 have no means of ascertaining. 
 
 The actual nature of the nebula in Orion offers a problem of 
 the greatest interest, especially when we consider the question as to 
 the materials of which this object is composed? This is a pro- 
 blem which for a long time presented a matter of controversy, 
 but happily the discoveries of Dr. Huggins have, in a great 
 measure, decided the matter. 
 
 We have already given a brief description of some of those 
 objects known as star clusters, and we have, in particular, referred to 
 the great cluster in Hercules. This is a superb object when seen in 
 an instrument of adequate power, but in a small instrument it is 
 not seen exactly as a cluster of stars; it is, rather, a dull hazy point 
 of light, not by any means unlike the appearance of some of the 
 nebulae seen in a great instrument. As it was found that many 
 apparently nebulous objects were shown to be really star clusters 
 when adequate telescopic power was applied, it seemed perhaps 
 plausible to contend that all nebula might be really only clusters 
 of stars ; sunk, however, to so great a depth in space, that even the 
 largest telescopes failed to disentangle the several rays of the diffe- 
 rent stars, and thus produced merely the hazy appearance of a nebula. 
 
 How far can this reasoning be applied to the great nebula in 
 Orion ? We have, fortunately, one or two very interesting observa- 
 tions bearing on this point. On a particularly fine night, when the 
 speculum of the great six-foot telescope at Parson st own was in its 
 finest order, the skilled eye of the late Earl of Rosse and of his 
 then assistant, Mr. Stoney, detected in the densest part of the 
 nebula myriads of minute stars, which had never before been 
 recognised by human eye. Unquestionably, the commingled rays 
 of these stars contributes not a little to the brilliancy of the 
 E E 
 
466 THE STORY OF THE HEAVENS. 
 
 nebula. But there still remains the question as to whether the 
 entire luminosity of the great nehula can be so explained, or 
 whether the light thereof may not partly arise from some other 
 source. This question is one which must necessarily be forced on 
 the attention of any observer who has ever enjoyed the privilege 
 of viewing the great nebula through a telescope of power really 
 adequate to render justice to its beauty. It seems impossible to 
 believe that the bluish light of such delicately graduated shades has 
 really arisen merely from stellar points. The object is so soft and 
 so continuous might we not almost say so ghost-like? that 
 it is impossible not to believe that we are really looking at some 
 gaseous matter. 
 
 But here a difficulty may be suggested. The nebula is a 
 luminous body, but ordinary gas is invisible. We do not see the 
 gases which surround us and form the atmosphere in which we live. 
 How, then, if the nebula consisted merely of gaseous matter, would 
 we see it shining on the far distant heavens ? A well-known expe- 
 riment will at once explain this difficulty. We take a tube con- 
 taining a very small quantity of some gas : for example, hydrogen ; 
 this gas is usually invisible : no one could tell that there was any 
 gas in the tube, or still less could the kind of gas be known ; but 
 pour a stream of electricity through the tube, and instantly the 
 gas begins to glow with a violet light. What has the electricity 
 done for us in this experiment ? Its sole effect has been to heat 
 the gas. It is, indeed, merely a convenient means of heating the 
 gas and making it glow. It is not the electricity which we see, it 
 is rather the gas heated by the electricity. We infer, then, that if 
 the gas be heated it becomes luminous. The gas does not burn in 
 the ordinary sense of the word; no chemical change has taken 
 place. The tube contains exactly the same amount of hydrogen 
 after the experiment that it did before. It glows with the heat 
 just as red-hot iron glows. If, then, we could believe that in 
 the great nebula in Orion there were vast volumes of rarefied gas 
 in the same physical condition as the gas in the tube while the 
 electricity was passing, then we should expect to find that this gas 
 would actually glow. 
 
PLATE XV. 
 
 THE GREAT NEBULA IN ANDROMEDA. 
 

STAR CLUSTERS AND NEBULA. 467 
 
 To settle the question as to the real nature of the nebulae, we 
 must call in the aid of that refined method of investigation known 
 as spectrum analysis. We have explained in a previous chapter 
 the principle of this method in its application to the stars ; let us 
 now see how it can Le applied to the nebulae. The spectrum of a 
 gaseous nebula is a truly remarkable sight. Instead of the con- 
 tinuous band of colours, crossed by dark lines, which is characteristic 
 of the spectrum of a star, the nebular spectrum consists of but three 
 or four bright lines. Two of these lines correspond to the spectrum 
 of hydrogen, another belongs to nitrogen, while the fourth seems 
 to belong to some element not hitherto identified with any terres- 
 trial substance. 
 
 This remarkable discovery made by Dr. Huggins has been 
 corroborated and extended in a very interesting manner by the 
 more recent labours of the same astronomer, who has actually 
 succeeded in obtaining a photograph of the spectrum of the great 
 nebula in Orion. The photograph shows a strong line in that 
 invisible part of the spectrum extending beyond the violet rays, 
 and this line has been proved to be in all probability identical with 
 one due to hydrogen. 
 
 The labours of Dr. Huggins have thus solved to a considerable 
 extent the important problem as to the constitution of the nebula 
 in Orion. We see that it consists in part of stars, making up, 
 perhaps, in number for what they want in size. These stars are 
 bathed in and surrounded by a stupendous mass of glowing gas, 
 partly consisting of that gas which enters so largely into the com- 
 position of our ocean, namely, hydrogen, and partly of that which 
 is so important an ingredient in our atmosphere, namely, nitrogen ; 
 and further, that these are mingled with some other gaseous sub- 
 stance of a nature at present unknown. 
 
 Space will only admit of a brief reference to a few other 
 nebulae, and among these, a chief place must be given to the 
 great nebula in Andromeda. This is visible to the unaided eye, 
 and has, indeed, not unfrequently been mistaken for a comet. Its 
 telescopic appearance is shown in Plate XV._, which has been 
 copied from one of Mr. Trouvelot's beautiful drawings made at 
 E E 2 
 
468 THE STORY OF THE HEAVENS. 
 
 Harvard College Observatory. Two dark channels in the nebula 
 cannot fail to be noticed, and the number of faint stars scattered 
 over its surface is also a point to which attention may be drawn. 
 To find this object, we must look out for Cassiopeia and the Great 
 Square of Pegasus, and then the nebula will be easily perceived in 
 the position shown on p. 376. It differs in a very remarkable degree 
 from the gaseous nebula of which that in Orion is a type. When 
 we examine the nebula in Andromeda with the spectroscope, we 
 find a faint continuous band of light, of a totally different character 
 to that shown in the four bright lines of Orion. In fact, the 
 spectroscopic evidence would seem to indicate that there was no 
 glowing gas in the nebula in Andromeda, and that it may really 
 be due to the existence of a vast cluster of minute stellar points. 
 Those who are experienced in the observation of nebulas, will be 
 aware of a peculiar bluish hue observable in the gaseous nebulas, 
 which is not present in those of the Andromeda type. In fact, the 
 difference is so marked, that it is quite possible to say to which of 
 the two classes the nebula really belongs, even before the crucial 
 test of the spectroscope has been applied. 
 
 Among the various other classes of nebulae, perhaps the most 
 striking are those known as the Annular, or the Ring Nebulas. The 
 most celebrated of these objects is found in the constellation of 
 Lyra, and its position can be readily determined from the annexed 
 sketch of the principal stars in the constellation (Fig. 87). This 
 annular nebula belongs to the gaseous class of objects, though the 
 lines in its spectrum are not so numerous as those in the nebula 
 of Orion. We here give two attempts to delineate this curious 
 object (Fig. 88). The drawing will, at all events, serve the pur- 
 pose of enabling the student to identify the nebula when he meets 
 with it in the heavens. The smaller picture shows it as seen in an 
 instrument of moderate size ; the larger one represents it as seen 
 in one of our more powerful telescopes. The latter view discloses 
 a number of stellar points; it also shows the remarkable fringe 
 surrounding the ring, while the interior of the ring is seen not to 
 be absolutely dark. 
 
 The nebula in Lyra is the most conspicuous ring nebula in the 
 
STAR CLUSTERS AND NEBULA. 
 
 469 
 
 heavens, but it is not to be supposed that it is the only member of 
 this class. Altogether, there are about a dozen of these objects. It 
 seems difficult to form any adequate conception of the nature of 
 
 Fig. 87. Lyra, with the Annular Nebula. 
 
 such a body. It is, however, impossible to view the annular 
 nebulae without being-, at all events, reminded of those elegant 
 objects known as vortex rings. Who has not noticed a graceful 
 
 Fig. 88. The Annular Nebula in Lyra. 
 
 ring of steam which occasionally escapes from the funnel of a 
 locomotive, and ascends high into the air, only dissolving some time 
 after the steam not so specialised has disappeared ? Such vortex 
 rings can be produced artificially by a cubical box, one open side of 
 which is covered with canvas, while on the opposite side of the box 
 is a circular hole. A tap on the canvas will cause a vortex ring to 
 start from the hole ; and if the box be filled with smoke, this ring 
 
470 THE STORY OF THE HEAVENS. 
 
 will be visible for many feet of its path. It would certainly be far 
 too much to assert that the annular nebulae have any real analogy 
 to vortex rings ; but if they have not, there is, at all events, no 
 other object known to us with which they can be compared. 
 
 The heavens contain a number of minute but brilliant objects 
 known as the planetary nebulae. They can only be described as 
 globes of glowing bluish-coloured gas, small enough to be often 
 mistaken for a star when viewed through a telescope. One of the 
 most remarkable of them lies in the constellation Draco, and can 
 be found half-way between the Pole Star and the star 7 Draconis. 
 Some of the more recently discovered planetary nebulae are ex- 
 tremely small, and they have indeed only been distinguished from 
 small stars by the spectroscope. It is also to be noticed that such 
 objects are a little out of the stellar focus in the refracting tele- 
 scope in consequence of their blue colour. This remark does not 
 apply to a reflecting telescope, as this instrument conducts all the 
 rays to a common focus. 
 
 There are many other forms of nebulae : there are long nebu- 
 lous rays; there are the wondrous spiral nebulae which have 
 been disclosed in Lord Rosse's great reflector; there are the 
 double nebulae; there are some very mysterious variable nebulae. 
 But all these various objects we must merely dismiss with this 
 passing reference. There is a great difficulty in making pictorial 
 representations of such nebulae. Most of them are very faint so 
 faint, indeed, that they can only be seen with close attention even 
 in powerful instruments. In making drawings of these objects, it 
 is impossible to avoid intensifying the features if an intelligible 
 picture is to be made. With this caution, however, we present 
 Plate XVI., which exhibits several of the more remarkable nebulae 
 as seen through Lord Rosse's great telescope. 
 
 There is one problem of the very greatest interest with regard 
 to the nebulae, which astronomers often turn over in their thoughts, 
 and which they as often despair of seeing satisfactorily settled. 
 That problem is this how to find the distance of a nebula. The 
 difficulties of finding the distance of a star are so great, that it is 
 only by the most lavish expenditure of time and of patience that 
 
PLATE XVI. 
 
 NEBULXE 
 OBSERVED WITH LORD ROSSE's GREAT TELESCOPE. 
 
STAR CLUSTERS AND NEBULA. 471 
 
 it can be accomplished ; but the difficulties are very much greater 
 and apparently insurmountable in the case of the nebulae, and no 
 method has yet been devised which will enable us to solve this 
 mighty problem. Our knowledge on the subject is merely of a 
 negative character. We cannot tell how great the distance may 
 be, but we are able in some cases to assign a minor limit to 
 that distance. Our ordinary measures of miles are quite unsuit- 
 able for such distances as we shall have to encounter. We must 
 call in a far longer measuring tape, and, fortunately, we have one 
 well suited for the purpose. The most appropriate unit for such 
 magnitudes is the velocity of light, which sweeps along with the 
 prodigious speed of 180,000 miles a second. Moving at this rate, 
 how long will the journey take from the nebula to the earth? 
 It is believed that some of these nebula3 are sunk in space to 
 such an appalling distance, that the light takes centuries before it 
 reaches the earth. We see these nebulae, not as they are now, 
 but as they were centuries ago. At this climax we bid farewell 
 to the nebulae. We have reached a point where man's intellect 
 begins to fail to yield him any more light, and where his 
 imagination has succumbed in the endeavour to realise what he 
 has gained. 
 
CHAPTER XXIV. 
 
 THE PRECESSION AND NUTATION OF THE EARTH^S AXIS. 
 
 The Pole is not a Fixed Point Its Effect on the Apparent Places of the Stars 
 The Illustration of the Peg-top The Disturbing Force which acts on the 
 Earth Attraction of the Sun on a Globe The Protuberance at the Equator 
 The Attraction of the Protuberance by the Sun and by the Moon produces 
 Precession The Efficiency of the Precessional Agent varies inversely as the 
 Cube of the Distance The Kelative Efficiency of the Sun and the Moon How 
 the Pole of the Earth's Axis Eevolves round the Pole of the Ecliptic. 
 
 THE position of the pole of the heavens is most conveniently indi- 
 cated by the bright star known as the Pole Star, which lies in its 
 immediate vicinity. Around this pole the whole heavens appear to 
 rotate once in a sidereal day ; and we have hitherto always referred 
 to the pole as though it were a fixed point in the heavens. This 
 language is sufficiently correct when we only embrace a moderate 
 period of time in our review. It is no doubt true that the pole lies 
 near the Pole Star at the present time. It did so during the lives 
 of the last generation, and it will do so during the lives of the next 
 generation. All this time, however, the pole is steadily moving in 
 the heavens, so that the time will at length come when the pole 
 will have departed a long way from the present Pole Star. This 
 movement of the pole is incessant. It can be easily detected and 
 measured by the instruments in our observatories, and astronomers 
 are familiar with the fact that in all their calculations it is neces- 
 sary to hold special account of this movement of the pole. It 
 produces an apparent change in the place of a star, which is known 
 by the term " precession." 
 
 The movement of the pole is very clearly shown in the accom- 
 panying figure, for which I am indebted to the kindness of 
 Professor C. Piazzi Smyth. The circle shows the track along 
 which the pole moves among the stars. The centre of the circle in 
 the constellation of Draco is the pole of the ecliptic. A complete 
 
PRECESSION OF THE EARTIPS AXIS. 
 
 473 
 
 journey of the pole occupies the considerable period of about 
 25,867 years. The drawing shows the position of the pole at the 
 
 ,''.*''';' 
 
 Star-m&p , representing tiif processionals 
 of iht/ GxZ&saaZ/ Polts of rotation; and/ tspeciatyr marfary ii/ 
 
 iron/ ikes year WOO B.C. to itu/fear 2000 A. D . 
 SvMbdU aJopttds to ryruuU' ifu/ maaTiitiubs or IrujlOiuisa tfOu/ start, til 1 ,- 27^O, Jr*A, 4 f^B . 
 
 Fig. 89. 
 
 several dates from 4000 B.C. to 2000 A.D. A glance at this map 
 brings prominently before us how casual is the proximity of the 
 pole to the Pole Star. At present, indeed, the distance of the two 
 is actually lessening, but afterwards the distance will increase, until 
 
474 THE STORY OF THE HEAVENS. 
 
 when half of the revolution has been accomplished, the pole will be 
 at a distance of twice the radius of the circle from the Pole Star. 
 It will then happen that the pole will be near the bright star Vega 
 or a Lyra, so that our successors some 12,000 years hence may 
 make use of Vega for many of the purposes for which the Pole Star 
 is at present employed ! Looking back into past ages, we see that 
 some 2,000 or 3,000 years B.C. the star a Draconis was suitably 
 placed to serve as the Pole Star, when ft and S of the Great Bear 
 served as pointers. It need hardly be added, that since the birth 
 of accurate astronomy the course of the pole has only been observed 
 over a very small part of the mighty circle. We are not, however, 
 entitled to doubt that the motion of the pole will continue to 
 pursue the same path. This will be made abundantly clear when 
 we proceed to render an explanation of this very interesting pheno- 
 menon. 
 
 The north pole of the heavens is the point of the celestial sphere 
 towards which the northern end of the axis about which the earth 
 rotates is directed. It therefore follows that this axis must be con- 
 stantly changing its position. The character of the movement of 
 the earth, so far as its rotation is concerned, may be illustrated by a 
 very common toy with which every boy is familiar. "When a peg- 
 top is set spinning, it has, of course, a very rapid rotation around 
 its axis, but besides this rotation there is usually another motion, 
 whereby the axis of the peg-top does not remain in a constant 
 direction, but moves in a conical path around the vertical line. 
 The adjoining figure gives a view of the peg-top. It is, of course, 
 rotating with great rapidity around its axis, while the axis itself 
 revolves around the vertical line with a very deliberate motion. If 
 we could imagine a vast peg-top which rotated on its axis once a 
 day, and if that axis were inclined at an angle of twenty- three and 
 a half degrees to the vertical, and if the slow conical motion of the 
 axis were such that the revolution of the axis were completed in 
 about 26,000 years, then the movements would resemble those 
 actually made by the earth. The illustration of the peg-top comes, 
 indeed, very close to the actual phenomenon of precession. In each 
 case the rotation about the axis is incomparably more rapid than 
 
PRECESSION OF THE EARTH'S AXIS. 475 
 
 that of the revolution of the axis itself; in each case also the slow 
 movement is due to an external interference. Looking at the figure 
 of the peg-top we may ask the question, Why does it not fall 
 down ? The obvious effect of gravity would seem to say that it 
 is impossible for the peg-top to be in the position shown in the 
 figure. Yet everybody knows that this is possible so long as the 
 top is spinning. If the top were not spinning, it would, of course, 
 fall. It therefore follows that the effect of the rapid rotation of 
 the top so modifies the effect of gravitation, that the latter, instead 
 of producing its apparently obvious consequence, causes the 
 
 Fig. 90. Illustration of the motion of Precession. 
 
 slow conical motion of the axis of rotation. This, is, no doubt, 
 a dynamical question of some difficulty, but it is easy to verify 
 experimentally that it is the case. If a top be constructed so that 
 the point about which it is spinning, and which supports it, shall 
 coincide with the centre of gravity, then there is no effect of gravi- 
 tation on the top, and there is no conical motion perceived. 
 
 If the earth were subject to no external interference, then the 
 direction of the axis about which it rotates must remain for ever 
 constant ; but as the direction of the axis does not remain constant, 
 it is necessary to seek for a disturbing force adequate to the pro- 
 duction of the phenomena which are observed. We have invariably 
 found that the dynamical phenomena of astronomy can be accounted 
 for by the law of universal gravitation. It is therefore natural to 
 inquire how far gravitation will render an account of the pheno- 
 menon of precession ; and to put the matter in its simplest form, 
 
476 THE STORY OF THE HEAVENS. 
 
 let us consider the effect which a distant attracting body can have 
 upon the rotation of the earth. 
 
 To answer this question, it becomes necessary to define pre- 
 cisely what we mean by the earth ; and as for most purposes of 
 astronomy we regard the earth as a spherical globe, we shall 
 commence with this assumption. It seems also certain that the 
 interior of the earth is, on the whole, heavier than the outer 
 portions. It is therefore reasonable to assume that the density 
 increases as we descend ; nor is there any sufficient ground for 
 thinking that the earth is much heavier in one part than at any 
 other part equally remote from the centre. It is therefore usual in 
 such calculations to assume that the earth is formed of concentric 
 spherical shells, each one of which is of uniform density ; while 
 the density decreases from each shell to the one exterior thereto. 
 
 A globe of this constitution being submitted to the attraction 
 of some external body, let us examine the effects which that external 
 body can produce. Suppose, for instance, the sun attracts a globe 
 of this character, what movements will be the result ? The first 
 and most obvious result is that which we have already so frequently 
 discussed, and which is expressed by Kepler's laws : the attraction 
 will compel the earth to revolve around the sun in an elliptic path, 
 of which the sun is in the focus. With this movement we are, 
 however, not at this moment concerned. We must inquire how 
 far the sun's attraction will modify the earth's rotation around its 
 axis. It can be demonstrated that the attraction of the sun would 
 be powerless to derange the rotation of the earth so constituted. 
 This is a result which can be formally proved by mathematical 
 calculation. It is, however, sufficiently obvious that the force 
 of attraction of any distant point on a symmetrical globe must pass 
 through the centre of that globe; and as the sun is only an 
 enormous aggregate of attracting points, it can only produce a 
 corresponding multitude of attractive forces ; each of these forces 
 passes through the centre of the earth, and consequently the 
 resultant force which expresses the joint result of all the individual 
 forces must also be directed through the centre of the earth. A 
 force of this character, whatever other potent influence it may have. 
 
PRECESSION OF THE EARTH'S AXIS. 477 
 
 will, at all events, be powerless to affect the rotation of the earth. 
 If the earth be rotating on an axis, the direction of that axis 
 would be invariably preserved ; so that as the earth revolves around 
 the sun, it would still continue to rotate around an axis which 
 always remained parallel to itself. Nor would the attraction of the 
 earth by any other body prove more efficacious than that of the 
 sun. If the earth really were the symmetrical globe we have 
 supposed, then the attraction of the sun and moon, and even the 
 influence of all the planets as well, would never be competent to 
 make the earth's axis of rotation swerve for a single second from 
 its original direction. 
 
 We have thus narrowed very closely the search for the cause of 
 the " precession." If the earth were a perfect sphere, precession 
 would be inexplicable. We are therefore forced to seek for an 
 explanation of precession in the fact that the earth is not a perfect 
 sphere. This we have already demonstrated to be the case. We 
 have shown that the equatorial axis of the earth is longer than 
 the polar axis, so that there is a protuberant zone girdling the 
 equator. The attraction of external bodies is able to grasp this 
 protuberance, and thereby force the earth's axis of rotation to 
 change its direction. 
 
 There are only two bodies in the universe which sensibly con- 
 tribute to the precessional movement of the earth's axis : these 
 bodies are the sun and the moon. The shares in which the labour 
 is borne by the sun and the moon are not what might have been 
 expected from a hasty view of the subject. This is a point on 
 which it will be desirable to dwell, as it illustrates a point in the 
 theory of gravitation which is of very considerable importance. 
 
 The law of gravitation asserts that the intensity of the attrac- 
 tion which a body can exercise is directly proportional to the mass 
 of that body, and inversely proportional to the square of the 
 distance of the attracted point. We can thus compare the 
 attraction exerted upon the earth by the sun and by the moon. 
 The mass of the sun exceeds the mass of the moon in the 
 proportion of about 26,000,000 to 1. On the other hand, the moon 
 is at a distance which, on an average, is about one-386th part of 
 
478 THE STORY OF THE HEAVENS. 
 
 that of the sun. It is thus an easy calculation to show that the 
 efficiency of the sun's attraction on the earth is about 175 times 
 as great as the attraction of the moon. Hence it is, of course, 
 that the earth obeys the supremely important attraction of the 
 sun, and pursues an elliptic path around the sun, bearing the 
 moon as an appendage. 
 
 But when we come to that particular effect of attraction which 
 is competent to produce precession, we find that the law by which 
 the efficiency of the attracting body is computed assumes a different 
 form. The measure of efficiency is, in this case, to be found by 
 taking the mass of the body and dividing it by the cube of the 
 distance. The complete demonstration of this statement must be 
 sought in the formulae of mathematics, and cannot be introduced 
 into these pages : we may, however, adduce one consideration 
 which will enable the reader in some degree to understand the 
 principle, though without pretending to be a demonstration of its 
 accuracy. It will be obvious that the nearer the disturbing body 
 approaches to the earth the greater is the leverage (if we may 
 use the expression) which is afforded by the protuberance at the 
 equator. The efficiency of a given force will, therefore, on this 
 account alone, increase in the inverse proportion of the distance. 
 The actual intensity of the force itself augments in the inverse 
 square of the distance, and hence the capacity of the attracting 
 body for producing precession will, for a double reason, increase 
 when the distance decreases. Suppose, for example, that the 
 disturbing body is brought to half its original distance from the 
 disturbed body, the leverage is by this means doubled, while the 
 actual intensity of the force is at the same time quadrupled accord- 
 ing to the law of gravitation. It will follow that the effect pro- 
 duced in the latter case must be eight times as great as in the 
 former case. And this is merely equivalent to the statement that 
 the precession-producing capacity of a body varies inversely as the 
 cube of its distance. 
 
