Great Astronomers: Isaac NewtonBall, Robert Stawell

Published: 1907Categorie(s): Non-Fiction, Biography & autobiography,Science & Technology, Science and Technics, Science, Astro-nomySource: The Internet Archive http://archive.org/details/greatastronomers00balluoft

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About Ball:Sir Robert Stawell Ball, Fellow of The Royal Society, (1 July

1840, Dublin – 25 November 1913, Cambridge) was an Irish as-tronomer. He worked for Lord Rosse from 1865 to 1867. In1867 he became Professor of Applied Mathematics at the RoyalCollege of Science in Dublin. In 1874 Ball was appointed RoyalAstronomer of Ireland and Andrews Professor of Astronomy inthe University of Dublin at Dunsink Observatory.[1] In 1892 hewas appointed Lowndean Professor of Astronomy and Geo-metry at Cambridge University at the same time becoming dir-ector of the Cambridge Observatory. His lectures, articles, andbooks (e.g. Starland and The Story of the Heavens) weremostly popular and simple in style. However, he also publishedbooks on mathematical astronomy such as A Treatise on Spher-ical Astronomy. His main interest was mathematics and he de-voted much of his spare time to his "Screw theory". He servedfor a time as President of the Quaternion Society. His work TheStory of the Heavens is mentioned in the "Lestrygonians"chapter of James Joyce's Ulysses. He was the son of naturalistRobert Ball and Amelia Gresley Hellicar. (Biography from Wik-pedia: http://en.wikipedia.org/wiki/Robert_Stawell_Ball)

Also available on Feedbooks for Ball:• Great Astronomers: Galileo Galilei (1907)• Great Astronomers: Nicolaus Copernicus (1907)• Great Astronomers: Johannes Kepler (1907)• Great Astronomers: Edmond Halley (1907)• Great Astronomers: Tycho Brahe (1907)• Great Astronomers: Pierre-Simon Laplace (1907)• Great Astronomers: William Herschel (1907)• Great Astronomers: William Rowan Hamilton (1907)• Great Astronomers: James Bradley (1907)• Great Astronomers: John Flamsteed (1907)

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Chapter 1IT was just a year after the death of Galileo, that an infant

came into the world who was christened Isaac Newton. Eventhe great fame of Galileo himself must be relegated to a secondplace in comparison with that of the philosopher who first ex-pounded the true theory of the universe.

Isaac Newton was born on the 25th of December (old style),1642, at Woolsthorpe,1 in Lincolnshire, about a half-mile fromColsterworth, and eight miles south of Grantham. His father,Mr. Isaac Newton, had died a few months after his marriage toHarriet Ayscough, the daughter of Mr. James Ayscough, ofMarket Overton, in Rutlandshire. The little Isaac was at first soexcessively frail and weakly that his life was despaired of. Thewatchful mother, however, tended her delicate child with suchsuccess that he seems to have thriven better than might havebeen expected from the circumstances of his infancy, and heultimately acquired a frame strong enough to outlast the ordin-ary span of human life.

For three years they continued to live at Woolsthorpe, thewidow's means of livelihood being supplemented by the incomefrom another small estate at Sewstern, in a neighbouring partof Leicestershire. In 1645, Mrs. Newton took as a second hus-band the Rev. Barnabas Smith, and on moving to her newhome, about a mile from Woolsthorpe, she entrusted littleIsaac to her mother, Mrs. Ayscough. In due time we find thatthe boy was sent to the public school at Grantham, the name ofthe master being Stokes. For the purpose of being near hiswork, the embryo philosopher was boarded at the house of Mr.Clark, an apothecary at Grantham. We learn from Newton him-self that at first he had a very low place in the class lists of theschool, and was by no means one of those model school-boys

1.England

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who find favour in the eyes of the school-master by attention toLatin grammar. Isaac's first incentive to diligent study seemsto have been derived from the circumstance that he wasseverely kicked by one of the boys who was above him in theclass. This indignity had the effect of stimulating youngNewton's activity to such an extent that he not only attainedthe desired object of passing over the head of the boy who hadmaltreated him, but continued to rise until he became the headof the school.

The play-hours of the great philosopher were devoted to pur-suits very different from those of most school-boys. His chiefamusement was found in making mechanical toys and variousingenious contrivances. He watched day by day with great in-terest the workmen engaged in constructing a windmill in theneighbourhood of the school, the result of which was that theboy made a working model of the windmill and of its ma-chinery, which seems to have been much admired, as indicat-ing his aptitude for mechanics. We are told that Isaac also in-dulged in somewhat higher flights of mechanical enterprise.He constructed a carriage, the wheels of which were to bedriven by the hands of the occupant, while the first philosoph-ical instrument he made was a clock, which was actuated bywater. He also devoted much attention to the construction ofpaper kites, and his skill in this respect was highly appreciatedby his schoolfellows. Like a true philosopher, even at this stagehe experimented on the best methods of attaching the string,and on the proportions which the tail ought to have. He alsomade lanthorns2 of paper to provide himself with light as hewalked to school in the dark winter mornings.

