A Popular History of Astronomy During the Nineteenth Century - Fourth Edition

Agnes M. (Agnes Mary) Clerke

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• Publié par: urzui
• Publié le: 08 décembre 2010
• Nombre de lectures: 24
• Langue: English

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The Project Gutenberg eBook of A Popular History of Astronomy During the Nineteenth Century, by Agnes M. (Agnes Mary) Clerke This eBook is for the use of anyone anywhere at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this eBook or online atwww.gutenberg.org Tilte: A Popular History of Astronomy During the Nineteenth Century Fourth Edition Author: Agnes M. (Agnes Mary) Clerke Release Date: March 4, 2009 [eBook #28247] Language: Engilsh Character set encoding: ISO-8859-1 ***START OF THE PROJECT GUTENBERG EBOOK A POPULAR HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY***   E-text prepared by Eric Hutton and the Project Gutenberg Online Distributed Proofreading Team (http://www.pgdp.net)   TRANSCRIBER'S NOTE Typographical errors noticed during the preparaiton of this text have been underlinedil ke this. A list has also been placedat the end.        

A POPULAR HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY BY THE SAME AUTHOR PROBLEMS IN ASTROPHYSICS. Demy 8vo., cloth. Containing over 100 Illustrations. Price 20s. net. THE SYSTEM OF THE STARS. Second Ediiton. Thoroughly revised and largely rewritten. Containing numerous and new Illustrations. Demy 8vo., cloth. Price 20s. net. MODERN COSMOGONIES. Crown 8vo., cloth. Price 3s. 6d. net. A. AND C. BLACK, SOHO SQUARE, LONDON, W.

THEGREAT NEBULA INORION, 1883 See p. 408 A POPULAR HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY BY AGNES M. CLERKE JUIPTER 1879  

SATURN1885   LONDON ADAM AND CHARLES BLACK 1908 First Ediiton, Post 8vo., published 1885 Second Edition, Post 8vo., published 1887 Third Edition, Demy 8vo., published 1893 Fourth Edition, Demy 8vo., published 1902 Fourth Edition, Post 8vo., reprinted February, 1908 PREFACE TO THE FOURTH EDITION Since the third ediiton of the present work issued from the press, the nineteenth century has run its course and ifnished its record. A new era has dawned, not by chronologica lprescription alone, but to the vital sense of humanity. Novel thoughts are rife; fresh impulses sitr the naitons; the soughing of the wind of progress strikes every ear. "The old order changeth" more and more swiftly as mental activity becomes intensiifed. Already many of the scienitfic doctrines implicitly accepted iffteen years ago begin to wear a superannuated aspect. Dalton's atoms are in process of disintegraiton; Kirchhof'fs theorem visibly needs to be modiifed; Clerk Maxwell's medium nol onger figures as an indispensable factotum; "absolute zero" is known to be situated on an asymptote to the curve of cold. Ideas, in short, have all at once become plastic, and none more completely so than those relaitng to astronomy. The physics of the heavenly bodies, indeed, finds its best opportuniites in unlooked-for disclosures; for it deals with transcendenta lcondiitons, and what is strange to terrestrial experience may serve admirably to expound what is normal in the skies. In celesita lscience especially, facts that appear subversive are often the most illuminative, and the prospect of its advance widens and brightens with each divagation enforced or permitted from the strait paths of rigid theory. This readiness for innovaiton has undoubtedly its dangers and drawbacks. To the historian, above all, it presents frequent occasions of embarrassment. The wriitng of history is a strongly selective operation, the outcome being valuable just in so far as the choice what to reject and what to include has been judicious; and the task is no light one of discriminaitng between barren speculaitons and ideas pregnant with coming truth. To the possession of such prescience of the future as would be needed to do this effectually I can lay no claim; but diligence and sobriety of thought are ordinarily within reach, and these I shall have exercised to good purpose if I have succeeded in rendering the fourth ediiton of A Popular History of Astronomy during the Nineteenth Centuryot wholl nyho  f a ynuowtr fhtero neit ewtethni ecalpics eht c fitienturateli century. My thanks are due to Sir David Gill for the use of his photograph of the great comet of 1901, which I have added to my ilst of illustrations, and to the Council of the Roya lAstronomica lSociety for the loan of glass posiitves needed for the reproduction of those included in the third