A History of Science, VOL 3 Modern Development of Physical Sciences - Henry Smith Williams (1904)

7/28/2019 A History of Science, VOL 3 Modern Development of Physical Sciences - Henry Smith Williams (1904)

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Digitized by the Internet Arciiivein 2010 witii funding from

Boston Library Consortium IVIember Libraries


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THE RESULTS OF EROSION BY RUNNING WATER


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HISTORY OF SCIENCEBY


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CONTENTSBOOK IIICHAPTER I

THE SUCCESSORS OF NEWTON IN ASTRONOMYThe work of Johannes Hevelius, p. 3Halley and Hevelius, p.4Halley 's observation of the transit of Mercury, and his meth-od of determining the parallax of the planets, p. 5Halley 's ob-servation of meteors, p. 7His inability to explain these bodies,p. 10The important work of James Bradley, p. 11Lacaille'smeasurement of the arc of the meridian, p. 13The determinationof the question as to the exact shape of the earth, p. 14D'Alem-bert and his influence upon science, p. 15Delambre's History ofAstronomy, p. 16The astronomical work of Euler, p. 17.

CHAPTER IITHE PROGRESS OF MODERN ASTRONOMY

The work of William Herschel, p. 19His discovery of Uranus, p.20His discovery that the stars are suns, p. 21His conceptionof the universe, p. 24His deduction that gravitation has causedthe grouping of the heavenly bodies, p. 25The nebulse hypothe-sis, p. 25Immanuel Kant's conception of the formation of theworld, p. 26Defects in Kant's conception, p. 30Laplace's finalsolution of the problem, p. 31His explanation in detail, p.32Change in the mental attitude of the world since Bruno, p.39 Asteroids and satellites, p. 40 Discoveries of Olbers, p.41 The mathematical calculations of Adams and Leverrier, p.42The discovery of the inner ring of vSaturn, p. 44Clerk-Maxwell's paper on the stability of Saturn's rings, p. 45Helm-holtz's conception of the action of tidal friction, p. 49Professor

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CONTENTSG. H. Darwin's estimate of the consequences of tidal action, p. 49Comets and meteors, p. 51Bredichin's cometary theory, p.53The final solution of the structure of comets, p. 55New-comb's estimate of the amount of cometary dust swept up daily bythe earth, p. 56The fixed stars, p. 56John Herschel's studiesof double stars, p. 58Fraunhofer's perfection of the refractingtelescope, p. 60Bessel's measurement of the parallax of a star,p. 60-Henderson's measurements, p. 61KirchhofT and Bun-sen's perfection of the spectroscope, p. 62Wonderful revelationsof the spectroscope, p. 63Lord Kelvin's estimate of the time thatwill be required for the earth to become completely cooled, p. 65Alvan Clark's discovery of the companion star of Sirius, p. 66-^The advent of the photographic film in astronomy, p. 67Dr.Huggins's studies of nebula, p. 69Sir Norman Lockyer's "cos-mogonic guess," p. 70CroU's pre-nebular theory, p. 72.

CHAPTER IIITHE NEW SCIENCE OF PALEONTOLOGY

William Smith and fossil shells., p. 74His discovery that fossilrocks are arranged in regular systems, p. 75Smith's inquiriestaken up by Cuvier, p. 77His Osseinents Fossiles containing thefirst description of hairy elephant, p. 78His contention that fos-sils represent extinct species only, p. 79Dr. Buckland's studiesof English fossil-beds, p. 82Charles Lyell combats catastrophism,p. 84Elaboration of his ideas with reference to the rotation ofspecies, p. 86The establishment of the doctrine of uniformitarian-ism, p. 92Darwin's Origin of Species, p. 93Fossil man, p. 98Dr. Falconer's visit to the fossil-beds in the valley of the Somme,p. 99Investigations of Prestwich and Sir John Evans, p. loiDiscovery of the Neanderthal skull, p. 103Cuvier's rejection ofhuman fossils, p. 104The finding of prehistoric carving on ivory,p. 104The fossil-beds of America, p. 105Professor Marsh'spaper on the fossil horses in America, p. 107The Warren masto-don, p. 113The Java fossil. Pithecanthropus Erectus, p. 113.

CHAPTER IVTHE ORIGIN AND DEVELOPMENT OF MODERN GEOLOGY

James Hutton and the study of the rocks, p. 116His theory of theearth, p. 120His belief in volcanic cataclysms in raising and form-ing the continents, p. 121His famous paper before the Royal So-

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CONTENTSciety of Edinburgh, 1781, p. 121His conclusions that all strata ofthe earth have their origin at the bottom of the sea, p. 124Hisdeduction that heated and expanded matter caused the elevationof land above the sea-level, p. 128Indifference at first shown thisremarkable paper, p. 129Neptunists versus Plutonists, p. 131Scrope's classical work on volcanoes, p. 132Final acceptance ofHutton's explanation of the origin of granites, p. 141Lyell andunifomiitarianism, p. 142Observations on the gradual elevationof the coast-lines of Sweden and Patagonia, p. 142Observationson the enormous amount of land erosion constantly taking place, p.143Agassiz and the glacial theory, p. 144Perraudin the chamois-hunter, and his explanation of perched bowlders, p. 145De Char-pentier's acceptance of Perraudin's explanation, p. 146Agassiz'spaper on his Alpine studies, p. 147His conclusion that the Alpswere once covered with an ice-sheet, p. 153Final acceptance ofthe glacial theory, p. 154The geological ages, p. 155The workof Murchison and Sedgwick, p. 157Formation of the Americancontinents, p. 161Past, present, and future, p. 164

CHAPTER VTHE NEW SCIENCE OF METEOROLOGY

Biot's investigations of meteors, p. 169The observations ofBrandes and Benzenberg on the velocity of falling stars, p. 170Professor Olmstead's observations on the meteoric shower of 1833,p. 171Confirmation of Chladni's hypothesis of 1794, p. 172Theaurora borealis, p. 172 Franklin's suggestion that it is of elec-trical origin, p. 173Its close association with terrestrial magnet-ism, p. 174Evaporation, cloud-formation, and dew, p. 177Dal-ton's demonstration that water exists in the air as an independentgas, p. 178Hutton's theory of rain, p. 178Luke Howard's paperon clouds, p. 182Observations on dew, by Professor Wilson andMr. Six, p. 184Dr. Wells's essay on dew, p. 187His observa-tions on several appearances connected with dew, p. 188Isothermsand ocean currents, p. 192Humboldt and the science of compara-tive climatology, p. 193His studies of ocean currents, p. 195Maury's theory that gravity is the cause of ocean currents, p. 196Dr. Croll on Climate and Time, p. 197Cyclones and anti-cyclones,p. 199Dove's studies in climatology, p. 199Professor Ferrel'smathematical law of the deflection of winds, p. 200Tyndall's es-timate of the amount of heat given off by the liberation of a poundof vapor, p. 203Meteorological observations and weather pre-dictions, p. 204.


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CONTENTSCHAPTER VI

MODERN THEORIES OF HEAT AND LIGHTJosiah Wedgwood and the clay pyrometer, p. 206Count Rumfordand the vibratory theory of heat, p. 208His experiments withboring cannon to determine the nature of heat, p. 209Causingwater to boil by the friction of the borer, p. 213His final deter-mination that heat is a form of motion, p. 215Thomas Youngand the wave theory of light, p. 215His paper on the theory oflight and colors, p. 217His exposition of the colors of thin plates,p. 218Of the colors of thick plates, and of striated surfaces, p.219Arago and Fresnel champion the wave theory, p. 225Oppo-sition to the theory by Biot, p. 226The French Academy's tacitacceptance of the correctness of the theory by its admission of Fres-nel as a member, p. 227.

CHAPTER VIITHE MODERN DEVELOPMENT OF ELECTRICITY AND MAGNETISM

Galvani and the beginning of modern electricity, p. 229The con-struction of the voltaic pile, p. 230Nicholson's and Carlisle's dis-covery that the galvanic current decomposes water, p. 232De-composition of various substances by Sir Humphry Davy, p. 233His construction of an arc-light, p. 234The deflection of the mag-netic needle by electricity demonstrated by Oersted, p. 237Ef-fect of this important discovery, p. 238Ampere creates the scienceof electro-dynamics, p. 239 Joseph Henry's studies of electro-magnets, p. 239Michael Faraday begins his studies of electro-magnetic induction, p. 240His famous paper before the RoyalSociety, in 1 831, in which he demonstrates electro-magnetic induc-tion, p. 241His explanation of Arago 's rotating disk, p. 244Thesearch for a satisfactory method of storing electricity, p. 247Roentgen rays, or X-rays, p. 247.