The only love affair in Newton's life appears to have com-menced while he was still of tender years. The incidents arethus described in Brewster's " Life of Newton," a work to whichI am much indebted in this chapter.

" In the house where he lodged there were some femaleinmates, in whose company he appears to have takenmuch pleasure. One of these, a Miss Storey, sister to Dr.Storey, a physician at Buckminster, near Colsterworth,was two or three years younger than Newton, and to

2.Lanthorn: an archaic term for a lantern.

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great personal attractions she seems to have addedmore than the usual allotment of female talent. The soci-ety of this young lady and her companions was alwayspreferred to that of his own schoolfellows, and it was oneof his most agreeable occupations to construct for themlittle tables and cupboards, and other utensils for hold-ing their dolls and their trinkets. He had lived nearly sixyears in the same house with Miss Storey, and there isreason to believe that their youthful friendship graduallyrose to a higher passion; but the smallness of her por-tion, and the inadequacy of his own fortune, appear tohave prevented the consummation of their happiness.Miss Storey was afterwards twice married, and underthe name of Mrs. Vincent, Dr. Stukeley visited her at Gr-antham in 1727, at the age of eighty-two, and obtainedfrom her many particulars respecting the early history ofour author. Newton's esteem for her continued unabatedduring his life. He regularly visited her when he went toLincolnshire, and never failed to relieve her from littlepecuniary difficulties which seem to have beset herfamily."

The schoolboy at Grantham was only fourteen years of agewhen his mother became a widow for the second time. Shethen returned to the old family home at Woolsthorpe, bringingwith her the three children of her second marriage. Her meansappear to have been somewhat scanty, and it was consequentlythought necessary to recall Isaac from the school. His recently-born industry had been such that he had already made goodprogress in his studies, and his mother hoped that hewould now lay aside his books, and those silent meditations towhich, even at this early age, he had become addicted. It wasexpected that, instead of such pursuits, which were deemedquite useless, the boy would enter busily into the duties of thefarm and the details of a country life. But before long it be-came manifest that the study of nature and the pursuit ofknowledge had such a fascination for the youth that he couldgive little attention to aught else. It was plain that he wouldmake but an indifferent farmer. He greatly preferred experi-menting on his water-wheels to looking after labourers, while

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he found that working at mathematics behind a hedge wasmuch more interesting than chaffering about the price of bul-locks in the marketplace. Fortunately for humanity his mother,like a wise woman, determined to let her boy's genius have thescope which it required. He was accordingly sent back to Gr-antham school, with the object of being trained in the know-ledge which would fit him for entering the university ofCambridge.

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Chapter 2It was the 5th of June, 1660, when Isaac Newton, a youth of

eighteen, was enrolled as an undergraduate of Trinity College,Cambridge. Little did those who sent him there dream that thisboy was destined to be the most illustrious student who everentered the portals of that great seat of learning. Little couldthe youth himself have foreseen that the rooms near the gate-way which he occupied would acquire a celebrity from the factthat he dwelt in them, or that the ante-chapel of his collegewas in good time to be adorned by that noble statue, which isregarded as one of the chief art treasures of CambridgeUniversity, both on account of its intrinsic beauty and the factthat it commemorates the fame of her most distinguished alum-nus, Isaac Newton, the immortal astronomer. Indeed, his ad-vent at the University seemed to have been by no means auspi-cious or brilliant. His birth was, as we have seen, comparat-ively obscure, and though he had already given indication ofhis capacity for reflecting on philosophical matters, yet heseems to have been but ill-equipped with the routine know-ledge which youths are generally expected to take with them tothe Universities.

From the outset of his college career, Newton's attentionseems to have been mainly directed to mathematics. Here hebegan to give evidence of that marvellous insight into the deepsecrets of nature which more than a century later led so dis-passionate a judge as Laplace to pronounce Newton's immortalwork as pre-eminent above all the productions of the human in-tellect. But though Newton was one of the very greatest math-ematicians that ever lived, he was never a mathematician forthe mere sake of mathematics. He employed his mathematicsas an instrument for discovering the laws of nature. His in-dustry and genius soon brought him under the notice of theUniversity authorities. It is stated in the University records

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that he obtained a Scholarship in 1664. Two years later we findthat Newton, as well as many residents in the University, hadto leave Cambridge temporarily on account of the breaking outof the plague. The philosopher retired for a season to his oldhome at Woolsthorpe, and there he remained until he was ap-pointed a Fellow of Trinity College, Cambridge, in 1667. Fromthis time onwards, Newton's reputation as a mathematicianand as a natural philosopher steadily advanced, so that in1669, while still but twenty-seven years of age, he was appoin-ted to the distinguished position of Lucasian Professor ofMathematics at Cambridge. Here he found the opportunity tocontinue and develop that marvellous career of discoverywhich formed his life's work.