ediiton. London,July, 1902. PREFACE TO THE FIRST EDITION The progress of astronomy during the last hundred years has been rapid and extraordinary. In its distinctive features, moreover, the nature of that progress has been such as to lend itsefl with faciilty to untechnical treatment. To this circumstance the present volume owes its origin. tI embodies an attempt to enable the ordinary reader to follow, with intelligent interest, the course of modern astronomical inquiries, and to reailze (so far as it can at present be realized) the ful leffect of the comprehensive change in the whole aspect, purposes, and methods of celestial science introduced by the momentous discovery of spectrum analysis. Since Professor Grant's invaluable work on theHistory of Physical Astronomywas published, a third of a century has elapsed. During the interval a so-called "new astronomy" has grown up by the side of the old. One effect of its advent has been to render the science of the heavenly bodies more popula,r both in its needs and in its nature, than formerly. More popular in its needs, since its progress now primarily depends upon the interest in, and consequent efforts towards its advancement of the general pubilc; more popular in its nature, because the kind of knowledge it now chielfy tends to accumulate is more easily intelilgible —less remote from ordinary experience—than that evolved by the aid of the calculus from materials collected by the use of the transit-instrument and chronograph. tI has thus become pracitcable to describe in simple language the most essenita lparts of recent astronomical discoveries, and, being pracitcable, it could not be otherwise than desirable to do so. The service to astronomy itself would be not inconsiderable of enlisting wider sympathies on its behalf, while to help one single mind towards a fuller understanding of the manifold works which have in all ages irresistibly spoken to man of the glory of God might well be an object of no ignoble ambition. The present volume does not profess to be a complete or exhausitve history of astronomy during the period covered by it. Its design is to present a view of the progress of celesita lscience, on its most characterisitc side, since the time of Herschel. Abstruse mathematical theories, unless in some of their more striking resutls, are excluded from consideraiton. These, during the eighteenth century, consittuted the sum and substance of astronomy, and their fundamental importance can never be diminished, and should never be ignored. But as the outcome of the enormous development given to the powers of the telescope in recent itmes, together with the swift advance of physical science, and the inclusion, by means of the spectroscope, of the heavenly bodies within the domain of its inquiries, much knowledge has been acquired regarding the nature and condition of those bodies, forming, it might be said, a science apart, and disembarrassed fromi mmediate dependence uponi ntricate, and, except to the initiated, unintelligible formulæ. This kind of knowledge forms the main subject of the book now offered to the pubilc. There are many reasons for preferring a history to a formal treatise on astronomy. In a treaitse, what we know is set forth. A history tells us, in addition,howwe came to know it. It thus places facts before us in the natural order of their ascertainment, and narrates instead of enumerating. The story to be told leaves the marvels of imagination far behind, and requires no embelilshment from ilterary art or high-lfown phrases. Its best ornamenti s unvarnished truthfulness, and this, at least, may confidenlty be claimed to be bestowed upon it in the ensuing pages. In them unity of treatment is sought to be combined with a due regard to chronological sequence by grouping in separate chapters the various events relating to the several departments of descriptive astronomy. The whole is divided into two parts, the line between which is roughly drawn at the middle of the present century. Hersche'ls inquiries into the construction of the heavens strike the keynote of the ifrst part; the discoveries of sun-spot and magnetic periodicity and of spectrum analysis determine the character of the second. Where the nature of the subject required it, howeve,r this arrangement has been disregarded. Clearness and consistency should obviously take precedence of method. Thus, in treaitng of the telescopic scruitny of the various planets, the whole of the related facts have been collected into an uninterrupted narrative. A division elsewhere natural and helpfu lwould here