 It is this consideration which gives to the moon an importance 
 as a precession-producing agent which its mere attractive capacity 
 would not have entitled it to. Even though the mass of the sun 
 
PRECESSION OF THE EARTH'S AXIS. 479 
 
 be 26,000,000 times as great as the mass of the moon, yet when 
 this number is divided by the cube of the relative value of the 
 distances of the bodies (386), it is seen that the efficiency of the 
 moon is more than twice as great as that of the sun. In other 
 words, we may say that one- third of the movement of precession is 
 due to the sun, and two-thirds to the moon. 
 
 For the study of the joint precessional effect due to the sun and 
 the moon acting simultaneously, it will be advantageous to consider 
 the effect produced by the two bodies separately ; and as the case of 
 the sun is the simpler of the two, we shall take it first. As the 
 earth rotates in its annual path around the sun, the axis of the 
 earth is directed to a point in the heavens which is always 23 J 
 from the pole of the ecliptic. The precessional effect of the sun is 
 to cause the pole of the earth to revolve, always preserving the 
 same angular distance; and thus we have a motion of the type 
 represented in the diagram. As the ecliptic occupies a position 
 which for our present purpose we may regard as fixed in space, it 
 follows that the pole of the ecliptic is a fixed point on the surface 
 of the heavens ; so that the path of the pole of the earth must be 
 a small circle in the heavens, fixed in its position relatively to the 
 surrounding stars. In this we find a motion strictly analogous to 
 that of the peg-top. It is the gravitation of the earth acting upon 
 the peg-top which forces it into the conical motion. The immediate 
 effect of the gravitation is so modified by the rapid rotation of the 
 top, that, in obedience to a profound dynamical principle, the axis 
 of the top revolves in a cone rather than fall down, as it would do 
 were the top not spinning. In a similar manner the immediate 
 effect of the sun's attraction on the protuberance at the equator 
 would be to bring the pole of the earth's axis towards the pole of 
 the ecliptic, but the rapid rotation of the earth modifies this into 
 the conical rotation of precession. 
 
 The circumstances with regard to the moon are much more 
 complicated. The moon describes a certain orbit around the 
 earth; that orbit lies in a certain plane, and that plane has, of 
 course, a certain pole on the celestial sphere. The precessional 
 effect of the moon would accordingly tend to make the pole of 
 
480 THE STOEY OF THE HEAVENS. 
 
 the earth's axis describe a circle around that point in the heavens 
 which is the pole of the moon's orbit. This point is about 5 
 from the pole of the ecliptic. The pole of the earth is therefore 
 solicited by two different movements one a revolution around 
 the pole of the ecliptic, the other a revolution about another 
 point 5 distant, which is the pole of the moon's orbit. It would 
 thus seem that the earth's pole should make a certain composite 
 movement due to the two separate movements. This is really 
 the case, but there is a point to be very carefully attended to 
 which at first seems almost paradoxical. We have shown how the 
 potency of the moon as a precessional agent exceeds that of 
 the sun, and therefore it might be thought that the composite 
 movement of the earth's pole would conform more nearly to a 
 rotation around the pole of the plane of the moon's orbit than 
 to a rotation around the pole of the ecliptic ; but this is not the 
 case. The precessional movement is found to be almost absolutely 
 represented by a revolution around the pole of the ecliptic, as is 
 shown in the figure. Here is a point which merits our careful 
 attention, for in it lies the germ of one of the most exquisite of 
 astronomical discoveries. 
 
 The plane in which the moon revolves does not occupy a con- 
 stant position. We are not here specially concerned with the causes 
 of this change in the plane of the moon, but the character of the 
 movement must be enunciated. The inclination of this plane to 
 the ecliptic is about 5, and this inclination does not vary; but the 
 line of intersection of the two planes does vary, and, in fact, varies 
 so quickly that it completes a revolution in about 18f years. This 
 movement of the plane of the moon's orbit necessitates a corre- 
 sponding change in the position of its pole. We thus see that the 
 pole of the moon's orbit must be actuajly revolving around the pole 
 of the ecliptic, always remaining at the same distance of 5, and 
 completing its revolution in 18f years. It will, therefore, be obvious 
 that there is a profound difference between the precessional effect of 
 the sun and of the moon in their action on the earth. The sun 
 invites the earth's pole to describe a circle around a fixed centre ; 
 the moon invites the earth's pole to describe a circle around a centre 
 
PRECESSION OF THE EARTH'S AXIS. 481 
 
 which is itself in constant motion. It fortunately happens that 
 the circumstances of the case are such as to considerably reduce the 
 complexity of the problem. The movement of the moon's plane, 
 only occupying about 18f years, is a very rapid motion compared 
 with the whole precessional movement, which occupies about 26,000 
 years. It follows that by the time the earth's axis has completed 
 one circuit of its majestic cone, the pole of the moon's plane will 
 have gone round about 1,400 times. Now, as this pole really only 
 describes a comparatively small cone of 5 U in radius, we may for 
 a first approximation take the average position which it occupies ; 
 but this average position is, of course, the centre of the circle which 
 it describes that is, the pole of the ecliptic. 
 
 We thus see that the average precessional effect of the moon 
 simply conspires with that of the sun to produce a revolution 
 around the pole of the ecliptic. The grosser phenomena of the 
 movements of the earth's axis are to be explained by the uni- 
 form revolution of the pole in a circular path ; but if we make 
 a minute examination of the track of the earth's axis, we shall 
 find that though it, on the whole, conforms with the circle, yet 
 that it really traces out a delicate sinuous line, sometimes on 
 the inside and sometimes on the outside of the circle. This 
 delicate movement arises from the continuous change in the 
 place of the pole of the moon's orbit. The period of these 
 undulations is 18f years, agreeing exactly with the period of the 
 revolution of the moon's nodes. The amount by which the pole 
 departs from the circle on either side is only about 9'2 seconds a 
 quantity rather less than the twenty-thousandth part of the radius of 
 the circle. This phenomenon, known as " nutation," was discovered 
 by the beautiful telescopic researches of Bradley in 1747. Whether 
 we look at the theoretical interest of the subject or at the refine- 
 ment of the observations involved, this achievement of the " Vir 
 incomparabilis," as Bradley has been called by Bessel, is one of 
 the masterpieces of astronomical genius. 
 
 F F 
 
CHAPTER XXV. 
 
 THE ABERRATION OF LIGHT. 
 
 The Real and Apparent Movements of the Stars How they can be Discriminated 
 Aberration produces Effects dependent on the Position of the Stars The Pole 
 of the Ecliptic Aberration make Stars se^ms to Move in a Circle An Ellipse 
 or a Straight Line according to Position All the Ellipses have Equal Major 
 Axes How is this Movement to be Explained ? How to be Distinguished 
 from Annual Parallax The Apex of the Earth's Way How this is to be 
 Explained by the Velocity of Light How the Scale of the Solar System can 
 be Measured by the Aberration of Light. 
 
 WE have in this chapter to narrate a discovery of a recondite 
 character, which illustrates in a very forcible manner some of 
 the great fundamental truths of Astronomy. Our discussion of it 
 will naturally be divided into two parts. In the first part we 
 must describe the nature of the phenomenon, and then we must 
 give the extremely elegant explanation afforded by the properties 
 of light. The telescopic discovery of aberration, as well as its 
 explanation, are both due to the illustrious Bradley. 
 
 The expression fixed star, so often used in astronomy, is to be 
 received in a very qualified sense. The stars are, no doubt, well 
 fixed in their places, so far as coarse observation is concerned. The 
 lineaments of the constellations remain unchanged for centuries, 
 and in contrast with the ceaseless movements of the planets, the stars 
 are not inappropriately called fixed. We have, however, had more 
 than one occasion to show throughout the course of this work that 
 the expression " fixed star''' is not an accurate one when minute quan- 
 tities are held in estimation. With the exact measures of modern 
 instruments, many of these quantities are so perceptible, that they 
 have to be always reckoned with in astronomical inquiry. We can 
 divide the movements of the stars into two great classes : the real 
 movements and the apparent movements. The proper motion of 
 the stars and the movements of revolution of the binary stars coii- 
 
THE ABERRATION OF LIGHT. 483 
 
 stitute the real movements of these bodies. These movements are 
 special to each star, so that two stars, although close together in 
 the heavens, may differ in the widest degree as to the real move- 
 ments which they possess. It may, indeed, sometimes happen, as 
 Mr. Proctor has pointed out, that stars in a certain region are 
 animated with a common movement. In this phenomenon, which 
 has been called star-drift by its discoverer, we have traces of a real 
 movement shared in by a number of stars in a certain group. 
 With this exception, however, the real movements of the stars are 
 governed by no systematic law, and the rapidly rotating binary 
 stars and the other rapidly moving stars are scattered here and 
 there indiscriminately over the heavens. 
 
 The apparent movements of the stars have a different character, 
 inasmuch as we find the movement of each star determined by the 
 place which it occupies in the heavens and not by the individual 
 features of the particular object. It is by this means that we can 
 readily discriminate the real movements of the star from its 
 apparent movements, and examine the character of both. 
 
 In the present chapter we are concerned with the apparent 
 movements only, and of these there are three, due respectively to 
 precession, to nutation, and to aberration. Each of these apparent 
 movements obeys laws peculiar to itself, and thus it becomes 
 possible to analyse the total apparent motion, and to discriminate 
 the proportions in which the precession, the nutation, and the 
 aberration have severally contributed. We are thus enabled to 
 isolate the effect of aberration as completely as if it were the sole 
 agent of apparent displacement, so that, by an alliance between 
 mathematical calculation and astronomical observation, we can 
 study the effects of aberration as clearly as if the stars were 
 affected by no other motions. 
 
 Concentrating our attention solely on the phenomena of aberra- 
 tion, we shall describe its particular effect upon stars in different 
 regions of the sky, and thus ascertain the laws according to which 
 the effects of aberration are exhibited. When this step has been 
 taken, we shall be in a position to give the beautiful explanation of 
 those laws, dependent upon the velocity of light. 
 F F 2 
 
484 THE STORY OF THE HEAVENS. 
 
 At one particular region of the heavens the effect of aberration 
 has a degree of simplicity which is not manifested anywhere else. 
 This region lies in the constellation Draco, at the pole of the 
 ecliptic. At this pole, or in its immediate neighbourhood, each 
 star, in virtue of aberration, describes a circle in the heavens. This 
 circle is very minute : it would take something like 2,000 of these 
 circles together to form an area equal to the area of the moon. 
 Expressed in the usual astronomical language, we would say that the 
 diameter of this small circle is about 40*9 seconds of arc. This is a 
 quantity which, though small to the unaided observation, is really of 
 great relative magnitude in the present state of telescopic research. 
 It is not only large enough to be perceived, but it can be measured 
 with an accuracy which actually does not admit of a doubt, to the 
 hundredth part of the whole. It is also observed that each star 
 describes its little circle in precisely the same period of time j and 
 that period is one year, or, in other words, the time of the revolu- 
 tion of the earth around the sun. It is found that for all stars 
 in this region, be they large stars or small, single or double, white 
 or coloured, the circles appropriate to each have all the same size, 
 and are all described in the same time. Even from this alone it 
 would be manifest that the cause of the phenomenon cannot lie 
 in the star itself. This unanimity in stars of every magnitude 
 and distance requires some simpler explanation. 
 
 Further examination of stars in different regions sheds new 
 light on the subject. As we proceed from the pole of the ecliptic, 
 we still find that each star exhibits an annual movement of the 
 same character as the stars just considered. In one respect, how- 
 ever, there is a difference. The apparent path of the star is no 
 longer a circle; it has become an ellipse. It is, however, soon 
 perceived that the shape and the position of this ellipse is governed 
 by the simple law, that the further the star is from the pole of the 
 ecliptic the greater is the eccentricity of the ellipse. The stars at the 
 same distance from the pole have equal eccentricity, and of the axes 
 of the ellipse, the shorter is always directed to the pole, the longer 
 being of course perpendicular thereto. It is, however, found that 
 no matter how great the eccentricity may become, the major axis 
 
THE ABERRATION OF LIGHT. 485 
 
 always retains its original length. It is always equal to about 
 40 '9 seconds, that is, to the diameter of the circle of aberration at 
 the pole itself. As we proceed further and further from the pole of 
 the ecliptic, we find that each star describes a path more and more 
 eccentric, until at length, when we examine a star on the ecliptic, 
 the ellipse has become so attenuated that it has flattened into 
 a line. Each star which happens to lie on the ecliptic oscillates to 
 and fro along the ecliptic through an amplitude of 40'9 seconds. 
 Half-a-year accomplishes the journey one way, and the other half 
 of the year restores the star to its original position. When we 
 come to stars on the other side of the ecliptic, we see the same 
 series of changes proceed in an inverse order. The ellipse, from 
 being actually linear, gradually grows in width, though still pre~ 
 serving the same length of major axis, until at length the stars 
 near the southern pole of the ecliptic are each found to describe a 
 circle equal to the paths pursued by the stars at the north pole of 
 the ecliptic. 
 
 The circumstance that the major axes of all these ellipses are of 
 equal length suggests a still further simplification. Let us suppose 
 that every star, either at the pole of the ecliptic or elsewhere, 
 pursues an absolutely circular path, and that all these circles agree, 
 not only in magnitude, but also in being all parallel to the plane of 
 the ecliptic : it is easy to see that this simple supposition will 
 account for the observed facts. The stars at the pole of the ecliptic 
 will, of course, show their circles turned fairly towards us, and we 
 shall see that they pursue circular paths. The circular paths of the 
 stars remote from the pole of the ecliptic will, however, be only 
 seen edgewise, and thus the apparent paths will be elliptical, as we 
 actually find them. We can even calculate the degree of ellipticity 
 which this surmise would require, and we find that it coincides with 
 the observed ellipticity. Finally, when we observe stars actually 
 moving in the ecliptic, the circles they follow would be seen 
 edgewise, and thus the stars would have merely the linear move- 
 ment which they are seen to possess. All the observed pheno- 
 mena are thus found to be completely consistent with the suppo- 
 sition that every star of all the millions in the heavens describes 
 
486 THE STORY OF THE HEAVENS. 
 
 once each year a circular path ; and that, whether the star be far or 
 near, this circle has always the same apparent diameter, and lies in 
 a plane always parallel to the plane of the ecliptic. 
 
 We have now wrought the facts of observation into a form 
 which enables us to examine into the cause of a movement so 
 systematic. Why is it that each star should seem to describe a 
 small circular path? Why should that path be parallel to the 
 ecliptic ? Why should it be completed exactly in a twelvemonth ? 
 We are at once referred to the motion of the earth around the sun. 
 That movement takes place in the ecliptic. It is completed in a 
 year. The coincidences are so obvious that we feel almost necessarily 
 compelled to connect in some way this apparent movement of the 
 stars with the annual movement of the earth around the sun. If 
 there were no such connection, it would be in the highest degree 
 improbable that the planes of the circles should be all parallel to 
 the ecliptic, or that the time of revolution of each star in its circle 
 should equal that of the revolution of the earth around the sun. 
 As both these conditions are fulfilled, the probability of the con- 
 nection rises to a value almost infinite. 
 
 The important question has then arisen as to why the move- 
 ment of the earth around the sun should be associated in so 
 remarkable a manner with this universal star movement. There is 
 here one obvious point to be noticed and to be dismissed. We have 
 in a previous chapter discussed the important question of the annual 
 parallax of stars, and we have shown how, in virtue of annual 
 parallax, each star describes an ellipse. It can further be demon- 
 strated that these ellipses are really circles parallel to the ecliptic ; 
 so that here we might hastily assume that annual parallax was the 
 cause of the phenomenon discovered by Bradley. A single circum- 
 stance will dispose of this suggestion. The circle described by a 
 star in virtue of annual parallax has a magnitude dependent on 
 the distance of the star, so that the circles described by various 
 stars are all of various dimensions, corresponding to the varied 
 distances of different stars. The phenomena of aberration, how- 
 ever, distinctly assert that the circular path of each star is of the 
 same size, quite independently of what its distance may be, and 
 
THE ABERRATION OF LIGHT. 487 
 
 hence annual parallax will not afford an adequate explanation. 
 It should also be noticed that the movements of a star produced 
 by annual parallax are very much smaller than those due to aberra- 
 tion. There is not any known star whose circular path due to annual 
 parallax has a diameter one-twentieth part of that of the circle due 
 to aberration ; indeed, in the great majority of cases the parallax 
 of the star is a quantity absolutely insensible. 
 
 There is, however, a still graver and quite insuperable dis- 
 tinction between the paral lactic path and the aberrational path. 
 Let us, for simplicity, think of a star situated near the pole of 
 the ecliptic, and thus appearing to revolve annually in a circle, 
 whether we regard either the phenomenon of parallax or of aber- 
 ration. As the earth revolves, so does the star revolve ; and thus 
 to each place of the earth in its orbit corresponds a certain place 
 of the star in its circle. If the movement arise from annual 
 parallax, it is easy to see w r here the place of the star will be for 
 any position of the earth. It is, however, found that in the 
 movement discovered by Bradley the star never has the position 
 which parallax assigns to it, but is, in fact, a quarter of the cir- 
 cumference distant therefrom. 
 
 A simple rule will find the position of the star due to aberration. 
 Draw from the centre of the circle a radius parallel to the direction 
 in which the earth is moving at the moment in question, then the 
 extremity of this radius gives the point on the circle where the star 
 is to be found. Tested at all seasons, and with all stars, this law 
 is found to be always verified, and by its means we are conducted 
 to the true explanation of the phenomenon. 
 
 We can enunciate the effects of aberration in a somewhat 
 different manner, which will show even more forcibly how the 
 phenomenon is connected with the motion of the earth in its orbit. 
 As the earth pursues its annual course around the sun, its move- 
 ment at any moment may be regarded as directed towards a 
 certain point of the ecliptic. From day to day, and even from hour 
 to hour, the point gradually moves along the ecliptic, so as to 
 complete the circuit in a year. At each moment, however, there is 
 always a certain point in the heavens towards which the earth's 
 
43S THE STORY OF THE HEAVENS. 
 
 motion is directed. It is, in fact, the point on the celestial sphere 
 towards which the earth would travel continuously, if, at the 
 moment, the attraction of the sun could be annihilated. It is found 
 that this point is intimately connected with the phenomenon of 
 aberration. In fact, the aberration is really equivalent to drawing 
 each star from its mean place towards the apex of the earth's way, 
 as the point is sometimes termed. It can also be shown by obser- 
 vation that the amount of aberration depends upon the distance 
 from the apex. A star which happened to lie on the ecliptic will 
 not be at all deranged by aberration from its mean place when 
 it happens that the apex coincides with the star. All the stars 
 10 Q from the apex will be displaced each by the same amount, 
 and all directly in towards the apex. A star 20 from the apex 
 will undergo a larger degree of displacement, though still in the 
 same direction, exactly towards the apex ; and all stars at the same 
 distance will be displaced by the same amount. Proceeding thus 
 from the apex, we come to stars at a distance of 90 there- 
 from. Here the amount of displacement will be a maximum. 
 Each one will be about twenty seconds from its average place ; 
 but in every case the imperative law will be obeyed, that the 
 displacement of the star from its mean place lies towards the apex 
 of the earth's way. We have thus given two distinct descriptions 
 of the phenomenon of aberration. In the first we found it con- 
 venient to speak of a star as describing a minute circular path ; 
 in the other we have regarded aberration as merely amounting 
 to a derangement of the star from its mean place in accordance 
 with specified laws. These descriptions are not inconsistent : they 
 are, in fact, geometrically equivalent ; but the latter is rather the 
 more perfect, inasmuch as it assigns completely the direction and 
 extent of the derangement caused by aberration in any particular 
 star at any particular moment. 
 
 The question has now been narrowed to a very definite form. 
 What is it which makes each star seem to close in towards the 
 point towards which the earth is travelling? The answer will 
 be found when we make a minute inquiry into the circumstances 
 under which we view a star in the telescope. 
 
THE ABERRATION OF LIGHT. 489 
 
 The beam of rays from a star falls on the object-glass of a 
 telescope ; those rays are parallel, and after they pass through the 
 object-glass they converge to a focus near the eye end of the instru- 
 ment. Let us first suppose that the telescope is at rest ; then if the 
 telescope be pointed directly towards the star, the rays will converge 
 to a point at the centre of the field of view where a pair of cross 
 wires are placed, whose intersection defines the axis of the telescope. 
 The case will, however, be altered if the telescope be moved after 
 the light has passed through the objective ; the rays of light in the 
 interior of the tube will pursue a direct path, as before, and will 
 proceed to a focus at the same precise point as before. As, how- 
 ever, the telescope has moved, it will, of course, have carried with 
 it the pair of cross wires ; they will no longer be at the same point 
 as at first, and consequently the image of the star will not now 
 coincide with their intersection. 
 
 The movement of the telescope arises from its connection with 
 the earth; for as the earth hurries along at a speed of eighteen 
 miles a second, the telescope is necessarily displaced with this 
 velocity. It might at first be thought, that in the incredibly 
 small fraction of time necessary for light to pass from the 
 object-glass to the eye-piece, the change in the position of the 
 telescope must be too minute to be appreciable. Let us suppose, 
 for instance, that the star is situated near the pole of the ecliptic, 
 then the telescope will be conveyed by the earth's motion in a 
 direction perpendicular to its length. If the tube of the instru- 
 ment be about twenty feet long, it can be readily demonstrated 
 that during the time the light travels down the tube the move- 
 ment of the earth will convey the telescope through a distance of 
 about one-fortieth of an inch.* This is a quantity very distinctly 
 measurable with the magnifying power of the eye-piece, and hence 
 this derangement of the star's place is very appreciable. It there- 
 fore follows that if we wish the star to be shown at the centre 
 
 * As the earth carries on the telescope at the rate of 18 miles a second, and as 
 light moves with the velocity of 180,000 miles a second very nearly, it follows that 
 the velocity of the telescope is about one ten -thousandth part of that of light. 
 While the light moves down the tube 20 feet long, the telescope will therefore have 
 moved the ten-thousandth part of 20 feet i.e., the fortieth of an inch. 
 
490 THE STORY OF THE HEAVENS. 
 
 of the instrument, the telescope is not to be pointed directly at 
 the star, as it would have to be were the earth at rest, but the 
 telescope must be pointed a little in advance of the star's true 
 position; and as we determine the apparent place of the star 
 by the direction in which the telescope is pointed, it follows 
 that the apparent place of the star is altered by the motion of the 
 earth. 
 
 Every circumstance of the change in the star's place admits of 
 complete explanation in this manner. Take, for instance, the small 
 circular path which each star appears to describe. We shall, for 
 simplicity, refer only to the star at the pole of the ecliptic. Suppose 
 that the telescope is pointed truly to the place of the star, then, as 
 we have shown, the image of the star will be at a distance of 
 one-fortieth of an inch from the cross wires. This distance will 
 remain constant, but each night the direction of the star from the 
 cross wires will change, so that in the course of the year it com- 
 pletes a circle, and returns to its original position. We shall not 
 pursue the calculations relative to other stars ; suffice it here to say 
 that the movement of the earth has been found adequate to account 
 for the phenomena, and thus the doctrine of the aberration of light 
 is demonstrated. 
 
 It remains to allude to one point of the utmost interest and 
 importance. We have seen that the magnitude of the aberration 
 can be measured by astronomical observation. This aberration 
 depends upon the velocity of light, and on the velocity with which 
 the earth's motion is performed. We can measure the velocity of 
 light by independent measurements, in the manner already ex- 
 plained in Chapter XII. We are thus enabled to calculate what 
 the velocity of the earth must be, for there is only one particular 
 velocity for the earth which, when combined with the measured 
 velocity of light, will give the measured value of aberration. The 
 velocity of the earth being thus ascertained, and the length of the 
 year being known, it is easy to find the circumference of the earth's 
 path, and therefore its radius ; that is, the distance from the earth 
 to the sun. 
 