The earliest of Newton's great achievements in natural philo-sophy was his detection of the composite character of light.That a beam of ordinary sunlight is, in fact, a mixture of a verygreat number of different-coloured lights, is a doctrine now fa-miliar to every one who has the slightest education in physicalscience. We must, however, remember that this discovery wasreally a tremendous advance in knowledge at the time whenNewton announced it.

We here give the little diagram3 originally drawn by Newton,to explain the experiment by which he first learned the com-position of light. A sunbeam is admitted into a darkened roomthrough an opening, H, in a shutter. This beam when not in-terfered with will travel in a straight line to the screen, andthere reproduce a bright spot of the same shape as the hole inthe shutter. If, however, a prism of glass, A B C, be introducedso that the beam traverse it, then it will be seen at once thatthe light is deflected from its original track. There is, however,a further and most important change which takes place. Thespot of light is not alone removed to another part of the screen,but it becomes spread out into a long-band beautifully col-oured, and exhibiting the hues of the rainbow. At the top arethe violet rays, and then in descending order we have the in-digo, blue, green, yellow, orange, and red.

3.Editor's note: The illustrations have been omitted from this eBook. Con-sult the original source for the images. See the attributions section of thiseBook.

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The circumstance in this phenomenon which appears to haveparticularly arrested Newton's attention, was the elongationwhich the luminous spot underwent in consequence of its pas-sage through the prism. When the prism was absent the spotwas nearly circular, but when the prism was introduced thespot was about five times as long as it was broad. To ascertainthe explanation of this was the first problem to be solved. Itseemed natural to suppose that it might be due to the thick-ness of the glass in the prism which the light traversed, or tothe angle of incidence at which the light fell upon the prism.He found, however, upon careful trial, that the phenomenoncould not be thus accounted for. It was not until after much pa-tient labour that the true explanation dawned upon him. Hediscovered that though the beam of white light looks so pureand so simple, yet in reality it is composed of differently col-oured lights blended together. These are, of course, indistin-guishable in the compound beam, but they are separated ordisentangled, so to speak, by the action of the prism. The raysat the blue end of the spectrum are more powerfully deflectedby the action of the glass than are the rays at the red end.Thus, the rays variously coloured red, orange, yellow, green,blue, indigo, violet, are each conducted to a different part ofthe screen. In this way the prism has the effect of exhibitingthe constitution of the composite beam of light. To us this nowseems quite obvious, but Newton did not adopt it hastily. Withcharacteristic caution he verified the explanation by many dif-ferent experiments, all of which confirmed his discovery. Oneof these may be mentioned. He made a hole in the screen atthat part on which the violet rays fell. Thus a violet ray was al-lowed to pass through, all the rest of the light being intercep-ted, and on this beam so isolated he was able to try further ex-periments. For instance, when he interposed another prism inits path, he found, as he expected, that it was again deflected,and he measured the amount of the deflection. Again he triedthe same experiment with one of the red rays from the oppos-ite end of the coloured band. He allowed it to pass through thesame aperture in the screen, and he tested the amount bywhich the second prism was capable of producing deflection.He thus found, as he had expected to find, that the secondprism was more efficacious in bending the violet rays than in

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bending the red rays. Thus he confirmed the fact that the vari-ous hues of the rainbow were each bent by a prism to a differ-ent extent, violet being acted upon the most, and red the least.

Not only did Newton decompose a white beam into its con-stituent colours, but conversely by interposing a second prismwith its angle turned upwards, he reunited the different col-ours, and thus reproduced the original beam of white light. Inseveral other ways also he illustrated his famous proposition,which then seemed so startling, that white light was the resultof a mixture of all hues of the rainbow. By combining painters'colours in the proper proportion he did not indeed succeed inproducing a mixture which would ordinarily be called white,but he obtained a grey pigment. Some of this he put on thefloor of his room for comparison with a piece of white paper.He allowed a beam of bright sunlight to fall upon the paperand the mixed colours side by side, and a friend whom hecalled in for his opinion pronounced that under these circum-stances the mixed colours looked the whiter of the two.