CHAPTER VIIITHE CONSERVATION OF ENERGY

Faraday narrowly misses the discovery of the doctrine of conserva-tion, p. 253Camot's belief that a definite quantity of work canbe transformed into a definite quantity of heat, p. 255The workof James Prescott Joule, p. 256Investigations begun by Dr.Mayer, p. 257Mayer's paper of 1842, p. 259His statement of the

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CONTENTSlaw of the conservation of energy, p. 266Mayer and Helmholtz,p. 267Joule's paper of 1843, p. 269Joule or Mayer, p. 272LordKelvin and the dissipation of energy, p. 274The final unification,p. 279.

CHAPTER IXTHE ETHER AND PONDERABLE MATTER

James Clerk-Maxwell's conception of ether, p. 283Thomas Youngand " Luminiferous ether," p. 285Young's and Fresnel's concep-tion of transverse luminiferous undulations, p. 285Faraday's ex-periments pointing to the existence of ether, p. 287ProfessorLodge's suggestion of two ethers, p. 288Lord Kelvin's calcula-tion of the probable density of ether, p. 289The vortex theory ofatoms, p. 290Helmholtz's calculations in vortex motions, p. 290Professor Tait's apparatus for creating vortex rings in the air, p.291The ultimate constitution of matter as conceived by Bosco-vich, p. 293Davy's speculations as to the changes that occur inthe substance of matter at different temperatures, p. 294Clau-sius's and Maxwell's investigations of the kinetic theory of gases,p. 295Lord Kelvin's estimate of the size of the molecule, p. 299Studies of the potential energy of molecules, p. 300Action ofgases at low temperatures, p. 304.APPENDIX 307


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ILLUSTRATIONSTHE RESULTS OF EROSION BY RUNNING WATER . . . FrontispieceEDMUND HALLEY Facing p- 6HERSCHEL AND HIS SISTER AT THE TELESCOPE ..." 24PIERRE SIMON DE LAPLACE " 34FRIEDRICH "WILHELM BESSEL " 42HEINRICH WILHELM MATTHIAS OLBERS ...... " 525,ORD ROSSe's TELESCOPE . " 68WILLIAM SMITH " 76BARON DE CUVIER " 80THE EVOLUTION OF A HORSE's FOOT AND OF A HORSe's

HEAD " 108JAMES HUTTON . " 118A LANDSCAPE AND MAMMAL OF THE TERTIARY AGE . " 130A LANDSCAPE AND TERRESTRIAL REPTILE OF THE MESO-

ZOIC TIME " 142JAMES DWIGHT DANA ^LOUIS JEAN RODOLPH AGASSIZ ICHARLES LYELLADAM SEDGWICKWATER-SPOUTS IN MID-ATLANTIC " 1 54MANHATTAN ISLAND IN THE QUATERNARY AGE THEMASTODON " 164ALEXANDER VON HUMBOLDT " 196

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ILLUSTRATIONSA SAND-STORM ON THE MOJAVE DESERT Facing p. 202BENJAMIN THOMPSON (cOUNT RUMFORd) . . .THOMAS YOUNGMICHAEL FARADAYSIR HUMPHRY DAVYHERMANN LUDWIG FERDINAND HELMHOLTZ . ,JAMES PRESCOTT JOULE 'WILLIAM THOMPSON (LORD KELVIN)JULIUS ROBERT MAYERJOHN TYNDALL

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A HISTORY OF SCIENCEBOOK III

MODERN DEVELOPMENT OF THE PHYSICALSCIENCES

WITH the present book we enter the field of thedistinctively modern. There is no precise dateat which we take up each of the successive stories,but the main sweep of development has to do in eachcase with the nineteenth century. We shall see atonce that this is a time both of rapid progress and ofgreat differentiation. We have heard almost nothinghitherto of such sciences as paleontology, geology, andmeteorology, each of which now demands full attention.Meantime, astronomy and what the workers of theelder day called natural philosophy become wonder-fully diversified and present numerous phases thatwould have been startling enough to the star-gazersand philosophers of the earlier epoch.

Thus, for example, in the field of astronomy, Her-schel is able, thanks to his perfected telescope, to dis-cover a new planet and then to reach out into the

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A HISTORY OF SCIENCEdepths of space and gain such knowledge of stars andnebulse as hitherto no one had more than dreamed of.Then, in rapid sequence, a whole coterie of hithertounsuspected minor planets is discovered, stellar dis-tances are measured, some members of the starry-galaxy are timed in their flight, the direction of move-ment of the solar system itself is investigated, thespectroscope reveals the chemical composition even ofsuns that are unthinkably distant, and a tangibletheory is grasped of the universal cycle which includesthe birth and death of worlds. -

Similarly the new studies of the earth's surface re-veal secrets of planetary formation hitherto quite in-scrutable. It becomes known that the strata of theearth's surface have been forming throughout untoldages, and that successive populations differing utterlyfrom one another have peopled the earth in differentgeological epochs. The entire point of view of thought-ful men becomes changed in contemplating the his-tory of the world in which we livealbeit the newestthought harks back to some extent to those dayswhen the inspired thinkers of early Greece dreamedout the wonderful theories with which our earlierchapters have made our readers familiar.

In the region of natural philosophy progress is noless pronounced and no less striking. It suffices here,however, by way of anticipation, simply to name thegreatest generalization of the century in physicalsciencethe doctrine of the conservation of energy.


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I

THE SUCCESSORS OF NEWTON IN ASTRONOMYHEVELIUS AND HALLEY

STRANGELY enough, the decade immediately fol-lowing Newton was one of comparative barren-

ness in scientific progress, the early years of the eigh-teenth century not being as productive of great as-tronomers as the later years of the seventeenth, or, forthat matter, as the later years of the eighteenth cen-tury itself. Several of the prominent astronomers ofthe later seventeenth century lived on into the open-ing years of the following century, however, and theyounger generation soon developed a coterie of as-tronomers, among whom Euler, Lagrange, Laplace,and Herschel, as we shall see, were to accomplish greatthings in this field before the century closed.One of the great seventeenth-century astronomers,

who died just before the close of the century, wasJohannes Hevelius (1611-1687), of Dantzig, who ad-vanced astronomy by his accurate description of theface and the spots of the moon. But he is remem-bered also for having retarded progress by his influ-ence in refusing to use telescopic sights in his observa-tions, preferring until his death the plain sights longbefore discarded by most other astronomers. The

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A HISTORY OF SCIENCEadvantages of these telescope sights have been dis-cussed under the article treating of Robert Hooke, butno such advantages were ever recognized by Hevelius.So great was Hevelius's reputation as an astronomerthat his refusal to recognize the advantage of the tele-scope sights caused many astronomers to hesitate be-fore accepting them as superior to the plain ; and eventhe famous Halley, of whom we shall speak further ina moment, was sufficiently in doubt over the matterto pay the aged astronomer a visit to test his skill inusing the old-style sights. Side by side, Hevelius andHalley made their observations, Hevelius with his oldinstrument and Halley with the new. The resultsshowed slightly in the younger man's favor, but notenough to make it an entirely convincing demonstra-tion. The explanation of this, however, did not lie inthe lack of superiority of the telescopic instrument,but rather in the marvellous skill of the aged Hevelius,whose dexterity almost compensated for the defect ofhis instrument. What he might have accomplishedcould he have been induced to adopt the telescope canonly be surmised.

Halley himself was by no means a tyro in mattersastronomical at that time. As the only son of awealthy soap-boiler living near London, he had beengiven a liberal education, and even before leaving col-lege made such novel scientific observations as that ofthe change in the variation of the compass. At nine-teen years of age he discovered a new method of de-termining the elements of the planetary orbits whichwas a distinct improvement over the old. The yearfollowing he sailed for the Island of St. Helena to make


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SUCCESSORS OF NEWTON IN ASTRONOMYobservations of the heavens in the southern hemi-sphere.

It was while in St. Helena that Halley made hisfamous observation of the transit of Mercury over thesun's disk, this observation being connected, indi-rectly at least, with his discovery of a method of de-termining the parallax of the planets. By parallaxis meant the apparent change in the position of an ob-ject, due really to a change in the position of the ob-server. Thus, if we imagine two astronomers makingobservations of the sun from opposite sides of theearth at the same time, it is obvious that to theseobservers the sun will appear to be at two differentpoints in the sky. Half the angle measuring this dif-ference would be known as the sun's parallax. Thiswould depend, then, upon the distance of the earthfrom the sun and the length of the earth's radius.Since the actual length of this radius has been de-termined, the parallax of any heavenly body enablesthe astronomer to determine its exact distance.The parallaxes can be determined equally well, how-

ever, if two observers are separated by exactly knowndistances, several hundreds or thousands of miles apart.In the case of a transit of Venus across the sun's disk,for example, an observer at New York notes the imageof the planet moving across the sun's disk, and notesalso the exact time of this observation. In the samemanner an observer at London makes similar obser-vations. Knowing the distance between New Yorkand London, and the different time of the passage, it isthus possible to calculate the difference of the paral-laxes of the sun and a planet crossing its disk. The

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A HISTORY OF SCIENCEidea of thus determining the parallax of the planetsoriginated, or at least was developed, by Halley, andfrom this phenomenon he thought it possible to con-clude the dimensions of all the planetary orbits. Aswe shall see further on, his views were found to becorrect by later astronomers.