 Here is indeed a singular result, and one which shows how 
 
THE ABERRATION OF LIGHT. 491 
 
 profoundly the various phenomena of science are interwoven. We 
 make experiments in our laboratory, and find the velocity of light. 
 We observe the fixed stars, and measure the aberration. We com- 
 bine these results, and deduce therefrom the distance from the 
 earth to the sun ! Although this method of finding the sun's 
 distance is one of very great elegance, and admits of a certain 
 amount of precision, yet it -cannot be relied upon as a perfectly 
 unimpeachable method of deducing the great constant. A perfect 
 method must be based on the operations of mere surveying, and 
 ought not to involve recondite physical considerations. We cannot, 
 however, fail to regard the' discovery of aberration as a most 
 pleasing and beautiful achievement, for it not only greatly improves 
 the calculations of practical astronomy, but links together several 
 physical phenomena of the greatest interest. 
 
CHAPTER XXVI. 
 
 THE ASTRONOMICAL SIGNIFICANCE OF HEAT. 
 
 Heat and Astronomy Distribution of Heat The Presence of Heat in the Earth 
 Heat in other Celestial Bodies Varieties of Temperature The Law of Cooling 
 The Heat -of the Sun Can its Temperature be Measured ? Eadiation con- 
 nected with the Sun's Bulk Can the Sun be Exhausting his Eesources ? No 
 marked Change has Occurred Geological Evidence as to the Changes of the 
 Sun's Heat Doubtful The Cooling of the Sun The Sun cannot be merely an 
 lucandescent Solid Cooling Combustion will not Explain the matter Somo 
 Heat is obtained from Meteoric Matter, but this is not Adequate to the Main- 
 tenance of the Sun's Heat The Contraction of a Heated Globe of Gas An 
 Apparent Paradox The Doctrine of Energy The Nebular Theory Evidence 
 in Support of this Theory Sidereal Evidence of the Nebular Theory 
 Herschel's View of Sidereal Aggregation The Nebulae do not Exhibit the 
 Changes within the Limits of our Observation. 
 
 THAT a portion of a work on astronomy should bear the title 
 placed at the head of this chapter, will perhaps strike some of our 
 readers as unusual, if not actually inappropriate. Is not heat, it 
 may be said, a question merely of experimental physics ? and how 
 can it be legitimately introduced into a discussion of the heavenly 
 bodies and their movements? Whatever weight such objections 
 might have once had, need not now be considered. The recent 
 researches on heat have shown not only that heat has important 
 bearings on astronomy, but that it has really been one of the 
 chief agents by which the universe has been moulded into its 
 actual form. At the present time no work on astronomy could 
 be complete without some account of the remarkable connection 
 between the laws of heat, and the astronomical consequences 
 which follow from obedience to those laws. 
 
 In discussing the planetary motions and the laws of Kepler, 
 or in discussing the movements of the moon, the proper motions 
 of the stars, or the revolutions of the binary stars, we proceed on 
 the supposition that the bodies we are dealing with are rigid 
 
ASTRONOMICAL SIGNIFICANCE OF HEAT. 493 
 
 particles, and the question as to whether these particles are hot or 
 cold does not seem to have any especial bearing. No doubt the 
 ordinary periodic phenomena of our system, such as the revolution 
 of the planets in conformity with Kepler's laws, will be observed 
 for countless ages, whether the planets be hot or cold, or whatever 
 may be the heat of the sun. It must, however, be admitted that 
 the laws of heat introduce certain modifications into the statement 
 of these laws. The effects of heat may not be immediately per- 
 ceptible, but they exist, they are constantly acting, and in the 
 progress of time they are adequate to effecting the mightiest 
 changes throughout the universe. 
 
 Let us briefly recapitulate the circumstances of our system 
 which give to heat its potency. Look first at our earth, which at 
 present seems on its surface, at all events to be a body devoid of 
 heat ; but a closer examination will at once dispel this idea. Have 
 we not the phenomena of volcanoes, of geysers, and of hot springs, 
 which show that in the interior of the earth heat must exist in 
 far greater intensity than we find on the surface ? These pheno- 
 mena are found in widely different regions of the earth. Their 
 origin is, no doubt, involved in a good deal of obscurity, but yet 
 no one can deny that they indicate vast reservoirs of heat. It 
 would indeed seem that heat is to be found everywhere in the 
 deep inner regions of the earth. If we take a thermometer down 
 a deep mine, we find it records a temperature higher than at the 
 surface. The deeper we descend the higher is the temperature; and 
 if the same rate of progress should be maintained through those 
 depths of the earth which we are not able to penetrate, it can be 
 demonstrated that at twenty or thirty miles below the surface the 
 temperature must be as great as that of red-hot iron. 
 
 We find in the other celestial bodies abundant evidence of the 
 present or the past existence of heat. Our moon, as we have 
 already mentioned, affords a very striking instance of a body which 
 must once have been very highly heated. The extraordinary vol- 
 canoes on its surface render this beyond any doubt. It is equally 
 certain that thc.se volcanoes have been silent for ages, so that, 
 whatever may be the interior condition of the moon, the surface 
 
494 THE STORY OF THE HEAVENS. 
 
 has now cooled down. Extending our view further, we see in the 
 great planets Jupiter and Saturn evidence that they are still 
 endowed with a temperature far in excess of that which the earth 
 has retained ; while when we look at our sun, we see a body in a 
 state of brilliant incandescence, and glowing with a fervour to 
 which we cannot approximate in our mightiest furnaces. The 
 various fixed stars are bodies which glow with heat, like our sun ; 
 while we have in the nebulae objects whose existence is hardly 
 intelligible to us unless we admit that they are possessed of a vast 
 store of heat. 
 
 From this rapid survey of the different bodies in our universe, 
 one conclusion is obvious. We may have great doubts as to the 
 actual temperature of any individual body of the system ; but it 
 cannot be doubted that there is a wide range of temperature among 
 the different bodies. Some are hotter than others. The stars and 
 suns are perhaps the hottest of all, but it is not improbable that they 
 may be immeasurably outnumbered by the cold and dark bodies of 
 the universe, which are to us invisible, and only manifest their 
 existence in an indirect and casual manner. 
 
 The law of cooling tells us that every body radiates heat, and 
 that the quantity of heat which it radiates increases when the 
 temperature of the body increases relatively to the surrounding 
 medium. This law appears to be universal. It is obeyed on the 
 earth, and it would seem that it must be equally obeyed by every 
 other body in space. We thus see that each of the planets and 
 each of the stars is continuously pouring forth in all directions a 
 never-ceasing stream of heat. 
 
 This radiation of heat is productive of very momentous conse- 
 quences. Let us study them, for instance, in the case of the sun. 
 Our great luminary pours forth a mighty flood of radiant heat 
 in all directions. A minute fraction of that heat is intercepted by 
 our earth, and is directly or indirectly the source of all life, and 
 of nearly all movement, on our earth. To pour forth heat as the 
 sun does, it is necessary that his temperature be enormously high. 
 And there are some facts which permit us to form an estimate of 
 what that temperature must actually be. 
 
ASTRONOMICAL SIGNIFICANCE OF HEAT. 495 
 
 Every one is acquainted with the use of a burning-glass, by 
 which we can condense the sun's rays to a focus, and produce 
 incandescence or set objects on fire. Large burning-glasses have 
 been constructed, in the focus of which an extraordinary tem- 
 perature has been obtained. It can, however, be proved that the 
 temperature at the focus cannot be greater, cannot be even equal, 
 to the temperature at the source of heat itself. The effect of a 
 burning-glass is merely equivalent to making a closer approach 
 towards the sun. The rule is indeed a simple one. The tempera- 
 ture at the focus of the burning-glass is the same as that of a point 
 placed at such a distance from the sun that the solar disc would 
 seem just as large as the lens itself viewed from its own focus. 
 The greatest burning-glass which has ever been constructed 
 virtually transports an object at its focus to within 250,000 miles 
 of the sun's surface : in other words, to a distance of about 
 l-400th part of its present amount. In this focus it was found 
 that the most refractory substances, agate, cornelian, platinum, 
 fire-clay, the diamond itself, were melted or even dissipated into 
 vapour. There can be no doubt that if the sun were to come as 
 near to us as the moon, the solid earth itself would melt like wax. 
 
 It is difficult to form any numerical statement of the actual 
 temperature of the sun. The intensity of that temperature vastly 
 transcends the greatest artificial heat, and any attempt to clothe 
 such estimates in figures is necessarily very precarious. But assum- 
 ing the greatest artificial temperature to be about 4,000 Fahr., we 
 shall probably be well within the truth if we state the effective 
 temperature of the sun to be about 18,000 Fahr. This is, 
 indeed, vastly below many of the estimates which have been made. 
 Secchi, for instance, has estimated the sun's temperature to be 
 nearly one thousand times that here given. 
 
 The copious outflow of heat from the sun corresponds with its 
 enormous temperature. We can express the amount of heat in 
 various ways, but it must be remembered that considerable un- 
 certainty still attaches to such measurements. The old method of 
 measuring heat by the quantity of ice melted may be used as an 
 illustration. It is computed that a shell of ice 43 feet thick 
 
496 THE STORY OF THE HEAVENS. 
 
 surrounding the whole sun would in one minute be melted by the 
 sun's heat underneath. A somewhat. more elegant illustration was 
 also given by Sir John Herschel, who showed that if a cylindrical 
 glacier forty-five miles in diameter were to be continually flowing 
 into the sun with the velocity of light, the end of that glacier 
 would be melted as quickly as it advanced. From each square foot 
 on the surface of the sun emerges a quantity of heat as great as 
 could be produced by the daily combustion of sixteen tons of coal. 
 This is, indeed, an amount of heat which, properly transformed into 
 work, would keep an engine of many hundreds of horse-power 
 running from one year's end to the other. The heat radiated from 
 a few acres on the sun would be adequate to drive all the steam- 
 engines in the world. When we reflect on the vast intensity of 
 the radiation from each square foot of the sun's surface, and when 
 we combine with this the stupendous dimensions of the sun, 
 imagination fails to realise how vast must be the actual expen- 
 diture of heat. 
 
 In one way the enormous intensity of the radiation from each 
 unit of sun surface is a consequence of its bulk. Imagine for a 
 moment two suns, one of which had a diameter double the other. 
 If these two suns were of analogous constitution, their stores of 
 heat may be taken as proportional to their volumes. The larger 
 sun would thus have a store of heat eight times as great as the 
 smaller one. But the ratio of the surfaces of the two suns is only four 
 to one. Hence it follows that by the time both suns cooled down, 
 twice as much heat per unit of area must have passed through the 
 surface of the large sun as through the surface of the small one. 
 To emphasise the contrast still more, suppose our present sun 
 compared with a fictitious sun not larger than our earth. If the 
 two suns started under equal circumstances, then before they had 
 both cooled down to the same temperature, nearly one hundred times 
 as much heat must be transmitted through each square foot of the 
 sun's surface as through each square foot of the earth's surface. 
 
 In presence of the beneficent, if prodigal, expenditure of the 
 sun's heat, we are tempted to ask a question which has the most 
 vital interest for the earth and its inhabitants. We live from 
 
ASTRONOMICAL SIGNIFICANCE OF HEAT. 497 
 
 hour to hour by the sun's splendid generosity ; and, therefore, it is 
 important for us to know what security we possess for the con- 
 tinuance of his favours. When we witness the terrific expenditure 
 of the sun's heat each hour, we are compelled to ask whether the 
 sun may not be exhausting its resources ; and if so, what are 
 the prospects of the future ? This question we can partly answer. 
 The whole subject is indeed of surpassing interest, and redolent 
 with the spirit of modern scientific thought. 
 
 Our first attempt to examine this question must lie in an appeal 
 to the facts which are attainable. We want to know whether the 
 sun is showing any symptoms of decay. Are the days as warm 
 and as bright now as they were last year, ten years ago, one 
 hundred years ago ? We can find no evidence of any change since 
 the beginning of authentic records. If the sun's heat had 
 perceptibly changed within the last two thousand years, we should 
 expect to find corresponding changes in the distribution of plants 
 and of animals ; but no such changes have been detected. There is 
 no reason to think that the climate of ancient Greece or of ancient 
 Rome was appreciably different from the climates of the Greece 
 and the Rome that we know at this day. The vine and the olive 
 grow now where they grew two thousand years ago. 
 
 We must not, however, lay too much stress on this argument; 
 for the effects of slight changes in the sun's heat may have 
 been neutralised by corresponding adaptations in the pliable 
 organisms of cultivated plants. All we can certainly conclude 
 is that no marked change has taken place in the heat of the 
 sun during historical time. But when we come to look back 
 
 o 
 
 into vastly earlier ages, we find the most copious evidence that 
 the earth has undergone great changes in climate. Geological 
 records can on this question hardly be misinterpreted. Yet it 
 is curious to note that these changes are hardly such as could 
 arise from the gradual exhaustion of the sun's radiation. No 
 doubt, in very early times we have evidence that the earth's 
 climate must have been much warmer than at present. We had 
 the great carboniferous epoch, when the temperature must almost 
 have been tropical in Arctic latitudes. Yet it is hardly possible 
 G a 
 
498 THE STORY OF THE HEAVENS. 
 
 to cite this as evidence that the sun was then much more power- 
 ful; for we are immediately reminded of the glacial epoch, 
 when our temperate zones were encased in sheets of solid ice, as 
 Northern Greenland is at present. If we suppose the sun to have 
 been hotter than it is at present to account for the vegetation which 
 produced coal, then we ought to assume the sun to be colder than 
 it is now to account for the glacial epoch. It is not reasonable 
 to attribute such phenomena to such oscillations in the radiation 
 from the sun. The glacial epochs prove that we cannot appeal to 
 geology in aid of the doctrine that a secular cooling of the sun is 
 now in progress. The geological variations of climate may have 
 been caused by changes in the earth itself, by changes in the posi- 
 tion of its axis, by changes in its actual orbit ; but however they 
 have been caused, they hardly tell us much with regard to the past 
 history of our sun. 
 
 The heat of the sun has lasted for countless ages ; yet we cannot 
 credit the sun with the power of actually creating heat. We must 
 apply even to the majestic mass of the sun the same laws which we 
 have found by our experiments on the earth. We must ask, whence 
 comes the heat sufficient to supply this tremendous outgoing ? Let 
 us briefly recount the various suppositions that have been made. 
 
 Place two red-hot spheres of iron side by side, a large one and 
 a small one. They have been taken from the same fire ; they were 
 both equally hot ; they are both cooling, but the small sphere cools 
 more rapidly. It speedily becomes dark, while the large sphere is 
 still glowing, and would continue to do so for some minutes. The 
 larger the sphere, the longer it will take to cool ; and hence it has 
 been supposed that a mighty sphere of the prodigious dimensions 
 of our sun would, if once heated, cool gradually, but the dura- 
 tion of the cooling would be so long, that for thousands and for 
 millions of years it could continue to be a source of light and heat 
 to the revolving system of planets. This suggestion will not bear 
 the test of arithmetic. If the sun had no source of heat beyond 
 that indicated by its high temperature, we can show that radiation 
 would cool the sun a few degrees every year. Two thousand 
 years would then witness a very great decrease in the sun's heat. 
 
ASTRONOMICAL SIGNIFICANCE OF HEAT. 499 
 
 We are certain that no such decrease can have taken place. The 
 source of the sun's radiation cannot be found in the mere cooling 
 of an incandescent mass. 
 
 Can the fires in the sun be maintained by combustion, analogous 
 to that which goes on in our furnaces ? Here we would seem to 
 have a source of gigantic heat ; but arithmetic also disposes of this 
 supposition. We know that if the sun were made of even solid coal 
 itself, and if that coal were burning in pure oxygen, the heat that 
 could be produced would only suffice for 6,000 years. If the sun 
 which shone upon the builders of the great Pyramid had been solid 
 coal from surface to centre, it must by this time have been in great 
 part burned away in the attempt to maintain its present rate of 
 expenditure. We are thus forced to look to other sources for the 
 supply of the sun's heat, since neither the heat of incandescence 
 nor the heat of combustion will suffice. 
 
 There is probably indeed, we may say certainly one external 
 source from which the heat of the sun is recruited. It will be 
 necessary for us to consider this source with some care, though I 
 think we shall find it to be merely an auxiliary of comparatively 
 trifling moment. According to this view, the solar heat receives 
 occasional accessions from the fall upon the sun's surface of masses 
 of meteoric matter. There can be hardly a doubt that such masses 
 do fall upon the sun ; there is certainly no doubt that if they do, 
 the sun must gain some heat thereby. We have experience on the 
 earth of a very interesting kind, which illustrates the development 
 of heat by meteoric matter. There lies a world of philosophy in a 
 shooting star. Some of these myriad objects rush into our atmo- 
 sphere and are lost ; others, no doubt, rush into the sun, with the 
 same result. We also admit that the descent of a shooting star into 
 the atmosphere of the sun must be attended with a flash of light 
 and of heat. The heat acquired by the earth from the flashing of 
 shooting stars through our air is quite insensible. It has been 
 supposed, however, that the heat accruing to the sun from the 
 same cause may be quite sensible nay, it has been even supposed 
 that the sun may be re-invigorated from this source. 
 
 Here, again, we must apply the cold principles of weights and 
 G G 2 
 
500 THE STORY OF THE HEAVENS. 
 
 measures to estimate the plausibility of this suggestion. "We first 
 calculate the actual weight of meteoric indraught to the sun which 
 would be adequate to sustain the fires of the sun at their present 
 vigour. The mass of matter that would be required is so enormous 
 that we cannot usefully express it by imperial weights ; we must deal 
 with masses of imposing magnitude. It fortunately happens that 
 the weight of our moon is a convenient unit. Conceive that our 
 moon a huge globe, 2,000 miles in diameter were crushed into a 
 myriad of fragments, and that these fragments were allowed to 
 rain in on the sun ; there can be no doubt that this tremendous 
 meteoric shower would contribute to the sun rather more heat than 
 would be required to supply his radiation for a whole year. If we 
 take our earth itself, conceive it comminuted into dust, and allow 
 that dust to fall on the sun as a mighty shower, each frag- 
 ment would instantly give out a quantity of heat, and the whole 
 would add to the sun a supply of heat adequate to sustain the 
 present rate of radiation for nearly one hundred years. The mighty 
 mass of Jupiter treated in the same way would generate a meteoric 
 display greater in the ratio in which the mass of Jupiter exceeds 
 the mass of the earth. Were Jupiter to fall into the sun, enough 
 heat would be thereby produced to scorch the whole solar system ; 
 while all the planets together would be capable of producing 
 heat which, if properly economised, would supply the radiation 
 of the sun for 45,000 years. 
 
 Here, then, is certainly a plausible source for the supply of 
 the sun's heat; but it must be remembered that though the 
 moon could supply one year's heat, and Jupiter 30,000 years' 
 heat, yet the practical question is not whether the solar system 
 could supply the sun's heat, but whether , it does. Is it likely 
 that meteors equal in mass to the moon fall into the sun every 
 year? This is the real question, and I think we are bound to 
 reply to it in the negative. It can be shown that the quantity 
 of meteors which could be caught by the sun in any one year 
 can be only an excessively minute fraction of the total amount. 
 If, therefore, a moon-weight of meteors were caught every year, 
 there must be an incredible mass of meteoric matter roaming at 
 
ASTRONOMICAL SIGNIFICANCE OF HEAT. 501 
 
 large through the system. There must be so many meteors that 
 the earth would be incessantly pelted with them, and heated to 
 such a degree as to be rendered uninhabitable. There are also other 
 reasons which preclude the supposition that a stupendous quantity 
 of meteoric matter exists in the vicinity of the sun. Such matter 
 would produce an appreciable effect on the movement of the planet 
 Mercury. There are, no doubt, some irregularities in the move- 
 ments of Mercury not yet fully explained, but these irregularities 
 are very much less than would be the case if meteoric matter 
 existed in quantity adequate to the sustentation of the sun. 
 Astronomers, then, believe that though meteors may be a rate in 
 aid of the sun's current expenditure, yet that the greater portion of 
 that expenditure must be defrayed from other resources. 
 
 It is one of the achievements of modern science to have effected 
 the solution of the problem to have shown how it is that, not- 
 withstanding the stupendous radiation, the sun still maintains 
 its temperature. The question is not free from difficulty in its 
 exposition, but the matter is one of such very great importance that 
 we are compelled to make the attempt. 
 
 Let us imagine a vast globe of heated gas in space. This is 
 not an entirely gratuitous supposition, inasmuch as there are globes 
 apparently of this character ; they have been already alluded to as 
 planetary nebulae. This globe will radiate heat, and we shall 
 suppose that it emits more heat than it receives from the radia- 
 tion of other bodies. The globe will accordingly lose heat, or what 
 is equivalent thereto, but it will be incorrect to assume that the 
 globe will necessarily fall in temperature. That the contrary is, 
 indeed, the case is a result almost paradoxical at the first glance ; 
 but yet it can be readily shown to be a necessary consequence of 
 the laws of heat and of gases. 
 
 Let us fix our attention on a portion of the gas lying on the 
 surface of the globe. This is, of course, attracted by all the 
 rest of the globe, and thus tends in towards the centre of 
 the globe. If equilibrium subsists, this tendency must be neu- 
 tralised by the pressure of the gas beneath; so that the greater 
 the gravitation, the greater is the pressure. When the globe 
 
502 THE STORY OF THE HEAVENS. 
 
 of gas loses heat by radiation, let us suppose that it grows colder 
 that its temperature accordingly falls ; then, since the pressure 
 of a gas decreases when the temperature falls, the pressure 
 beneath the superficial layer of the gas will decrease, while the 
 gravitation is unaltered. The consequence will inevitably be that 
 the gravitation will now conquer the pressure, and the globe of 
 gas will accordingly contract. There is, however, another way 
 in which we can look at the matter. We know that heat is 
 equivalent to energy, so that when the globe radiates forth heat, 
 it must expend energy. A part of the energy of the globe will be 
 that due to its temperature ; but another, and in some respects a 
 more important, part is that due to the separation of its particles. 
 If we allow the particles to come closer together, we shall diminish 
 the energy due to separation, and the energy thus set free can take 
 the form of heat. But this drawing in of the particles necessarily 
 involves a shrinking of the globe ; and thus we find that whatever 
 way the matter be viewed, the radiation of heat from the globe 
 must be attended with contraction. 
 
 And now for the remarkable consequence, which seems to have 
 a very important application in Astronomy. As the globe contracts, 
 a part of its energy of separation is changed into heat ; that heat is 
 partly radiated away, but it is not radiated away as rapidly as it is 
 produced by the contraction. The consequence is, that although 
 the globe is really losing heat and really contracting, yet that its 
 temperature is actually rising. A simple case will suffice to demon- 
 strate this result, paradoxical as it may at first seem. Let us 
 suppose that by contraction of the sphere it had diminished to one- 
 half its diameter ; and let us fix our attention on a cubic inch of 
 the gaseous matter in any point of the mass. After the contraction 
 had taken place each edge of the cube would be reduced to half an 
 inch, and the volume would therefore be reduced to one-eighth part 
 of its original amount. The law of gases tells us that if the tem- 
 perature be unaltered the pressure varies inversely as the volume, 
 and consequently the internal pressure in the cube will be increased 
 eight-fold. As, however, the distance between every two particles 
 is reduced to one-half, it will follow that the gravitation between 
 
ASTRONOMICAL SIGNIFICANCE OF HEAT. 503 
 
 every two particles is increased four-fold, and as the area is also 
 reduced to one-fourth it will follow that the pressure inside the 
 reduced cube is increased sixteen-fold ; but we have already seen 
 that with a constant temperature it only increases eight-fold, and 
 hence the temperature cannot be constant but must rise with the 
 contraction. 
 
 We thus have the somewhat astonishing result that a gaseous 
 globe in space radiating heat, and thereby growing smaller, is all 
 the time actually increasing in temperature. But, it may be said, 
 surely this cannot go on for ever. Are we to suppose that the 
 gaseous mass will go on contracting and contracting with a tem- 
 perature ever fiercer and fiercer, and actually radiating out more 
 and more heat the more it loses ? where lies the limit to such a 
 prospect ? As the body contracts, its density must increase, until 
 it either becomes a liquid, or a solid, or, at any rate, until it 
 ceases to obey the laws of a purely gaseous body which we have 
 supposed. Once these laws cease to be observed, the argument dis- 
 appears ; the loss of heat may then really be attended with a loss 
 of temperature, until, in the course of time, the body has sunk to 
 the temperature of space itself. 
 