By repeated demonstrations Newton thus established hisgreat discovery of the composite character of light. He at onceperceived that his researches had an important bearing uponthe principles involved in the construction of a telescope.Those who employed the telescope for looking at the stars, hadbeen long aware of the imperfections which prevented all thevarious rays from being conducted to the same focus. But thisimperfection had hitherto been erroneously accounted for. Ithad been supposed that the reason why success had not beenattained in the construction of a refracting telescope was dueto the fact that the object glass, made as it then was of a singlepiece, had not been properly shaped. Mathematicians hadabundantly demonstrated that a single lens, if properly figured,must conduct all rays of light to the same focus, provided allrays experienced equal refraction in passing through the glass.Until Newton's discovery of the composition of white light, ithad been taken for granted that the several rays in a whitebeam were equally refrangible. No doubt if this had been thecase, a perfect telescope could have been produced by prop-erly shaping the object glass. But when Newton had demon-strated that light was by no means so simple as had been sup-posed, it became obvious that a satisfactory refracting

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telescope was an impossibility when only a single object lenswas employed, however carefully that lens might have beenwrought. Such an objective might, no doubt, be made to con-duct any one group of rays of a particular shade to the same fo-cus, but the rays of other colours in the beam of white lightmust necessarily travel somewhat astray. In this way Newtonaccounted for a great part of the difficulties which had hithertobeset the attempts to construct a perfect refracting telescope.

We now know how these difficulties can be, to a great extent,overcome, by employing for the objective a composite lensmade of two pieces of glass possessing different qualities. Tothese achromatic object glasses, as they are called, the greatdevelopment of astronomical knowledge, since Newton's time,is due. But it must be remarked that, although the theoreticalpossibility of constructing an achromatic lens was investigatedby Newton, he certainly came to the conclusion that the diffi-culty could not be removed by employing a composite objectivewith two different kinds of glass. In this his marvellous saga-city in the interpretation of nature seems for once to havedeserted him. We can, however, hardly regret that Newtonfailed to discover the achromatic objective, when we observethat it was in consequence of his deeming an achromatic ob-jective to be impossible that he was led to the invention of thereflecting telescope.

Finding, as he believed, that the defects of the telescopecould not be remedied by any application of the principle of re-fraction, he was led to look in quite a different direction for theimprovement of the tool on which the advancement of astro-nomy depended. The refraction of light depended, as he hadfound, upon the colour of the light. The laws of reflection were,however, quite independent of the colour. Whether rays be redor green, blue or yellow, they are all reflected in precisely thesame manner from a mirror. Accordingly, Newton perceivedthat if he could construct a telescope the action of which de-pended upon reflection, instead of upon refraction, the diffi-culty which had hitherto proved an insuperable obstacle to theimprovement of the instrument would be evaded.

For this purpose Newton fashioned a concave mirror from amixture of copper and tin, a combination which gives a surfacewith almost the lustre of silver. When the light of a star fell

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upon the surface, an image of the star was produced in the fo-cus of this mirror, and then this image was examined by amagnifying eye-piece. Such is the principle of the famous re-flecting telescope which bears the name of Newton.4 The littlereflector which he constructed, represented in the adjoiningfigure, is still preserved as one of the treasures of the RoyalSociety. The telescope tube had the very modest dimension ofone inch in diameter. It was, however, the precursor of a wholeseries of magnificent instruments, each outstripping the otherin magnitude, until at last the culminating point was attainedin 1845, by the construction of Lord Rosse's mammoth reflect-or of six feet in aperture.5

Newton's discovery of the composition of light led to an em-bittered controversy, which caused no little worry to the greatphilosopher. Some of those who attacked him enjoyed consid-erable and, it must be admitted, even well-merited repute inthe ranks of science. They alleged, however, that the elonga-tion of the coloured band which Newton had noticed was dueto this, to that, or to the other—to anything, in fact, rather thanto the true cause which Newton assigned. With characteristicpatience and love of truth, Newton steadily replied to eachsuch attack. He showed most completely how utterly his ad-versaries had misunderstood the subject, and how slight in-deed was their acquaintance with the natural phenomenon inquestion. In reply to each point raised, he was ever able to citefresh experiments and adduce fresh illustrations, until at lasthis opponents retired worsted from the combat.

4.Newtonian telescope.5.Leviathan of Parsonstown is the unofficial name of the Rosse six foottelescope. This is a historic reflecting telescope of 72 in (1.8 m) aperture,which was the largest telescope in the world from 1845 until the con-struction of the 100 in (2.5 m) Hooker Telescope in 1917. The Rosse sixfoot telescope was built by William Parsons, 3rd Earl of Rosse on his es-tate, Birr Castle, at Parsonstown (now Birr in County Offaly, Ireland). (ht-tp://en.wikipedia.org/wiki/Leviathan_of_Parsonstown)

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Chapter 3It has been often a matter for surprise that Newton,throughout his whole career, should have taken so muchtrouble to expose the errors of those who attacked his views.He used even to do this when it plainly appeared that his ad-versaries did not understand the subject they were discussing.A philosopher might have said, " I know I am right, and wheth-er others think I am right or not may be a matter of concern tothem, but it is certainly not a matter about which I needtrouble. If after having been told the truth they elect to remainin error, so much the worse for them; my time can be betteremployed than in seeking to put such people right." This,however, was not Newton's method. He spent much valuabletime in overthrowing objections which were often of a very fu-tile description. Indeed, he suffered a great deal of annoyancefrom the persistency, and in some cases one might almost sayfrom the rancour, of the attacks which were made upon him.Unfortunately for himself, he did not possess that capacity forsublime indifference to what men may say, which is often thehappy possession of intellects greatly inferior to his.