In 1 72 1 Halley succeeded Flamsteed as astronomerroyal at the Greenwich Observatory. Although sixty-four years of age at that time his activity in astronomycontinued unabated for another score of years. AtGreenwich he undertook some tedious observationsof the moon, and during those observations was firstto detect the acceleration of mean motion. He wasunable to explain this, however, and it remained forLaplace in the closing years of the century to do so,as we shall see later.

Halley's book, the Synopsis AstronomicB CometiccE,is one of the most valuable additions to astronomicalliterature since the time of Kepler. He was first toattempt the calculation of the orbit of a comet, havingrevived the ancient opinion that comets belong to thesolar system, moving in eccentric orbits round the sun,and his calculation of the orbit of the comet of 1682 ledhim to predict correctly the return of that comet in1758. Halley's Study of Meteors.Like other astronomers of his time he was greatly

puzzled over the well-known phenomena of shooting-stars, or meteors, making many observations himself,and examining carefully the observations of otherastronomers. In 17 14 he gave his views as to theorigin and composition of these mysterious visitorsin the earth's atmosphere. As this subject will be

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EDMUND HALLEY(From a painting ascribed to Dahl in the possession of the Royal Society.)


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SUCCESSORS OF NEWTON IN ASTRONOMYagain referred to in a later chapter, Halley's views,representing the most advanced views of his age, areof interest.

" The theory of the air seemeth at present," he says,"to be perfectly well understood, and the differingdensities thereof at all altitudes; for supposing thesame air to occupy spaces reciprocally proportional tothe quantity of the superior or incumbent air, I haveelsewhere proved that at forty miles high the air israrer than at the surface of the earth at three thousandtimes; and that the utmost height of the atmosphere,which reflects light in the Crepuscuhim, is not fullyforty - five miles, notwithstanding which 'tis stillmanifest that some sort of vapors, and those in nosmall quantity, arise nearly to that height. An in-stance of this may be given in the great light thesociety had an account of (vide Transact. Sep., 1676)from Dr. Wallis, which was seen in very distant coun-ties almost over all the south part of England. Ofwhich though the doctor could not get so particular arelation as was requisite to determine the height there-of, yet from the distant places it was seen in, it couldnot but be very many miles high.

" So likewise that meteor which was seen in 1708, onthe 31st of July, between nine and ten o'clock at night,was evidently between forty and fifty miles perpendicu-larly high, and as near as I can gather, over Sherenessand the buoy on the Nore. For it was seen at Londonmoving horizontally from east by north to east bysouth at least fifty degrees high, and at Redgrove, inSuffolk, on the Yarmouth road, about twenty milesfrom the east coast of England, and at least forty miles

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A HISTORY OF SCIENCEto the eastward of London, it appeared a little to thewestward of the south, suppose south by west, andwas seen about thirty degrees high, sliding obliquelydownward. I was shown in both places the situationthereof, which was as described, but could wish somejjerson skilled in astronomical matters had seen it,that we might pronounce concerning its height withmore certainty. Yet, as it is, we may securely concludethat it was not many more miles westerly than Red-grove, which, as I said before, is about forty miles moreeasterly than London. Suppose it, therefore, whereperpendicular, to have been thirty-five miles east fromLondon, and by the altitude it appeared at in Londonviz., fifty degrees, its tangent will be forty-two miles,for the height of the meteor above the surface of theearth; which also is rather of the least, because thealtitude of the place shown me is rather more thanless than fifty degrees ; and the like may be concludedfrom the altitude it appeared in at Redgrove, nearseventy miles distant. Though at this very greatdistance, it appeared to move with an incrediblevelocity, darting, in a very few seconds of time, forabout twelve degrees of a great circle from north tosouth, being very bright at its first appearance; andit died away at the east of its course, leaving for sometime a pale whiteness in the place, with some remainsof it in the track where it had gone; but no hissingsound as it passed, or bounce of an explosion wereheard.

" It may deserve the honorable society's thoughts,how so great a quantity of vapor should be raised tothe top of the atmosphere, and there collected, so

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SUCCESSORS OF NEWTON IN ASTRONOMYas upon its ascension or otherwise illumination, togive a light to a circle of above one hundred milesdiameter, not much inferior to the light of the moonso as one might see to take a pin from the ground inthe otherwise dark night. 'Tis hard to conceive whatsort of exhalations should rise from the earth, eitherby the action of the sun or subterranean heat, so as tosurmount the extreme cold and rareness of the air inthose upper regions : but the fact is indisputable, andtherefore requires a solution."From this much of the paper it appears that there

was a general belief that this burning mass washeated vapor thrown off from the earth in somemysterious manner, yet this is unsatisfactory to Hal-ley, for after citing various other meteors thathave appeared within his knowledge, he goes on tosay

" What sort of substance it must be, that couldbe so impelled and ignited at the same time; therebeing no Vidcano or other Spiraculum of subterra-neous fire in the northeast parts of the world, thatwe ever yet heard of, from whence it might be pro-jected." I have much considered this appearance, and thinkit one of the hardest things to account for that I haveyet met with in the phenomena of meteors, and I aminduced to think that it mmst be some collection ofmatter formed in the ther, as it were, by somefortuitous concourse of atoms, and that the earth metwith it as it passed along in its orb, tlien but newlyformed, and before it had conceived any great impetusof descent towards the sun. For the direction of it

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A HISTORY OF SCIENCEwas exactly opposite to that of the earth, which madean angle with the meridian at that time of sixty-sevengr., that is, its course was from west southwest to eastnortheast, wherefore the meteor seemed to move thecontrary way. And besides falling into the power ofthe earth's gravity, and losing its motion from theopposition of the medium, it seems that it descendedtowards the earth, and was extinguished in the Tyr-rhene Sea, to the west southwest of Leghorn. Thegreat blow being heard upon its first immersion intothe water, and the rattling like the driving of a cartover stones being what succeeded upon its quenching;something like this is always heard upon quenching avery hot iron in water. These facts being past dispute,I would be glad to have the opinion of the learned there-on, and what objection can be reasonably made againstthe above hypothesis, which I humbly submit to theircensure."^These few paragraphs, coming as they do from a

leading eighteenth-century astronomer, convey moreclearly than an}^ comment the actual state of themeteorological learning at that time. That this ballof fire, rushing " at a greater velocity than the swiftestcannon-ball," was simply a mass of heated rock passingthrough our atmosphere, did not occur to him, or atleast was not credited. Nor is this surprising when wereflect that at that time universal gravitation had beenbut recently discovered; heat had not as yet beenrecognized as simply a form of motion; and thunderand lightning were unexplained mysteries, not to beexplained for another three - quarters of a century.In the chapter on meteorology we shall see how the

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SUCCESSORS OF NEWTON IN ASTRONOMYsolution of this mystery that puzzled Halley and hisassociates all their lives was finally attained.

BRADLEY AND THE ABERRATION OF LIGHTHalley was succeeded as astronomer royal by a man

whose useful additions to the science were not tobe recognized or appreciated fully until brought tolight by the Prussian astronomer Bessel early in thenineteenth century. This was Dr. James Bradley, anecclesiastic, who ranks as one of the most eminentastronomers of the eighteenth century. His most re-markable discovery was the explanation of a peculiarmotion of the pole-star, first observed, but not explain-ed, by Picard a century before. For many years asatisfactory explanation was sought unsuccessfully byBradley and his fellow-astronomers, but at last he wasable to demonstrate that the star 7 Draconis, on whichhe was making his observations, described, or appearedto describe, a small ellipse. If this observation wascorrect, it afforded a means of computing the aberrationof any star at all times. The explanation of thephysical cause of this aberration, as Bradley thought,and afterwards demonstrated, was the result of thecombination of the motion of light with the annualmotion of the earth. Bradley first formulated thistheory in 1728, but it was not until 1748twenty yearsof continuous struggle and observation by himthat hewas prepared to communicate the results of his effortsto the Royal Society. This remarkable paper isthought by the Frenchman, Delambre, to entitle itsauthor to a place in science beside such astronomers asHipparchus and Kepler,

