A Letter from the Reverend Mr. James Bradley Savilian Professor of Astronomy at Oxford, and F.R.S....

A Letter from the Reverend Mr. James Bradley Savilian Professor of Astronomy at Oxford,and F.R.S. to Dr.Edmond Halley Astronom. Reg. &c. Giving an Account of a New DiscoveredMotion of the Fix'd Stars.Author(s): James BradleySource: Philosophical Transactions (1683-1775), Vol. 35 (1727 - 1728), pp. 637-661Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/103725 .

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( 636 ) was always fotnething warlting tQ render the SucceSs of the Operation certainX Tlze w}<ole Secret then of Blanching lies entirely in the PreDaration of this Sllet & And tllis lle at laR diScovered tQ confi{} only in firR -frying and burning it; wllich not only glves it t11e Cow lourX but puts it lnto a Condition to give the Iron a DiEpofition to be tinnedX whicll it does furpri(inglsr The Tin itSelf ougllt to have a certain Degree of EIeat > for if it is not Ilor enough it will not Itick to tlle Iron; sf it is too hors it will cover it witll too thin a Coat, and rlle Plates will have feveral ColoursX as a Mixture of reds blue, and yellow) and the whole appear of a villain ous yello-w CaR. To presrent tllis, by knowing when the 'rin has a proper Degree of Heat, tlley tnight firl} anake an ESay witli ftnall Pieces of rlle fcaled Platessand they would le.arn from theln when the Tin is in proper Order: But generally fpeaking, tlley dip the Plates inW to Tin that is more or lefs hot, according to the Thick nefW they would have the Coat to be of. Some Plates they::onlygiveone Layerto, and tlleSetlleyplunge inw to Tin, that has a lefl>er Degree of Heat than--that irz_ to wllich they plunge thofe Plates which tlley nvould have take two Layers; as alrO sxrhen tlley gisre thefe rhe fecond Layer, they put tllern into Tin that has not fo great a Degree of Heat, as that into which they were put tlle firl Time : Befides Wllich) it is to be obirved, that the Tin, which is to give tlle fecond Coat, ought to lte freEh covered svith Suer,but only with the cotutnon Sort wlthout Preparation i for melted Tin is fuffici ently difpoSed to attacll it felf to folid Tin ; and in this Cafe it is to tin itSelf, to s^rIzicll tlle new Tin is to be ioined. As to the Choice of the Tln, the Man- wr of making it i6 a-s bright as poil1blg wlth a Nulu-

ber

( 636 ) was always fotnething warlting tQ render the SucceSs of the Operation certainX Tlze w}<ole Secret then of Blanching lies entirely in the PreDaration of this Sllet & And tllis lle at laR diScovered tQ confi{} only in firR -frying and burning it; wllich not only glves it t11e Cow lourX but puts it lnto a Condition to give the Iron a DiEpofition to be tinnedX whicll it does furpri(inglsr The Tin itSelf ougllt to have a certain Degree of EIeat > for if it is not Ilor enough it will not Itick to tlle Iron; sf it is too hors it will cover it witll too thin a Coat, and rlle Plates will have feveral ColoursX as a Mixture of reds blue, and yellow) and the whole appear of a villain ous yello-w CaR. To presrent tllis, by knowing when the 'rin has a proper Degree of Heat, tlley tnight firl} anake an ESay witli ftnall Pieces of rlle fcaled Platessand they would le.arn from theln when the Tin is in proper Order: But generally fpeaking, tlley dip the Plates inW to Tin that is more or lefs hot, according to the Thick nefW they would have the Coat to be of. Some Plates they::onlygiveone Layerto, and tlleSetlleyplunge inw to Tin, that has a lefl>er Degree of Heat than--that irz_ to wllich they plunge thofe Plates which tlley nvould have take two Layers; as alrO sxrhen tlley gisre thefe rhe fecond Layer, they put tllern into Tin that has not fo great a Degree of Heat, as that into which they were put tlle firl Time : Befides Wllich) it is to be obirved, that the Tin, which is to give tlle fecond Coat, ought to lte freEh covered svith Suer,but only with the cotutnon Sort wlthout Preparation i for melted Tin is fuffici ently difpoSed to attacll it felf to folid Tin ; and in this Cafe it is to tin itSelf, to s^rIzicll tlle new Tin is to be ioined. As to the Choice of the Tln, the Man- wr of making it i6 a-s bright as poil1blg wlth a Nulu-

ber



( 637 ) ber of littleArticles necelEary to thePra&iceXthe Attho refers them to another TilneX as lnore properly belongv ing to tl1e Defcription of the wh-ole Art, than to a WIetnoir in uthicll lle ot}ly gives tlle Principles of it.

IV. X Letter from the !telered Mr. James Brad ley Sauilian fProfelfor of AJ}ronomy at Oxford} ctnd F*g?^ S to 1)r.Edtnond Halley A{Eronoms Reg. &c. gitt}Zg an aC087Zt of a nev dst- colered l!Motion of the Fix'd Stars.

S I R, 0 U I1aving been pleafed to exprefs your S.-itis- 1 faEion with what I had an Opportunity folne

time .ngo, of telling you in ConverEation) concerning fome ObSerrration¢, that were tnaking by olar late wor thy and ingenious Friend the laonourable Svm?wel Mo1ynew ESquire, and wllich llaxre fiace been conti

nued and repeated by Iny felfv in order to detertnine tlle MParallax of the fxt St6lrr ; I Skall now beg 1eave to lay before you a tnoIe particular Account of tlletn.