The subject of optics still continuing to engross Newton's at-tention, he followed up his researches into the structure of thesunbeam by many other valuable investigations in connectionwith light. Every one has noticed the beautiful colours manifes-ted in a soap-bubble. Here was a subject which not unnaturallyattracted the attention of one who had expounded the coloursof the spectrum with such success. He perceived that similarhues were produced by other thin plates of transparent materi-al besides soap-bubbles, and his ingenuity was sufficient to de-vise a method by which the thicknesses of the different filmscould be measured. We can hardly, indeed, say that a like suc-cess attended his interpretation of these phenomena to thatwhich had been so conspicuous in his explanation of the

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spectrum. It implies no disparagement to the sublime genius ofNewton to admit that the doctrines he put forth as to thecauses of the colours in the soap-bubbles can be no longer ac-cepted. We must remember that Newton was a pioneer in ac-counting for the physical properties of light. The facts that heestablished are indeed unquestionable, but the explanationswhich he was led to offer of some of them are seen to be unten-able in the fuller light of our present knowledge.

Had Newton done nothing beyond making his wonderful dis-coveries in light, his fame would have gone down to posterityas one of the greatest of Nature's interpreters. But it was re-served for him to accomplish other discoveries, which havepushed even his analysis of the sunbeam into the background ;it is he who has expounded the system of the universe by thediscovery of the law of universal gravitation.

The age had indeed become ripe for the advent of the geniusof Newton. Kepler had discovered with marvellous penetrationthe laws which govern the movements of the planets aroundthe sun, and in various directions it had been more or lessvaguely felt that the explanation of Kepler's laws, as well as ofmany other phenomena, must be sought for in connection withthe attractive power of matter. But the mathematical analysiswhich alone could deal with this subject was wanting; it had tobe created by Newton.

At Woolsthorpe, in the year 1666, Newton's attention ap-pears to have been concentrated upon the subject of gravita-tion. Whatever may be the extent to which we accept the moreor less mythical story as to how the fall of an apple first direc-ted the attention of the philosopher to the fact that gravitationmust extend through space, it seems, at all events, certain thatthis is an excellent illustration of the line of reasoning which hefollowed. He argued in this way. The earth attracts the apple ;it would do so, no matter how high might be the tree fromwhich that apple fell. It would then seem to follow that thispower which resides in the earth by which it can draw all ex-ternal bodies towards it, extends far beyond the altitude of theloftiest tree. Indeed, we seem to find no limit to it. At thegreatest elevation that has ever been attained, the attractivepower of the earth is still exerted, and though we cannot byany actual experiment reach an altitude more than a few miles

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above the earth, yet it is certain that gravitation would extendto elevations far greater. It is plain, thought Newton, that anapple let fall from a point a hundred miles above this earth'ssurface, would be drawn down by the attraction, and wouldcontinually gather fresh velocity until it reached the ground.From a hundred miles it was natural to think of what wouldhappen at a thousand miles, or at hundreds of thousands ofmiles. No doubt the intensity of the attraction becomes weakerwith every increase in the altitude, but that action would stillexist to some extent, however lofty might be the elevationwhich had been attained.

It then occurred to Newton, that though the moon is at a dis-tance of two hundred and forty thousand miles from the earth,yet the attractive power of the earth must extend to the moon.He was particularly led to think of the moon in this connection,not only because the moon is so much closer to the earth thanare any of the other celestial bodies, but also because themoon is an appendage to the earth, always revolving around it.The moon is certainly attracted to the earth, and yet the moondoes not fall down ; how is this to be accounted for ? The ex-planation was to be found in the character of the moon'spresent motion. If the moon were left for a moment at rest,there can be no doubt that the attraction to the earth wouldbegin to draw the lunar globe in towards our globe. In thecourse of a few days our satellite would come down on theearth with a most fearful crash. This catastrophe is averted bythe circumstance that the moon has a movement of revolutionaround the earth. Newton was able to calculate from theknown laws of mechanics, which he had himself been mainlyinstrumental in discovering, what the attractive power of theearth must be, so that the moon shall move precisely as we findit to move. It then appeared that the very power which makesan apple fall at the earth's surface, is the power which guidesthe moon in its orbit.

Once this step had been taken, the whole scheme of the uni-verse might almost be said to have become unrolled before theeye of the philosopher. It was natural to suppose that just asthe moon was guided and controlled by the attraction of theearth, so the earth itself, in the course of its great annual pro-gress, should guided and controlled by the supreme attractive

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power of the sun. If this were so with regard to the earth, thenit would be impossible to doubt that in the same way the move-ments of the planets could be explained to be consequences ofsolar attraction.