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A HISTORY OF SCIENCEBradley's studies led him to discover also the libra-

tory motion of the earth's axis. "As this appear-ance of 7 Draconis indicated a diminution of theinclination of the earth's axis to the plane of theecliptic," he says; "and as several astronomers havesujjposed that inclination to diminish regularly ; if thisphenomenon depended upon such a cause, nd amount-ed to i8'' in nine years, the obliquity of the eclipticwould, at that rate, alter a whole minute in thirtyyears; which is much faster than any observations,before made, would allow. I had reason, therefore, tothink that some part of this motion at the least, if notthe whole, was owing to the moon's action upon theequatorial parts of the earth ; which, I conceived, mightcause a libratory motion of the earth's axis. But as Iwas unable to judge, from only nine years observations,whether the axis would entirely recover the sameposition that it had in the year 1727, I found itnecessary to continue my observations through awhole period of the moon's nodes; at the end ofwhich I had the satisfaction to see, that the stars re-turned into the same position again; as if there hadbeen no alteration at all in the inclination of the earth'saxis; which fully convinced me that I had guessedrightly as to the cause of the phenomena. This cir-cumstance proves likewise, that if there be a gradualdiminution of the obliquity of the ecliptic, it does notarise only from an alteration in the position of theearth's axis, but rather from some change in the planeof the ecliptic itself ; because the stars, at the end of theperiod of the moon's nodes, appeared in the sameplaces, with respect to the equator, as they ought to

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SUCCESSORS OF NEWTON IN ASTRONOMYhave done, if the earth's axis had retained the sameinclination to an invariable plane." ^

FRENCH ASTRONOMERSMeanwhile, astronomers across the channel were by-

no means idle. In France several successful observerswere making many additions to the already long listof observations of the first astronomer of the RoyalObservatory of Paris, Dominic Cassini (162 5-1 7 12),whose reputation among his contemporaries wasmuch greater than among succeeding generations ofastronomers. Perhaps the most deserving of thesesuccessors was Nicolas Louis de Lacaille (171 3-176 2),a theologian who had been educated at the expenseof the Duke of Bourbon, and who, soon after com-pleting his clerical studies, came under the patronageof Cassini, whose attention had been called to theyoung man's interest in the sciences. One of Lacaille'sfirst undertakings was the remeasuring of the Frencharc of the meridian, which had been incorrectly meas-ured by his patron in 1684. This was begun in 1739,and occupied him for two years before successfullycompleted. As a reward, however, he was admittedto the academy and appointed mathematical professorin Mazarin College.

In 1 75 1 he went to the Cape of Good Hope for thepurpose of determining the sun's parallax by observa-tions of the parallaxes of Mars and Venus, and inci-dentally to make observations on the other southernhemisphere stars. The results of this undertakingwere most successful, and were given in his Ccelumaustrale stelligerum, etc., published in 1763. In this he

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A HISTORY OF SCIENCEshows that in the course of a single year he had ob-served some ten thousand stars, and computed theplaces of one thousand nine hundred and forty-two ofthem, measured a degree of the meridian, and mademany observations of the moonproductive industryseldom equalled in a single year in any field. Theseobservations were of great service to the astronomers,as they afforded the opportunity of comparing the starsof the southern hemisphere with those of the northern,which were being observed simultaneously by Lelandeat Berlin.

Lacaille's observations followed closely upon thedetermination of an absorbing question which oc-cupied the attention of the astronomers in theearly part of the century. This question was asto the shape of the earthwhether it was actuallyflattened at the poles. To settle this question oncefor all the Academy of Sciences decided to make theactual measurement of the length of two degrees, oneas near the pole as possible, the other at the equator.Accordingly, three astronomers, Godin, Bouguer, andLa Condamine, made the journey to a spot on theequator in Peru, while four astronomers, Camus,Clairaut, Maupertuis, and Lemonnier, made a voyageto a place selected in Lapland. The result of theseexpeditions was the determination that the globe isoblately spheroidal.A great contemporary and fellow-countryman ofLacaille was Jean Le Rond d'Alembert (17 17-1783),who, although not primarily an astronomer, did so muchwith his mathematical calculations to aid that sciencethat his name is closely connected with its progress

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SUCCESSORS OF NEWTON IN ASTRONOMYduring the eighteenth century. D'Alembert, whobecame one of the best - known men of science ofhis day, and whose services were eagerly soughtby the rulers of Europe, began life as a found-ling, having been exposed in one of the markets ofParis. The sickly infant was adopted and cared forin the family of a poor glazier, and treated as a mem-ber of the family. In later years, however, after thefoundling had become famous throughout Europe, hismother, Madame Tencin, sent for him, and acknowl-edged her relationship. It is more than likely thatthe great philosopher believed her story, but if so hedid not allow her the satisfaction of knowing his be-lief, declaring always that Madame Tencin could "notbe nearer than a step-mother to him, since his motherwas the wife of the glazier."D'Alembert did much for the cause of science by hisexample as well as by his discoveries. By living aplain but honest hfe, declining magnificent offers ofpositions from royal patrons, at the same time refusingto grovel before nobility, he set a worthy example toother philosophers whose cringing and pusillanimousattitude towards persons of wealth or position hadhitherto earned them the contempt of the upperclasses.

His direct additions to astronomy are several, amongothers the determination of the mutation of the axisof the earth. He also determined the ratio of the at-tractive forces of the sun and moon, which he foundto be about as seven to three. From this he reachedthe conclusion that the earth must be seventy timesgreater than the moon. The first two volumes of his

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A HISTORY OF SCIENCEResearches on the Systems of the World, published in1754, are largely devoted to mathematical and astro-nomical problems, many of them of little importancenow, but of great interest to astronomers at thattime.Another great contemporary of D'Alembert, whose

name is closely associated and frequently confoundedwith his, was Jean Baptiste Joseph Delambre (I749-I822). More fortunate in birth as also in his educa-tional advantages, Delambre as a youth began hisstudies under the celebrated poet Delille. Later he wasobliged to struggle against poverty, supporting himselffor a time by making translations from Latin, Greek,Italian, and English, and acting as tutor in privatefamilies. The turning-point of his fortune came whenthe attention of Lalande was called to the young manby his remarkable memory, and Lalande soon showedhis admiration by giving Delambre certain difficultastronomical problems to solve. By performing thesetasks successfully his future as an astronomer be-came assured. At that time the planet Uranus hadjust been discovered by Herschel, and the Acad-emy of Sciences offered as the subject for one ofits prizes the determination of the planet's orbit.Delambre made this determination and won theprize a feat that brought him at once into prom-inence.By his writings he probably did as much towards

perfecting modern astronomy as any one man. HisHistory of Astronomy is not merely a narrative of prog-ress of astronomy but a complete abstract of all thecelebrated works written on the subject. Thus he

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SUCCESSORS OF NEWTON IN ASTRONOMYbecame famous as an historian as well as an as-tronomer.

LEONARD EULERStill another contemporary of D'Alembert and Del-

ambre, and somewhat older than either of them, wasLeonard Euler (i 707-1 783), of Basel, whose fame as aphilosopher equals that of either of the great French-men. He is of particular interest here in his capacityof astronomer, but astronomy was only one of themany fields of science in which he shone. Surely some-thing out of the ordinary was to be expected of theman who could "repeat the Mneid of Virgil from thebeginning to the end without hesitation, and indicatethe first and last line of every page of the edition whichhe used." Something was expected, and he fulfilledthese expectations.

In early life he devoted himself to the st'dy oftheology and the Oriental languages, at the request ofhis father, but his love of mathematics proved toostrong, and, with his father's consent, he finally gaveup his classical studies and turned to his favoritestudy, geometry. In 1727 he was invited by Cath-arine I. to reside in St. Petersburg, and on acceptingthis invitation he was made an associate of the Acade-my of Sciences. A little later he was made professorof physics, and in 1733 professor of mathematics. In1735 he solved a problem in three days which someof the eminent mathematicians would not undertakeunder several months. In 1741 Frederick the Greatinvited him to Berlin, where he soon became a memberof the Academy of Sciences and professor of mathe-

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A HISTORY OF SCIENCEmatics; but in 1766 he returned to St. Petersburg.Towards the close of his Hfe he became virtually blind,being obliged to dictate his thoughts, sometimes topersons entirely ignorant of the subject in hand.Nevertheless, his remarkable memory, still furtherheightened by his blindness, enabled him to carry outthe elaborate computations frequently involved.

Euler's first memoir, transmitted to the Academy ofSciences of Paris in 1747, was on the planetary per-turbations. This memoir carried off the prize thathad been offered for the analytical theory of the mo-tions of Jupiter and Saturn. Other memoirs followed,one in 1749 and another in 1750, with further expan-sions of the same subject. As some slight errors werefound in these, such as a mistake in some of the for-mulse expressing the secular and periodic inequalities,the academy proposed the same subject for the prizeof 1752. Euler again competed, and won this prizealso. The contents of this memoir laid thefoimda-tion for the subsequent demonstration of the perma-nent stability of the planetary system by Laplace andLagrange.