Before I proceed to give you the HiRory of the Obe fervations themfelvesX it may be proper tO let 5't.2 knouz, that they were atfirll begtln in locpes of srrifying and confirming tllofe) that Dr. Hook fortnerly co-,lnunica.t^- ed to tlle publickX whiclu feetned to lte attrzrlded srith circatnaances vllat prolriid greater Exa&nefs lrl tlleme than could be expeAed in any other) tl<at llad bAen made andpublifhed onthe fame AccoWlnt. And as llis Atteinpt was xvhat principally ga!e Rife to t}li¢3 fo llis Method ln tnaking the Oh*etv;ations xJas in fome

Et?ts



( 638 )

Aleafore tlzat xvllich Mr. Molye8x followed: For lae Inade Clloice of the fame Start and his lnltruTnenle

was conLtruAed upon altnoR tile fatne Principles. 12ut if it llad not greatly exceeded tlle Dv2or's in P:x adtrlefs, Ns e lMi,sllt yet 11ave relnained itl grear Unceid

tainty as to tlle SParall4softhe lExt Stars i as you will perceive upon the Col-np-lriCon of the two Experitnent¢¢

Tllis indeed was chiefly ossting to our curious MeLn- ber, WIr. George GrahamX to N7llotn tlae Lovers of Aflronotzly are alfo not a little indebted for feareral O ther e;ad and utell-contrisred InRruments. TIle N>- ceS1ty of fuch will fcarce be difputed by thofe tlaat h.ave had any E;xperience in tnakinD Altronotnical Obw fervationsj and the Inconr1{tencar, Wl1icl1 is to be Inet xvith alnong differwnt lAutllors in their Atternpts to detertnine fTllall ngleCs particularly tlle annual Paralw Gax of tllefixt Stars) tnay be a filfficient Proof of it --to others. Their DiSagreement indeed in this Article is not noxv CO mucl1 to be wondered at, ftnce I doubt IlOt, but it will appear very probable, that the Inb lEruments comrnonly made uSe of by theln, were liable to greater Errors thatl many times that Pa- rallax \vill amoullt to.

The Succefs then of this Experiment evidently depetlding very much on the Accuratenefs of the Irl {trumert that ivas principally to be taken Care of: In xvhat Manner this xvas dotle, is not my prefent PurpoSe tO tell yoll; but if from the Refult of rhe Oblervatiolls xvhich I nor felld you, it Illall be 3udged nece{Eary to conlmunicate to tlle Curious the Manner of makirlg them, I may hereafter perhaps give them a particular DeScription, not only cf NIr. Molyttetwx's IllIlrumes, but alSo os llzy o>s,

sv-hich



( 639 ) which hath fi1llce been ereded for the fanze Purpofe md upon the like Principles, though it 15 rOme\Shat diSerent in its Conflrudtions for a Reafon you will wect with preSently

Mr. Molyneus's Spparatgs was compleated and Etted for obServig about the End of Novesnber I7z5, and on the third Day of iDecem6er following, the bright Star in ehe Head of 5Draro (marked > by Bayer ) ras for the firft Time obServed) as it paffied near the i:enith,- alld itS Situation carefully taken w-ith the Inflrument. The like ObServations wrere made on the sth, Ixth, and xwth Days of the fame Month, and there appearing no material DifFerence in the Place of the Star, a farther Repetition of them at ehis Seafon Seemed needlefs, it being a Part of the Year, wntherein no fenf1ble Al-teration of Parallax in this Star could foon be exped;ted. Ic xvas chiefly therefore Curiollty that tempted me (being then at

Kew, where the Infirument was fixed) to prepare for obServing the Star on S:Derem6er t7th) xvlaen having adjutted the Inttrument as uSual, I perceived that it paSed a little more Southerly this Day than when it was obferved before. Slor fufpeding any other GauSe of this Appearance, xve frft concluded, tghat it was owing to the Uncertaitlty of tlle ObServ vations, alld that etther tllis or the foregoing x^tere not fo exadt as we had before fuppofed; for svhich Reafotl we purpoXed to repeat thc ObServation again, in order to deterazine from whe-nce this D-iSerence proceeded; and upon doing it on (13ecetnber zothX I found tllat the Star paSed ftill more Sou-therly than tn the former ObServations. This fenllS!e Alteration the more Surprized usX in that it w:-as tlze cosu¢trary

\-t; 3,y



( 64° ) way {lotn ̂ hat it would have beens had it prbz cceded from arl annual Parallax of the Star: But being now pretty well fatisfied, that it could not be

wely owing to the want of 13xadEtlefs in the Ob-_ fervations; and having no Notion of any thing elfe, tllat could cauSe Such a11 apparent Motion as thts in the Star; wre began to think that fome rChange in the Materials, Gc. of ehe Infirument itEelf, might lave occafioned it. Under thefe Apprehenfions ute rellaaine-d feme time, but being at length fully con- arinced, by feveral Trials, of the great ExadtneSs of the Irlfirllment, and finding by the gradual IncrcaSe of th;e Stars DiRat}ce from the Pole, that there muR lJe Sorne regular-CauSe that produced it; we took care to exaInine DiceJys at the Time c+;each ObEer vation, how much it utas: and about the Begini]tzg of Marah t7z6, the Star was found to be to more Southerly thatl at the Time of the lSrR ObEervation. 1t nosr indeed feetned to have arrived at itS utmoft

LiInit Southsstard, becauSe in Several TriMs made a- bout this Time, no Senflble Diflierence was-obEerlred in its Situation. By the Middle of Spril it appearw ed to be returllilzg back again towards the North ; and about the Beginlling of 7ane, it pa{Ned at the fame lDillance from the tZetlith as it had done in Deremv bes; z?hell it was firR obEcrved.

From the quick Altexaiiosll of this Stat's Declinaw ation about this Tilne (it increafing a $econd in

three Days ) it was concluded, that it would llOW proceedS7orthw-ard, as it before had gone Southward of its- preSent Situatioll g and it happened as was coll- JeEtured: for the Star continued to moYe Northward |ffi1 Sepreber following, hetl it again became lta

txonaryX



( 641 ) tionary, being thetl ncar zott more Nortleriy tFsa- is yse, and rso lefs than 39" nzore Nortllerly- than it was in MarahF From SePtember the Star rertlrnedl towards the South} till 1r arrived in q:)eresn6er ro the falne Situation it xvas ill at that time tvelve Months, allowing for the Diffierence of Deciination on accollnt of the PreceMon of th-e Equinox.