It was at this point that the great laws of Kepler became es-pecially significant. Kepler had shown how each of the planetsrevolves in an ellipse around the sun, which is situated on oneof the foci. This discovery had been arrived at from the inter-pretation of observations. Kepler had himself assigned no reas-on why the orbit of a planet should be an ellipse rather thanany other of the infinite number of closed curves which mightbe traced around the sun. Kepler had also shown, and hereagain he was merely deducing the results from observation,that when the movements of two planets were compared to-gether, the squares of the periodic times in which each planetrevolved were proportional to the cubes of their mean dis-tances from the sun. This also Kepler merely knew to be trueas a fact, he gave no demonstration of the reason why natureshould have adopted this particular relation between the dis-tance and the periodic time rather than any other. Then, too,there was the law by which Kepler, with unparalleled ingenu-ity, explained the way in which the velocity of a planet varies atthe different points of its track, when he showed how the linedrawn from the sun to the planet described equal areas aroundthe sun in equal times. These were the materials with whichNewton set to work. He proposed to infer from these the actuallaws regulating the force by which the sun guides the planets.Here it was that his sublime mathematical genius came intoplay. Step by step Newton advanced until he had completelyaccounted for all the phenomena.

In the first place, he showed that as the planet describesequal areas in equal times about the sun, the attractive forcewhich the sun exerts upon it must necessarily be directed in astraight line towards the sun itself. He also demonstrated theconverse truth, that whatever be the nature of the force whichemanated from a sun, yet so long as that force was directedthrough the sun's centre, any body which revolved around itmust describe equal areas in equal times, and this it must do,whatever be the actual character of the law according to whichthe intensity of the force varies at different parts of the

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planet's journey. Thus the first advance was taken in the expos-ition of the scheme of the universe.

The next step was to determine the law according to whichthe force thus proved to reside in the sun varied with the dis-tance of the planet. Newton presently showed by a most su-perb effort of mathematical reasoning, that if the orbit of aplanet were an ellipse and if the sun were at one of the foci ofthat ellipse, the intensity of the attractive force must vary in-versely as the square of the planet's distance. If the law hadany other expression than the inverse square of the distance,then the orbit which the planet must follow would not be an el-lipse ; or if an ellipse, it would, at all events, not have the sunin the focus. Hence he was able to show from Kepler's lawsalone that the force which guided the planets was an attractivepower emanating from the sun, and that the intensity of this at-tractive power varied with the inverse square of the distancebetween the two bodies.

These circumstances being known, it was then easy to showthat the last of Kepler's three laws must necessarily follow. If anumber of planets were revolving around the sun, then suppos-ing the materials of all these bodies were equally affected bygravitation, it can be demonstrated that the square of the peri-odic time in which each planet completes its orbit is propor-tional to the cube of the greatest diameter in that orbit.

These superb discoveries were, however, but the starting-point from which Newton entered on a series of researches,which disclosed many of the profoundest secrets in the schemeof celestial mechanics. His natural insight showed that not onlylarge masses like the sun and the earth, and the moon, attracteach other, but that every particle in the universe must attractevery other particle with a force which varies inversely as thesquare of the distance between them. If, for example, the twoparticles were placed twice as far apart, then the intensity ofthe force which sought to bring them together would be re-duced to one-fourth. If two particles, originally ten miles asun-der, attracted each other with a certain force, then, when thedistance was reduced to one mile, the intensity of the attrac-tion between the two particles would be increased one-hundred-fold. This fertile principle extends throughout thewhole of nature. In some cases, however, the calculation of its

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effect upon the actual problems of nature would be hardly pos-sible, were it not for another discovery which Newton's geniusenabled him to accomplish.

In the case of two globes like the earth and the moon, wemust remember that we are dealing not with particles, but withtwo mighty masses of matter, each composed of innumerablemyriads of particles. Every particle in the earth does attractevery particle in the moon with a force which varies inverselyas the square of their distance. The calculation of such attrac-tions is rendered feasible by the following principle. Assumingthat the earth consists of materials symmetrically arranged inshells of varying densities, we may then, in calculating its at-traction, regard the whole mass of the globe as concentrated atits centre. Similarly we may regard the moon as concentratedat the centre of its mass. In this way the earth and the mooncan both be regarded as particles in point of size, each particlehaving, however, the entire mass of the corresponding globe.The attraction of one particle for another is a much moresimple matter to investigate than the attraction of the myriaddifferent points of the earth upon the myriad different points ofthe moon.