It was Euler also who demonstrated that withincertain fixed limits the eccentricities and places of theaphelia of Saturn and Jupiter are subject to constantvariation, and he calculated that after a lapse of aboutthirty thousand years the elements of the orbits ofthese two planets recover their original values.


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II

THE PROGRESS OF MODERN ASTRONOMYANEW epoch in astronomy begins with the work

of William Herschel, the Hanoverian, whom Eng-land made hers by adoption. He was a man with apositive genius for sidereal discovery. At first a mereamateur in astronomy, he snatched time from hisduties as music-teacher to grind him a telescopic mir-ror, and began gazing at the stars. Not content withhis first telescope, he made another and another, andhe had such genius for the work that he soon possesseda better instrument than was ever made before. Hispatience in grinding the curved reflective surface wasmonumental. Sometimes for sixteen hours togetherhe must walk steadily about the mirror, polishing it,without once removing his hands. Meantime his sister,always his chief lieutenant, cheered him with her pres-ence, and from time to time put food into his mouth.The telescope completed, the astronomer turned nightinto day, and from sunset to sunrise, year in and yearout, swept the heavens unceasingly, unless preventedby clouds or the brightness of the moon. His sistersat always at his side, recording his observations.They were in the open air, perched high at the mouth ofthe reflector, and sometimes it was so cold that the inkfroze in the bottle in Caroline Herschel' s hand ; but thetwo enthusiasts hardly noticed a thing so common-

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A HISTORY OF SCIENCEplace as terrestrial weather. They were living in dis-tant worlds.The results ? What could they be ? Such enthusi-asm would move mountains. But, after all, the mov-

ing of mountains seems a liliputian task comparedwith what Herschel really did with those wonderfultelescopes. He moved worlds, stars, a universeeven, if you please, a galaxy of universes; at least heproved that they move, which seems scarcely less won-derful ; and he expanded the cosmos, as man conceivesit, to thousands of times the dimensions it had before.As a mere beginning, he doubled the diameter of thesolar system by observing the great outlying planetwhich we now call Uranus, but which he christenedGeorgium Sidus, in honor of his sovereign, and whichhis French contemporaries, not relishing that name,preferred to call Herschel.

This discovery was but a trifle compared with whatHerschel did later on, but it gave him world-wide repu-tation none the less. Comets and moons aside, thiswas the first addition to the solar system that had beenmade within historic times, and it created a veritablefuror of popular interest and enthusiasm. IncidentallyKing George was flattered at having a world namedafter him, and he smiled on the astronomer, and camewith his court to have a look at his namesake. Theinspection was highly satisfactory; and presently theroyal favor enabled the astronomer to escape thethraldom of teaching music and to devote his entiretime to the more congenial task of star-gazing.Thus relieved from the burden of mundane em-

barrassments, he turned with fresh enthusiasm to the20


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PROGRESS OF MODERN ASTRONOMYskies, and his discoveries followed one another in be-wildering profusion. He found various hitherto un-seen moons of our sister planets; he made specialstudies of Saturn, and proved that this planet, with itsrings, revolves on its axis ; he scanned the spots on thesun, and suggested that they influence the weather ofour earth ; in short, he extended the entire field of solarastronomy. But very soon this field became too smallfor him, and his most important researches carriedhim out into the regions of space compared with whichthe span of our solar system is a mere point. With hisperfected telescopes he entered abysmal vistas whichno human eye ever penetrated before, which no humanmind had hitherto more than vaguely imagined. Hetells us that his forty-foot reflector will bring him lightfrom a distance of "at least eleven and three-fourthsmillions of millions of millions of miles"light whichleft its source two million years ago. The smalleststars visible to the unaided eye are those of the sixthmagnitude; this telescope, he thinks, has power toreveal stars of the 1^,426. magnitude.

But what did Herschel learn regarding these awfuldepths of space and the stars that people them ? Thatwas what the world wished to know. Copernicus,Galileo, Kepler, had given us a solar system, but thestars had been a mystery. What says the great re-flectorare the stars points of light, as the ancientstaught, and as more than one philosopher of the eigh-teenth century has still contended, or are they suns, asothers hold? Herschel answers, they are suns, eachand every one of all the millionssuns, many of them,larger than the one that is the centre of our tiny system,

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A HISTORY OF SCIENCENot only so, but they are moving suns. Instead ofbeing fixed in space, as has been thought, they are whirl-ing in gigantic orbits about some common centre. Isour sun that centre ? Far from it. Our sun is only astar like all the rest, circling on with its attendantsatellitesour giant sun a star, no different frommyriad other stars, not even so large as some; a mereinsignificant spark of matter in an infinite shower ofsparks.Nor is this all. Looking beyond the few thousandstars that are visible to the naked eye, Herschel sees

series after series of more distant stars, marshalled ingalaxies of millions; but at last he reaches a distancebeyond which the galaxies no longer increase. Andyetso he thinkshe has not reached the limits of hisvision. What then ? He has come to thebounds of thesidereal systemseen to the confines of the imiverse.He believes that he can outline this system, this uni-verse, and prove that it has the shape of an irregularglobe, oblately flattened to almost disklike proportions,and divided at one edgea bifurcation that is revealedeven to the naked eye in the forking of the Milky Way.

This, then, is our universe as Herschel conceives ita vast galaxy of suns, held to one centre, revolving,poised in space. But even here those marvellous tele-scopes do not pause. Far, far out beyond the confinesof our universe, so far that the awful span of our ownsystem might serve as a unit of measure, are revealedother systems, other universes, like our own, each com-posed, as he thinks, of myriads of suns, clustered likeour galaxy into an isolated systemmere islands ofmatter in an infinite ocean of space. So distant from

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PROGRESS OF MODERN ASTRONOMYour universe are these new universes of Herschel's dis-covery that their light reaches us only as a dim, nebu-lous glow, in most cases invisible to the unaided eye.About a hundred of these nebula were known whenHerschel began his studies. Before the close of thecentury he had discovered about two thousand more ofthem, and many of these had been resolved by hislargest telescopes into clusters of stars. He believedthat the farthest of these nebulae that he could seewas at least three hundred thousand times as distantfrom us as the nearest fixed star. Yet that neareststarso more recent studies proveis so remote thatits light, travelling one hundred and eighty thousandmiles a second, requires three and one-half years toreach our planet.As if to give the finishing touches to this novel

scheme of cosmology, Herschel, though in the mainvery little given to unsustained theorizing, allows him-self the privilege of one belief that he cannot call uponhis telescope to substantiate. He thinks that all themyriad suns of his numberless systems are instinct withlife in the human sense. Giordano Bruno and a longline of his followers had held that some of our sisterplanets may be inhabited, but Herschel extends thethought to include the moon, the sun, the starsall theheavenly bodies. He believes that he can demonstratethe habitability of our own sun, and, reasoning fromanalogy, he is firmly convinced that all the suns of allthe systems are "well supplied with inhabitants." Inthis, as in some other inferences, Herschel is misled bythe faulty physics of his time. Future generations,working with perfected instruments, may not sustain

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A HISTORY OF SCIENCEhim all along the line of his observations, even, let alonehis inferences. But how one's egotism shrivels andshrinks as one grasps the import of his sweepingthoughts

Continuing his observations of the innumerable nebu-lae, Herschel is led presently to another curious specula-tive inference. He notes that some star groups aremuch more thickly clustered than others, and he is ledto infer that such varied clustering tells of varyingages of the different nebulae. He thinks that at firstall space may have been evenly sprinkled with thestars and that the grouping has resulted from theaction of gravitation.

" That the Milky Way is a most extensive stratum ofstars of various sizes admits no longer of lasting doubt,"he declares, "and that our sun is actually one of theheavenly bodies belonging to it is as evident. I havenow viewed and gauged this shining zone in almostevery direction and find it composed of stars whosenumber . . . constantly increases and decreases in pro-portion to its apparent brightness to the naked eye.

" Let us suppose numberless stars of various sizes,scattered over an indefinite portion of space in sucha manner as to be almost equally distributed through-out the whole. The laws of attraction which no doubtextend to the remotest regions of the fixed stars willoperate in such a manner as most probably to producethe following effects:

" In the first case, since we have supposed the starsto be of various sizes, it will happen that a star, beingconsiderably larger than its neighboring ones, will at-tract them more than they will be attracted by others

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HERSCHEL AND HIS SISTER AT THE TELESCOPE


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PROGRESS OF MODERN ASTRONOMYthat are immediately around them; by which meansthey will be, in time, as it were, condensed about acentre, or, in other words, form themselves into a clus-ter of stars of almost a globular figure, more or lessregular according to the size and distance of the sur-rounding stars. . . .