This xvas a Sudicient Proofv that the In{Eralment had not been the CauSe of this apparent Motion nf the Star, a£ld to find one adequate eo fach an Effed feemed a Diiculty. A Nutation of the Eartil8s Axis 557as one of the firR thillgs that oSered icKelit upon this Occafton, bllt it was Soon found to be irlfuicient n for though it might have accounted for the changeofDeclination in > 7Oraroniw yer it would not at the -fame time agree xvith tbe Phznonzena in. other Stars ; particularly in a fmall one almoA oppofite ill right ACcenfion to > fZ)racoXis, at about the fame DiAance from the North Pole of the Equator: For, though this Star feemed to move the fanze xsay, as a Nalt-atiorl of the Eartll's Axis xrotlld 11ave Inade its yet it chat3g,ing ies Declinatto£s .but abottt half as much as > tDrar01^lgs ln the ianwe ttme ( as appeared uponcomparingthe Obfe<vatsens of beth made upon the fame Days, at dxirent Seatonis of the Year ) this platnly proved) that the ap3;w^rent Motion of the S£S.S Nx,!aS tlot occaXioned by a real Ntltatiotl t1llce iiS that had bee£w the CauSex the Aiteratioll in both tStArS

xrould hvve been near equal. The great Regulatity of tiwe ObServations lelk ao

roorn to doubt, but t1iat there XVaS torslU<- regtllar caue tlzat prodfuced tt;s unexpeded NIotlotl, u; hiclr diad not depend on the {JIcCtt3.ty or Vraety of }1¢

t Q q (AA S;rafour



( 642 ) Seafons of the Year. Upon comparing the ObSer--- vations with each other, it was diScovered, that in both the fore-mentioned Stars, the apparent Dif- ference of Declination from the Maxisna, was aI- ways nearly proportional to the lrerEed Sine of the Sun's DiRance from the Equinodtial Points This wras an Inducemetlt tO think, that the CauSe, whatever it was, had fome 1?elation tO the Sun's Sitasa- tion nvith refipedr tO thofe Points. But not being able tO frame any Hypotheis at that Time, fufli- cient to folve all the Phznomena, and beirlg very def1rous to fearch a little farther into this Matter, I began to think of credring an InItrument for mt Self at EYanJ?ed, that having it always at FIand, I Jnight xvith the more EaKe and Certainty, enquire into the Laxvs of this new Motion. The Confide- ration likeuSife of being able by another In{irument, to confirm the Truth of the ObServations hitherto made with Mrv MoSynenc's, was rio fmall Induce ment to me; but the Chief of all sras, the Oppor- tunity I £aould thereby have of trying, in what Manner other Stars were aSeded by the Iame CauSe, xvhatever It utas. For Mr. Mo.lywe2nc's InIlrument being originally defigned for obEerving > i)raconis ( in order, as I faid betore, to try xvhether it had any fenfible Parallax ) was fo contrived, as to be capabJe of but littie Alteration in itS Diredrion, llot above Seven or eight Minutes of a Degree : alld there being fexv Stars within half that DiRance from the Zenith of Kev, bright enough to be well obEervedX hc could not, with his laRrtlments throughly examine hosr this CauSe aSedted Stars diSerently fltuated vvith

reEpedr



( 643 )

reSped to the equinod;tial and folflitial Points of t1<e Ecliptick.

TheSe Con&derations determined me; and by the Contrivance and OireXtion of the fame illgentous Perion, Mr. Graharw, my Illllrument was fixed up ,g x9, s7X7. As I had no convellient Place wherfe I could make uSe of fo long a TeleScope as Mr. Mo/ynex's, ! contented my fielf xvith one of but little more than half the Length of his (vix. of about Itt- Fes, his being 144) judging from eheX Bxperience xvhich I had already had, that this Ra- dius would be long enough to ad jull the InRrument to a fufTicient Degree of Exadnefs, and I have had nt Reafon fince to change xny Opinion: for from all the Trials I have yer made, I as very well fatisfieds that when it is carefully redtiSed, itS Situation may

be fecurely depended upon to half a Second. As the I?lace where my InArument was to be hung, iu fome MeaSure determined itS Radius, fo did it alSO rhe Length of the Archj or Limb, on which the Divifions w>ere made tO adjuR it: For the Arch could not con- ^7eniently be extended farther, than tO reach to about 6s° on each Side my Xenith. This indeed was Suffi- cient, fillce it gave me an Opportunity of making Choice of feveral Stars, very difEerellt both in Mag- nitude and Situation; there being more thals two hundred inSerted irs the BritiJb Catalogue,that may be obServed with it. I neededtlot to have extended the kimb fo far, but that I was willing to take in Cape/Ea, the ouly Star of the {;rR Magnitude that comes fo, near my Zenith.

My InRrument being fixed, I- immediateIy begaIl to obServe fuch Stars as I judged moR proper to

Qq q q ? gisc



( 644 ) gse -Xut Jighe nto the CauSe of the Motion already nacntioned. Tllere was Variety enoalgh of Email ones; -atld not ieSs than twelve, ehat I collld obEerve rllrougb all the ;eafons of ehe \tear; they- being bright erlough to be feen in the Day-timeX when IleareR the Su£1^ I had not been long obSexvillg, be- fioxe I perctived that the Notion we had betore env tertained of tllC starS beingEartheR lSorth and South, rhen the Sun was about the Equinoxes, was only trut ofthoSe that w ere near the fol0tial Colure: And after I had continued my Obforvations a few Mo-nths, I diScovered, what I-then apprehended to be a genew ral Lasr, obEerved by all the Stars, viz. That each of them became {tationaryX or WaS fartheR North or South, wlletI they paSed over my Zenith at fix of the Clock, either in the Morning or Evenillg. I perceived {ikexvife, that whatever Situation the Stars were in witlS reSpedEt to the cardinal Points of the Ecliptick, the apparent Motion of every one tend- ed the fame Way, when they paSed my In&rument about the fame Hour of the Day or Night ; for they all luoved Southward, while they paed in the DayX and Northward in the Night; fo that each- xvas far- theft lKlorFh, xvhen it came about Six of the Clock in the Evening, and firtheIt South, wllen it came a bout Six in the Mortling.