 It is not pretended that this reasoning can be applied in all its 
 completeness to the present state of the sun. The sun's density is 
 now so great that the laws of gases cannot be there strictly fol- 
 lowed. There is, however, good reason to believe that the sun was 
 once more gaseous than at present; possibly at one time he may 
 have been quite gaseous enough to admit of this reasoning in all 
 its fulness. At present the sun appears to be in some intermediate 
 stage of its progress from the gaseous condition to the solid condi- 
 tion. We cannot, therefore, say that the temperature of the sun is 
 now increasing in correspondence with the process of contraction. 
 This may be true or it may not be true; we have no means of 
 deciding the point. We may, however, feel certain that the sun 
 is still sufficiently gaseous to experience in some degree the rise 
 of temperature associated with the contraction. That rise in tem- 
 perature may be partly or wholly obscured by the fall in tempera- 
 ture, which would be the more obvious consequence of the radiation 
 
504 THE STORY OF THE HEAVENS. 
 
 of heat from the partially solid body. It will, however, be mani- 
 fest that the cooling- of the sun may be enormously protracted if 
 the fall of temperature from the one cause be nearly compensated 
 by the rise of temperature from the other. It can hardly be 
 doubted that in this we find the real explanation of the fact, that 
 we have no historical evidence of any appreciable alteration in the 
 radiation of heat from the sun. 
 
 This question is one of such interest that it may be worth 
 while to look at it from a slightly different point of view. The 
 sun contains a certain store of energy, part of which is con- 
 tinually disappearing in the form of radiant heat. The energy 
 remaining in the sun is partly transformed in character; some 
 of it is transformed into heat, which goes wholly or partly to 
 supply the loss by radiation. The total energy of the sun must, 
 however, be decreasing; and hence it would seem the sun must at 
 some time or other have its energy exhausted, and cease to be a 
 source of light and of heat. It is true that the rate at which the 
 sun contracts is very slow. We are, indeed, not able to measure 
 with certainty the decrease in the sun's bulk. It is a quantity so 
 minute, that the contraction since the birth of accurate astronomy 
 is not large enough to be perceptible in our telescopes. It is, 
 however, possible to compute what the contraction of the sun's 
 bulk must be, on the supposition that the energy lost by that 
 contraction just suffices to supply the daily radiation of heat. The 
 change is very small when we consider the present size of the sun. 
 At the present time the sun's diameter is about 860,000 miles. If 
 each year this diameter decreases by about 220 feet, sufficient 
 energy will be yielded to account for the entire radiation. This 
 gradual decrease is always in progress. 
 
 These considerations are of considerable interest when we apply 
 them retrospectively. If it be true that the sun is at this moment 
 shrinking, then in past times his globe must have been greater 
 than it is at present. Assuming the figures already given, it 
 follows that one hundred years ago the diameter of the sun must 
 have been four miles greater than it is now; one thousand years 
 ago the diameter was forty miles greater ; ten thousand years ago 
 
ASTRONOMICAL SIGNIFICANCE OF HEAT. 505 
 
 the diameter of the sun was four hundred miles greater than it is 
 to-day. When man first trod this earth it would seem that the 
 sun must have been many hundreds, perhaps many thousands, of 
 miles greater than it is at this time. 
 
 We must not, however, over-estimate the significance of this 
 statement. The diameter of the sun is so great, that a diminution 
 of 10,000 miles would be but little more than the hundredth part 
 of its diameter. If it were suddenly to shrink to the extent of 
 10,000 miles, the change would not be appreciable to ordinary ob- 
 servation, though a much smaller change would not elude delicate 
 astronomical measurement. It does not necessarily follow that the 
 climates on our earth in these early times must have been very 
 different from those which we find at this day, for the question of 
 climate depends upon other matters besides sunbeams. 
 
 Yet we need not abruptly stop our retrospect at any epoch, 
 however remote. We may go back earlier and earlier, through the 
 long ages which geologists claim for the deposition of the stratified 
 rocks; and back again still further, to those very earliest epochs 
 when life began to dawn on the earth. Still we can find no reason 
 to suppose that the law of the sun's decreasing heat is not main- 
 tained; and thus we would seem bound by our present know- 
 ledge to suppose, that the sun grows larger and larger the further 
 our retrospect extends. We cannot assume that the rate of that 
 growth is always the same. No such assumption is required ; it is 
 sufficient for our purpose that we find the sun growing larger and 
 larger the further we peer back into the remote abyss of time past. 
 If the present order of things in our universe has lasted long 
 enough, then it would seem that there was a time when the sun 
 must have been twice as large as it is at present ; it must once 
 have been ten times as large. How long ago that was no one 
 can venture to say. But we cannot stop at the stage when the sun 
 was even ten times as large as it is at present ; the arguments will 
 still apply in earlier ages. We see the sun swelling and swell- 
 ing, with a corresponding decrease in its density, until at length 
 we find, instead of our sun as we know it, a mighty nebula filling 
 a gigantic region of space. 
 
506 THE STORY OF THE HEAVENS. 
 
 Such is, in fact, the doctrine of the origin of our system which 
 has been advanced in that celebrated speculation known as the 
 nebular theory. Nor can it be ever more than a speculation; it 
 cannot be established by observation, nor can it be proved by cal- 
 culation. It is merely a conjecture, more or less plausible, but 
 perhaps in some degree necessarily true, if our present laws of heat, 
 as we understand them, admit of the extreme application here 
 required, and if also the present order of things has reigned for 
 sufficient time without the intervention of any influence at present 
 unknown to us. This nebular theory is not confined to the history 
 of our sun. Precisely similar reasoning may be extended to the 
 individual planets : the farther we look back, the hotter and the 
 hotter does the whole system become. It has been thought that 
 if we could look far enough back, we should see the earth too hot 
 for life; back further still, we should find the earth and all the 
 planets red-hot; and back further still, to an incredibly remote 
 epoch, when the planets would be heated just as much as our sun 
 is now. In a still earlier stage the whole solar system is thought 
 to have been one vast mass of glowing gas, from which the pre- 
 sent forms of the sun, with the planets and their satellites, have been 
 gradually solved. We cannot be sure that the course of events 
 has been what is here indicated, but there are very sufficient 
 grounds for thinking that this doctrine substantially represents 
 what has actually occurred. 
 
 Many of the features in the solar system harmonise with the 
 supposition that the origin of the system has been that suggested 
 by the nebular theory. We have already had occasion in an 
 earlier chapter to allude to the fact that all the planets perform 
 their revolutions around the sun in the same direction. It is also 
 to be observed that the rotation of the planets on their axes, as 
 well as the movements of the satellites around their primaries, all 
 follow the same law, with one slight exception in the case of the 
 Uranian system. A coincidence so remarkable naturally suggests 
 the necessity of some physical explanation. Such an explanation is 
 offered by the nebular theory. Suppose that countless ages ago a 
 mighty nebula was slowly rotating and slowly contracting. In the 
 
ASTRONOMICAL SIGNIFICANCE OF HEAT. 507 
 
 process of contraction, portions of the condensed matter of the 
 nebula would be left behind. These portions would still revolve 
 around the central mass, and each portion would rotate on its axis 
 in the same direction. As the process of contraction proceeded, it 
 would follow from dynamical principles that the velocity of rotation 
 would increase ; and thus at length these portions would consolidate 
 into planets, while the central mass would gradually contract to 
 form the sun. By a similar process on a smaller scale the systems 
 of satellites were evolved from the contracting primary. These 
 satellites would also revolve in the same direction, and thus the 
 characteristic features of the solar system could be accounted for. 
 
 The nebular origin of the solar system receives considerable 
 countenance from the study of the sidereal heavens. We have 
 already dwelt upon the resemblance between the sun and the stars. 
 If, then, our sun has passed through such changes as the nebular 
 theory requires, may we not anticipate that similar phenomena 
 should be met with in other stars ? If this be so, it is reasonable to 
 suppose that the evolution of some of the stars may not have pro- 
 gressed so far as has that of the sun, and thus we may be able 
 actually to witness stars in the earlier phases of their development. 
 Let us see how far the telescope responds to these anticipations. 
 
 The field of view of a large telescope usually discloses a number 
 of stars scattered over a black background of sky ; but the black- 
 ness of the background is not uniform : the practised eye of the 
 skilled observer will detect in some parts of the heavens a faint 
 luminosity. This will sometimes be visible over the whole extent 
 of the field, or it may even occupy several fields. Years may pass 
 on, and still there is no perceptible change. There can be no 
 illusion, and the conclusion is irresistible that the object is a stu- 
 pendous mass of faintly luminous glowing gas or vapour. This is 
 the simplest type of nebula ; it is characterised by extreme faint- 
 ness, and seems composed of matter of the utmost tenuity. "We 
 are, on the other hand, occasionally presented with the beautiful and 
 striking phenomenon of a definite and brilliant star surrounded 
 by a luminous atmosphere. Between these two extreme types of 
 a faint diffused mass on the one hand, and a bright star with a 
 
508 THE STOET OF THE HEAVENS. 
 
 nebula surrounding it on the other, Herschel thought that a 
 graduated series of various other nebulae could be arranged. We 
 thus have a series of links passing by imperceptible gradations 
 from the most faintly diffused nebulae on the one side, into stars on 
 the other. It was the perception of this which led Sir William 
 Herschel to his theory of sidereal aggregation. 
 
 The nebulae seemed to Herschel to be vast masses of phos- 
 phorescent vapour. This vapour gradually cools down, and ulti- 
 mately condenses into a star, or a cluster of stars. When the 
 varied forms of nebulae were classified, it almost seemed as if the 
 different links in the process could be actually witnessed. In the 
 vast faint nebulae the process of condensation had just begun ; in 
 the smaller and brighter nebulae the condensation had advanced 
 farther ; while in others, the star, or stars, arising from the conden- 
 sation had already become visible. 
 
 But, it may be asked, how did Herschel know this ? what is his 
 evidence ? Let us answer this question by an illustration. Go into 
 a forest, and look at a noble old oak which has weathered the storm 
 for centuries ; have we any doubt that the oak-tree was once a 
 young small plant, and that it grew stage by stage until it reached 
 maturity ? Yet no one has ever followed an oak-tree through its 
 various stages, the brief span of life has not been long enough to 
 do so. The reason why we believe the oak-tree to have passed 
 through all these stages is, because we are familiar with oak-trees 
 of every gradation in size, from the seedling up to the noble 
 veteran. Having seen this gradation in a vast multitude of trees, 
 we are convinced that each individual has passed through all these 
 stages. 
 
 It was by a similar train of reasoning that Herschel was led to 
 adopt the view of the origin of the stars which we have endeavoured 
 to describe. The astronomer's life is not long enough, the life of 
 the human race might not be long enough, to watch the process by 
 which the nebula condenses down so as to form a solid body. But 
 by looking at one nebula after another, the astronomer thinks he is 
 able to detect the various stages which connect the nebula in its 
 original form with the final form. He is thus led to believe that 
 
ASTRONOMICAL SIGNIFICANCE OF HEAT. 509 
 
 each of the nebulee passes, in the course of ages, through these 
 stages. And thus Herschel adopted the opinion that stars some, 
 many, or all have each originated from what was once a glowing 
 nebula. 
 
 Such a speculation may captivate the imagination, but it must 
 be carefully distinguished from the truths of astronomy, properly so 
 called. Remote posterity may perhaps obtain evidence on the sub- 
 ject which to us is inaccessible : our knowledge of nebulse is too 
 recent. There has not yet been time enough to have seen any 
 appreciable changes : for the study of nebula3 can only be said to 
 date from Messier's Catalogue in 1771. 
 
 Since Herschel's time, no doubt, many careful drawings and 
 observations of the nebulae have been obtained; but still the 
 interval has been much too short, and the earlier observations are 
 too imperfect, to enable any changes in the nebulae to be investi- 
 gated with sufficient accuracy. If the human race lasts for very 
 many centuries, and if our present observations are preserved during 
 that time for comparison, then Herschel's theory may perhaps be 
 satisfactorily tested. 
 
CHAPTER XXVII. 
 
 THE TIDES.* 
 
 Mathematical Astronomy Recapitulation of the Facts of the Earlier Researches 
 Another great Step has been taken Lagrange's Theories, how far they are 
 really True The Solar System not Made of Rigid Bodies Kepler's Laws 
 True to Observation, but not Absolutely True when the Bodies are not Rigid 
 The Errors of Observation Growth of certain Small Quantities Periodical 
 Phenomena Some Astronomical Phenomena are not Periodic The Tides 
 How the Tides were Observed Discovery of the Connection between the Tides 
 and the Moon Solar and Lunar Tides Work done by the Tides Whence do 
 the Tides obtain the Power to do the Work Tides are Increasing the Length 
 of the Day Limit to the Shortness of the Day Early History of the Earth- 
 Moon System Unstable Equilibrium Ratio of the Month to the Day The 
 Future Course of the System Equality of the Month and the Day The 
 Future Critical Epoch. The Constant Face of the Moon accounted forThe 
 other Side of the Moon The Satellites of Mars Their Remarkable Motions 
 Have the Tides Possessed Influence in Moulding the Solar System generally ? 
 Moment of Momentum Tides have had little or no appreciable Effect on 
 the Orbit of Jupiter Conclusion. 
 
 AT various points of our progress through this volume it has been 
 necessary to detail the methods of research whjch have conducted 
 astronomers to those discoveries by whicli astronomy has been 
 developed into a branch of science. We have explained how the 
 telescope has been employed to scrutinize the heavenly bodies, to 
 survey their positions, to weigh and to measure, and to enable the 
 artist to draw the features by which they are characterised. We 
 have also indicated how another whole class of discoveries has 
 arisen in a different manner. We have shown how observations 
 afford to the mathematician the basis on which his calculations can be 
 rested ; we have shown how he can discover truths of the most wide- 
 spreading generality truths which seem to have no bounds, either 
 in time or in space. We have even shown how the mathematician 
 
 * The theory of Tidal Evolution sketched in this chapter is mainly due to the 
 researches of Professor G. H. Darwin, F.R.S. 
 
THE TIDES, 511 
 
 at his desk has felt the unknown planet, and guided the telescope 
 of the astronomer to the point of the heavens in which it lay. "We 
 have in this chapter to unfold another most remarkable modern 
 development of astronomical research, which has enabled us in 
 some degree to explore the recesses of nature in departments 
 entirely closed to every other method of inquiry. These modern 
 researches are entirely based on mathematics ; they have, in fact, 
 quite anticipated observation. 
 
 To appreciate the importance of these modern researches, it will 
 be well to recall clearly the standpoint which the problems of 
 gravitational astronomy occupied before the new departure was 
 undertaken. We may illustrate the point most simply by taking 
 an example from our solar "system. Let us recapitulate briefly the 
 history of our knowledge of the motions of the planets around 
 the sun. The steps of that history are fourfold. We have first 
 the establishment of the Copernican theory, which demonstrated 
 that the planets revolve around the sun. The next great epoch is 
 marked by Kepler's famous discovery, that the orbit of each planet 
 was an ellipse, with the sun in the focus. The third epoch is that of 
 the Newtonian discovery of gravitation, by which the motion of each 
 planet in an ellipse was shown to be a consequence of the law of uni- 
 versal gravitation. Another step brings us to the labours of the great 
 French mathematicians at the close of the last century. By these 
 researches it seemed that the theory of gravitation was well-nigh 
 completed. It was shown that though the planets mutually dis- 
 turbed each other, the effect of these disturbances must be always 
 small. The eccentricity of the ellipse which each planet was 
 describing was shown not to be constant; it was found that that 
 eccentricity was for ever changing now waxing, now waning 
 but always confined between very narrow limits. It was shown 
 that the orbits of the planets were inclined to each other at angles 
 which were always changing ; but these changes are merely oscil- 
 lations, for the angles constantly fluctuate about a mean value, from 
 which they never depart to any considerable distance. All these 
 movements are periodic, like the motion of the planet itself ; and 
 after oscillating through small limits for a majestic period of 
 
512 THE STORY OF THE HEAVENS. 
 
 innumerable years, the system would regain almost its original 
 position, again to start on a series of oscillations which will last 
 for untold ages. 
 
 These sublime discoveries were regarded as the crowning 
 triumph of gravitational astronomy. The investigations which led 
 to them originated in consequence of the attention of mathema- 
 ticians being called to apparent irregularities in the motion of the 
 heavenly bodies which demanded explanation. Observation was 
 thus always in advance of theory ; but when, at length, the theory 
 had overtaken the results of observation when, indeed, in some cases 
 (as in the discovery of Neptune) 3 theory had actually outstripped 
 observation it was naturally supposed that the theory was perfect. 
 No doubt, here and there a small difficulty remained unexplained : 
 the moon would not come up to the meridian within a second of 
 the right time, a planet would be slightly out of its calculated 
 place, or a satellite not be eclipsed at the expected moment. Men 
 thought and often truly thought that these were merely minute 
 imperfections, which closer examination would remove ; and thus 
 they concluded that gravitational astronomy was fundamentally 
 perfect, and that the trifling discrepancies remaining would be 
 ultimately cleared away. 
 
 Dazzled by these brilliant discoveries, theoretical astronomers 
 had done but little for many years. They dwelt with admiration 
 on the great classical achievements, and endeavoured here and 
 there to patch up the small flaws and still outstanding difficulties 
 of the theory. At length, however, theoretical astronomers have 
 been aroused, and have taken another great stride. It is, in- 
 deed, curious to note what has been the origin of these modern 
 researches. The energies of a Laplace were called forth to explain 
 the long irregularities of Jupiter and Saturn; the genius of an 
 Adams or a Le Verrier was evoked by the unexplained perturbations 
 of Uranus. In such cases theoretical labours have arisen, in the hope 
 of reconciling the discrepancies between theory and observation. 
 The motives which have excited this new outburst of discovery have 
 been wholly different. There is now no notorious discrepancy or 
 irregularity, to remain a reproach to astronomy until it is accounted 
 
THE TIDES. 513 
 
 for : the theory to which we wish now to draw attention is entirely, 
 or almost entirely, in advance of astronomical observation. 
 
 That the great discoveries of Lag-range on the stability of the 
 planetary system are correct, is in one sense strictly true. No one 
 has ever ventured to impeach the mathematics of Lagrange. Given 
 the planetary system in the form which Lagrange assumed, and the 
 stability of that system is assured for all time. There is, however, 
 one assumption which Lagrange makes, and on which his whole 
 theory was founded : his assumption is that the planets are rigid 
 bodies. 
 
 No doubt our earth seems a rigid body. What can be more 
 solid and unyielding than the mass of rocks and metals which form 
 the earth, so far as it is accessible to us ? In the wide realms of 
 space the earth is but as a particle ; it surely was a natural and a 
 legitimate assumption to suppose that that particle was a rigid 
 body. If the earth were absolutely rigid if every particle of the 
 earth were absolutely at a fixed distance from every other particle 
 if under no stress of forces, and under no conceivable circum- 
 stance, the earth could experience even the minutest change of 
 form if the same could be said of the sun and of all the other 
 planets then Lagrange's prediction of the eternal duration of our 
 system must be fulfilled. 
 
 But what are the facts of the case ? Is the earth really rigid ? 
 We know from experiment that a rigid body in the mathematical 
 sense of the word does not exist. Rocks are not rigid ; steel is not 
 rigid; even a diamond is not perfectly rigid. The whole earth is 
 far from being rigid even on the surface, while the interior is still, 
 perhaps, more or less fluid. The earth cannot be called a rigid 
 body ; still less can the larger bodies of our system be called rigid. 
 Jupiter and Saturn are perhaps hardly even what could be called 
 solid bodies. The solar system of Lagrange consisted of a rigid 
 sun and a number of minute rigid planets ; the actual solar system 
 consists of a sun which is in no sense rigid, and planets which are 
 only partially so. 
 
 The question then arises as to whether the discoveries of the 
 great mathematicians of the last century will apply, not only to the 
 H u 
 
514 THE STORY OF TEE HEAVENS. 
 
 ideal solar system which they conceived, but to the actual solar 
 system in which our lot has been cast. There can be no doubt that 
 these discoveries are approximately true : they are, indeed, so near 
 the absolute truth, that observation has not yet satisfactorily shown 
 any departure from them. 
 
 But in the present state of science we can no longer overlook 
 the important questions which arise when we deal with bodies not 
 rigid, in the mathematical sense of the word. Let us, for instance, 
 take the simplest of the laws to which we have referred, the great 
 law of Kepler, which asserts that a planet will revolve for ever in 
 an elliptic path of which the sun is one focus. This is seen to be 
 verified by actual observation : indeed, it was established by obser- 
 vation before any theoretical explanation of that movement was 
 propounded. If, however, we state the matter with a little more 
 precision, we shall find that what Newton really demonstrated was, 
 that if two rigid particles attract each other by a law of force which 
 varies with the inverse square of the distance between the particles, 
 then each of the particles will describe an ellipse with the com- 
 mon centre of gravity in the focus. The earth is, to some extent, 
 rigid, and hence it was natural to suppose that the relative be- 
 haviour of the earth and the sun would, to a corresponding 
 extent, observe the simple elliptic law of Kepler ; as a matter of 
 fact they do observe it with such fidelity, that if we make allow- 
 ance for other causes of disturbance, we cannot, by our most 
 careful observations, detect the slightest variation in the motion 
 of the earth arising from its want of rigidity. 
 
 There is, however, a subtlety in the investigations of mathe- 
 matics which, in this instance at all events, immeasurably 
 transcends the most delicate observations which our instruments 
 enable us to make. The principles of mathematics tell us that 
 though Kepler's laws may be true for bodies which are absolutely 
 and mathematically rigid, yet that if the sun or the planets be 
 either wholly, or even in their minutest part, devoid of perfect 
 rigidity, then Kepler's laws can be no longer true. Do we not 
 seem here to be in the presence of a contradiction ? Observation 
 tells us that Kepler's laws are true in the planetary system; 
 
THE TIDES. 515 
 
 theory tells us that these laws cannot be true in the planetary 
 system, because the bodies in that system are not perfectly rigid. 
 How is this discrepancy to be removed? or is there really a 
 discrepancy at all? There is not. When we say that Kepler's 
 laws have been proved to be true by observation, we must reflect on 
 the nature of the proofs which are attainable. We observe the 
 places of the planets with the instruments in our observatories; 
 these places are measured by the help of our clocks and of the 
 graduated circles on the instruments. These observations are no 
 doubt wonderfully accurate ; but they do not, they cannot, possess 
 absolute accuracy in the mathematical sense of the word. We can, 
 for instance, determine the place of a planet with such precision 
 that it is certainly not one second of arc wrong ; and one second is 
 an extremely small quantity. A foot-rule placed at a distance of 
 about forty miles subtends an angle of a second, and it is surely 
 a delicate achievement to measure the place of a planet, and feel 
 confident that no error greater than this can have intruded into 
 our result. 
 
 When we compare the results of observation with the calcu- 
 lations conducted on the assumption of the truth of Kepler's laws, 
 and when we pronounce on the agreement of the observations with 
 the calculations, there is always a reference, more or less explicit, to 
 the inevitable errors of the observations. If the calculations and 
 observations agree so closely that the differences between the two 
 are minute enough to have arisen in the errors inseparable from the 
 observations, then we are satisfied with the accordance ; for, in fact, 
 no closer agreement is attainable, or even conceivable. Now, the 
 influence which the want of rigidity exercises on the fulfilment of 
 the laws of Kepler can be estimated by calculation ; it is found, as 
 might be expected, to be extremely small; so small, in fact, as to be 
 contained within that slender margin of error by which observations 
 are liable to be affected. We are thus not able to discriminate by 
 actual measurement the effects due to the absence of rigidity; they 
 are inextricably hid among the errors of observation. 
 