" The next case, which will also happen almost as fre-quently as the former, is where a few stars, though notsuperior in size to the rest, may chance to be rathernearer one another than the surrounding ones, . . . andthis construction admits of the utmost variety ofshapes. ...

" From the composition and repeated conjunction ofboth the foregoing formations, a third may be derivedwhen many large stars, or combined small ones, arespread in long, extended, regular, or crooked rows,streaks, or branches; for they will also draw the sur-rounding stars, so as to produce figures of condensedstars curiously similar to the former which gave rise tothese condensations."We may likewise admit still more extensive com-binations; when, at the same time that a cluster ofstars is forming at the one part of space, there may beanother collection in a different but perhaps not far-distant quarter, which may occasion a mutual approachtowards their own centre of gravity."In the last place, as a natural conclusion of the

former cases, there will be formed great cavities orvacancies by the retreating of the stars towards thevarious centres which attract them."^

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A HISTORY OF SCIENCEwhen all the suns of a system will be drawn togetherand destroyed by impact at a common centre. Al-ready, it seems to Herschel, the thickest clusters have"outlived their usefulness" and are verging towardstheir doom.But again, other nebulae present an appearance sug-

gestive of an opposite condition. They are not re-solvable into stars, but present an almost uniform ap-pearance throughout, and are hence believed to becomposed of a shining fluid, which in some instances isseen to be condensed at the centre into a glowing mass.In such a nebula Herschel thinks he sees a sun inprocess of formation.

THE NEBULAR HYPOTHESIS OF KANTTaken together, these two conceptions outline a ma-

jestic cycle of world formation and world destructiona broad scheme of cosmogony, such as had been vague-ly adumbrated two centuries before by Kepler and inmore recent times by Wright and Swedenborg. Thisso-called "nebular hypothesis" assumes that in thebeginning all space was uniformly filled with cosmicmatter in a state of nebular or "fire-mist" diffusion,"formless and void." It pictures the condensationcoagulation, if you willof portions of this mass toform segregated masses, and the ultimate developmentout of these masses of the sidereal bodies that we see.

Perhaps the first elaborate exposition of this ideawas that given by the great German philosopher Im-manuel Kant (born at Konigsberg in 1724, died in1804), known to every one as the author of the Critiqueof Pure Reason. Let us learn from his own words how

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PROGRESS OF MODERN ASTRONOMYthe imaginative philosopher conceived the world tohave come into existence."I assume," says Kant, "that all the material ofwhich the globes belonging to our solar systemallthe planets and cometsconsist, at the beginning ofall things was decomposed into its primary elements,and filled the whole space of the tmiverse in which thebodies formed out of it now revolve. This state ofnature, when viewed in and by itself without any ref-erence to a system, seems to be the very simplest thatcan follow upon nothing. At that time nothing hasyet been formed. The construction of heavenly bod-ies at a distance from one another, their distances reg-ulated by their attraction, their form arising out of theequilibrium of their collected matter, exhibit a laterstate. ... In a region of space filled in this manner, auniversal repose could last only a moment. The ele-ments have essential forces with which to put eachother in motion, and thus are themselves a source oflife. Matter immediately begins to strive to fashionitself. The scattered elements of a denser kind, bymeans of their attraction, gather from a sphere aroundthem all the matter of less specific gravity ; again, theseelements themselves, together with the material v/hichthey have united with them, collect in those pointswhere the particles of a still denser kind are found;these in like manner join still denser particles, and soon. If we follow in imagination this process by whichnature fashions itself into form through the whole ex-tent of chaos, we easily perceive that all the results ofthe process would consist in the formation of diversmasses which, when their formation was complete,

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A HISTORY OF SCIENCEwould by the equality of their attraction be at restand be forever unmoved.

" But nature has other forces in store which arespecially exerted when matter is decomposed into fineparticles. They are those forces by which these par-ticles repel one another, and which, by their conflictwith attractions, bring forth that movement which is,as it were, the lasting life of nature. This force of re-pulsion is manifested in the elasticity of vapors, theeffluences of strong-smelling bodies, and the diffusionof all spirituous matters. This force is an uncontest-able phenomenon of matter. It is by it that the ele-ments, which may be falling to the point attractingthem, are turned sideways promiscuously from theirmovement in a straight line; and their perpendicularfall thereby issues in circular movements, which en-compass the centre towards which they were falling.In order to make the formation of the world more dis-tinctly conceivable, we will limit our view by withdraw-ing it from the infinite universe of nature and directingit to a particular system, as the one which belongs toour sun. Having considered the generation of thissystem, we shall be able to advance to a similar con-sideration of the origin of the great world-systems, andthus to embrace the infinitude of the whole creation inone conception."From what has been said, it will appear that if a

point is situated in a very large space where the at-traction of the elements there situated acts more strong-ly than elsewhere, then the matter of the elementaryparticles scattered throughout the whole region will fallto that point. The first effect of this general fall is

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PROGRESS OF MODERN ASTRONOMYthe formation of a body at this centre of attraction,which, so to speak, grows from an infinitely small nu-cleus by rapid strides ; and in the proportion in whichthis mass increases, it also draws with greater forcethe surrounding particles to unite with it. When themass of this central body has grown so great that thevelocity with which it draws the particles to itself withgreat distances is bent sideways by the feeble degreeof repulsion with which they impede one another, andwhen it issues in lateral movements which are capableby means of the centrifugal force of encompassing thecentral body in an orbit, then there are producedwhirls or vortices of particles, each of which by itselfdescribes a curved line by the composition of the at-tracting force and the force of revolution that had beenbent sideways. These kinds of orbits all intersectone another, for which their great dispersion in thisspace gives place. Yet these movements are in manyways in conflict with one another, and they naturallytend to bring one another to a uniformitythat is,into a state in which one movement is as little ob-structive to the other as possible. This happens intwo ways : first by the particles limiting one another'smovement till they all advance in one direction; and,secondly, in this way, that the particles limit theirvertical movements in virtue of which they are ap-proaching the centre of attraction, till they all movehorizontally i. e., in parallel circles round the sun astheir centre, no longer intercept one another, and bythe centrifugal force becoming equal with the fallingforce they keep themselves constantly in free circularorbits at the distance at which they move. The result,

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A HISTORY OF SCIENCEfinally, is that only those particles continue to move inthis region of space which have acquired by their falla velocity, and through the resistance of the other par-ticles a direction, by which they can continue to main-tain a free circular movement. . . .

" The view of the formation of the planets in this sys-tem has the advantage over every other possible theoryin holding that the origin of the movements, and theposition of the orbits in arising at that same point oftimenay, more, in showing that even the deviationsfrom the greatest possible exactness in their determina-tions, as well as the accordances themselves, becomeclear at a glance. The planets are formed out of par-ticles which, at the distance at which they move, haveexact movements in circular orbits ; and therefore themasses composed out of them will continue the samemovements and at the same rate and in the same direc-tion."^

It must be admitted that this explanation leaves agood deal to be desired. It is the explanation of ametaphysician rather than that of an experimentalscientist. Such phrases as "matter immediately be-gins to strive to fashion itself," for example, have noplace in the reasoning of inductive science. Never-theless, the hypothesis of Kant is a remarkable con-ception; it attempts to explain along rational linessomething which hitherto had for the most part beenconsidered altogether inexplicable.But there are various questions that at once suggestthemselves which the Kantian theory leaves unan-

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PROGRESS OF MODERN ASTRONOMYmass which gave birth to our solar system was dividedinto several planetary bodies instead of remaining asingle mass ? Were the planets struck from the sun bythe chance impact of comets, as Buffon has suggested ?or thrown out by explosive volcanic action, in accord-ance with the theory of Dr. Darwin? or do they owetheir origin to some unknown law ? In any event, howchanced it that all were projected in nearly the sameplane as we now find them ?

LAPLACE AND THE NEBULAR HYPOTHESISIt remained for a mathematical astronomer to solve

these puzzles. The man of all others competent totake the subject in hand was the French astronomerLaplace. For a quarter of a century he had devotedhis transcendent mathematical abilities to the solu-tion of problems of motion of the heavenly bodies.Working in friendly rivalry with his countryman La-grange, his only peer among the mathematicians of theage, he had taken up and solved one by one the prob-lems that Newton left obscure. Largely through theefforts of these two men the last lingering doubts as tothe solidarity of the Newtonian hypothesis of universalgravitation had been removed. The share of Lagrangewas hardly less than that of his co-worker ; but Laplacewill longer be remembered, because he ultimatelybrought his completed labors into a system, and, in-corporating with them the labors of his contemporaries,produced in the Mecanique Celeste the undisputedmathematical monument of the century, a fitting com-plement to the Principia of Newton, which it supple-ments and in a sense completes.