Many great discoveries now crowded in upon Newton. Hefirst of all gave the explanation of the tides that ebb and flowaround our shores. Even in the earliest times the tides hadbeen shown to be related to the moon. It was noticed that thetides were specially high during full moon or during new moon,and this circumstance obviously pointed to the existence ofsome connection between the moon and these movements ofthe water, though as to what that connection was no one hadany accurate conception until Newton announced the law ofgravitation. Newton then made it plain that the rise and fall ofthe water was simply a consequence of the attractive powerwhich the moon exerted upon the oceans lying upon our globe.He showed also that to a certain extent the sun produces tides,and he was able to explain how it was that when the sun andthe moon both conspire, the joint result was to produce espe-cially high tides, which we call " spring tides " ; whereas if thesolar tide was low, while the lunar tide was high, then we hadthe phenomenon of "neap" tides.

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But perhaps the most signal of Newton's applications of thelaw of gravitation was connected with certain irregularities inthe movements of the moon. In its orbit round the earth oursatellite is, of course, mainly guided by the great attraction ofour globe. If there were no other body in the universe, then thecentre of the moon must necessarily perform an ellipse, andthe centre of the earth would lie in the focus of that ellipse.Nature, however, does not allow the movements to possess thesimplicity which this arrangement would imply, for the sun ispresent as a source of disturbance. The sun attracts the moon,and the sun attracts the earth, but in different degrees, and theconsequence is that the moon's movement with regard to theearth is seriously affected by the influence of the sun. It is notallowed to move exactly in an ellipse, nor is the earth exactly inthe focus. How great was Newton's achievement in the solutionof this problem will be appreciated if we realise that he notonly had to determine from the law of gravitation the nature ofthe disturbance of the moon, but he had actually to constructthe mathematical tools by which alone such calculations couldbe effected.

The resources of Newton's genius seemed, however, to proveequal to almost any demand that could be made upon it. Hesaw that each planet must disturb the other, and in that way hewas able to render a satisfactory account of certain phenomenawhich had perplexed all preceding investigators. That mysteri-ous movement by which the pole of the earth sways aboutamong the stars had been long an unsolved enigma, but New-ton showed that the moon grasped with its attraction the pro-tuberant mass at the equatorial regions of the earth, and thustilted the earth's axis in a way that accounted for the phe-nomenon which had been known but had never been explainedfor two thousand years. All these discoveries were brought to-gether in that immortal work, Newton's "Principia."

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Chapter 4Down to the year 1687, when the "Principia" was published,

Newton had lived the life of a recluse at Cambridge, being en-tirely occupied with those transcendent researches to whichwe have referred. But in that year he issued from his seclusionunder circumstances of considerable historical interest. KingJames the Second attempted an invasion of the rights and priv-ileges of the University of Cambridge by issuing a commandthat Father Francis, a Benedictine monk, should be received asa Master of Arts in the University, without having taken theoaths of allegiance and supremacy. With this arbitrary com-mand the University sternly refused to comply. The Vice-Chan-cellor was accordingly summoned to answer for an act of con-tempt to the authority of the Crown. Newton was one of ninedelegates who were chosen to defend the independence of theUniversity before the High Court. They were able to show thatCharles the Second, who had issued a mandamus under some-what similar circumstances, had been induced after due con-sideration to withdraw it. This argument appeared satisfactory,and the University gained their case. Newton's next step inpublic life was his election, by a narrow majority, as memberfor the University, and during the years 1688 and 1689, heseems to have attended to his parliamentary duties with con-siderable regularity.

An incident which happened in 1692 was apparently thecause of considerable disturbance in Newton's equanimity, ifnot in his health. He had gone to early morning chapel, leavinga lighted candle among his papers on his desk. Tradition as-serts that his little dog "Diamond" upset the candle; at allevents, when Newton came back he found that many valuablepapers had perished in a conflagration. The loss of thesemanuscripts seems to have had a serious effect. Indeed, it hasbeen asserted that the distress reduced Newton to a state of

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mental aberration for a considerable time. This has, appar-ently, not been confirmed, but there is no doubt that he experi-enced considerable disquiet, for in writing on September 13th,1693, to Mr. Pepys, he says:

" I am extremely troubled at the embroilment I am in, andhave neither ate nor slept well this twelvemonth, nor have myformer consistency of mind."

Notwithstanding the fame which Newton had achieved bythe publication of his "Principia," and by all his researches, theState had not as yet taken any notice whatever of the most il-lustrious man of science that this or any other country has everproduced. Many of his friends had exerted themselves to pro-cure him some permanent appointment, but without success. Ithappened, however, that Mr. Montagu, who had sat with New-ton in Parliament, was appointed Chancellor of the Exchequerin 1694. Ambitious of distinction in his new office, Mr.Montagu addressed himself to the improvement of the currentcoin, which was then in a very debased condition. It fortunatelyhappened that an opportunity occurred of appointing a new of-ficial in the Mint; and Mr. Montagu on the 19th of March,1695, wrote to offer Mr. Newton the position of warden. Thesalary was to be five or six hundred a year, and the businesswould not require more attendance than Newton could spare.The Lucasian professor accepted this post, and forthwithentered upon his new duties.