Though I ha^7e fince diScovered, that the Matitna in mof.t of thefe Stars do not happen exadtly whexl they come to my InRrun:lent at thofe Hours, yet nor being able at that time tO prove the contrary, and fuppofing that they did, I endeantoured tO ISnd out what Proportion the greateR Alterations of Decli- narioil in diSerent Star-s bore to each other; it being

* \-ery



( 45 ) rery evidents that they did not a1t change their I)eb clination eqllally. I have before taken notice, tllat it appeared from Mr. Malgneanc's ObServatiols7 that > iOraaonis altered its Declination about -tWiCE as much as the fore-mentioned frnall Star almoft op_ pof1te tO it; but examining ehe matter more particu larly, I found that the greate(tAlteration of Deciina- tion in thefe Stars, was as the Sine of the Latieude of each refpedEtively. This made me fuMpedr that there nzight be the like Proportion between the Maximoz of other Stars; but finding, that the Ob Servations of fome of them would not perfedtly correfpond with fuch an Hypothefis, and not knowing whether the fmall DiSererlce I mer with, might noc be owing tO the Ullcertainty and Exror of the Ob*- fervations, I deferred the farther Examination into the Truth of this Hypothef1s, till I nould be fur- nifhed with a Series of ObServa-tiorls made in all Parts of the Year; which might enable meX noe ouly to determine what Errors the ObEerva- tions are liable to, or how far they may fafely be depended upon; but alSo to judge, wheeher there had been any Senfible Change in the Parts of the InRret ment itEelf.

Upon thefeConfiderations, I Iaid afide all Thougtlts at that Time about ehe CauSe of the fore-nzentioned Phsenomena, hopil}g that I llzeald the eafter diScovet it, when I was better provided with proper Means tO determine more preci-fely what they s^tere.

When the Year xras compleated; I began ro examine and sompare my ObServatiolls, and Ilavillg pretz

ty Well fatisEed my felf as to the gelleral Lavvs of tiue sPhaomena, 1 then endeavours-d to find out the

Caufe



( 646 ) CauSe of them. I nvas already convinced, that the apparent Motion of the Stars was not oWing to a ^3utation of the Earth's Axis. The next Thing that ofEered itWelf, ^7as an Alteration in the Dired;tion of the Plumb-line, utith wthich the Inflrument was con- Rantly redified; but this upon Trial proved inSuS- cient. Then I colilldered xvhat Refrad;tion might do, but here alfo nothing fatisfaftory occurred. At laR X conjeEtured, that all the Thnomexa hitherto mentioned, proceeded from the progrefrlVe Motion of Lsght and the Earth's a-anual Motion in itS Orbit. For I perceived, thatX if Light was prc)pagated in Time, the apparent Place of a fixt Objeds would nor be ehe fame when the Eye is Xt Re*, as when it is moving in any other Diredion, than that-of theLine patWing through the Eye and Obj-e&; and that, when the Eye is moving in diSerent Diredtions, the appa rent Place of the Objedt wollld be diSerent.

C I confizdered this Matter in the fol *< lowing Manner. I imagined CA to / \ be a Ray of Light, falling perpencii- - \ <:ularly uponth¢Line BD; thell if / . \ the Eye is at reIl at As the Objed;t { \ muIt appear in the Dired;tion A C, ! " \ xthether Light se propagated in Time t * \ or in an InItant. Buit if the E:ye is j -. \ moving from lJ towards A, and Light fi - \ is propagated in Time, with a Velo 8 - \ city that is to the Velocity of the \ ¢ \ Eye) as C A- to B A; then Light mov-

DL;X3, ing firom C to A, whilR the Eye moves Som B to A, that Particlt of

atZ



( 647 ) it, by urhich the ObjeA will be diScerned, when the F;ye in its Motion comes to A, is at C when the Eyeis atB. Joining the Points B,C, I fuppoSed the Line CB, to be a Tube (inclined to the Line B D in

the Angle D B C ) of fiuch a Diameter, as to admic of but olle Particle of Light; tllen it was cary to conceive, that the Particle of Light at C (by which the (:)bjeA muI} be feen wilen the Eye, as ic moves along, arrives at A) would paSs through the Tube B C, if it is inclined to B D in the Ale D B C, andaccompanies the Eye in itS Motion from B toA; arld that it could not come to the Eye, placed behind fuch a Tube, if ie had any other Inclination tO the ldine BD. If inRead of luppefing CB fo fmall a Tube, we imagine it to be the Axis of a larger; the for the farne ReaSon, the Particle of Light at C,could or paSs through that Axis, unlefs it iS inclined to B D,

in the Angle CBD. In like manner, if the Eye moved the contrary ways from D tonvards A, xvith the fame Velocity ; tcN the Tube muR be illclined in the Angle B D C. Although therefore the true or

real Place of an Objeds is perpendicular to the Line in which the Eye is moving, yet the vifible Place Will BOt be fo, fince that, no doubtX muR be in the Diredrion of the Tube; but the Diffierence betw-een the true and apparent Place will be (cereris pari6Xw) greater or lefs, according tO the diflRerent Proportio3 between the Yelocity of Light and rl]Xtof the Byee So that if we couldluppofe that Light x^Zas propagated in an Inflarlt, then there wtould be tRO DiSerence between the real and vifible Place of an Objed, altheE the Eye were in Motion, for in that caSe, AC be- zllg inEnite snrith ReMpedt tO A B, the Angle A. C B (the

2 D i t



( 6G8 )

ference between th--e true and vifible Place)- vianiffies But if Light be propagated in Time (which I preSume will readily be allowed by moR of the PhiloSophers of ehis Age3 rllen it is evodent from the foregoing Confiderations, that there will be always a DiSetence between the real and viFlble Place of StR Objed, un- lels the Eye is moving either diredly toxvards or from the Objed. And in all CaSes, the Sine of the Difw ferexicc betnveen the toal and vif1ble Place of the ObX ied;t, will be to the Sine of the vifi1ble Inclination of the ObjeEt roH the Line in svhich the Eye is moving} as the Velocity of the- Bye to the Velocity of Light.