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A HISTORY OF SCIENCEIn the closing years of the eighteenth century La-

place took up the nebular hypothesis of cosmogony, towhich we have just referred, and gave it definite pro-portions; in fact, made it so thoroughly his ownthat posterity will always link it with his name.Discarding the crude notions of cometary impactand volcanic eruption, Laplace filled up the gaps inthe hypothesis with the aid of well-known laws ofgravitation and motion. He assumed that the primi-tive mass of cosmic matter which was destined toform our solar system was revolving on its axiseven at a time when it was still nebular in character,and filled all space to a distance far beyond thepresent limits of the system. As this vaporous masscontracted through loss of heat, it revolved moreand more swiftly, and from time to time, through bal-ance of forces at its periphery, rings of its substancewere whirled off and left revolving there, subsequentlyto become condensed into planets, and in their turnwhirl off minor rings that became moons. The mainbody of the original mass remains in the present as thestill contracting and rotating body which we call thesun.Let us allow Laplace to explain all this in detail"In order to explain the prime movements of the

planetary system," he says, "there are the five follow-ing phenomena: The movement of the planets in thesame direction and very nearly in the same plane ; themovement of the satellites in the same direction asthat of the planets; the rotation of these dift'erentbodies and the sim in the same direction as their revo-lution, and in nearly the same plane; the slight eccen-

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PROGRESS OF MODERN ASTRONOMYtricity of the orbits of the planets and of the satellitesand, finally, the great eccentricity of the orbits of thecomets, as if their inclinations had been left to chance.

" Buffon is the only man I know who, since the dis-covery of the true system of the world, has endeavoredto show the origin of the planets and their satellites.He supposes that a comet, in falling into the sun, drovefrom it a mass of matter which was reassembled at adistance in the form of various globes more or lesslarge, and more or less removed from the sun, and thatthese globes, becoming opaque and solid, are now theplanets and their satellites.

"This hypothesis satisfies the first of the five pre-ceding phenomena; for it is clear that all the bodiesthus formed would move very nearly in the planewhich passed through the centre of the sun, and in thedirection of the torrent of matter which was producedbut the four other phenomena appear to be inexplicableto me by this means. Indeed, the absolute move-ment of the molecules of a planet ought then to be inthe direction of the movement of its centre of gravitybut it does not at all follow that the motion of the ro-tation of the planets should be in the same direction.Thus the earth should rotate from east to west, butnevertheless the absolute movement of its moleculesshould be from east to west; and this ought also toapply to the movement of the revolution of the satel-lites, in which the direction, according to the hypoth-esis which he offers, is not necessarily the same as thatof the progressive movement of the planets."A phenomenon not only very difficult to explainunder this hypothesis, but one which is even contrary

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A HISTORY OF SCIENCEto it, is the slight eccentricity of the planetary orbits.We know, by the theory of central forces, that if a bodymoves in a closed orbit around the sun and touches it,it also always comes back to that point at every revo-lution ; whence it follows that if the planets were origi-nally detached from the sun, they would touch it ateach return towards it, and their orbits, far from beingcircular, would be very eccentric. It is true that a massof matter driven from the sun cannot be exactly com-pared to a globe which touches its surface, for the im-pulse which the particles of this mass receive from oneanother and the reciprocal attractions which they ex-ert among themselves, could, in changing the directionof their movements, remove their perihelions from thesun; but their orbits would be always most eccentric,or at least they would not have slight eccentricitiesexcept by the most extraordinary chance. Thus wecannot see, according to the hypothesis of Buffon,why the orbits of more than a hundred comets alreadyobserved are so elliptical. This hypothesis is there-fore very far from satisfying the preceding phenomena.Let us see if it is possible to trace them back to theirtrue cause.

"Whatever may be its ultimate nature, seeing that ithas caused or modified the movements of the planets,it is necessary that this cause should embrace everybody, and, in view of the enormous distances whichseparate them, it could only have been a fluid of im-mense extent. In order to have given them an almostcircular movement in the same direction around thesun, it is necessary that this fluid should have envel-oped the sun as in an atmosphere. The consideration

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PIERRE SIMON DE LAPLACE(From a painting by Nedeon )


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PROGRESS OF MODERN ASTRONOMYof the planetary movements leads us then to thinkthat, on account of excessive heat, the atmosphere ofthe stm originally extended beyond the orbits of allthe planets, and that it was successively contracted toits present limits,

" In the primitive condition in which we suppose thesun to have been, it resembled a nebula such as^thetelescope shows is composed of a nucleus more or lessbrilliant, surrounded by a nebulosity which, on con-densing itself tow^ards the centre, forms a star. If it isconceived by analogy that all the stars were formed inthis manner, it is possible to imagine their previouscondition of nebulosity, itself preceded by other statesin which the nebulous matter was still more diffused,the nucleus being less and less luminous. By goingback as far as possible, we thus arrive at a nebulosityso diffused that its existence could hardly be sus-pected.

" For a long time the peculiar disposition of certainstars, visible to the unaided eye, has struck philo-sophical observers, Mitchell has already remarkedhow little probable it is that the stars in the Pleiades,for example, could have been contracted into the smallspace which encloses them by the fortuity of chancealone, and he has concluded that this group of stars,and similar groups which the skies present to us, arethe necessary result of the condensation of a nebula,with several nuclei, and it is evident that a nebula, bycontinually contracting towards these various nuclei,at length would form a group of stars similar to thePleiades. The condensation of a nebula with twonuclei would form a system of stars close together,

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A HISTORY OF SCIENCEturning one upon the other, such as those double starsof which we already know the respective movements." But how did the solar atmosphere determine themovements of the rotation and revolution of the plan-ets and satelhtes ? If these bodies had penetrated verydeeply into this atmosphere, its resistance would havecaused them to fall into the sun. We can thereforeconjecture that the planets were formed at their suc-cessive limits by the condensation of a zone of vaporswhich the sun, on cooling, left behind, in the plane ofhis equator.

" Let us recall the results which we have given ina preceding chapter. The atmosphere of the sun couldnot have extended indefinitely. Its limit was the pointwhere the centrifugal force due to its movement ofrotation balanced its weight. But in proportion asthe cooling contracted the atmosphere, and those mole-cules which were near to them condensed upon thesurface of the body, the movement of the rotation in-creased ; for, on account of the Law of Areas, the sumof the areas described by the vector of each moleculeof the sun and its atmosphere and projected in theplane of the equator being always the same, the rota-tion should increase when these molecules approach thecentre of the sun. The centrifugal force due to thismovement becoming thus larger, the point where theweight is equal to it is nearer the sun. Supposing,then, as it is natural to admit, that the atmosphereextended at some period to its very limits, it should,on cooling, leave molecules behind at this limit andat limits successively occasioned by the increased ro-tation of the sun. The abandoned molecules would

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PROGRESS OF MODERN ASTRONOMYcontinue to revolve around this body, since their cen-trifugal force was balanced by their weight. But thisequilibrium not arising in regard to the atmosphericmolecules parallel to the solar equator, the latter, onaccount of their weight, approached the atmosphereas they condensed, and did not cease to belong to ituntil by this motion they came upon the equator.

" Let us consider now the zones of vapor successivelyleft behind. These zones ought, according to appear-ance, by the condensation and mutual attraction oftheir molecules, to form various concentric rings ofvapor revolving around the sun. The mutual gravita-tional friction of each ring would accelerate some andretard others, until they had all acquired the sameangular velocity. Thus the actual velocity of themolecules most removed from the sun would be thegreatest. The following cause would also operate tobring about this difference of speed. The moleculesfarthest from the sun, and which by the effects ofcooling and condensation approached one another toform the outer part of the ring, would have alwaysdescribed areas proportional to the time since thecentral force by which they were controlled has beenconstantly directed towards this body. But this con-stancy of areas necessitates an increase of veloc-ity proportional to the distance. It is thus seenthat the same cause would diminish the velocityof the molecules which form the inner part of thering.

" If all the molecules of the ring of vapor continuedto condense without disuniting, they would at lengthform a ring either solid or fluid. But this formation

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A HISTORY OF SCIENCEwould necessitate such a regularity in every part ofthe ring, and in its cooling, that this phenomenon isextremely rare; and the solar system affords us, in-deed, but one examplenamely, in the ring of Saturn.In nearly every case the ring of vapor was broken intoseveral masses, each moving at similar velocities, andcontinuing to rotate at the same distance around thesun. These masses would take a spheroid form with arotatory movement in the direction of the revolution,because their inner molecules had less velocity thanthe outer. Thus were formed so many planets in a con-dition of vapor. But if one of them were powerfulenough to reunite successively by its attraction all theothers around its centre of gravity, the ring of vaporwould be thus transformed into a single spheroidicalmass of vapor revolving around the sun with a rota-tion in the direction of its revolution. The latter casehas been that which is the most common, but never-theless the solar system affords us an instance of thefirst case in the four small planets which move be-tween Jupiter and Mars ; at least, if we do not sup-pose, as does M. Gibers, that they originally formeda single planet which a mighty explosion broke upinto several portions each moving at different veloc-ities.