 The great law of universal gravitation has been recently applied 
 to astronomical discovery in the direction we have indicated. By 
 H H 2 
 
516 THE STOEY OF THE HEAVENS. 
 
 its means we are enabled to peer back into the abyss of past time, 
 and trace to some extent the changes which our system has under- 
 gone. Let us attempt to render some account of these memorable 
 researches. 
 
 At the spring of the year the advent of the birds which visit 
 us yearly, the bursting of the trees into leaf, and the gradual 
 opening of the flowers, bring before us in a most pleasing manner 
 the principle of periodicity, which is of such prevalence in nature. 
 The astronomical phenomena which are most familiar to us are, 
 indeed, all periodic. We have the rising and setting of the sun, 
 we have the changes of the moon, we have the changes of the 
 seasons, as very simple instances ; but there are many other 
 periodical phenomena with which astronomers are acquainted. 
 Take, for instance, that great cycle which is accomplished in the 
 precession of the equinoxes. The pole describes a majestic circle 
 in the heavens in a period of about 26,000 years; but even this 
 long period seems brief when compared with some other periodic 
 phenomena. There is a stupendous period in our system when the 
 planets, having gone through all their mutual perturbations, com- 
 mence a new cycle. All periodic phenomena have one great feature 
 in common : at the close of a cycle the condition of the system is 
 the same as it was at the commencement. For our present purpose 
 phenomena of this kind are immaterial : they are not the source of 
 the mighty changes now to occupy our attention. 
 
 There are some astronomical phenomena in progress which are 
 not periodic : they do not increase, and then decrease, with the 
 rhythm of the more obvious phenomena. It behoves us to look 
 very closely into any phenomena of this character : for they are the 
 real architects of the universe. 
 
 There are two elements in determining the effect which such a 
 cause can produce. One of those elements is the efficiency of the 
 cause itself ; the other element is the time during which that cause 
 has acted. The latter factor is capable of indefinite increase. Even 
 if the cause be extremely small, yet if the time during which that 
 cause has acted be extremely great, the result attained may possess 
 stupendous dimensions. We propose to illustrate one of these 
 
THE TIDES. 517 
 
 constantly acting- influences, which has had some share in moulding 
 our solar system generally, and has specially had a paramount share 
 in the development of our earth and our moon. 
 
 The argument on which we are to base our researches is really 
 founded on a very familiar phenomenon. There is no one who has 
 ever visited the sea-side, who is not familiar with that rise and 
 fall of the sea which we call the tide. Twice every twenty-four 
 hours the sea advances on the beach to produce high tide; twice 
 every day the sea again retreats to produce low tide. These tides 
 are not merely confined to the coasts; they penetrate for miles 
 up the courses of rivers ; they periodically inundate great estuaries. 
 In a maritime country the tides are of the most profound practical 
 importance : they also possess a significance of a far less obvious 
 character, which it is our object now to investigate. 
 
 These daily pulses of the ocean have long ceased to be a 
 mystery. It was in the earliest times perceived that there was 
 a connection between the tides and the moon. Ancient writers, 
 such, as Pliny and Aristotle, have referred to the alliance be- 
 tween the times of high water and the age of the moon. I 
 think we sometimes do not give the ancient astronomers as much 
 credit as their shrewdness really entitles them to. We have all 
 read we have all been taught that the moon and the tides are 
 connected together; but how many of us are in a position to say 
 that we have actually noticed that connection by direct personal 
 observation ? The first man who really studied this matter, with 
 sufficient attention to convince himself and to convince others 
 of its reality, was a great philosopher. We know not his name, 
 we know not his nation, we know not the age in which he lived ; 
 but our admiration of his discovery must be increased by the 
 reflection that he had not the theory of gravitation to guide him. 
 A philosopher of the present day who had never seen the sea could 
 still predict the necessity of tides as a consequence of the law of 
 universal gravitation ; but the primitive astronomer, who knew not 
 of the invisible bond by which all bodies in the universe are drawn 
 together, made a splendid indeed, a typical inductive discovery, 
 when he ascertained the relation between the moon and the tides. 
 
518 THE STORY OF THE HEAVENS. 
 
 We can surmise that this discovery, in all probability, first 
 arose from the observation of experienced navigators. In all 
 matters of entering port or of leaving port, the state of the 
 tide is of the utmost concern to the sailor. Even in the open 
 sea he has sometimes to shape his course in accordance with the 
 currents produced by the tides ; or, in guiding his course by taking 
 soundings, he has always to bear in mind that the depth varies with 
 the tide. All matters relating to the tide would thus come under 
 his daily observation. His daily work, the success of his occupation, 
 the security of his life, depend often on the tides ; and hence he 
 would be solicitous to learn from his observation all that would be 
 useful to him in the future. To the coasting sailor the question of 
 the day is the time of high water. That time varies from day to 
 day ; it is an hour or more later to-morrow than to-day, and there 
 is no very simple rule which can be enunciated. The sailor would 
 therefore welcome gladly any rule which would guide him in a 
 matter of such importance. We can make a conjecture as to the 
 manner in which such a rule was first discovered. Let us suppose 
 that a sailor at Calais, for example, is making for harbour. He 
 has a beautiful night the moon is full ; it guides him on his way ; 
 he gets safely into harbour ; and the next morning he finds the 
 tide high between 11 and 12.* He often repeats the same voyage, 
 but he finds sometimes a low and inconvenient tide in the morning. 
 At length, however, it occurs to him that when he has a moonlight 
 night he has a high tide at 11. This occurs once or twice : he 
 thinks it is but a chance coincidence. It occurs again and again. 
 At length he finds it always occurs. He tells the rule to other 
 sailors; tKey try it too. It is invariably found that when the 
 moon is full, the high tide always recurs at the same hour at the 
 same place. The connection between the moon and the tide is thus 
 established, and the intelligent sailor will naturally compare other 
 phases of the moon with the times of high water. He finds, for 
 example, that the moon at the first quarter always gives high 
 water at the same hour of the day ; and finally he obtains a practical 
 
 * The hour varies with the locality: it would be 11.49 at Calais; at Liver- 
 pool, 11.23 : at Swansea Bay, 5.56, &c. 
 
THE TIDES. 519 
 
 rule, by which, from the state of the moon, he can at once tell 
 the time when the tide will be high at the port where his occu- 
 pation lies. A diligent observer will trace a still further con- 
 nection between the moon and the tides : he will observe that some 
 high tides rise higher than others, that some low tides fall lower 
 than others. This is a matter of great practical importance. When 
 a dangerous bar has to be crossed, the sailor will feel much 
 additional security in knowing that he is carried over it on the top 
 of a spring tide; or if he has to contend against tidal currents, 
 which in some places have enormous force, he will naturally prefer 
 for his voyage the neap tides, in which the strength of these 
 currents is less than usual. The spring tides and the neap tides 
 will become familiar to him, and he will perceive that the spring 
 tides occur when the moon is full or new or, at all events, that 
 the spring tides are within a certain constant number of days of 
 the full or new moon. It was, no doubt, by reasoning such as this, 
 that in primitive times the connection between the moon and the 
 tides came to be perceived. 
 
 It was not, however, until the great discoveiy of Newton had 
 disclosed the law of universal gravitation that it became possible to 
 give a physical explanation of the tides. It was then seen how the 
 moon attracts the whole earth and every particle of the earth. It 
 was seen how the fluid particles which form the oceans on the 
 earth, were enabled to obey the attraction in a way that the solid 
 parts could not. When the moon is overhead it draws the water 
 up, as it were, into a heap underneath, and thus gives rise to the 
 high tide. The water on the opposite side of the earth is also 
 affected in a way that might not be at first anticipated. The moon 
 attracts the solid body of the earth with greater intensity than it 
 attracts the water at the other side, which lies more distant from it. 
 The earth is thus drawn away from the water, which accordingly 
 exhibits a high tide as well on the side of the earth away from 
 the moon as on that towards the moon. The low tides occupy the 
 intermediate positions. 
 
 The sun also excites tides on the earth, but owing to the 
 great distance of the sun, the difference of its attraction on the 
 
520 THE STORY OF THE HEAVENS. 
 
 sea and on the solid interior of the earth is not so appreciable. The 
 solar tides are thus much less than the lunar tides. "When the two 
 conspire, they cause a spring tide ; when the solar and lunar tides 
 are opposed, we have the neap tide. 
 
 There are, however, a multitude of circumstances to be taken 
 into account when we attempt to apply this general reasoning to 
 the conditions of a particular case. Owing to local peculiarities 
 the tides vary enormously at the different parts of the coast. In 
 a confined area like the Mediterranean Sea, the tides have only a 
 comparatively small range, varying at different places from one 
 foot to a few feet. In mid-ocean also the tidal rise and fall is 
 not large, amounting, for instance, to a range of three feet at 
 St. Helena. Near the great continental masses the tides become 
 very much modified by the coasts. We find at London a tide of 
 eighteen or nineteen feet; but the most remarkable tides in the 
 British Islands are those in the Bristol Channel, where, at Chep- 
 stow or Cardiff, there is a rise and fall during spring tides to the 
 height of thirty-seven or thirty-eight feet, and at neap tides to a 
 height of twenty-eight or twenty-nine. These tides are surpassed 
 in magnitude at other parts of the .world. The greatest of all tides 
 are those in the Bay of Fundy, at some parts of which the rise and 
 fall at spring tides is not less than fifty feet. 
 
 It will, of course, be obvious that the rising and falling of the 
 tide is attended with the formation of currents. Such currents are, 
 indeed, well known, and in some of our great rivers they are of the 
 utmost consequence. These currents of water can, like water- 
 streams of any other kind, be made to do useful work. We can, 
 for instance, impound the rising water in a reservoir, and as the 
 tide falls we can compel the enclosed water to work a water-wheel 
 before it returns to the sea. We have, indeed, here a source of 
 actual power ; but it is only under very unusual circumstances that 
 it would be found economical to use the tides for this purpose. 
 The question can be submitted to calculation, and the area of the 
 reservoir can be computed which would retain sufficient water to 
 work a water-wheel of given horse-power. It can be shown that 
 the area of the reservoir necessary to impound water enough to 
 
THE TIDES. 521 
 
 produce 100 horse-power would be 40 acres. The whole question is 
 then reduced to the simple one of expense : would the construction 
 and the maintenance of this reservoir be more or less costly than 
 the erection and the maintenance of a steam-engine of equivalent 
 power? In most cases it would seem that the latter would be 
 by far the cheaper; at all events, we do not practically find tidal 
 engines in use, so that the power of the tides is now running to 
 waste, no less than the power of Niagara itself. The economical 
 aspects of the case may, however, be very profoundly altered at 
 some remote epoch, when our stores of fuel, now so lavishly 
 expended, give appreciable signs of approaching exhaustion. 
 
 The tides are, however, doing work of one kind or another. A 
 tide in a river estuary will sometimes scour away a bank and carry 
 its materials elsewhere. We have here work done and energy con- 
 sumed, just as much as if the same task had been accomplished by 
 engineers directing the powerful arms of navvies. We know that 
 work cannot be done, without the consumption of energy in some of 
 its forms; whence, then, comes the energy which supplies the power 
 of the tides ? At a first glance the answer to this question seems a 
 very obvious one. Have we not said that the tides are caused by 
 the moon? and must not the energy, therefore, be derived from the 
 moon ? This seems obvious enough, but unfortunately, it is not true. 
 It is one of those cases by no means infrequent in Dynamics, where 
 the truth is widely different from that which seems to be obviously 
 the case. An illustration will perhaps make the matter clearer. 
 W T hen a rifle is fired, it is the finger of the rifleman that pulls the 
 trigger; but are we, then, to say that the energy by which the 
 bullet has been driven off has been supplied by the rifleman? 
 Obviously not; the energy is, of course, due to the gunpowder, 
 and all the rifleman did was to provide the means by which the 
 energy stored \ip in the powder could be liberated. To a certain 
 extent we may compare this with the tidal problem; the tides 
 raised by the moon are the originating cause whereby a certain 
 store of energy is drawn upon and applied to do such work as the 
 tides are competent to perform. This store of energy, strange to 
 say, does not lie in the moon; it is in the earth itself. Indeed, it 
 
522 THE STORY OF THE HEAVENS. 
 
 is extremely remarkable that the moon actually gains energy 
 from the tides, by itself absorbing some of the store which exists 
 in the earth. This is not put forward as an obvious result; in- 
 deed, it is very far from being obvious, as it depends upon a refined 
 dynamical theorem. 
 
 Let us, however, clearly understand the nature of this mighty 
 store of energy from which the tides draw their power, and on 
 which even the moon herself makes large and incessant drafts. In 
 what sense does the earth possess a store of energy ? Do we not 
 know that the earth rotates on its axis once every day ? It is 
 this rotation which is the source of the energy. Let us compare 
 the rotation of the earth with the rotation of the fly-wheel be- 
 longing to a steam-engine. The rotation of the fly-wheel is 
 really a reservoir, into which the engine pours energy at each 
 stroke of the piston. The various machines in the mill worked 
 by the engine merely draw on the store of energy accumulated 
 in the fly-wheel. The earth may be likened to a gigantic fly- 
 wheel detached from the engine, though still connected with 
 the machines in the mill. From its stupendous dimensions 
 and from its rapid velocity, that great fly-wheel possesses an 
 enormous store of energy, and that energy must be expended 
 before the fly-wheel comes to rest. Hence it is that though the 
 tides are caused by the moon, yet the energy they require is 
 obtained by simply drawing on the vast supply ready to hand in 
 the rotation of the earth. 
 
 There is, however, a distinction of a very fundamental character 
 between the earth and the fly-wheel of an engine. As the energy 
 is withdrawn from the fly-wheel and consumed by the various 
 machines in the mill, it is continually replaced by fresh energy, 
 which flows in from the exertions of the steam-engine, and thus 
 
 O ' 
 
 the velocity of the fly-wheel is maintained. But the earth is the 
 fly-wheel without the engine. When the tides draw on the store 
 of energy and expend it in doing work, that energy is not replaced. 
 The consequence is irresistible : the energy in the rotation of the 
 earth must be decreasing. This leads to a consequence of the very 
 utmost significance. If the engine be cut off from the fly-wheel, 
 
THE TIDES. 523 
 
 then, as every one knows, the massive fly-wheel may still give a few 
 rotations, but it will speedily come to rest. A similar inference 
 must be made with regard to the earth ; but its store of energy is 
 so enormous, in comparison with the demands which are made 
 upon it, that the earth is able to hold out. Ages of countless 
 duration must elapse before the energy of the earth's rotation can 
 be completely exhausted by such drafts as the tides are capable of 
 making. Nevertheless, it is necessarily true that the energy is 
 decreasing; and if it be decreasing, then the speed of the earth's 
 rotation must be surely, if slowly, abating. Now we have arrived 
 at a consequence of the tides which admits of being stated in the 
 simplest language. If the speed of rotation be abating, then the 
 length of the day must be increasing ; and hence we are conducted 
 to the following most important statement; that the tides are 
 increasing the length of the day. 
 
 To-day is longer than yesterday to-morrow will be longer than 
 to-day. The difference is so small that even in the course of ages 
 it can hardly be said to have been distinctly established by obser- 
 vation. We do not pretend to say how many centuries have elapsed 
 since the day was even one second shorter than it is at present ; 
 but centuries are not the units which we employ in tidal evolution. 
 A million years ago it is quite probable that the divergence of the 
 length of the day from its present value may have been very con- 
 siderable. Let us take a profound glance back into the depths of 
 times past, and see what the tides have to tell us. If the present 
 order of things has lasted, the day must have been shorter and 
 shorter the farther we look back into the dim past. The day is 
 now twenty-four hours ; it was once twenty hours, once ten hours ; 
 it was once six hours. How much farther can we go ? Once the 
 six hours is past, we begin to approach a limit which must at some 
 point bound our retrospect. The shorter the day, the more is the 
 earth bulged at the equator ; the more the earth is bulged at the 
 equator, the greater is the strain put upon the materials of the 
 earth by the centrifugal force of its rotation. If the earth were to 
 go too fast, it would be unable to cohere together ; it would separate 
 into pieces, just as a grindstone driven too rapidly is rent asunder 
 
524 THE STORY OF THE HEAVENS. 
 
 with violence. Here, therefore, we discern in the remote past a 
 barrier which stops the present argument. There is a certain 
 critical velocity which is the greatest that the earth could have 
 without risk of rupture, but the exact amount of that velocity is a 
 question not very easy to answer. It depends upon the nature of 
 the materials of the earth : it depends upon the temperature : it 
 depends upon the effect of pressure, and on other details not accu- 
 rately known to us. An estimate of the critical velocity has, how- 
 ever, been made, and it has been shown that the shortest period of 
 rotation which the earth could have, without flying into pieces, is 
 about three or four hours. The doctrine of tidal evolution has 
 thus conducted us to the conclusion that, at some inconceivably 
 remote epoch, the earth was spinning round its axis in a period 
 approximating to three or four hours. 
 
 We thus learn tha,t we are indebted to the moon for the 
 gradual elongation of the day from its primitive value up to 
 twenty-four hours. In obedience to one of the most profound 
 laws of nature, the earth has reacted on the moon, and the re- 
 action of the earth has taken a tangible form. It has simply 
 consisted in gradually driving the moon away from the earth. 
 We may observe that this driving away of the moon resembles 
 a piece of retaliation on the part of the earth. "You raise tides 
 on me," says the earth, " and I do not like to be troubled with 
 the tides, so I gradually push you away. If you will stop raising 
 the tides on me, then I will stop pushing you away, and we can 
 revolve round each other uniformly and for ever/' The conse- 
 quence of the retreat of the moon is sufficiently remarkable. The 
 path in which the moon is revolving has at the present time a 
 radius of 240,000 miles. This radius must be constantly growing 
 larger, in consequence of the tides. .Provided with this fact, let us 
 now glance back into the past history of the moon. As the moon's 
 distance is increasing when we look forwards, so we find it decreasing 
 when we look backwards. The moon must have been nearer the 
 earth yesterday than it is to-day: the difference is no doubt 
 inappreciable in years, in centuries, or in thousands of years; but 
 when we come to millions of years, the moon must have been 
 
THE TIDES. 525 
 
 significantly closer than it is at present : until at length we find 
 that its distance, instead of being 240,000 miles, has dwindled 
 down to 40,000, to 20,000, to 10,000 miles. Nor need we stop 
 nor can we stop until we find the moon actually close to the 
 earth's surface. If the laws of nature have lasted long enough, 
 and if there has been no external interference, then it cannot be 
 doubted that the moon and the earth were once in immediate 
 proximity. It is easy to calculate the time in which the moon 
 must have been revolving round the earth. The nearer the moon 
 is, the quicker it must revolve ; and at the wondrous epoch, when 
 the moon was close to the earth, it must have completed each 
 revolution in about three or four hours. 
 
 This has led to one of the most daring speculations that has 
 ever been made in astronomy. We cannot refrain from enunciating 
 it ; but it must be remembered that it is only a speculation, and to 
 be received with corresponding reserve. The speculation is intended 
 to answer the question, What brought the moon into that position, 
 close to the surface of the earth ? We will only say that there is 
 the gravest reason to believe that the moon was, at some very early 
 period, fractured off from the earth when the earth was in a soft or 
 plastic condition. 
 
 At the beginning of the history we found the earth and the moon 
 close together. We found that the rate of rotation of the earth was 
 only a few hours instead of twenty -four hours. We found that the 
 moon completed- its journey round the primitive earth in exactly 
 the same time as the primitive earth rotated on its axis, so that the 
 two bodies were then constantly face to face. Such a state of 
 things formed what a mathematician would describe as a case of 
 unstable dynamical equilibrium. It could not last. It may be 
 compared to the case of a needle balanced on its point ; the needle 
 must fall to one side or the other. In the same way, the moon 
 could not continue to preserve this position. There were two 
 courses open : the moon must either have fallen back on the earth, 
 and been re-absorbed into the mass of the earth, or it must have 
 commenced its outward journey. Which of these courses was the 
 moon to adopt ? We have no means, perhaps, of knowing exactly 
 
526 THE STORY OF THE HEAVENS. 
 
 what it was which determined the moon to one course rather than to 
 another, but as to the course which was actually taken there can be 
 no doubt. The fact that the moon exists, shows that it did not 
 return to the earth, but commenced its outward journey. As the 
 moon recedes from the earth, it must, in conformity with Kepler's 
 laws, require a longer time to complete its revolution. It has thus 
 happened that from the original period of only a few hours, the 
 duration has increased until it has reached the present number 
 of 656 hours. The rotation of the earth has, of course, also 
 been modified, in accordance with the retreat of the moon. 
 Once the moon had commenced to recede, the earth was released 
 from the obligation which required it constantly to direct the same 
 face to the moon. When the moon had receded to a certain 
 distance, the earth would complete the rotation in less time than 
 that required by the moon for one revolution. Still the moon gets 
 further and further away, and the duration of the revolution 
 increases to a corresponding extent, until three, four, or more days 
 (or rotations of the earth) are identical with the month (or revo- 
 lution of the moon) . Although the number of days in the month 
 increases, yet we are not to suppose that the rate of the earth's 
 rotation is increasing ; indeed, the contrary is the fact. The earth's 
 rotation is getting slower, and so is the revolution of the moon, 
 but the retardation of the moon is much greater than that of the 
 earth. Even though the period of rotation of the earth has greatly 
 increased from its primitive value, yet the period of the moon 
 has increased, so that it is several times as large as that of the 
 rotation of the earth. As ages roll on the moon recedes further 
 and further, its orbit increases, the duration of the revolution 
 augments, until at length a very noticeable epoch is attained, 
 which is, in one sense, a culminating point in the career of the 
 moon. At this epoch the revolutions of the moon, when mea- 
 sured in rotations of the earth, attain their greatest value. It 
 would seem that at this time the month was about twenty-nine 
 days. It is not, of course, meant that the month and the day at 
 that epoch were the month and the day as our clocks now measure 
 time. Both were shorter then than now. But what we mean is, 
 
THE TIDES. 527 
 
 that at this epoch the earth rotated twenty-nine times on its axis 
 while the moon completed one circuit. 
 
 This epoch has now been passed. No attempt can be made at 
 present to evaluate the date of that epoch in our ordinary units 
 of measurement. At the same time, however, no doubt can be 
 entertained as to the immeasurable antiquity of the event, in com- 
 parison with all historic records ; but whether it is to be reckoned 
 in hundreds of thousands of years, in millions of years, or in tens 
 of millions of years, must be left in great degree to conjecture. 
 
 This remarkable epoch once passed, we find that the course of 
 events in the earth-moon system begins to shape itself towards 
 that remarkable final stage which has points of resemblance to the 
 initial stage. The moon still continues to revolve in an orbit with 
 a diameter, steadily, though very slowly, growing. The length of 
 the month is accordingly increasing, and the rotation of the earth 
 being still constantly retarded, the length of the day is also con- 
 tinually growing. But the ratio of the length of the month to the 
 length of the day now exhibits a change. That ratio had gradually 
 increased, from unity at the commencement, up to the maximum 
 value of somewhere about twenty-nine at the epoch just referred to. 
 The ratio now begins again to decline, until we find the earth 
 makes only twenty-eight rotations, instead of twenty-nine, in one 
 revolution of the moon. The decrease in the ratio continues until 
 the number twenty-seven expresses the days in the month. Here, 
 again, we have an epoch which it is impossible for us to pass 
 without special comment. In all that has hitherto been said, we 
 have been dealing with events in the distant past ; here we have 
 at length arrived at the present state of the earth-moon system. 
 The days at this epoch are our well-known days, the month is the 
 well-known period of the revolution of our moon. At the present 
 time the month is about twenty-seven of our days, and this relation 
 has remained sensibly true for thousands of years past. It will 
 continue to remain sensibly true for thousands of years to come, 
 but it will not remain true indefinitely. It is merely a stage 
 in this grand transformation; it may possess the attributes of 
 permanence to our ephemeral view, just as the wings of a gnat 
 
528 THE STORY OF THE HEAVENS. 
 
 seem at rest when illuminated by the electric spark, but when we 
 contemplate the mighty history with conceptions of time spacious 
 enough for astronomy, we realise how the present condition of the 
 earth-moon system can have DO greater permanence than any other 
 stage in the history. 
 