The knowledge of physics which Newton had acquired by hisexperiments was of much use in connection with his duties atthe Mint. He carried out the re-coinage with great skill in thecourse of two years, and as a reward for his exertions, he wasappointed, in 1697, to the Mastership of the Mint, with a salarybetween £1,200 and £1,500 per annum. In 1701 his duties atthe Mint being so engrossing, he resigned his Lucasian pro-fessorship at Cambridge, and at the same time he had to sur-render his fellowship at Trinity College. This closed his connec-tion with the University of Cambridge. It should, however, beremarked that at a somewhat earlier stage in his career he wasvery nearly being appointed to an office which might have en-abled the University to retain the great philosopher within itsprecincts. Some of his friends had almost succeeded in secur-ing his nomination to the Provostship of King's College,

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Cambridge ; the appointment, however, fell through, inasmuchas the statute could not be evaded, which required that theProvost of King's College should be in holy orders.

In those days it was often the custom for illustrious mathem-aticians, when they had discovered a solution for some newand striking problem, to publish that problem as a challenge tothe world, while withholding their own solution. A famous in-stance of this is found in what is known as the Brachistochroneproblem, which was solved by John Bernouilli. The nature ofthis problem may be mentioned. It was to find the shape of thecurve along which a body would slide down from one point (A)to another point (B) in the shortest time. It might at first bethought that the straight line from A to B, as it is undoubtedlythe shortest distance between the points, would also be thepath of quickest descent; but this is not so. There is a curvedline, down which a bead, let us say, would run on a smoothwire from A to B in a shorter time than the same bead wouldrequire to run down the straight wire. Bernouilli's problem wasto find out what that curve must be. Newton solved it correctly;he showed that the curve was a part of what is termed a cyc-loid—that is to say, a curve like that which is described by apoint on the rim of a carriage-wheel as the wheel runs alongthe ground. Such was Newton's geometrical insight that hewas able to transmit a solution of the problem on the day afterhe had received it, to the President of the Royal Society.

In 1703 Newton, whose world-wide fame was now estab-lished, was elected President of the Royal Society. Year afteryear he was re-elected to this distinguished position, and histenure, which lasted twenty-five years, only terminated withhis life. It was in discharge of his duties as President of theRoyal Society that Newton was brought into contact withPrince George of Denmark. In April, 1705, the Queen paid avisit to Cambridge as the guest of Dr. Bentley, the then Masterof Trinity, and in a court held at Trinity Lodge on April 15th,1705, the honour of knighthood was conferred upon the dis-coverer of gravitation.

Urged by illustrious friends, who sought the promotion ofknowledge, Newton gave his attention to the publication of anew edition of the "Principia." His duties at the Mint, however,added to the supreme duty of carrying on his original

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investigations, left him but little time for the more ordinarytask of the revision. He was accordingly induced to associatewith himself for this purpose a distinguished young mathem-atician, Roger Coates, a Fellow of Trinity College, Cambridge,who had recently been appointed Plumian Professdr of Astro-nomy. On July 27th, 1713, Newton, by this time a favourite atCourt, waited on the Queen, and presented her with a copy ofthe new edition of the " Principia."

Throughout his life Newton appears to have been greatly in-terested in theological studies, and he specially devoted his at-tention to the subject of prophecy. He left behind him amanuscript on the prophecies of Daniel and the Apocalypse ofSt. John, and he also wrote various theological papers. Manyother subjects had from time to time engaged his attention. Hestudied the laws of heat; he experimented in pursuit of thedreams of the Alchymist; while the philosopher who had re-vealed the mechanism of the heavens found occasional relaxa-tion in trying to interpret hieroglyphics. In the last few years ofhis life he bore with fortitude a painful ailment, and onMonday, March 20th, 1727, he died in the eighty-fifth year ofhis age. On Tuesday, March 28th, he was buried in Westmin-ster Abbey.

Though Newton lived long enough to receive the honour thathis astonishing discoveries so justly merited, and though formany years of his life his renown was much greater than thatof any of his contemporaries, yet it is not too much to say that,in the years which have since elapsed, Newton's fame has beenever steadily advancing, so that it never stood higher than itdoes at this moment.

We hardly know whether to admire more the sublime discov-eries at which he arrived, or the extraordinary character of theintellectual processes by which those discoveries werereached. Viewed from either standpoint, Newton's " Principia "is incomparably the greatest work on science that has ever yetbeen produced.

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Attributions

The cover image is from http://commons.wikimedia.org/wiki/File:GodfreyKneller-IsaacNewton-1689.jpg in the public do-main. This image is in the public domain in the UnitedStates, and those countries with a copyright term of life of theauthor plus 100 years or less.

The text was taken from Great Astronomers, by Sir RobertStawell Ball (1840-1913), second edition(1907),at http://archive.org/details/greatastronomers00balluoft .

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