"According to our hypothesis, the comets are stran-gers to our planetary system. In considering them,as we have done, as minute nebulosities, wanderingfrom solar system to solar system, and formed bythe condensation of the nebulous matter everywhereexistent in profusion in the universe, we see that whenthey come into that part of the heavens where the sun

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PROGRESS OF MODERN ASTRONOMYis all-powerful, he forces them to describe orbits eitherelliptical or hyperbolic, their paths being equally pos-sible in all directions, and at all inclinations of theecliptic, conformably to what has been observed. Thusthe condensation of nebulous matter, b}^ which wehave at first explained the motions of the rotation andrevolution of the planets and their satellites in the samedirection, and in nearly approximate planes, explainsalso why the movements of the comets escape thisgeneral law."^

The nebular hypothesis thus given detailed comple-tion by Laplace is a worthy complement of the grandcosmologic scheme of Herschel. Whether true or false,the two conceptions stand as the final contributions ofthe eighteenth century to the history of man's ceaselessefforts to solve the mysteries of cosmic origin and cos-mic structure. The world listened eagerly and withoutprejudice to the new doctrines ; and that attitude tellsof a marvellous intellectual growth of our race. Markthe transition. In the year 1600, Bruno was burnedat the stake for teaching that our earth is not the cen-tre of the universe. In 1700, Newton was pronounced"impious and heretical" by a large school of philoso-phers for declaring that the force which holds the plan-ets in their orbits is universal gravitation. In 1800,Laplace and Herschel are honored for teaching thatgravitation built up the system which it still controlsthat our universe is but a minor nebula, our sun buta minor star, our earth a mere atom of matter, ourrace only one of myriad races peopling an infinityof worlds. Doctrines which but the span of two hu-

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A HISTORY OF SCIENCEman lives before would have brought their enimcia-tors to the stake were now pronounced not impious,but sublime.

ASTEROIDS AND SATELLITESThe first day of the nineteenth century was fittingly

signalized by the discovery of a new world. On theevening of January i, 1801, an ItaHan astronomer,Piazzi, observed an apparent star of about the eighthmagnitude (hence, of course, quite invisible to the un-aided eye), which later on was seen to have moved,and was thus shown to be vastly nearer the earth thanany true star. He at first supposed, as Herschel haddone when he first saw Uranus, that the unfamiliarbody was a comet; but later observation proved it atiny planet, occupying a position in space betweenMars and Jupiter. It was christened Ceres, after thetutelary goddess of Sicily.Though unpremeditated, this discovery was not un-

expected, for astronomers had long surmised the exist-ence of a planet in the wide gap between Mars and Ju-piter. Indeed, they were even preparing to make con-certed search for it, despite the protests of philosophers,who argued that the planets could not possibly exceedthe magic number seven, when Piazzi forestalled theirefforts. But a surprise came with the sequel; for thevery next year Dr. Olbers, the wonderful physician-astronomer of Bremen, while following up the courseof Ceres, happened on another tiny moving star, sim-ilarly located, which soon revealed itself as planetary.Thus two planets were found where only one was ex-pected.

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PROGRESS OF MODERN ASTRONOMYThe existence of the superniunerary was a puzzle, but

Gibers solved it for the moment by suggesting thatCeres and Pallas, as he called his captive, might befragments of a quondam planet, shattered by internalexplosion or by the impact of a comet. Other sim-ilar fragments, he ventured to predict, would befound when searched for. William Herschel sanc-tioned this theory, and suggested the name asteroidsfor the tiny planets. The explosion theory was sup-ported by the discovery of another asteroid, by Hard-ing, of Lilienthal, in 1804, and it seemed clinchedwhen Gibers himself found a fourth in 1807. Thenew-comers were named Juno and Vesta respec-tively.There the case rested till 1845, when a Prussian

amateur astronomer named Hencke found anotherasteroid, after long searching, and opened a new epochof discovery. From then on the finding of asteroidsbecame a commonplace. Latterly, with the aid ofphotography, the list has been extended to above fourhimdred, and as yet there seems no dearth in the sup-ply, though doubtless all the larger members have beenrevealed. Even these are but a few hundreds of milesin diameter, while the smaller ones are too tiny formeasurement. The combined bulk of these minorplanets is believed to be but a fraction of that of theearth.

Glbers's explosion theory, long accepted by astrono-mers, has been proven open to fatal objections. Theminor planets are now believed to represent a ring ofcosmical matter, cast off from the solar nebula like therings that went to form the major planets, but prevent-

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A HISTORY OF SCIENCEed from becoming aggregated into a single body by theperturbing mass of Jupiter.

The Discovery of NeptuneAs we have seen, the discovery of the first asteroid

confirmed a conjecture; the other important planetarydiscovery of the nineteenth century fulfilled a predic-tion, Neptune was found through scientific prophecy.No one suspected the existence of a trans-Uranianplanet till Uranus itself, by hair-breadth departuresfrom its predicted orbit, gave out the secret. No onesaw the disturbing planet till the pencil of the mathe-matician, with almost occult divination, had pointedout its place in the heavens. The general predicationof a trans-Uranian planet was made by Bessel, the greatKonigsberg astronomer, in 1 840 ; the analysis that re-vealed its exact location was undertaken, half a dec-ade later, by two independent workersJohn CouchAdams, just graduated senior wrangler at Cambridge,England, and U. J. J. Leverrier, the leading Frenchmathematician of his generation.Adams's calculation was first begun and first com-

pleted. But it had one radical defectit was the workof a young and untried man. So it found lodgment in apigeon-hole of the desk of England's Astronomer Royal,and an opportunity was lost which English astrono-mers have never ceased to mourn. Had the searchbeen made, an actual planet would have been seenshining there, close to the spot where the pencil of themathematician had placed its hypothetical counter-part. But the search was not made, and while theprophecy of Adams gathered dust in that regrettable

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PROGRESS OF MODERN ASTRONOMYpigeon-hole, Leverrier's calculation was coming on, histentative results meeting full encouragement fromArago and other French savants. At last the labori-ous calculations proved satisfactory, and, confident ofthe result, Leverrier sent to the Berlin observatory,requesting that search be made for the disturber ofUranus in a particular spot of the heavens. Dr. Gallereceived the request September 23, 1846. That verynight he turned his telescope to the indicated region,and there, within a single degree of the suggested spot,he saw a seeming star, invisible to the unaided eye,which proved to be the long-sought planet, henceforthto be known as Neptune. To the average mind, whichfinds something altogether mystifying about abstractmathematics, this was a feat savoring of the miraculous.

Stimulated by this success, Leverrier calculated anorbit for an interior planet from perturbations of Mer-cury, but though prematurely christened Vulcan, thishypothetical nursling of the sun still haunts the realmof the undiscovered, along with certain equally hypo-thetical trans-Neptunian planets whose existence hasbeen suggested by "residual perturbations" of Uranus,and by the movements of comets. No other veritableadditions of the sun's planetary family have been madein our century, beyond the finding of seven small moons,which chiefly attest the advance in telescopic powers.Of these, the tiny attendants of our Martian neighbor,discovered by Professor Hall with the great Washing-ton refractor, are of greatest interest, because of theirsmall size and extremely rapid flight. One of them ispoised only six thousand miles from Mars, and whirlsabout him almost four times as fast as he revolves,

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A HISTORY OF SCIENCEseeming thus, as viewed by the Martian, to rise in thewest and set in the east, and making the month onlyone-fourth as long as the day.

The Rings of SaturnThe discovery of the inner or crape ring of Saturn,

made simultaneously in 1850 by William C. Bond, atthe Harvard observatory, in America, and the Rev.W. R. Dawes in England, was another interesting op-tical achievement; but our most important advancesin knowledge of Saturn's unique system are due to themathematician. Laplace, like his predecessors, sup-posed these rings to be solid, and explained their sta-bility as due to certain irregularities of contour whichHerschel had pointed out. But about 1851 ProfessorPeirce, of Harvard, showed the untenability of this

A History of Science, VOL 3 Modern Development of Physical Sciences - Henry Smith Williams (1904)

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Transcript of A History of Science, VOL 3 Modern Development of Physical Sciences - Henry Smith Williams (1904)