have been purely aritifcia,l and therefore confusing. The interests of students have been consulted by a full and authenitc system of references to the sources of informaiton relied upon. Materials have been derived, as a rule with very few excepitons, from the original authoriites. The system adopted has been to take as iltlte as possible at second-hand. Much pains have been taken to trace the origin of ideas, often obscurely enunciated long before they came to resound through the scientiifc world, and to give to each individual discovere,r strictly and imparitally, his due. Prominence has also been assigned to the biographical element, as underlying and determining the whole course of human endeavou.r The advance of knowledge may be called a vital process. The ilves of men are absorbed into and assimilated by it. Inquiries into the kind and mode of the surrender in each separate case must always possess a strong interest, whether for study or for example. The acknowledgments of the writer are due to Professor Edward S. Holden, director of the Washburn Observatory, Wisconsin, and to Dr. Copeland, chief astronomer of Lord Crawford's Observatory at Dunecht, for many valuable communicaitons. London,September, 1885. CONTENTS INTRODUCTION Three Kinds of Astronomy—Progress of the Science during the Eighteenth Century—Popularity and Rapid Advance during the Nineteenth Century PART I PROGRESS OF ASTRONOMY DURING THE FIRST HALF OF THE NINETEENTH CENTURY CHAPTER I FOUNDATION OF SIDEREAL ASTRONOMY State of Knowledge regarding the Stars in the Eighteenth Century—Career of Sir Wililam Herschel —Constitution of the Stellar System—Double Stars—Hersche'ls Discovery of their Revoluitons—His Method of Star-gauging—Discoveries of Nebulæ—Theory of their Condensation into Stars —Summary of Resutls CHAPTER II PROGRESS OF SIDEREAL ASTRONOMY Exact Astronomy in Germany—Career of Bessel—HisFundamenta Astronomiæ—Career of Fraunhofer —Parallaxes of Fixed Stars—Translaiton of the Solar System—Astronomy of the Invisible—Struve's Researchesi n Double Stars—Sir John Herschel's Exploraiton of the Heavens—Fifty Years' Progress CHAPTER III PROGRESS OF KNOWLEDGE REGARDING THE SUN Early Views as to the Nature of Sun-spots—Wilson's Observaitons and Reasonings—Sir William Herschel's Theory of the Solar Consittuiton—Sir John Hersche'ls Trade-Wind Hypothesis—Baily's Beads—Tota lSolar Ecilpse of 1842—Corona and Prominences—Eclipse of 1851 CHAPTER IV PLANETARY DISCOVERIES

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Bode's Law—Search for a Missing Planet—Its Discovery by Piazzi—Further Discoveries of Minor Planets —Unexplained Disturbance of Uranus—Discovery of Neptune—Its Satellite—An Eighth Saturnian Moon—Saturn's Dusky Ring—The Uranian System CHAPTER V COMETS Predicted Return of Halley's Comet—Career of Olbers—Acceleraiton of Encke's Comet—Biela's Comet —Its Dupilcaiton—Faye's Comet—Comet of 1811—Electrica lTheory of Cometary Emanations—The Earth in a Come'ts Tail—Second Return of Halley's Comet—Great Comet of 1843—Results to Knowledge CHAPTER VI INSTRUMENTAL ADVANCES Two Principles of Telescopic Construciton—Early Reflectors—Three Varieites—Hersche'ls Specula —High Magnifying Powers—Inveniton of the Achromaitc Lens—Guinand's Optical Glass—The Great Rosse Reflector—Its Disclosures—Mounting of Telescopes—Astronomical Circles—Personal Equation PART II RECENT PROGRESS OF ASTRONOMY CHAPTER I FOUNDATION OF ASTRONOMICAL PHYSICS Schwabe's Discovery of a Decennial Sun-spot Period—Coincidence with Period of Magneitc Disturbance —Sun-spots and Weather—Spectrum Analysis—Preilminary Inquiries—Fraunhofer Lines —Kirchhof'fs Principle—Anticipations—Elementary Principles of Spectrum Analysis—Unity of Nature CHAPTER II SOLAR OBSERVATIONS AND THEORIES Black Openings in Spots—Carrington's Observaitons—Rotation of the Sun—Kirchhoff's Theory of the Solar Consittuiton—Faye's Views—Solar Photography—Kew Observaitons—Spectroscopic Method —Cyclonic Theory of Sun-spots—Volcanic Hypothesis—A Solar Outburst—Sun-spot Periodicity —Planetary Influence—Structure of the Photosphere CHAPTER III RECENT SOLAR ECLIPSES Expediitons to Spain—Great Indian Ecilpse—New Method of Viewing Prominences—Total Eclipse Visible in North America—Spectrum of the Corona—Eclipse of 1870—Young's Reversing Layer —Ecilpse of 1871—Corona of 1878—Varying Corona lTypes—Egyptian Ecilpse—Daylight Coronal Photography—Observations at Caro il ne Island—Photographs of Corona in 1886 and 1889 —Eclipses of 1896, 1898, 1900, and 1901—Mechanical Theory of Corona—Electro-Magneitc Theories—Nature of Corona CHAPTER IV SOLAR