 Our narrative must, however, now assume a different form. 
 We have been speaking of the past ; we have been conducted to the 
 present; can we say anything of the future? Here, again, the 
 tides come to our assistance. If we have rightly comprehended the 
 truth of dynamics (and who is there now that can doubt them ?) , 
 we shall be enabled to make a forecast of the further changes of 
 the earth-moon system. If there be no interruption from any 
 external source at present unknown to us, we can predict in out- 
 line, at all events the subsequent career of the moon. We can 
 see how the moon will still follow its outward course. The path in 
 which it revolves will grow with incredible slowness, but yet it will 
 always grow ; the progress will not be reversed, at all events, before 
 the final stage of our history has been attained. We shall not 
 now delay to dwell on the intervening stages; we will rather 
 attempt to sketch the ultimate type to which our system tends. In 
 the dim future countless millions of years to come this final 
 stage will be approached. The ratio of the month to the day, whose 
 decline we have already referred to, will continue to decline. The 
 period of revolution of the moon will grow longer and longer, but 
 the length of the day will increase much more rapidly than the 
 increase in the duration of the moon's period. From the month of 
 twenty-seven days we shall pass to a month of twenty-six days, 
 and so on, until we shall reach a month of ten days, and, finally, a 
 month of one day. 
 
 Let us clearly understand what we mean by a month of one 
 day. We mean that the time in which the moon revolves around 
 the earth will be equal to the time in which the earth rotates around 
 its axis. The length of this day will, of course, be vastly greater 
 than our day. The only element of uncertainty in these inquiries 
 arises when we attempt to give numerical accuracy to the state- 
 ments. It seems to be as true as the laws of dynamics, that a state 
 
THE TIDES. 529 
 
 of the earth-moon system in. which the day and the month are 
 equal must be ultimately attained ; but when we attempt to state 
 the length of that day, we introduce a hazardous element into the 
 inquiry. In giving, then, any estimate of its length, it must be 
 understood that the magnitude is stated with great reserve. It 
 may be erroneous to some extent, though, perhaps, not to any con- 
 siderable amount. The length of this great day would seem to be 
 about equal to fifty-seven of our days. In other words, at some 
 critical time in the incredibly distant future, the earth will take 
 something like 1,400 hours to perform a rotation, while the moon 
 will complete its journey precisely in the same time. 
 
 We thus see how, in some respects, the first stage of the earth- 
 moon system and the last stage resemble each other. In each case 
 we have the day equal to the month. In the first case the day and 
 the month were only a small fraction of our day ; in the last stage 
 the day and month are each a large multiple of our day. There is, 
 however, a profound contrast between the first critical epoch and 
 the last. We have already mentioned that the first epoch was one 
 of instability it could not last ; but this second state is one of 
 dynamical stability. Once that state has been acquired, it must 
 be permanent, and would endure for ever if the earth and the moon 
 could be isolated from all external interference. 
 
 There is one special feature which characterises the movement 
 when the month is equal to the day. A little reflection will show 
 that when this is the case, the earth must constantly direct the 
 same face towards the moon. If the day be equal to the month, 
 then the earth and moon must revolve together, as if bound by 
 invisible bands ; and whatever hemisphere of the earth be directed 
 to the moon when this state of things commences, will remain there 
 so long as the day remains equal to the month. 
 
 At this point it is hardly possible to escape being reminded of 
 that characteristic feature of the moon's motion which has been 
 observed from all antiquity. We refer, of course, to the fact that 
 the moon at the present time constantly turns the same face to 
 the earth. 
 
 It is incumbent on astronomers to provide a physical explanation 
 I I 
 
530 THE STORY OF THE HEAVENS. 
 
 of this remarkable fact. The moon revolves around our earth once 
 in a definite number of seconds. If the moon always turns the 
 same face to the earth, then it is demonstrated that the moon 
 rotates on its axis once in the same number of seconds also. Now, 
 this would be a coincidence wildly improbable unless there were some 
 physical cause to account for it. We have not far to seek for a 
 cause : it is the influence of the tides which has caused the pheno- 
 menon. We now find the moon has a rugged surface, which testifies 
 to the existence of intense volcanic activity in former times. Those 
 volcanoes are now silent the internal fires in the moon seem to 
 have become exhausted; but there was a time when the moon 
 must have been a heated and semi-molten mass. There was a time 
 when the materials of the moon were so hot as to be soft and 
 yielding, and in that soft and yielding mass the attraction of our 
 earth excited great tides. We have no historical record of these 
 tides (they were long anterior to the existence of telescopes ; they 
 were probably long anterior to the existence of the human race), 
 but we know that these tides once existed by the work they have 
 accomplished, and that work is seen to-day in the constant face 
 which the moon turns towards the earth. The gentle rise and fall 
 of the oceans which form our tides, present a picture widely dif- 
 ferent from the tides by which the moon was once agitated. The 
 tides on the moon were vastly greater than those of the earth. 
 They were greater because the weight of the earth is greater than 
 that of the moon, so that the earth was able to produce much more 
 powerful tides in the moon than the moon has ever been able to 
 raise on the earth. 
 
 That the moon should bend the same face to the earth depends 
 immediately upon the condition that the moon shall rotate on its 
 axis in precisely the same period as that which it requires to revolve 
 around the earth. The tides are a regulating power of the most un- 
 remitting efficiency to ensure that this condition should be observed. 
 If the moon rotated more slowly than it ought, then the great lava 
 tides would drag the moon round faster and faster, until it attained 
 the desired velocity ; and then, but not till then, they would give 
 the moon peace. Or if the moon were to rotate faster on its axis 
 
THE TIDES. 531 
 
 than on its orbit, again the tides would come furiously into play ; 
 but this time they would be engaged in retarding the moon's 
 rotation, until they had reduced the speed of the moon to one 
 rotation for each revolution. 
 
 Can the moon ever escape from the thraldom of the tides ? 
 This is not very easy to answer, but it seems perhaps not im- 
 possible that the moon may, at some future time, be freed from 
 tidal control. It is, indeed, obvious that the tides, even at present, 
 have not the extremely stringent control over the moon which they 
 once exercised. We now see no ocean on the moon, nor do the 
 volcanoes show any trace of molten lava. There can hardly be 
 tides on the moon, but there may be tides in the moon. It may be 
 that the interior of the moon is still hot enough to retain an 
 appreciable departure from solidity, and if so, the tidal control 
 would still retain the moon in its grip ; but the time will probably 
 come, if it have not come already, when the moon will be cold to 
 the centre cold as the temperature of space. If the materials of the 
 moon were what a mathematician would call absolutely rigid, there 
 can be no doubt that the tides could no longer exist, and the moon 
 would be emancipated from tidal control. It seems impossible to 
 predicate how far the moon can ever conform to the circumstances 
 of an actual rigid body, but it may be conceivable that at some 
 future time the tidal control shall have practically ceased. There 
 would then be no longer any necessary identity between the period 
 of rotation and that of revolution. A gleam of hope is thus pro- 
 jected over the astronomy of the distant future. We know that 
 the time of revolution of the moon is increasing, and so long as 
 the tidal governor could act, the time of rotation must increase 
 sympathetically. We have now surmised a state of things in 
 which the control is absent. There will then be nothing to prevent 
 the rotation remaining as at present, while the period of revolution 
 is increasing. The privilege of seeing the other side of the moon, 
 which has been withheld from all previous astronomers, may thus 
 in the distant future be granted to their successors. 
 
 The tides which the moon raises in the earth act as a brake on 
 the rotation of the earth. They now constantly tend to bring the 
 I i '2 
 
532 THE STORY OF THE HEAVENS." 
 
 period of rotation of the earth to coincide with the period of 
 revolution of the moon. As the moon revolves once in twenty- 
 seven days, the earth is at present going too fast, and consequently 
 the tidal control at the present moment endeavours to retard the 
 rotation of the earth. The rotation of the moon long since 
 succumbed to tidal control, hut that was because the moon was 
 comparatively small and the tidal power of the earth was enormous. 
 Now the matters are reversed. The earth is large and more massive 
 than the moon, the tides raised by the moon are but small and 
 weak, and the earth has not yet completely succumbed to the tidal 
 action. But the tides are constant, they never for an instant relax 
 the effort to control, and they are gradually tending to render the 
 day and the month coincident, though the process is a very slow one. 
 The theory of the tides leads us to look forward to an ultimate 
 state of things, in which the moon revolves around the earth in a 
 period equal to the day, so that the two bodies shall constantly 
 bend the same face to each other, provided the tidal control be still 
 able to guide the moon's rotation. So far as the mutual action of 
 the earth and the moon are concerned, such an arrangement pos- 
 sesses all the attributes of permanence. If, however, we venture to 
 project our view to the incredibly remote future, we can discern a 
 certain external cause which must prevent this mutual accommo- 
 dation between the earth and the moon from being eternal. The 
 tides raised by the moon on the earth are so much greater than 
 those raised by the sun, that we have, in the course of our previous 
 reasoning, held little account of the sun-raised tides. This is 
 obviously only an approximate method of dealing with the 
 question. The influence of the solar tide is appreciable, and its 
 importance relatively to the lunar tide will gradually rise as the 
 earth and moon approach the final critical stage. The solar tides 
 will have the effect of constantly applying a further brake to the 
 rotation of the earth. It will therefore follow that after the day 
 and the month have become equal, a still further retardation awaits 
 the length of the day. We thus see that in the remote future we 
 shall find the moon revolving around the earth in a shorter time 
 than that in which the earth rotates on its axis. 
 
THE TIDES. 533 
 
 A most instructive corroboration of these views is afforded by the 
 discovery of the satellites of Mars. The planet Mars is one of the 
 smaller members of our system. It has a mass which is only the 
 eighth part of the mass of the earth. A small planet like Mars has 
 much less energy of rotation to be destroyed than a larger one like 
 the earth. It may therefore be expected that the small planet will 
 proceed much more rapidly in its evolution than the large one ; we 
 might, therefore, anticipate that Mars and his satellites have at- 
 tained a more advanced stage of their history than is the case with 
 the earth and hei satellite. 
 
 When the discovery of the satellites of Mars startled the world 
 a few years ago, there was no feature which created so much 
 amazement as the periodic time of the interior satellite. We have 
 already pointed out in Chapter X. how Phobos revolves around 
 Mars in a period of 7 hours 39 minutes. The period of rotation of 
 Mars himself is 24 hours 37 minutes, and hence we have the fact, 
 unparalleled in the solar system, that the satellite is actually revolv- 
 ing three times as rapidly as the planet is rotating. There can 
 hardly be a doubt that the solar tides on Mars have abated its 
 velocity of rotation in the manner just suggested. 
 
 It has always seemed to me that the matter just referred to is 
 one of the most interesting and instructive in the whole history of 
 astronomy. We have, first, a very beautiful telescopic discovery of 
 the minute satellites of Mars, and we have a determination of the 
 very anomalous movement of one of them. We have then found a 
 very satisfactory physical explanation of the cause of this pheno- 
 menon, and we have shown it to be a striking instance of tidal 
 evolution. Finally, we have seen that the system of Mars and his 
 satellite is really a forecast of the destiny which, in the lapse of 
 incredible ages, awaits the earth-moon system. 
 
 It seems natural to inquire how far the influence of tides can 
 have contributed towards moulding the planetary orbits. The cir- 
 cumstances are here very different to those we have encountered in the 
 earth-moon system. Let us first enunciate the problem in a definite 
 shape. The solar system consists of the sun in the centre, and of 
 the planets revolving around the sun. These planets rotate on their 
 
534 TEE STORY OF THE HEAVENS. 
 
 axes; and circulating round some of the planets, we have their 
 system of satellites. For simplicity, we may suppose all the 
 planets and their satellites to revolve in the same plane, and the 
 planets to rotate about axes which are perpendicular to that plane. 
 In the study of the theory of tidal evolution, we must be mainly 
 guided by a profound dynamical principle, known as the conserva- 
 tion of the "moment of momentum/'' The proof of this great 
 principle is not here attempted; suffice it to say, that it can be 
 strictly deduced from the laws of motion, and is thus only second 
 in certainty to the fundamental truths of ordinary geometry or of 
 algebra. Take, for instance, the giant planet, Jupiter. In one 
 second he moves around the sun through a certain angle. If we 
 multiply the mass of Jupiter by that angle, and if we then multiply 
 the product by the square of the distance from Jupiter to the sun, 
 we obtain a certain definite amount. A mathematician calls this 
 quantity the " orbital " moment of momentum of Jupiter. In the 
 same way, if we take the planet Saturn, multiply the mass of 
 Saturn by the angle through which the planet moves in one second, 
 and into the square of the distance between the planet and the sun, 
 then we have the orbital moment of momentum of Saturn. In a 
 similar manner we ascertain the moment of momentum for each of 
 the other planets due to revolution around the sun. We have also 
 to define the moment of momentum of the planets around their 
 axes. In one second Jupiter rotates through a certain angle ; we 
 multiply that angle by the mass of Jupiter, and by the square of a 
 certain line which depends on his internal constitution : the product 
 forms the " rotational " moment of momentum. In a similar manner 
 we find the rotational moment of momentum for each of the other 
 planets. Each satellite revolves through a certain angle around its 
 primary in one second; we obtain the moment of momentum of 
 each satellite by multiplying its mass into the angle described in 
 one second, and then multiplying the product into the square of 
 the distance of the satellite from its primary. Finally, we compute 
 the moment of momentum of the sun due to its rotation. This we 
 obtain by multiplying the angle through which the sun turns in 
 one second by the whole mass of the sun, and then multiplying the 
 
THE TIDES. 535 
 
 product by the square of a certain line of prodigious length, which 
 depends upon the details of the sun's internal structure. 
 
 If we have succeeded in explaining what is meant by the moment 
 of momentum, then the statement of the great law is comparatively 
 simple. We are, in the first place, to observe that the moment of 
 momentum of any planet may alter. It would alter if the distance 
 of the planet from the sun changed, or if the velocity with which 
 the planet rotates upon its axis changed ; so, too, the moment of 
 momentum of the sun may change, and so may those of the 
 satellites. In the beginning, a certain total quantity of moment of 
 momentum was communicated to our system, and not one particle 
 of that total can the solar system, as a whole, squander or alienate 
 No matter what be the mutual actions of the various bodies of the 
 system, no matter what perturbations they may undergo what 
 tides may be produced, or even what mutual collisions may occur 
 the great law of the conservation of moment of momentum must be 
 obeyed. If some bodies in the solar system be losing moment of 
 momentum, then other bodies in the system must be gaining, so 
 that the total quantity shall remain for ever the same. This con- 
 sideration is one of supreme importance in connection with the tides. 
 The distribution of moment of momentum in the system is being 
 continually altered by the tides ; but however the tides may ebb 
 or flow, the total moment of momentum can never alter so long 
 as influences external to the system are absent. 
 
 We must here point out the contrast between the endowment of 
 our system with energy, and with moment of momentum. The 
 mutual actions of our system, in so far as they produce heat, tend 
 to squander the energy, a considerable part of which can be thus 
 dissipated and lost ; but the mutual actions have no power of dissi- 
 pating the moment of momentum. 
 
 The total moment of momentum of the solar system being taken 
 to be 100, this is at present distributed as follows : 
 
 Orbital moment of momentum of Jupiter ... ... 60 
 
 Saturn 24 
 
 ., Uranus 6 
 
 Neptune 8 
 
 Rotational moment of momentum of Sun 
 
 100 
 
536 THE STORY OF THE HEAVENS. 
 
 The contributions of the other items are excessively minute. The 
 orbital moments of momentum of the few interior planets contain 
 but little more than one thousandth part of the total amount. The 
 rotational contributions of all the planets and of their satellites is 
 very much less, being not more than one sixty-thousandth part of 
 the whole. When, therefore, we are studying the general effects 
 of tides on the planetary orbits, these trifling matters may be over- 
 looked. We shall, however, find it desirable to narrow the question 
 still more, and concentrate our attention on one splendid illustration. 
 Let us take the sun and the planet Jupiter, and supposing all other 
 bodies of our system to be absent, let us discuss the influence of 
 tides produced in Jupiter by the sun, and of tides in the sun by 
 Jupiter. 
 
 It might be hastily thought that, just as the moon was born of 
 the earth, so the planets were born of the sun and have gradually 
 receded by tides into their present condition. W T e have the means 
 of inquiry into this question by the figures just given, and we shall 
 show that it seems utterly impossible that Jupiter, or any of the 
 other planets, can ever have been very much closer to the sun than 
 they are at present. In the case of Jupiter and the sun, we have 
 the moment of momentum made up of three items. By far the 
 largest of these items is due to the orbital revolution of Jupiter, the 
 next is due to the sun, the third is due to the rotation of Jupiter on 
 its axis. We may put them in round numbers as follows : 
 
 Orbital moment of momentum of Jupiter 600,000 
 
 Rotational Sun 20,000 
 
 Jupiter 12 
 
 The sun produces tides in Jupiter, those tides retard the rotation 
 of Jupiter. They make Jupiter rotate more and more slowly, 
 therefore the moment of momentum of Jupiter is decreasing, 
 therefore its present value of 12 must be decreasing. Even the 
 mighty sun himself may be distracted by tides. Jupiter raises 
 tides in the sun, those tides retard the motion of the sun, and 
 therefore the moment of momentum of the sun is decreasing, and 
 it follows from both causes that the item of 600,000 must be 
 
THE TIDES. 537 
 
 increasing, in other words the orbital motion of Jupiter must be 
 increasing-, or Jupiter must be receding from the sun. To this 
 extent therefore, the sun-Jupiter system is analogous to the earth- 
 moon system. As the tides on the earth are driving away the 
 moon, so the tides in Jupiter and the sun are gradually driving 
 the two bodies apart. But there is a profound difference between 
 the two cases. It can be proved that the tides produced in Jupiter 
 by the sun are more effective than those produced in the sun by 
 Jupiter. The contribution of the sun may, therefore, be at present 
 omitted; so that, practically, the augmentations of the orbital 
 moment of momentum of Jupiter are achieved at the expense 
 of that stored up by Jupiter's rotation. But what is 12 com- 
 pared with 600,000. Even when the whole of Jupiter's rota- 
 tional moment of momentum and that of his satellites has become 
 absorbed into the orbital motion, there will hardly be an appre- 
 ciable difference in the latter. In ancient days we may indeed suppose 
 that Jupiter being hotter was larger than at present, and that he, 
 therefore, had more rotational moment of momentum. But it is 
 hardly credible that Jupiter can ever have had one hundred times 
 the moment of momentum that he has at present. Yet even if 
 1,200 units of rotational momentum had been transferred to the 
 orbital motion it would only correspond with the most trivial dif- 
 ference in the distance of Jupiter from the sun. We are hence 
 assured that the tides have not appreciably altered the dimensions 
 of the orbit of Jupiter, or of the other great planets. 
 
 The time will however, come, when the rotation of Jupiter on his 
 axis will be gradually abated by the influence of the tides. It will 
 then be found that the moment of momentum of the sun's rotation 
 will be gradually expended in increasing the orbits of the planets, 
 but as this reserve only holds about two per cent, of the whole 
 amount in our system, it cannot produce any considerable effect. 
 
 And now we must draw this chapter to a close, though there are 
 many other subjects that might be included. The theory of tidal 
 evolution is, indeed, one of quite exceptional interest. The earlier 
 mathematicians expended their labour on the determination of the 
 dynamics of a system which consisted of rigid bodies. "We are 
 
538 THE STORY OF THE HEAVENS. 
 
 indebted to contemporary mathematicians for opening up celestial 
 mechanics on the more real supposition that the bodies are not 
 rigid ; in other words, that they are subject to tides. The mathe- 
 matical difficulties are enormously enhanced, but the problem 
 is more true to nature, and has already led to some of the most 
 remarkable astronomical discoveries made in modern times. 
 
 Our Story of the Heavens has now been told. We commenced 
 this work with some account of the mechanical and optical aids 
 to astronomy; we have ended it with a brief description of an 
 intellectual method of research which reveals some of the celestial 
 phenomena that occurred ages before the human race existed. 
 We have spoken of those objects which are comparatively near to 
 us, and then step by step we have advanced to the distant nebulae 
 and clusters which seem to lie on the confines of the visible 
 universe. Yet, after all, how little is all we can see even with our 
 greatest telescopes, when compared with the whole extent of infinite 
 space ! No matter how vast may be the depth which our instru- 
 ments have sounded, there is yet a beyond of infinite extent. 
 Imagine a mighty globe described in space, a globe of such 
 stupendous dimensions that it shall include the sun and his system, 
 all the stars and nebulae, and even all the objects which our finite 
 capacities can imagine. Yet, after all, what must be the relation 
 of even this great globe to the whole extent of infinite space. The 
 globe will bear to that a ratio infinitely less than that which the 
 water in a single drop of dew bears to the water in the whole 
 Atlantic Ocean. 
 
 THE END. 
 
APPENDIX. 
 
 ASTRONOMICAL QUANTITIES. 
 
 THE SUN. 
 
 THE sun's mean distance from the earth is 92,700,000 miles; his 
 diameter is 865,000 miles; his density, as compared with water, is 
 1 '4 ; his ellipticity is insensible ; he rotates on his axis in a period 
 between 25 and 26 days. 
 
 THE MOON. 
 
 The moon's mean distance from the earth is 238,000 miles. The 
 least possible distance is 221,000 miles; the greatest is 260,000 miles. 
 The diameter of the moon is 2,160 miles; and her density, as compared 
 with water, is 3 '5. The time of a revolution around the earth is 
 27-322 days. 
 
 THE PLANETS. 
 
 
 Distnnce from Sun in 
 
 
 
 
 
 
 Millions of Miles. 
 
 Per'odic 
 Time 
 in Dajs. 
 
 Diameter 
 in Miles. 
 
 Axial Eotation. 
 
 Density 
 compared 
 with 
 Water. 
 
 
 
 
 
 Mean. 
 
 Least. 
 
 Greatest. 
 
 
 
 
 
 Mercury 
 
 35-9 
 
 28-6 
 
 43-3 
 
 87-969 
 
 2,992 
 
 h. m. eecs. 
 
 24 5 (1) 
 
 6-85 
 
 Venus - 
 
 67-0 
 
 66-6 
 
 67-5 
 
 224-70 
 
 7,660 
 
 23 21 (?) 
 
 4-81 
 
 Earth - 
 
 92-7 
 
 91-1 
 
 94-6 
 
 365-26 
 
 7,918 
 
 23 56 4-09 
 
 5-66 
 
 Mars 
 
 141 
 
 128 
 
 155 
 
 686-98 
 
 4,200 
 
 24 37 22-7 
 
 4-01 
 
 Jupiter - 
 
 482 
 
 459 
 
 505 
 
 4,332-6 
 
 85,000 
 
 9 55 
 
 1-38 
 
 Saturn - 
 
 884 
 
 834 
 
 936 
 
 10,759 
 
 71,000 
 
 10 14 23-8 
 
 0-75 
 
 Uranus - 
 
 1,780 
 
 1,700 
 
 1,800 
 
 30,687 
 
 31,700 
 
 Unknown. 
 
 1-28 
 
 Neptune 
 
 2,780 
 
 2,760 
 
 2,810 
 
 60,127 
 
 34,500 
 
 Unknown. 
 
 1-15 
 
 Name. 
 
 Phobos 
 Deimos 
 
 THE SATELLITES OF MARS. 
 
 Mean Distance from Centre 
 of Mars. 
 
 5,800 miles 
 14,500 
 
 Periodic Time, 
 hrs. mins. eecs 
 
 7 39 14 
 30 17 54 
 
540 
 
 THE STORY OF THE HEAVENS. 
 
 THE SATELLITES OF JUPITER. 
 
 Name. 
 
 T. 
 II. 
 III. 
 IV 
 
 Name. 
 