If Light moved but X ooo times fa{ter than the Eyes and an Objedt (fuppofed tQ be at all infinite DiRance) was really placed perpendicularly over the Plain in which the Eye is moving, it follows from what hath been already faid, that the apparent Place of fucll an Objedc will be always inclined to that Plain, in an Angle of 89° S682; fo that it will coullanely appear 3'^ from its true Place, and Seem fO much lefs inclill ed tO the Plain, that uzay towards which the EBye tends. That is, if AC is to AB(or AD) as IOOQ tO one, the Angle A B C will be 89° S6t Y, and A C B-3' , and B C D-z A C B = 7'. So that according to this Sup- pofition. the 57ifible or apparent Place of the Objedc will be altered 7s, if the Diredion of the Eye's Mo- tion is at one time contrary to what it is at ano -ther.

lf the Earth revolve round the Sun annually, and the Velecity of Idight were to the Velocity of the Earth's Motion in itS Oxbit (which I will at preSent fuppoSe to be a Circle) as IOC;O tO one X then tis eafy

tC}



( 649 ) tB conctive, that a Star really placed in ehe very Pole of the Ecliptick, woulds to an Eye carried along with the Earth, feem tO change its Place continuallys and (negledting the fimall DiSerence on e Account ot the li:arth's diurnal R.evolution on its Axis) xvould Seem to deScribe a Circle round that Poie, every Way diRant therefrom 3'. So that its Longitude would be varied through all the Points of the Ecliprick every Year s but its Latitude would always remain the fame. Its right Afcenfion would alfo chages and its Decli- nation, according to the difEerent Situation of the Sun irl refiped to the equinodial Points s and its apparent Diitance from the North Pole of the Eqllator would be 71 lefs at the Autumnal, than at the vernal Equinox.

The greateR Alteration of ehe Place of a Star in the Pole of the Ecliprick (or u7hich ill EtWed:t amounts to L-he fames the Proportion between the V¢loctty of lFight and the li:arth's Motion in itsOrbit) being known n it will not be difficult to find xvhat would be the Dtf- ference upon this Accountz between the true and apparent Place of any other Star at any time ; and on the coIltraryy the DiSerence between the true and appa retlt Place being given ; the Proportion betweetl the Velocity of Ligbt and ehe Earth's Motion in its Or- ltit may be found.

As I only obierved the apparent DifEerence of De- clination of the Stars) I all not now take any Sr- ther Notice tl what matltler fiuch a CauSe as I have here fuppofed would occafion an Alteration in their apparent Places in other Reedrs, but, iuppofing the Eartk to move equally in a Circle, it may be gather- ed from what hath been already faid, that a Star which

Rrrr ts



( t§50 ) is ncither in the Pole nor P-lain of ehe Ecltptick, witl feenz o defcribe about itS true Place a Figure, in-Senfi- bly diffieretlt from an Ellipfe, nvhofe Tranfverfe Axis is at Right-angle tO the Circle of Longitude pafling througll the Stars truc Place, and equal to the Diame- ter of the little Circle deScribed by a Star (as was before fiuppoSed) in the PoSe of the Bcliptick; and svhoSe Conjugate Axis is tO itS Tranfverfe Axis, as the Sine of the Stars Latittlde to the Radigs. Anzd allowing that a Stat by its apparent Motion does ex- airly defcribe fuch an Elliple, it will be foulld, tha-t if A be the Angle of Poftion (or the Angle at the Star made by two great Circles drawn from it, thro'8 the Poles of the E:cliptick and Equator) and B be another Allgle, whofe Tallgent is to the Tangent of A as Radius to the Sine of the Latitu-de of the Star; then B xvill be equal to the Oilierence of Longitude between the Sun and the Star, when the true and apparent Declirlation of the Srar are the farne And if the Suns Longitude in tlle Ecliptick be reckoned from that Polrst, Bherein it is when thisIlappens9 therl the DiSerence betveell the true and apparent Declination of the Star (on Accoutt of the CauSe I am now confidering) will be atsrays, as the Sine of £1ze Sun's Longitude f$om therlce. It will likewife be found, that the greate{} DiSerence of Declitlation tllae can be bet^rcen the true and apparent Place of the Star, xvill be to the Semi-Tranfiverfe Axis of the ;llipfe (or t-o the Semidiameter of the little Circle defcribed by a Star in the Pole of the E:cliptick) as the Sille of A to the Sine of B.

If the Star hath North Latitude, the Time, when sts £rue and apparent I)eclination are the Eame is lze

Sore



( 65SI )

fore the Sun comes in ConjunAion with or Oppofitia: to its if its Longitude be in tIle firR or laR Quadrant (siz. in the afcending Selnitcircle) of tlle Ecliptick ; and after tlletn, if in tlle defcending Seluicircle ; and it urill appear neareIi to the Nortll Pole of tIte Equator, at the trilne of that Maxir2m -(or when tlle greatefl: Differ ence between tlle trueandapparentDeclinationllappens) wl1ich precedes the Sunls ConjunAion tViti the Star.