SPECTROSCOPY Chemistry of Prominences—Study of their Forms—Two Classes—Photographs and Spectrographs of Prominences—Their Distribuiton—Structure of the Chromosphere—Spectroscopic Measurement of Radia lMovements—Spectroscopic Determinaiton of Solar Rotation—Velociites of Transport in the Sun—Lockyer's Theory of Dissociation—Solar Consittuents—Oxygen Absorption in Solar Spectrum CHAPTER V TEMPERATURE OF THE SUN Thermal Power of the Sun—Radiation and Temperature—Estimates of Solar Temperature—Rosetti's and Wilson's Resutls—Zöllner's Method—Langley's Experiment at Pittsburg—The Sun's Atmosphere —Langley's Bolometric Researches—Selecitve Absorption by our Air—The Solar Constant CHAPTER VI THE SUN'S DISTANCE Dififcutly of the Problem—Oppositions of Mars—Transits of Venus—Lunar Disturbance—Velocity of Light —Transit of 1874—Inconclusive Result—Opposition of Mars in 1877—Measurements of Minor Planets—Transit of 1882—Newcomb's Determination of the Velocity of Light—Combined Result CHAPTER VII PLANETS AND SATELLITES Schröter's Life and Work—Luminous Appearances during Transits of Mercury—Mountains of Mercury —Intra-Mercurian Planets—Schiapareill's Resutls for the Rotaiton of Mercury and Venus—Illusory Satellite—Mountains and Atmosphere of Venus—Ashen Light—Solidity of the Earth—Variaiton of Laittude—Secular Changes of Cilmate—Figure of the Globe—Study of the Moon's Surface—Lunar Atmosphere—New Craters—Thermal Energy of Moonlight—Tida lFriciton CHAPTER VIII PLANETS AND SATELLITES—(cnoitunde) Analogy between Mars and the Earth—Martian Snowcaps, Seas, and Conitnents—Climate and Atmosphere—Schiapareill's Canals—Discovery of Two Martian Sateliltes—Photographic Detection of Minor Planets—Orbit of Eros—Distribution of the Minor Planets—Their Collecitve Mass and Esitmated Diameters—Condition of Jupiter—His Spectrum—Transits of his Sateliltes—Discovery of a Fifth Satelilte—The Great Red Spot—Constituiton of Saturn's Rings—Period of Rotation of the Planet—Variabiilty of Japetus—Equatorial Markings on Uranus—His Spectrum—Rotaiton of Neptune —Trans-Neptunian Planets CHAPTER IX THEORIES OF PLANETARY EVOLUTION Origin of the World according to Kant—Laplace's Nebular Hypothesis—Maintenance of the Sun's Heat —Meteoric Hypothesis—Radiaiton as an Effect of Contraction—Regeneraitve Theory—Faye's Scheme of Planetary Development—Origin of the Moon—Effects of Tidal CHAPTER X RECENT COMETS Donait's Comet—The Earth again Involved in a Comet's Tail—Comets of the August and November Meteors—Star Showers—Comets and Meteors—Biela's Comet and the Andromedes—Holmes's Comet—Delfeciton of the Leonids—Orbits of Meteorites—Meteors with Staitonary Radiants —Spectroscopic Analysis of Cometary Light—Comet of 1901—Coggia's Comet CHAPTER XI RECENT COMETS—(deunitnco) Forms of Comets' Tails—Electrical Repulsion—Brédikhine's Three Types—Great Southern Comet —Supposed Previous Appearances—Tebbutt's Comet and the Comet of 1807—Successful Photographs—Schaeberle's Comet—Comet Wells—Sodium Blaze in Spectrum—Great Comet of 1882—Transit across the Sun—Relation to Comets of 1843 and 1880—Cometary Systems —Spectral Changes in Comet of 1882—Brooks's Comet of 1889—Swift's Comet of 1892—Origin of Comets CHAPTER XII STARS AND NEBULÆ Stellar Chemistry—Four Orders of Stars—Their Relaitve Ages—Gaseous Stars—Spectroscopic Star- Catalogues—Stellar Chemistry—Hydrogen Spectrumi n Stars—The Draper Catalogue—Velocities of Stars in Line of Sight—Spectroscopic Binaries—Eclipses of Algol—Catalogues of Variables—New Stars—Outbursts in Nebulæ—Nova Aurigæ—Nova Persei—Gaseous Nebulæ—Variable Nebulæ —Movements of Nebulæ—Stellar Persei—Gaseous Nebulæ—Variable Nebulæ—Movements of Nebulæ—Stellar and Nebular Photography —Nebulæ in the Pleiades—Photographic Star-charting—Stellar Parallax—Double Stars—Stellar Photometry—Status of Nebulæ—Photographs and Drawings of the Milky Way—Star Drift CHAPTER XIII METHODS OF RESEARCH Development of Telescopic Power—Silvered Glass Reflectors—Giant Refractors—Comparison with Reflectors—The Yerkes Telescope—Atmospheric Disturbance—The Lick Observatory—Mechanical Dififcutlies—The Equatorea lCoudé—The Photographic Camera—Retrospect and Conclusion APPENDIX Chronology, 1774-1893—Chemical Elements in the Sun (Rowland, 1891)—Epochs of Sun-spot Maximum and Minimum from 1610 to 1901—Movements of Sun and Stars—List of Great Telescopes—List of Observatories employed in the Construction of the Photographic Chart and Catalogue of the Heavens INDEX LIST OF ILLUSTRATIONS P HOTOGRAPH   OF   THE G REAT N EBULA I   N O RION , 1883   Frontispiece P HOTOGRAPHS   OF J UPITER , 1879, AND   OF S ATURN , 1885   Vignette P LATE I. P HOTOGRAPHS   OF   THE S OLAR C HROMOSPHERE   AND P ROMINENCES   T O   FACE   P . 