 Mimas 
 
 Enceladus 
 
 Tethys 
 
 Dione 
 
 Rhea 
 
 Titan 
 
 Hyperion 
 
 Japetus 
 
 Mean Distance from Centre 
 
 Periodic Time. 
 
 of Jupiter. 
 
 days. 
 
 Lrs. 
 
 mins. 
 
 sees. 
 
 262,000 miles 
 
 1 
 
 18 
 
 27 
 
 34 
 
 417,000 
 
 3 
 
 13 
 
 13 
 
 42 
 
 664,000 
 
 7 
 
 3 
 
 42 
 
 33 
 
 1,170,000 
 
 16 
 
 16 
 
 32 
 
 11 
 
 THE SATELLITES OF 
 
 SATURN. 
 
 
 
 
 Mean Distance from Centre 
 
 Periodic Time. 
 
 of Saturn. 
 
 days. 
 
 bis. 
 
 mins. 
 
 sees. 
 
 118,000 miles 
 
 
 
 22 
 
 37 
 
 27-9 
 
 152,000 
 
 1 
 
 8 
 
 53 
 
 6-7 
 
 188,000 
 
 1 
 
 21 
 
 18 
 
 25-7 
 
 241,000 
 
 2 
 
 17 
 
 41 
 
 8-9 
 
 337,000 
 
 4 
 
 12 
 
 25 
 
 10-8 
 
 781,000 
 
 15 
 
 22 
 
 41 
 
 25-2 
 
 946,000 
 
 21 
 
 7 
 
 7 
 
 40-8 
 
 2,280,000 
 
 79 
 
 7 
 
 54 
 
 40-4 
 
 Name. 
 
 Ariel 
 Umbriel 
 Titania 
 Oberon 
 
 THE SATELLITES OF URANUS. 
 
 Mean Distance from Centre 
 of Uranus. 
 
 119,000 miles 
 166,000 
 272,000 
 363,000 
 
 Periodic Time, 
 days. 
 
 2.520383 
 
 4-144181 
 
 8-705897 
 
 13-463269 
 
 Name. 
 Satellite 
 
 THE SATELLITE OF NEPTUNE. 
 
 Mean Distance from Centre Periodic Time, 
 
 of Neptune. days. 
 
 220,000 miles 5-87690 
 
INDEX. 
 
 Aberration, 4R2. 
 
 Bradley and, 482. 
 
 Cause of, 489. 
 
 Circle of, 485. 
 
 Effect on Stars, 487. 
 
 in Draco, 484. 
 Achromatic Object Glass, 11. 
 Adams, Discovery of Neptune, 282. 
 
 Path, of the Leonids, 342. 
 
 Perturbations of Uranus, 512. 
 
 and Prof. Challis, 286. 
 
 and Sir G. Airy, 285. 
 Aerolite, The Chaco, 356. 
 Air, Composition of, 441. 
 Airy, Sir Gr., and Prof. Adams, 285. 
 Alcor, 402. 
 
 Aldebaran, a Sun, 371. 
 Colour of, 380. 
 like Mars, 371. 
 Spectrum of, 451. 
 Algol, 397. 
 Almagest, The, 6. 
 Alphons, 61, 69. 
 
 Alps, Great Valley of the, 61, 65. 
 Altair, 387. 
 
 Aluminium in the Sun, 449. 
 Andrews, Prof., 367. 
 Andromeda, 377. 
 
 Nebula in, 467. 
 Andromedae, y, 494. 
 Andromedes,The,and Biela's Comet, 348. 
 Angular Measure, 206. 
 Antares, 386. 
 
 Apex of the Earth's Way, 488. 
 Apparent Size of the Sun, 92. 
 Aquarius, 376. 
 Archimedes, 61, 66. 
 Arcturus, 384. 
 
 ,, and Donati's Comet, 315. 
 Argelander's Catalogue, 396. 
 Ariel, 269. 
 
 Elements of, 540. 
 Aristarchus, 61, 67. 
 Aristillus, 61, 65. 
 Aristotle, 61, 65. 
 
 Depth of, 65. 
 Aristotle and the Tides, 517. 
 
 Astronomer Royal, The Firjt, 271. 
 Astronomy, New Phase of, 442. 
 Atmosphere, Height of, 77. 
 
 Lunar, Evidence against, 78. 
 
 Refractive Power of, 316. 
 Audubon, 335. 
 Aurija, 377. 
 
 Auwers, Motion of Sirius, 392. 
 Axis of the Earth, 474. 
 
 B. 
 
 Barium in the Sun, 149. 
 
 Bay of Fundy, Tides in, 520. 
 
 Beehive, The, 384. 
 
 Berlin Academy of Sciences, 288 
 
 Berlin Year Book, 205. 
 
 Bessel and Bradley, 481. 
 
 Distance of the Stars, 408, 412. 
 61 Cygni, 412, 415. 
 
 ,, Motion of Sirius, 392. 
 
 Prediction by, 393. 
 Bessemer Converter, The, 442. 
 Betelgueze, 380. 
 
 Spectrum of, 451. 
 Biela's Comet, Periodic Time of, 349, 
 
 Sir J. Herschel and, 315. 
 
 The Andromedes and, 348. 
 Biot, The L'Aigle Meteorites, 351. 
 Bode, List of Double Stars, 399. 
 Bode's Law, 197. 
 Bond, Prof., Nebula in Orion, 462. 
 
 Satellites of Saturn, 254. 
 
 Saturn's Rings, 244. 
 Bootes, 384. 
 Bradley, Aberration, 482. 
 
 and Bessel, 481. 
 
 Nutation, 481. 
 
 Observation of Uranus, 272. 
 Bredichin, Prof,, Comets' Tails, 322. 
 Breitenbach Iron, The, 306. 
 Brilliancy of Venus, 141. 
 Bristol Channel, Tides in, 520. 
 British Association, Reports of, 349. 
 British Museum, The Rowton Siderite, 
 354. 
 
542 
 
 INDEX. 
 
 Briinnow, Dr. Groombridge 1830, 430. 
 
 Parallax of Stars, 422. 
 Burnham, Mr., Double Stars, 405. 
 Burning Glasses, 495. 
 Burton, C. E., Views of Mars, 189. 
 Butler, Bishop, on Probability, 438. 
 Butsura Meteorite, The, 355. 
 
 C. 
 
 Cadmium in the Sun, 449. 
 Calcium in the Sun, 449 
 Cancri, 406. 
 Canis Major, 382. 
 Canopus, 384. 
 Capella, 377. 
 
 Spectrum of, 451. 
 Carboniferous Epoch, 497. 
 Cardiff, Tides at, 520. 
 Cassini, Double Stars, 399. 
 
 Satellites of Saturn, 252. 
 Cassiopeia, 375. 
 Castor, 382. 
 
 a binary Star, 402. 
 Catharina, 61, 69. 
 Celestial Mechanics, 538. 
 
 Objects, Webb's, 25. 
 Centauri, a, 384. 
 
 Distance of, 412. 
 
 ,, Henderson and, 425. 
 Ceres, Discovery of, 198. 
 
 and Meteorites, 363. 
 Cerium in the Sun, 449. 
 Chaco Meteorite, The, 356. 
 Challenger, Cruise of the, 367. 
 Challis, Prof., and Prof. Adams, 286. 
 
 Observation of Neptune, 291. 
 
 ,, Search for Neptune, 287. 
 Charts, Photographic, of the Heavens, 
 
 463. 
 
 Chepstow, Tides at, 520. 
 Cheseaux's Comet, 325. 
 Chladni, 350. 
 
 Chromium in the Sun, 449. 
 Clairaut and Halley's Comet, 302. 
 Clavius, 61, 68. 
 
 and Jupiter's Satellites, 231. 
 Cobalt in the Sun, 449. 
 Collision of large bodies, 433. 
 Colours, The Primary, 444. 
 Comet, Biela's, Sir J. Herschel on, 315. 
 
 Coggia's, 29. 
 Comets, 86, 296. 
 
 and Gravitation, 298. 
 
 ,, Carbon in, 327. 
 
 ,, Donati's, 313. 
 
 Direction of Tail, 319. 
 
 Disturbed by Planets, 304. 
 
 Encke's, 303, 304. 
 
 ,, Growth of Tails, 321. 
 
 Halley's, 301. 
 
 M Early appearance of, 303. 
 
 Comets, Halley's, in Bayeux Tapestry, 
 303. 
 
 Returns of, 301. 
 
 Identity of, 298. 
 
 Mass of, 316. 
 
 Mass of Jupiter from, 215. 
 
 ,, Movements of, 296. 
 
 ,, Non-periodic, 313. 
 
 Nucleus of, 297. 
 
 Number of, 328. 
 
 of 1680, 300. 
 
 of 1744, 325. 
 
 of 1843, 313. 
 
 near the Sun, 313. 
 
 of 1881, 314. 
 
 ,, Carbon in, 314. 
 
 Size of, 314. 
 
 ,, and the Spectroscope, 
 
 314. 
 
 of 1882, Constitution of, 314. 
 
 Parabolic Orbits of, 300. 
 
 ,, Periodic, 304. 
 
 ,, and Meteoric Showers, 
 
 347. 
 
 Periodic Return of, 298. 
 
 Planes of Orbits, 317. 
 
 Shapes of, 296. 
 
 Tailless, 327. 
 
 ,, Tails of, Decrease of, 326. 
 
 ,, Electricity in, 426. 
 
 ,, Prof. Bredichin and, 
 
 326. 
 
 ,, Prof. 0. Reynolds and 
 
 322. 
 
 Repulsive force in, 326. 
 
 ,, Types of, 323. 
 
 Telescopic, 297. 
 
 Typical, 297. 
 Common, Mr., Comet of 1882, 314. 
 
 Nebula in Orion, 462. 
 Comparative Sizes of the Planets, 93. 
 Conservation of Moment of Momentum, 
 
 534. 
 
 Cook, Captain, Transit of Venus, 154. 
 Copeland, Dr., Transit of Venus, 159. 
 Copernican Theory and Jupiter's Satel- 
 lites, 231. 
 Copernicus, 61, 67, 72. 
 
 System of, 7. 
 Copper in the Sun, 449. 
 Corona, The, 44. 
 
 Rifts in, 44. 
 
 ,, Inner, 44. 
 
 Borealis, 384. 
 Cor Scorpionis, 386. 
 Cosmical Dust, 368. 
 Cotopaxi, Eruption of, 359 
 Crab, The, 384. 
 Crabtree, "William, Transit of Venus, 
 
 150. 
 
 Crown, The, 386. 
 Currents of Water, 520. 
 
INDEX. 
 
 543 
 
 Cygni, 0, 404. 
 
 Cygni 61, A Double, 415. 
 
 Distance of, 412. 
 
 Large Proper Motion, 425. 
 
 Motion of, 426. 
 
 Mr. Gill and, 425. 
 
 O. Struve and, 420. 
 
 Parallax of, 415, 422. 
 
 in Declination, 423. 
 
 Prof. A. Hall and, 423. 
 
 Uncertainty of Distance, 420. 
 Cygnus, 387. 
 CyriUus, 61, 69. 
 
 D. 
 
 Darwin, Prof. G. H., on the Tides, 510. 
 
 Dawes, Saturn's Rings, 244. 
 
 Day, Length of, and the Tides, 523. 
 
 Sidereal, 167. 
 Deimos, Diameter of, 193. 
 
 Elements of, 539. 
 
 Magnitude of, 193. 
 Denehola, 386. 
 Dione, Elements of, 540. 
 Distance, 420. 
 Distance of Sun, Encke and, 154. 
 
 by Aberration of Light, 490. 
 
 by Jupiter's Satellites, 229. 
 
 by Sappho, 208. 
 
 by Small Planets, 206. 
 
 ,. by Victoria, 208. 
 Dog Star, The, 84. 
 Dog, The Little, 383. 
 Donati's Comet, 383. 
 
 and Arcturus, 315. 
 
 Size of, 314. 
 
 Tails of, 324. 
 Double Star, The First, 382. 
 Dragon, The, 378. 
 
 Draper, Prof., Nebula in Orion, 462. 
 Dunsink Observatory, 154. 
 Dynamics, Foundation of. 10. 
 
 E. 
 
 Eagle, The, 387. 
 Earth, The, a Planet, 88. 
 as a Fly-wheel, 521. 
 Cause of Shape of, 168. 
 Constitution of, 476. 
 Critical Velocity for, 360, 524. 
 Determination of Weight of, 
 
 173. 
 
 Diameter of Orbit, 89. 
 Drawn from the Sea, 519. 
 Early Ideas of, 3. 
 Early Plasticity of, 525. 
 Elements of, 539. 
 Internal Heat of, 168. 
 
 Earth, Is it Rigid? 513. 
 
 ,, Length of Orbit, 89. 
 
 Losing Heat, 170. 
 
 ,, Method of finding Radius, 164. 
 
 ,, Period of Revolution, 89. 
 
 ,, Period of Rotation, 532. 
 
 Periodic Time, 116. 
 
 . Polar Axis of, 167. 
 
 Shape of, 163. 
 
 Velocity of, 89. 
 
 Volcanic Energy on, 365. 
 Ecliptic, The, 5. 
 
 Pole cf the, 473. 
 Eclipses, Recurrence of, 58. 
 Eight-year Interval, The, 146. 
 Elements, The, 441. 
 Ellipse, The, 109. 
 
 Construction of, 1 10. 
 
 ,, Eccentricity of, 110. 
 
 Focus of, 110. 
 
 ,, Parallactic, 413. 
 Enceladus, Elements of, 540. 
 Encke, Distance of Sun, 154. 
 
 ,, Theory by, 312. 
 Encke's Comet, 303, 304. 
 
 Approach to Mercury, 307. 
 
 Brightness of, 307. 
 
 Disturbed Orbit of, 308. 
 
 Disturbed by the Earth, 3 J 0. 
 
 Eccentricity of, 308. 
 
 Irregularities of, 311. 
 
 ,, and Jupiter, 309. 
 
 near the Sun, 307. 
 
 Orbit of, 305. 
 
 Period of, 311. 
 
 Von Asten and, 309. 
 Ensisheim, Meteorite at, 352. 
 Eratosthenes, 61, 67. 
 Errors, Neutralisation of, 425 
 
 of Observation, 207. 
 Evening Star, The, 140. 
 Eye, Structure of the, 10. 
 
 F. 
 
 Faculae, 40. 
 
 Fire-ball of Nov., 1869, 332. 
 
 Fire-balls, 331. 
 
 First Law of Motion, 119. 
 
 Flamsteed, 271. 
 
 Fomalhaut, 376. 
 
 Force of Gravitation, 97. 
 
 Foucault's Experiments, 168. 
 
 G. 
 
 Galileo, The Telescope, 10. 
 the Pleiades, 380. 
 Jupiter's Satellites, 222. 
 Saturn's Rings, 236. 
 
544 
 
 INDEX. 
 
 Galle, Dr., Identification of Neptune, 
 289. 
 
 ,, Search for Neptune, 288. 
 Gassendi, 61, 68. 
 
 ,, Transit of Mercury, 137. 
 Gauss and Ceres, 199. 
 Gemini, 382, 402. 
 Geological Records, 497. 
 Geysers, 493. 
 Gill, David, and a Centauri, 425. 
 
 Juno, 209. 
 
 ,, Opposition of Mars, 186. 
 Glacial Epoch, 498. 
 Glass Prism, 444. 
 Globe of Heated Gas, 501. 
 Gravitation Dependent on Mass, 106. 
 
 ,, Effect on Small Bodies, 105. 
 
 Force of, 97. 
 
 ,, Intensity of, 105. 
 
 Law of, 96. 
 
 ,, on the Moon, 75. 
 
 and the Motion of the Planets, 
 102. 
 
 ,, and the Tides, 519. 
 Great Bear, The, 23, 206, 372. 
 
 Proper Motion in, 428. 
 Greatest Artificial Temperature, 495. 
 Greenwich Observatory, 271. 
 Grimaldi, 61, 67. 
 Groombiidge 1830, 429. 
 
 ,, Briinnow and, 430. 
 
 Orbit of, 431. 
 
 Proper Motion of, 429. 
 
 Struve and, 430. 
 
 ,, Velocity of, 431. 
 Guards, The, 375. 
 
 Hall, Prof. Asaph, 61 Cygni, 423. 
 
 Satellites of Mars, 192. 
 Halley and Transit of Venus, 151. 
 Halley's Method, 153. 
 Harkness, Prof. W., 45. 
 Harvard, Great Telescope at, 43. 
 Heat and Astronomy, 292. 
 
 in a Mine, 493. 
 Heliometer, The, 209, 419. 
 Henderson, Distance of o Centauri, 412, 
 
 425. 
 
 Hercules, Star Cluster in, 453, 465. 
 Herodotus, 67. 
 Herschel, Caroline, 258, 267, 457. 
 
 Sir J., 67. 
 
 Address by, 287, 412. 
 
 Biela's Comet, 315. 
 
 Letter by, 290. 
 
 Survey of Southern Sky, 
 
 458. 
 
 Sir W., Distances of Stars, 408. 
 
 Distance of Vega, 409. 
 
 I Herschel, Sir W., Double Stars, 399. 
 Flashes of Genius, 439. 
 Labours of, 267, 456. 
 Motion of the Sun, 435. 
 Motions of the Stars, 438. 
 not a Mathematician, 439. 
 Observations of Saturn, 24 1 . 
 Observing the Heavens, 457. 
 Reflecting Telescope, 253, 261. 
 Satellites of Saturn, 254. 
 Sidereal Aggregation, 508. 
 and Sir W. Watson, 262. 
 Sketch of Life, 258. 
 Sizes of Stars, 410. 
 Survey of the Heavens, 263, 
 
 457. 
 
 Revolution of Double Stars, 400. 
 Various Nebulae, 508. 
 and Nebulae, 455. 
 Hi ad, Mr., 155. 
 High Tide, Time of, 518. 
 at Calais, 518. 
 at Liverpool, 518. 
 ,, at Swansea, 518. 
 Hoedi, 377. 
 
 Horrocks and Transit of Venus, 150. 
 Hot Springs, 493. 
 Howard, the Benares Stone, 351. 
 Huen, Island of, 10. 
 Huggins, Mr., Colour of Cygni, 404. 
 ,, Nebula in Orion, 464. 
 Spectra of Nebulae, 466. 
 Spectroscopic Method, 404. 
 Huyghfns, Cipher by, 240. 
 ,, Nebula in Orion, 459. 
 Prediction by, 241. 
 Satellites of Saturn, 252. 
 ,. Saturn's Rings, 238. 
 Hyades, 381. 
 Hydrogen, Colour of .Heated, 466. 
 
 in the Sun, 449. 
 Hyginus, 61, 70, 71. 
 Hyperion, Elements of, 540. 
 
 Iberians, The, 3. 
 Iris, The, 10. 
 Iron in Sand, 368. 
 
 Spectrum of, 448. 
 
 turned to Vapour, 442. 
 Isochronism of Oscillations, 176. 
 
 in Heavenly Bodies, 441. 
 
 J. 
 
 Janssen, the Spectroscope, 450. 
 Transit of Venus, 148. 
 Japetus, Elements of, 540. 
 Juno, Discovery of, 199. 
 
INDEX. 
 
 545 
 
 Jupiter, 84, 211. 
 Bolts on, 218 
 Brilliancy of, 221. 
 ,, Bulk of, 214. 
 ,, Composition of, 216. 
 , Diameter of, 213. 
 v Distance of, 84. 
 Distance from Sun, 212. 
 Drawings by L. Trouvelot, 218. 
 Elements of, 539. 
 Elliptic Orbit, 212. 
 Great Red Spot on, 219. 
 Great Size of, 213. 
 ,, Laplace and, 512. 
 Life on, 221. 
 Light Reflected by, 221. 
 ,, Mass from Small Planet, 215. 
 Comets, 215, 310. 
 
 ,, Moment of Momentum, 534, 
 
 535. 
 
 ,, Moons of, 215. 
 not a Solid Body, 213, 513. 
 Period of Revolution, 213. 
 Rotation, 173, 213, 220. 
 
 Perturbed by Saturn, 276. 
 ,, Receding from Sun, 537. 
 Satellites of, 222. 
 
 Clavius on, 231. 
 Distance of, 224. 
 Eclipse of, 223. 
 Elements of, 540. 
 Laws of Motion, 230. 
 Occultation of, 223. 
 Periods of, 224. 
 Transit of, 224. 
 Shape of, 173, 213. 
 Shape of Orbit, 212. 
 Size, compared with Earth, 212. 
 Solar Tides on, 536. 
 Temperature of, 494. 
 Velocity of, 213. 
 Volcanoes in, 362. 
 
 K. 
 
 Kepler, 61, 67. 
 
 Elliptical Orbits, 111. 
 1st Law of Planetary Motion, 
 
 ^112. 
 
 2nd 114. 
 
 3rd 115. 
 
 Number of Comets, 328. 
 Planetary Orbits, 109. 
 Transit of Mercury, 136. 
 . Venus, 136. 
 
 Laws and Meteoric Orbits, 344. 
 and Rigidity of Planets, 
 
 515. 
 
 Kids, The, 377. 
 Kirkwood, Prof., Law of, 255. 
 Klinkerfues, Prof., and Mr. Pogson, 
 349. 
 
 J J 
 
 Lagrange, Permanence of our System, 
 
 281, 513. 
 
 Planetary Perturbation, 279. 
 Lalaude, Observation of Neptune, 292. 
 Landscapes, Lunar, 75. 
 Laplace, Motions of Jupiter and Saturn, 
 
 512. 
 
 Jupiter's Satellites, 230. 
 Lassell, Mr., Satellite of Neptune. 
 
 293. 
 
 Satellites of Saturn, 254. 
 Telescopes, 261. 
 Law of Gravitation, 96. 
 
 Planetary Motion, First, 112. 
 Load in the Sun, 449. 
 Lciigvr, Mr., Mercury, 135. 
 Lemonnier, Observations of Uranus 
 
 272. 
 Leonids, The, 340. 
 
 ,, Ancient Records of, 342. 
 Date Changing, 342. 
 
 Number Decreasing, 341. 
 Path of, 342. 
 
 Perturbation of Orbit, 343. 
 Prof. Schiaparelli and, 347. 
 Return of, 340. 
 Width of, 344. 
 Le Verrier, Discovery of Neptune, 282 
 
 236. 
 
 Mars, 186. 
 Mass of Mercury, 338, 
 Meteors, 345. 
 Perturbation of Uranus, 512. 
 Weight of Mercury, 303 
 Linne, 61, 65, 71. 
 Lion, The, 383. 
 Little Bear, 375. 
 Lloyd, Provost, 367. 
 Lockyer, The Spectroscope, 450. 
 London, Tides at, 520. 
 Lunar Atmosphere, Evidence against. 
 
 78. 
 
 Craters, 61. 
 Mr. Nasmyth and, 72. 
 
 Origin of, 72. 
 Distances, 59. 
 Landscapes, 75. 
 Mountains, 61. 
 Alps, 61. 
 Apennines, 61. 
 Carpathian, 61. 
 Caucasus, 61. 
 Cordilleras, 61. 
 D'Alembert, 61. 
 Doerfel, 61. 
 Leibnitz, 61. 
 Rook, 61. 
 
 Theory of Mottorites, 366. 
 Lyra; e, 403. 
 Lyre, The, 386. 
 
546 
 
 INDEX. 
 
 M. 
 
 Madler, 65, 66, 67, 68. 
 Magnesium in the Sun, 449. 
 Manganese in the Sun, 449. 
 Maps of the Stars, 463. 
 Mare crisium, 61. 
 
 ,, foecunditatis, 61. 
 humorum, 61. 
 ,, imbrium, 61. 
 nectaris, 61. 
 nubium, 61. 
 ,, serenitatis, 61. 
 sinus iridium, 61. 
 tranquillitatis, 61. 
 ,, vaporum, 61. 
 Mars, Aldebaran like, 371. 
 
 Apparent Movements, 183. 
 
 Colour of, 181. , 
 
 Diameter of, 188. 
 
 ,, Distance from Earth, 188. 
 
 Eccentricity of Orbit, 182. 
 
 Elements of, 539. 
 
 Gravity on, 190. 
 
 Ice on, 189. 
 