SI'lleCe Particulars being fufficient for my prefent PurpoSe, I ffiall not detain you with the llecital of any tnore, or witll any fartller Explication vf tllefe It onay be titne ellough to enlarge more upon this Heads wllen I give a lDefcription of tlle InRruments Sc. if that be Judged neceCary to be donei and ullen 1[ fhall find, what I now adsrances to be allowed of (as I flar ter Iny felf i-t will)as fomething more than a bare Hyt pothetis. 1 have purpofely otnitted fome tuatters of no great Moalzent, and confidered tlle Earth as tuoxring in a Circle, and not an Ellipfe, to arroid too perplexed a CaGcBls, whicll after all tlle Trouble of it urould not SenGbly differ frotn tllat whicll I lnake uCe ofX erpecia3 ly in thoSe ConCequences which I thall at prefient dranv froln tlle foregoing Hypotllefls. - This being pretnifed, I Ihall now proceed to deter-

mine froan tlle ObSerxrations,>tllat tlue real Proportion is between tlleVelocoty of Light and tlleVelocity of the iartllis annual Alotion in-itS Orbit; ueon Suppofltion tllat the 7Phsnotnena before tnentioned do depend upon the Caufes I llave here afllglzed. But 1 1nuk firR 1er you knoxv, tllat in all the ObServations hereafter tnenW tioned) I have made an Allosrance for tlle Cllange of the StarSs Decllnation on >Xccount of tlle Precfl:on of

RXX12 t12C



( 65^ ) tIze EquinoxX upon Suppofition vilat the Alteration tioin this (Caufe is prop3rtional to tile Tilne, and regu- llr tl1ro*lgh all the Parts of tlle Year. I have dedllced the real ann lal Alteration of Declination of each Star from tlae ObServations themfelves, and I tlle rather chooCe to depend ttpon thetn in this Article, becaufe a11 which I llave yet lnade, concur to prove, that tlle Stars near the EquinoAial Colure, change tlleir Declination at this tilne s/J g or ̂ ts in a Year tnore than tlley would do if the PreceflLon was o¢ly 50",> as is n-ow generally fupw poCed I have likewiCe met witb follle frnall Varieties in t11e Declination of other Stars in different Yearsy whKl1 do not Seetn to proceed from the fatne CauSe, parw ticularly in tllofe that are near tlle fol{titial Colure, xxrllicll on the contrary have altered their Declination rec tllan> they ougllt, if tlle PreceElon vas S3" But ssrtlether tlleCe Elnall Alterations proceed froln a regular CauSe, or are occafioned by arly Cllange irs tlle Mate_ rials W. of my InRrumente I aln not yet able fully to detertnine. Howesrer, I rllought it tnight not be a-- IniEs ju(t to mention to yoll how I have endeavoured toX allow fir them, tllougtl tlle Refult would llave been; nearly tlle falne,+ if I 11ad not conEdered tlletn at allM W11at that iv¢, I will ffiew, fir{} froln tlle ObEervations of > 7Dravonis, whicha was found to be 3911 tnore Southw crly in the Beginning of A.alarah, tilan in S.eptem6er..

Frotn what llatll been prelnifed it vwill appear that the greateR Alteratio-n of the apparent Declination. of > DraonisX on Account of rlle fubceiEve Propagation of Ligllt, would be tov the Diatneter of the little Circler

wllich a Star (as was lzeEvre remarked) would feeln to

defcribe about the Pole of the Ecliptick, as 39" to 4O") 4* Thfi lzalf of tllis is the Atzgle A C*B*(as repre

fented



( 653 ) fented in the «g.)This therefore being to" ^, A C will be to A B, that is, tlle Velocity of Light to the Velo- city of the Eye (whiclz in this CaCe may be fuppofed the fame as tlle Velocity ofthe E;artll's annllal Motion in its Orbit)- as-tozIo to Onev froln whence it would- follotrs that Light tnoves, or is propagated as far as frotn tlle Slln ta.tlle Eartll in 8' IXtl.

It is w ell known,that Mr. Romer, who firR attelnpred to accotlnt for an apparent Inequality in t11e Titnes of tTle Eclipf@s of 7gltitst's Satellites, by the Hypotllefis of the progrefl;ve Motion of Light, filppofed that it fpe-nt about xt' Minutes of Titne in its PaiCage frorn the Sun; to us : bllt it hatll flnce been concluded by ottlers firotn the like Eclipfes,^ that it is propag.ated as far in about: 7 Minutes. The llelocity of Ligllt therefore deduced trom the bregoing Hypotllefis, is as it were a Meag betw.ixt what 11ad at ditEerent titNes bem deterrnined from the Eclipfes of 7Bpit.erts Satellites.

Tllefe different Nletllods of fitading ttne Velocity o£ Light thus agreeing in tlie 3lelu1t, we lnay reaforably concllldeg. not onlv tllat thefe SPhnomena are owing; to the CauSes to whicll tlley have been atcribed i bllt alfo, tllat I>ight is propagated (in the fa.ne xlSeii&WEW)f

with tlle fatne Velonity after it hath been refleded as before: for tllisw will be vlle ConlSequence7 if we allow tllat the Light of the Sun is propagated svith tlle fatne Velocity, before it is -reZedrnd, as the Light of tlle jt Stars. And 1 imagine this will fcarce be queRiorled, if it can be made appear tizat the Velocity ot tlle Light of all thefit Stars is eqalal and tllat tlleir Liglst luoves or is propagated througll equal Spaces in equal Tilnes:, atall DiIlances from thwm: botll wlaich p3ints (as I ap- prchend) are fufficiently proved fsolnthe apparert Altev

ration..



( 4y4 ) tation of tl2e E)eclination of Stars of difEerent LuiRre ; fiOw t^nat ib not fenFlbly diffbPrent in fuc^ta Stars as Seetn r. zar toX¢t derX thougll they apper of very dffetent iiagni+edes. And whatever thelr Situations are(tfI proceed accordtng tO tile ioregoingHypc3thetIs) I find the ftlre Velocity of LSiigllt frotn ny ()iDitr<tiOnS of ftnall Stars of the fifih or Exthg as frOlil tIl()fe of the fecond and third M<agnitude, wtnrch in all ProbabiEity are placed at xrery diflherent D§ftanceb EOtn t1S. S ze Iintlll Star,for Exatnple-before fpoken ot tlza£ xs qlizoR oppoGte to > i)raconis (being the 35*tIl CaxweJopdrd. Hevelii in Mr. FlamJ?eed's Catalogue) wss Igl tnoxe Northerly about tlle Beginning of Marah than in Sep tember. liVIlence 1 conclude, according to my Hypo thef1sX that the Diatneter of tlle little Circle defcribed b) atStar in the Pole oftileEcliptickwould be4oll) ̂ .