198 P LATE  II . P HOTOGRAPH   OF   THE G REAT C OMET   OF M AY , 1901 (T AKEN   AT   THE R OYAL O BSERVATORY , C APE   OF G OOD H OPE ) P LATE III. T HE G REAT C OMET   OF S EPTEMBER , (P HOTOGRAPHED   AT   THE C APE   OF G OOD H OPE ) P LATE IV. P HOTOGRAPHS   OF S WIF ' T S C OMET , 1892 P LATE V. P HOTOGRAPHIC   AND V ISUAL S PECTRUM   OF N OVA A URIGÆ P LATE VI. P HOTOGRAPH   OF   THE M ILKY W AY I   N S AGITTARIUS HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY INTRODUCTION We can distinguish three kinds of astronomy, each with a different origin and history, but a llmutually dependent, and composing, in their fundamental unity, one science. First in order of time came the art of observing the returns, and measuring the places, of the heavenly bodies. This was the sole astronomy of the Chinese and Chaldeans; but to it the vigorous Greek mind added a highly complex geometrical plan of their movements, for which Copernicus substituted a more harmonious system, without as yet any idea of a compelilng cause. The planets revolved in circles because it was their nature to do so, just as laudanum sets to sleep because it possesses avirtus dormitivaand oldest branch is known as. This first "observaitona,l" or "practical astronomy." tIs business is to note facts as accurately as possible; and it is essentially unconcerned with schemes for connecting those facts in a manner saitsfactory to the reason. The second kind of astronomy was founded by Newton. Its nature is best indicated by the term "gravitational"; but it is also called "theoretical astronomy." [1]  It is based on the idea of cause; and the whole of its elaborate structure is reared according to the dictates of a single law, simple in itsefl, but the tangled web of whose consequences can be unravelled only by the subtle agency of an elaborate calculus. The third and last division of celestia lscience may properly be termed "physica land descripitve astronomy." tI seeks to know what the heavenly bodies are in themselves,l eaving the How? and the Wherefore? of their movements to be otherwise answered. Now, such inquiries became possible only through the invention of the telescope, so that Gailleo was, in point of fact, their originato.r But Herschel first gave them a prominence which the whole progress of science during the nineteenth century served to conifrm and render more exclusive. Inquisitions begun with the telescope have been extended and made effecitve in unhoped-for direcitons by the aid of the spectroscope and photographic camera; and a large part of our atteniton in the present volume wi llbe occupied with the brilliant resutls thus achieved. The unexpected development of this new physical-celestial science is the leading fact in recent astronomical history. tI was out of the regular course of events. In the degree in which it has actually occurred it could certainly not have been foreseen. It was a seizing of the prize by a competitor who had hardly been thought quailifed to enter the ilsts. Orthodox astronomers of the old schoo llooked with a certain contempt upon observers who spent their nights in scruitnising the faces of the moon and planets rather than in itming their transits, or devoted dayilght energies, not to reductions and computaitons, but to counting and measuring spots on the sun. They were regarded as irregular practitioners, to be tolerated perhaps, but certainly not encouraged. The advance of astronomy in the eighteenth century ran in general an even and logical course. The age succeeding Newton's had for its special task to demonstrate the universal validity, and trace the complex results, of the law of gravitation. The accomplishment of that task occupied just one hundred years. tI was virtually brought to a close when Laplace explained to the French Academy, November 19, 1787, the cause of the moon's accelerated moiton. As a mere machine, the solar system, so far as it was then known, was found to be complete and intelligible in all its parts; and in the Mécanique Céleste its mechanical perfections were displayed under a form of majestic unity which fitly commemorated the successive triumphs of analyitcal genius over problems amongst the most arduous ever deatl with by the mind of man. Theory, however, demands a practical test. Al lits data are derived from observaiton; and their insecurity becomes less tolerable as it advances nearer to perfection. Observation, on the other hand, is the pitiless criitc of