 Inner Satellite, 193. 
 
 Life on, 190. 
 
 like Betelgueze, 181. 
 
 Mass of, 533. 
 
 Opposition of, 181. 
 
 Orbit of, 1-81. 
 
 Outer Satellite, 193. 
 
 Period of Eevolution, 90. 
 
 Eotation, 189, 533. 
 
 Satellites of, 192. 
 
 Shape of Orbit, 107. 
 
 Snow on, 189. 
 
 Surface of, 181. 
 Views of, 189. 
 Water on, 189. 
 Mayer, Tobias, Observation of Uranus, 
 
 272. 
 
 Mediterranean Sea, Tides in, 520. 
 Mercury, Apparent Size of, 132. 
 
 as a Crescent, 133. 
 
 Average Velocity, 134. 
 
 Difficulty of Weighing, 308. 
 
 Discovery of, 128. 
 
 Distance from Sun, 124, 134. 
 
 Earliest Observation of, 129. 
 
 Eccentricity of Orbit, 308. 
 
 Elements of, 539. 
 
 Encke's Comet near, 307. 
 
 Heat on, 135. 
 
 Le Verrier and the Weight of, 
 308. 
 
 Light on, 135. 
 
 Mass of, 308. 
 
 Mr. Ledger on, 135. 
 
 not easily seen, 129. 
 
 Period of Revolution, 89, 134. 
 
 Phases of, 132. 
 
 Mercury, Shape of, 133. 
 
 Shape of Orbit, 107. 
 
 Transit of, 137. 
 ,. Weight of, 138. 
 Meridian Circle, 20. 
 
 The Ideal, 22. 
 
 The Real, 22. 
 
 Mcssior's Catalogue of Nebulae, 509. 
 Meteoric Showers and Periodic Comets, 
 
 347. 
 
 Meteorite, Analysis of, 442. 
 at Ensisheim, 352. 
 at L'Aigle, 351. 
 Fall of a, 349. 
 The Butsura, 355. 
 ,, The Chaco, 356. 
 The Wold Cottage, 350. 
 Meteorites, 349. 
 
 Ancient Records of, 351. 
 Carbonaceous, 357. 
 Ceres and, 363. 
 Characteristics of, 356. 
 Cotopaxi and, 359. 
 Explanation of, 358. 
 Explosion of, 355. 
 ,, from Benares, 351. 
 Italy, 351. 
 ,, the Moon, 361. 
 
 Lunar Theory of, 366. 
 Origin of, 357. 
 Small Planets and, 362. 
 and Star Showers, 358. 
 Structure of, 353. 
 Terrestrial Theory of, 366. 
 and Volcanoes, 359, 365. 
 Meteors, 331. 
 
 Method of least Squares, 439. 
 Micrometer, The, 63. 
 The Filar, 64. 
 Milky Way, The, 250, 378. 
 
 Richness of, 452. 
 
 Mimas, Elements of, 540. 
 Mizar, 462. 
 
 Moment of Momentum, 534. 
 Month of One Day, 529. 
 Moon, Apparent Size of, 53. 
 ,, Atmosphere of, 78. 
 Attraction on the Earth, 477. 
 Craters on, 65 70. 
 Critical Velocity for, 361. 
 Distance of, 52, 54, 62. 
 Eclipse of, 56. 
 Elements of, 539. 
 formerly very Hot, 493. 
 ,, Gravitation on, 75. 
 ,, Great Tides on, 530. 
 Influence on Tides, 49. 
 Lava Tides on, 530. 
 Light of, 50. 
 Light of FuU, 193. 
 ,, Measuring Heights on, 64. 
 Motion among the Stars, 6. 
 
INDEX. 
 
 547 
 
 Moon, No Life on, 76, 80. 
 
 ,, Period of Revolution, 55, 530. 
 
 Rotation, 530. 
 Phases of, 50, 55. 
 
 Plane of Orbit, 480. 
 
 and Precession, 477. 
 
 Proximity to the Earth, 52, 525. 
 
 ,, Receding from the Earth, 524. 
 
 Relative Brightness, 50. 
 Size of the, 53. 
 
 Weight of the, 53. 
 
 ,, Shadows on, 63. 
 
 Surface of, 60. 
 
 ,, and the Tides, 519. 
 
 and the Weather, 60. 
 Motion not Noticeahle, 436. 
 
 of Planets and Gravitation, 102. 
 Mutual Attraction of Bodies, 277. 
 
 N. 
 
 Nasmyth, Mr., Lunar Craters, 72. 
 Map of the Moon, 60. 
 the Moon, 51. 
 Nautical Almanac, The, 25, 155. 
 Neap Tides, 519. 
 Nebula, Annular, in Lyra, 468. 
 ,, in Andromeda, 467. 
 in Orion, 452, 459. 
 Change in, 464. 
 
 Colour of, 460. 
 Magnitude of, 464. 
 Nature of, 465. 
 Planetary, in Draco, 470. 
 Ring, 469. 
 
 Spectrum of a Gaseous, 467. 
 Nebulrc, 452, 454. 
 
 Condensation of, 508. 
 Distances of, 460, 470. 
 Double, 470. 
 Number of, 458. 
 Planetary, 470. 
 Spiral, 470. 
 Variable, 470. 
 ;; and W. Herschel, 455. 
 Nebular Theory, 506. 
 Neptune, Adams and, 282. 
 Atmosphere of, 293. 
 Density of, 293. 
 Diameter of, 293. 
 Disc of, 293. 
 Elements of, 339. 
 Le Verrier and, 282. 
 Moment of Momentum, 535. 
 Observation bv Laland", 292. 
 
 Prof. Challes, 291. 
 Revolution of, 293. 
 Satellite of, 293. 
 
 Elements of, 540. 
 Period of, 294. 
 Search for, 286. 
 
 Neptune, Weight of, 294. 
 
 Wonderful Discovery of, 275. 
 Newton, Prof., Path of the Leonids, 
 342. 
 
 Shooting Stars, 335. 
 
 Sir Isaac, and Gravitation, 99, 
 
 100, 514. 
 Niagara, Waste of Power, 521. 
 Nickel in the Sun, 449. 
 Nordenskjold, the Ovifak Meteorite, 366. 
 Nova Cygni, 432. 
 
 Cause of Brightness, 432. 
 
 Distance of, 434. 
 
 Lessening of Lustre, 435. 
 
 Parallax of, 434. 
 November Meteors, Change in Cha- 
 racter, 341. 
 
 G. J. Stonoy and, 346. 
 
 in 1866, 337. 
 
 Lord Rosse's Telescope, 337. 
 
 Number Decreasing, 341. 
 
 Period of, 336. 
 
 Perspective and, 339. 
 
 Radiant Point of, 338. 
 
 Return of, 340. 
 
 The Earth and, 336. 
 
 0. 
 
 Oberon, 269. 
 
 Elements of, 540. 
 Oceanus procellarum, 61. 
 Orgueil, Meteorite at, 357. 
 Orion, 373, 381. 
 
 Quadruple Star in, 278. 
 Orionis a, Spectrum of, 451. 
 
 6 a Multiple Star, 459. 
 Ovifak Meteorite, The, 366. 
 
 P. 
 
 Palermo Observatory, 197. 
 Pallas, 350. 
 
 Disco very of, 199. 
 Parabola, The, 298. 
 
 ,, and the Ellipse, 298. 
 Parabolic Orbit, 299. 
 Parallactic Ellipse, The, 412. 
 Parallax, 151. 
 
 Annual, 419. 
 Pegasus, 375. 
 Pendulum, The, 176. 
 
 Time of Vibration, 177. 
 Penumbra, 57. 
 Periodic Phenomena, 516. 
 
 Time, 112. 
 Perseids, 348. 
 
 Path of a Comet, 318 
 Perseus, 377. 
 Petavius,61, 70. 
 
548 
 
 INDEX. 
 
 Petavius, Size of, 74. 
 
 Peters, Prof., Motion of Sinus, 392. 
 
 Small Planets, 201. 
 Phases of Venus, 142. 
 
 Elements of, 539. 
 Phobos, Magnitude of, 193. 
 
 Period of Revolution, 533. 
 
 Size of, 194. 
 
 Photographs of the Heavens, 463. 
 Piazxi, first Minor Planet, 197. 
 Pickering, Prof., Jupiter's Satellites, 229. 
 Pico, 66. 
 
 Planet of Romance, 124. 
 Planetary Orbits, Kepler and, 109. 
 
 Perturbation of, 279. 
 
 System, Bounds of, 295. 
 Planets, The, according to the Ancients, 
 85. 
 
 Comparative Size of, 93, 211. 
 
 Critical Velocity for, 360. 
 
 First Recognition, 2. 
 
 Orbits of the Interior, 88. 
 
 Original Heat of, 218. 
 
 Places found Correctly, 515. 
 
 Periodic Time, 112. 
 
 Rigid Bodies, 513. 
 
 Shape of Orbits, 107. 
 
 Velocity not Uniform, 113. 
 Plato, 61, 63, 66, 72. 
 Pleiades, The, 206, 379 
 
 Galileo and, 380. 
 Pliny and the Tides, 517. 
 Plough, The, 23. 
 Pointers, The, 24, 374. 
 Pole, The, Elevation of, 166. 
 
 Moving, 472. 
 
 near a Draconis, 474. 
 
 near Vega, 474. 
 
 of the Ecliptic, 473. 
 Pole Star, The, 165, 374. 
 
 A Double, 403. 
 
 Distance from Pole, 374. 
 
 Fixity of, 374. 
 
 Slow Motion, 374. 
 Pollux, 382. 
 
 Pons, Encke's Comet, 304. 
 Posidonius, 61, 65. 
 
 Depth of, 65. 
 Position Angle, 420. 
 Potassium in the Sun, 449. 
 Praesepe, 384. 
 Precession, 472, 516. 
 
 Cause of, 477. 
 
 Explanation of, 475. 
 
 Illustration of, 475. 
 Prism, The, 444. 
 Proctor, Movement of Stars, 483. 
 
 ,, Rotation of Mars, 189. 
 Procyon, 383. 
 
 Ptolemaic Doctrine, The, 6. 
 Ptolemy, 61, 69. 
 Ptolemy's Catalogue, 428, 
 
 R. 
 
 Radiation of Heat, 494. 
 
 Rain, 382. 
 
 Rainbow, The, 444. 
 
 Ratio of Month and Day, 527. 
 
 Reflector, 16. 
 
 Mr. Common's, 20. 
 
 The Herschelian, 16. 
 Refraction, Effect of, 417. 
 
 of light, 445. 
 Rogulus, 383. 
 Retina, The, 11. 
 
 Reynolds, Prof. 0., Comets' Tails, 322. 
 Rhea, Elements of, 540. 
 Rigel, 380. 
 Rice Grains, 40. 
 Rosmer, Velocity of Light, 225. 
 Rosse, Lord, Nebula in Orion, 461, 465. 
 
 ,, Telescopes, 231. 
 Rotational Moment of Momentum, 534. 
 Rowton Siderite, The, 353. 
 
 Weight of, 354. 
 
 S. 
 
 Sand, Iron Particles in, 368. 
 Sappho, Sun's Distance by, 208. 
 Saturn, 232. 
 
 Beauty of, 232. 
 Brilliancy of, 233. 
 Bulk of, 232. 
 
 Compared with the Earth, 236. 
 Crape Ring of, 244. 
 Density of, 234. 
 Distance from Sun, 234. 
 Elements of, 539. 
 Equatorial Diameter of, 234. 
 First Satellite of, 252. 
 known to the Ancients, 233. 
 Laplace and, 512. 
 Mass of, 232. 
 Mean Diameter of, 234. 
 Moment of Momentum, 534. 
 not a Solid, 513. 
 Path of, 234. 
 
 Period of Revolution, 233. 
 First Satellite, 252. 
 Picturesqueness of, 251. 
 Polar Diameter of, 234. 
 Satellites of, Bond, Prof., 254. 
 
 Daily Motion, 255. 
 
 Elements of, 540. 
 
 Herschel, 254. 
 
 Mr. Lassell, 254. 
 
 Number of, 252. 
 
 Perturbed by Jupiter. 
 
 276. 
 
 ,, Revolution of, 254, 
 Temperature of, 494. 
 Velocity of, 234. 
 Velocity of Rotation, 234. 
 
INDEX. 
 
 Saturn, Volcanoes in, 362. 
 Saturnian System, Complexity of, 253. 
 Saturn's Rings, 233, 236. 
 Constitution of, 249. 
 ,, Dawes, 244. 
 Galileo, 236. 
 Gill, 241. 
 Huyghens, 238. 
 ,, not Self -Luminous, 245. 
 Prof. Bond, 244. 
 Rotation of, 248. 
 Unstable Equilibrium, 247. 
 Savary, Binary Stars, 401. 
 Scheiner, Sun Spots, 33. 
 SchiaparellijTath of Leonids, 347. 
 Schickard, 61, 68. 
 Schmidt, 65, 66. 
 
 Nova Cygni, 432. 
 Schrceter, 65. 
 
 Scorpion's Heart, The, 386. 
 Seelig-er, Mr., Cancri, 405. 
 Secchi, Temperature of Sun, 495. 
 Ship entering Harbour, 437. 
 Shooting Star Shoal, Bulk of, 335. 
 
 ,, Dimensions of, 335. 
 Showers of Celestial Dust, 367. 
 Sickle, The, 383. 
 Sidereal Aggregation, 508. 
 
 Day, 167. 
 Siderite, The Rowton, 354. 
 Siderites, 354. 
 
 ,, Iron and Nickel in, 355. 
 Siemens Furnace, The, 442. 
 Siiius, 84, 382. 
 
 Brilliancy of, 295, 388. 
 ,, Companion cf, 393. 
 Distance of, 389. 
 and Light of Moon, 388. 
 
 Sun, 388. 
 Light from, 390. 
 Mass of, 395. 
 
 not Moving Steadily, 392. 
 Proper Motion of, 391. 
 Revolution of Companion, 394. 
 Speed of, 392. 
 The Disturber of, 393. 
 Weight of, 394. 
 Whiteness of, 450. 
 Small Planets, Atmosphere on, 204. 
 Berlin Year Book, 205. 
 Critical Velocity, 204. 
 First, 197. 
 Gravity on, 202. 
 Method of Observing, 200. 
 and Meteorites, 362. 
 Nature of, 202. 
 Numbers of, 200. 
 Orbits of, 199. 
 Prof. Peters, 201. 
 Planets, Search for, 197. 
 Smith, Prof., C. P., Movement of the 
 Pole, 472. 
 
 Sodium, Colour of Flame, 446. 
 and Dark Lino D., 447. 
 in the Sun, 449. 
 Solar Eclipse, 56. 
 
 ,, Prominences, 42. 
 
 ,, Changes in, 43. 
 
 Dimensions of, 43. 
 
 Specially Interesting, 
 
 43. 
 
 Velocity of, 44. 
 System, 81. 
 Changes in, 516. 
 
 Origin of, 506. 
 
 Speed of, 439. 
 
 Stability of, 281. 
 
 Tides, 519. 
 Influence of, 532. 
 
 On Mars, 533. 
 
 South Equatorial at Dunsink, 12. 
 
 Pole, no Star at, 375. 
 Southern Cross, 453. 
 
 Fish, 376. 
 Speculation on Meteors, 345. 
 Spectroscope, The, 440. 
 
 and Comet of 1881, 314 
 Spectrum Analysis, 442, 446. 
 Dark Line D, 446. 
 Dark Lines of, 446. 
 ,, of Iron, 448. 
 Spica, 385. 
 Spring Tides, 519. 
 Star, The Dog, 84. 
 
 ,, Clusters in Hercules, 452. 
 Perseus, 453. 
 
 Drift, 483. 
 Stars, Apparent Motions, 438. 
 
 ,, Decomposition of Motion, 439. 
 ,, Distances of, 407. 
 Double, 399. 
 
 Bode's List, 399. 
 
 Cassini, 399. 
 
 Herscbel, 399. 
 
 Measures less Correct, 
 
 400. 
 
 ,, Revolution of, 400. 
 Savary, 401. 
 Shape of Orbit, 401. 
 Finding Distance of, 414. 
 Fixed, 482. 
 
 Gradually Spreading, 438. 
 have no Disc, 390. 
 Magnitudes of, 396. 
 Number of, 25. 
 
 ,, each Magnitude, 396. 
 Visible, 396. 
 Occultation of, 314. 
 Parallax of, 420. 
 Photographs of, 463 
 Rapidity of Motion, 428. 
 Krai Motions, 438. 
 Really Suns, 29. 
 Self -Luminous, 371. 
 
550 
 
 INDEX. 
 
 Stars, Size of, 410 
 
 Shooting, Cause of, 331. 
 
 Early Account of, 334. 
 
 ,, Enormous Profusion of, 
 
 330. 
 
 Great Shower of, 1866, 
 
 334. 
 
 and Meteors, 358. 
 
 Orbit of, 331. 
 Professor Newton, 335. 
 Shower of, 334. 
 Varied Size of, 331. 
 Velocity of, 331. 
 
 Triple, 405. 
 
 Variable, 397. 
 St. Helena, Tides at, 520. 
 Stoney, G. J. , November Meteors, 346. 
 Strontium, Colour of Flame, 445. 
 
 in the Sun, 449. 
 Struve, 0., 61 Cygni, 420. 
 
 Groombridge 1830, 430. 
 
 Vega, 412. 
 
 ,, the Younger, 61 Cygni, 417. 
 Sun, The, Annular Eclipse of, 56. 
 
 ,, Apparent Shape of, 172. 
 
 Size of, 92. 
 
 ,, Appearance of, 30. 
 
 Attraction of the Earth by, 477. 
 
 Benefits Derived from, 48. 
 
 Compared with the Earth, 27. 
 Stars, 28. 
 
 Composition of the Prominences, 
 450. 
 
 Surface, 36, 441. 
 
 Connection with the Seasons, 4. 
 
 Constitution of, 443. 
 
 Continuance of Heat from, 497. 
 
 ,, Contraction of, 504. 
 
 Convulsions on, 42. 
 
 Cooling of, 498. 
 
 Diameter of, 26. 
 
 Distance of, 28, 311. 
 
 Direction of Motion, 435. 
 
 Elements in, 449. 
 of, 539. 
 
 Fall of Meteors on, 499. 
 
 From Sirius, 391. 
 
 ,, Granulated Appearance of, 40. 
 
 Heat of, 46. 
 
 Jovian Tides in, 536. 
 
 ,, Light of, 443. 
 
 Moment of Momentum, 534, 535. 
 
 Motion among the Stars, 5. 
 
 Motion of, 435. 
 
 and Precession, 477. 
 
 ,, Prominences on, 41. 
 
 Radiation of Heat on, 494, 496. 
 
 Rising and Setting, 2. 
 
 Rotation of, 35. 
 
 Sources of Heat, 499. 
 
 Speed of, 436. 
 
 Temperature of, 27. 495. 
 
 Sun, The, Texture of, 40. 
 
 Total Eclipse of, 56 
 Sun-spots, 31. 
 
 and Terrestrial Magnetism, 37. 
 
 Duration of, 36. 
 
 ,. Epochs of Maximum, 38. 
 
 Motion of, 32. 
 
 Period of Revolution, 34. 
 
 Regions of, 36. 
 
 Scheiner's Observations on, 33. 
 Swan, The, 387. 
 Systems, Other, 370. 
 
 T. 
 
 Taurus, 380. 
 
 Tebbutt, Comet of, 1881, 314. 
 Telescope, Field of a Large, 455. 
 Invention of the, 10. 
 Lassell and the, 261. 
 Lord Rosse's, 18, 261. 
 Rays from a Star in, 489. 
 Reflecting, 11, 253. 
 Refracting, 11. 
 The Great Vienna, 14. 
 Temperature at a Great Depth, 4 93. 
 Tempests Caused by Sun, 220. 
 Tethys, Elements of, 540. 
 Theophilus, 61, 69. 
 Tidal Engines, 521. 
 Tides, The, 510. 
 
 at Bay of Fundy, 520. 
 at Cardiff, 520. 
 at Chepstow, 520. 
 at London, 520 
 at St. Helena, 520. 
 in Bristol Channel, 520. 
 in Mediterranean, 520. 
 and Length of Day, 523. 
 and Rotation of Jupiter, 537. 
 Store of Energy, 521. 
 Titan, Elements of, 540. 
 Titania, 269. 
 
 Elements of, 540. 
 Titanium in the Sun, 449. 
 Transit of Mercury, Kepler, 136. 
 Venus, Dr. Copeland, 159. 
 
 First Observation of, 
 
 149. 
 
 Halley, 151. 
 HaUey's Method, 153. 
 Horrocks, 150. 
 Janssen, 148. 
 Kepler, 136. 
 Recent, 154. 
 Sun from two Stations, 
 
 153. 
 
 Triesnecker, 61, 62, 70. 
 Trouvelot, L., 43. 
 
 Drawings of Jupiter, 218. 
 Nebula in Andromeda, 467. 
 
INDEX. 
 
 551 
 
 Trouvclot, L., View of Saturn, 241. 
 Tschcrrnak, Meteorites, 359. 
 Tycho Brahe, 9. 
 
 the Volcano, 61, 68. 
 
 U. 
 
 Umbriel, 269. 
 
 Elements of, 540. 
 Uraniberg, Observatory of, 10. 
 Uranium in the Sun, 449. 
 Uranus, 86. 
 
 Bradley, 272. 
 
 Density of, 268. 
 
 Diameter of, 268. 
 
 Discovery of, 265. 
 
 Elements of, 539. 
 
 Kepler's Law, 274. 
 
 ,, Lemonnier, 272. 
 
 ,, Moment of Momentum, 535. 
 
 Period of Revolution, 265, 273. 
 
 Perturbed by Jupiter, 282. 
 
 Saturn, 282. 
 
 ,, Remote Distance of, 268. 
 
 Satellites of, 269. 
 
 Elements of, 540. 
 
 Plane of Orbit, 270. 
 
 Orbit of, 269. 
 
 Tobias Mayer, 272. 
 
 Unaccountably Disturbed, 283, 
 512. 
 
 Volume of, 268. 
 Ursa Major, 23, 372. 
 
 not Changed, 373. 
 
 V. 
 
 Vega, 377, 386. 
 
 Distance of, 409, 412. 
 
 Light of, 193. 
 
 Whiteness of, 450. 
 Velocity of Light, 226. 
 
 Method of Finding, 226. 
 Venus, 83. 
 
 Atmosphere on, 142, 159. 
 
 Brilliancy of, 141. 
 
 Dark Lines of, 449. 
 
 Density of, 161. 
 
 Diameter of, 161. 
 
 Venus, Distance from Sun, 116, 162. 
 
 Elements of, 539. 
 
 Evening Star, 140. 
 
 Gravitation at, 161. 
 
 Mass of, 161. 
 
 Path across Sun, 149. 
 
 Periodic Time, 11G. 
 
 Period of Revolution, 89, 162. 
 
 Phases of, 142. 
 
 Shape of Orbit, 107, 162. 
 
 Time of Rotation, 161. 
 
 Transit of, 143. 
 
 Velocity of, 162. 
 
 Weight of, 161. 
 Vesta, Discovery of, 199. 
 Victoria, Sun's Distance by, 208. 
 Virginis a, 385. 
 Virgo, 385. 
 Volcanoes, 493. 
 
 and Meteorites, 365. 
 Von Asten, Encke's Comet, 309. 
 Theory, 312. 
 
 ,, Mass of Jupiter, 310. 
 
 ,, ,, Mercury, 319. 
 
 Sun's Distance, 311. 
 
 Weight of Mercury, 138. 
 Vortex Rings, 469. 
 Voyage into Space, 370. 
 
 W. 
 
 Wargcntin, 61, 68. 
 
 Water, Composition of, 441. 
 
 Watson, Sir W., and Herschel, 262. 
 
 White Colour, 443. 
 
 Willow Leaves, 40. 
 
 Young, Prof., 43. 
 
 Zinc in the Sun, 449. 
 Zodiac, The, 5. 
 
 Signs of, 5. 
 Zodiacal Light, 46. 
 
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