Iahe laR Star of the great Bear's-tail of tlle td Magnitude (tnarked " by Bayer) was 3 61/ more Southv erlSt about the Middle of yanry tllaIl in 7a#. lIence the Mancimgm, or greatei} Alteration of Decli- nation of a Star in the Pole of tlle E;clioticolt xarould 1ze 4Ot', 4, exadlfr the fatne as was before found frown tho Obforlrationsof > i)raconS.

T!-zeStar oftlle Stll tnagnitude in the ESead of EPe rerfAYS

innrked a bv Bayer, utas 2RSlJ tnore Aiortherly about the Fnw of Desember than on tlle 2gLh of 78Sy fol loxxting Hence ttle MSximam would be Xtt. This Star is not brigilt enolugll tXo be teen as it palies ovAr tny Zenit1n about th.e FXnd of 7sae nvllen it thould be ac cordng to the FIyptllefs firtheR South But becauSe

t ctin tI5OtC cCrtAitily unpond upon tlae greattIt Alterati otz of lDeclination of aOreZ SrsX xarlzich I 11ave frequent

b h ti-lsne; ,5.ffiz tley becotne Ilatiow o s2aryX



( 655 ) nary, witll reEpeA -t the Motion I am nt)Mr conliderZ lngj I will fet down a fet more [n0ances of fuct.ts from which you 1nay be able to judge how near it tnay be pofllble from theCe ()bServations, tO deterlutNe wit vlzat XJelocity tigllt is propagated.

X (:Perfei Bayero was t3tl more NortilerlyKlt .11e beginning of *7488ary than in 2ly. Hence the Maxie mtxm would be 40t't2. X C4gope was 34' more Northerly about tlle End of vDevember tharl ila 3tatwee Hence tlle Maxtmgm would be 4oltX 8. N iErva8 >tas 39t' more Nortllerly in ttlle beginting af SeptemO ber than in Maral) ) 11etlce tlle Mancitntn would be 4ot', z. Capell was about I6a/ tI<ore Southerly in SggN tllan in Febrgary; llence tl1e -Maxien xxrould be about 4O". But tllis Star being £irther frotn my Zenitll ttlan thofe I llave before tnade ufe of X I can- ]ot fo well depend upon tny OltServations of it, as of tlle otllers; becauSe I tneet Wltll fome fiillall LAlterations

of its Declination tllat do not feeln to proceed frolntlle CauSe I am nosv conlidering.

I have colnpared the ObServations of feveral otiler Star and tlley all confpire to prove tIlat tlle MasimR; ts altout 4oH or 41tt I will therefore fuppofe that it is 40"2i or (wllicll amounts to tlle fame) that Ligflt nove¢, or is prooagated as far as firom. tlle Sun to us in 8'- I3Bt. 'The near greetnent which I lnet With c}1UOnZ

ny (3bServations induces tne to tllinkX tllat the AlaxgO w (as I have 11ere fixedXit) cannot difFer fo Lnucll as a Second froln tlle Trutlzy and tllerefiore it is probable t-llat tlle Titne which Light fpends in paElng firorn tlne

Sun to u¢, tnty be detertnined by theSe Olfervratio.-wQ };ritilitl 5t or Iot82; wlaicll is fucll a degree of exacrtae{N ;nc we can never hope to attaing fiorn tlle EclipSes o i:**#a- piter's Satellitess F



( 656 ) vHaving tllus found the Masimxnw, or what the great°

eR Alteration of Declination would be n a Star plas

ced in the Pole of the E;cliptick, I will now deduce irOln it (according to the foregoing Hypothe&s) the Alteration of Declination in one or two StarCX at fuch tilues as tlley zsrere achually obSerrred, in order to fee how tile Hypothef1s will correfpond witll the PDZ#OJ

mena through a11 tile Parts of tlle Year. It would be too tedious to Set dorvrl tlle whole Se-

rles of lny ObServations j- I will tsllerefore tnake Choice only of fUCIl as are lllOS proper for 1ry preNent Pur- pofe, and will begin witl;z thoSe of > iOraconis.

Tllis Star appeared fartlleR North -about September 7th, I757, as it ought to have done according to lny Hypotherls. - The following 1zable fhews hoW mucll nore Solltherly the Star was found to be by ObServ2tiv on in Several Parts of tllC Year, and likewiCe how lnucil 1nore Soutllerly it ough-t tO be according to the By- potllefiS.



( 657 )

Hence st appears, that the HypotlleGls correEpon with the ObSerlrations of tlliS Star througll all Parts of the Year i for the ftnall Differences between tlleln feeta to arife froln the Uncertainty of ttue ObServation¢> urhich is occafioned (as I ilnagine) chiefly by the tte- mulous or undulating Motion of the ̂ Air, and of tlle Vapours in it; wllich cauSes the Stars folnetitnes to dance to alld fro, fo mucll that it is difficult to judge when they are exadly on the Middle of tlle XVire tllat is fixed in the comtnon Focus of the GlaSes sf tlle

Telefcope. I muI} confefs to yous tllat the Agreement of the

ObServations with each otluera as well as witli the HSt^ pothefi¢, is lnuch greater tIlan I expeded to find, be fore I llad colnpared thetlz; and it tuay pofilbly be thought to lte too great, by thofe wllo have been uSed to ARronomical Olzfersrations, and know how difficult it is to make fuch as are in all refpeds exaft. But if it would be any SatisfaEion to fuch PerSons (till I hantes an Opportunlty of defcribing my InRrutnent and the manner of ufing it) I could aXure theln, that in above 70 OltCervations whicll I lnade of this Star in a Yearf there is but one (and-that is noted as very dubious on account of Clouds) wllich differs from the foregaing EIypothefis more than zlt, and this does not diffier 3{t.