theory; it detects weak points, and provokes reforms which may be the beginnings of discovery. Thus, theory and observaiton mutually act and react, each atlernately taking the lead in the endless race of improvement. Now, while in France Lagrange and Laplace were bringing the gravitaitona ltheory of the solar system to completion, work of a very different kind, yet not less indispensable to the future weflare of astronomy, was being done in England. The Roya lObservatory at Greenwich is one of the few useful insittuitons which date their origin from the reign of Charles .II The leading position which it stil loccupies in the science of celestia lobservaiton was, for near a century and a hafl after its foundaiton, an exclusive one. Delambre remarked that, had all other materials of the kind been destroyed, the Greenwich records alone would suffice for the restoration of astronomy. The estabilshment was indeed absolutely without a rival. [2] Systematic observaitons of sun, moon, stars, and planets were during the whole of the eighteenth century made only at Greenwich. Here materials were accumulated for the secure correction of theory, and here reifnements were introduced by which the exquisite accuracy of modern pracitce in astronomy was eventually attained. The chief promoter of these improvements was James Bradley. Few men have possessed in an equal degree with him the power of seeing accurately, and reasoning on what they see. He let nothing pass. The silghtest inconsistency between what appeared and what was to be expected roused his keenest attention; and he never relaxed his mental grip of a subject until it had yielded to his persistent inquisition. It was to these qualiites that he owed his discoveries of the aberration of light and the nutaiton of the earth's axis. The first was announced in 1729. What is meant by it is that, owing to the circumstance of light not being instantaneously transmitted, the heavenly bodies appear shifted from their true places by an amount depending upon the ratio which the velocity of ilght bears to the speed of the earth in its orbit. Because light travels with enormous rapidity, the shiftingi s very silght; and each star returns to its origina lposiiton at the end of a yea.r Bradley's second great discovery was ifnally ascertained in 1748. Nutaiton is a rea l"nodding" of the terrestrial axis produced by the dragging of the moon at the terrestrial equatorial protuberance. From it resutls an apparentfo hcae ed meht ngbirisctlit l ailsp eleuo t ebatrueits  orsplaceme di etsra,stno  fht "mean" posiiton, in a period of nearly nineteen years. Now, an acquaintance with the fact and the laws of each of these minute irregularities is vital to the progress of observational astronomy; for without it the places of the heavenly bodies could never be accurately known or compared. So that Bradley, by their detection, at once raised the science to a higher grade of precision. Nor was this the whole of his work. Appointed Astronomer-Royal in 1742, he executed during the years 1750-62 a series of observations which formed the real beginning of exact astronomy. Part of their superiority must, indeed, be attributed to the co-operaiton of John Bird, who provided Bradley in 1750 with a measuring instrument of till then unequalled excellence. For not only was the art of observing in the eighteenth century a pecuilarly Engilsh art, but the means of observing were furnished almost exclusively by Briitsh artists. John Dollond, the son of a Spitalfields weave,ri nvented the achromaitc lensi n 1758, removing thereby the chief obstacle to the development of the powers of refracting telescopes; James Short, of Edinburgh, was without a riva lin the construciton of reflectors; the sectors, quadrants, and circles of Graham, Bird, Ramsden, and Cary were inimitable by Conitnenta lworkmanship. Thus pracitcal and theoretica lastronomy advanced on paralle lilnes in England and France respectively, the improvement of their several tools—the telescope and the quadrant on the one side, and the calculus on the other—keeping pace. The whole future of the science seemed to be theirs. The cessaiton of interest through a too speedy attainment of the perfection towards which each spurred the other, appeared to be the only danger it held in store for them. When all at once, a rival stood by their side—not, indeed, menacing their progress, but threatening to absorb their popularity. The rise of Herschel was the one conspicuous anomaly in the astronomical history of the eighteenth

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