This therefore being the FaA; - I cannot but think it very probable tllat the 5Phaonotnena proceed from the CauSe I halre aff*lgned, fince the foregoing ObServations make it fufficiently elrident, that tlle Effed of there,l CCauSe, whatever le is, varies in this Star, in the fame Proportion that it ought according to tlle Bypothefisv

But leaR "ut l)raronis may be thought not fo proper to {hew the Proportion, in whigh the apparent ftltera

Sfrf tion



* - -- -

- " - -

-

( C58 ) tion of Declination is inereafed or diminithed, as thofe Stars whicll lie near tlle Equinodial ColureX -I will girre you alfo tlle Cotnparifon between tlle Hypo- thefis and tlle Obfervations of n R)ra? Mporis, that wllicll wasfirtlleRSouth aboutthe 17tllDay of 7an_ ary I7z8, agreeable to the HypotheE1S. The following Taltle {hews laow tnucil arore Northerly it uras fioiJnd by ObSersration in fesZeral Parts of tl:le Year, and alSo wlaat tlle DiSerence fhould have been according to rlle lIypotllefls.

T:? s e X O H Hs a -l-}% so >

C <, <D j o n s

3 rt- Sp, o vF. i * O (D ;n O fD'

- p H :n > w.

rr C w n *<- o. o (D *-F X CO - C

I7z8 d. /t ',1t O

Jpril - I 6 I 8 t I 8

May - - f t4 2 L3 Zz

7gne - Sf3 3t 3 - zr 3r 342

yU/7 - - I 7 3 6 3 6

[ X<2g829 z 3 S 3 F 2

'Se,pteSiber - 20 26r 26t

172a d. Neptem6er- I4

- 4

O2ober _ _ X ¢ NoveenlAer X X December - 14 t7z8 Febrry W I 77 March _ z X

I find upon Examination, that the Hypothefis a- grees altogetber as exadtiy xvith the OSfervations of this Star, as the former ; foI itu about S° that were made of it in a Years I do not meer wvi£11 a llif: ^rence of Ib much as ;" except in one, which is

mark d

. (D _ SC Q 5 C n (D t n (D

0en t- rn t.

n 0nd t".m .

Ci D ' r n 9¢ n 9 n . O X te fD

r0 r0

I! m tJ o .

L9; 28z 42t zr2 t9 2t I92t

1l 2 IO

3 X 3 t

It,S Iof s

2



( 659 ) martcsd as dollStful on Account of the Undulation of the Air, eic Arld this does not diSer 3tt from the Hypothefts.

The Agreement betreen the Hypothefis and the ObServaticns of this Star is the more to be reguard ed, fince it proves that the Alteration of Declinations on account of the Proceffilon of the Equsnow is fas I before fuppoled) regular illro alJ Parts of tlie i'ear

fo far at leafE, as NOt tO occafron a DiSerense great enough tO 5t diScovered xvith tbis InArument Ic like- wiCe proves tlle other part of my former SuppoGtionX %niz. that the annual Alteratiotl of Declinatiotl irl Stars near tlle Equinod:tial Colure, is at this Titne greater tharl a Preceffion of ioa would occafioll: for this Star uas zO/J more Sotltherly ill Seprewber T7z8,

than in September 17z7, that is, abollt t" mXore thatl it would have beerl, if the PreceSlon *vas Lout 50'1. ENut I may hereafter, perhaps, be better able to determine this Point, from my ObServations of thofe Stars tllat lie near the E;quitlod;tial Colure, at about rhe fame DiRance frotn the North Pole of the Ew quator, and nearly oppofite in right Afcellfion.

I think it needlefs to give you the Coalparifon betweeli the Hypothefis and the ObSersrattons of any more Stars ; fonce the Agreemetlt in the foregoirlg is a kind of Demonfiratioll (Nvhetber it be aI!ored that I have diScoverefl the real Can{e of the (Phzesoase7? or not; ) that the Hypotllefis gi^7es at leaR the tru: Law of the Variation of Deciillatlon lrl different Stars, vlth Refped;t tO their diSerent sittlatiolls allul A1- peds with the Surl. And if tllis is the CafeX it muR be granted, tllat the Parallax of the fiXt Stars is mvacll fmaller, than hath beell hitherto fppoid by tilofe3



t 660 ) who have pretended to deduce it from their ObServatte ons I believe, that I may srenture to fayX that in either of the two Stars laft metltioned, it does Ilot amount tO zJ' I anz of Opinion, that if it u ere x" I ihould have perceived it, in the great number of Ob fervations that I made efipecially of > lDrvcagiw; -whicl-s agreeilog with the Hypothe{;s (utithout allowing any thing for Parallax) nearly as x^tell sxllen the Slltl was in Conjundrion with, as in Oppofition tol this Stars it Weems very probal)le that the Parallax of it iS not fo great as one fingle Second ; atld conSequent- ly that it is above 4ooooo times farther from us eha the Suna -

There appearing therefore after allX no fenf5le Parallax in the fixt Stars, the Snti-Copernirans have flcill room on that Account, to objed againR the Mo- tion of the li:arth; and they may have (if they pleafe) a much greater Objediorl again(t the Hy-potheftss by x:?hich I have endeavoured tO folve tlle fore men tionod MPhznomena ; by denying the progrefilve Mov tion of Light, as xvell as that of the Eart!i.

But as I do nor apprehend, -that either of thefe Po. Itulates xvill be denied me by the Generality of the ARronomers and PhiloSophers of the ptefent Age; fo I £zall not doubt of obtaining their ASellt tO the Cot)fcquences, >hich I have deduced from thenz i it they rare fusb as have the Approbatton of fo greaE

a Judge of ehenl as yotlrEel£ I axnX SirX Go&rzno.f Obedient

HanbAe Servaat J. B R A D L E Yt



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A Letter from the Reverend Mr. James Bradley Savilian Professor of Astronomy at Oxford, and F.R.S....

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