A Letter to the Right Honourable George Earl of Macclesfield concerning an Apparent Motion Observed...

A Letter to the Right Honourable George Earl of Macclesfield concerning an Apparent MotionObserved in Some of the Fixed Stars; By James Bradley D. D. Astronomer Royal, and F. R. S.Author(s): James BradleySource: Philosophical Transactions (1683-1775), Vol. 45 (1748), pp. 1-43Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/104516 .

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Philosophical Transactions

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[ t ]

I. A Letter to the Right hotoarAble George Earl of Macclesfield concernng an appa rent Motion obJeersed ir Jôene of he fixed Stars ; > James Bradley D. D. XJ?ronomer Roy6ll, and F. R. S.

MY LOR D, ReadataMeetIngrT>HE grcat ExaEtneSs, with

af the Royal Society, | Ngllich InEtruments are now icbm t4* I747* 1 conFruEted, hath enabled

the A0ronomers of the prefcnt Age to difcover fieveral Changes in the PoX fitions of the heavenly Bodies; which, by reafon of their S^nalEne.rs, had efcaped the Notice of their PreZ deceXors. And aleho' the CauSes of fuch Motions haare alsvays fubfilted) yCt Phllofophers had not Co fully conflderdN^hat the Effeds of thofe known GallSes would be, as to demonArate a prxori the EPhR?soZ wena they might produce; fo that Theory itSelf is here, as wcll as in many other Cafcs, indebted to Prad:tice, for the DiScovery of fome of its moR ele- tant Dedudions. This points out to us the great Advantagc of cultivating this, as well as every other Branch of Natuzal llnowledge, by a regular Series of Obfiervations and Experiments.

The ProgreSs of A0vronomy indeed has always bcen found, to ha.ve fo great a Dependence upon

A accurate



l 2 J accuratc Ol)Ecrs*v.tionsX tllat} till iuch werc madc, 1r ads?anccd but flonvly: for tlae fir(t confidcrable Im provcmctlts tht it recclvcd} ill point of Theory, mterc owing tQ tllc rcnossncd 1jcho BrvEe; NYllO ir ex- cceding thofe tllAt had gonc bcirc him) in tllc ExaRnclS of lais C)bScrvations} cllabled tlle fagacious Jfir to fild oue fomc of thc principaL Laws, re- Sating to ttic Motion of thc hcavcnly 130dics. Tllc Inxcrltiorl of Tclefcopes al d PellduLunl-Clocks afo fording propcr lMcalls of Ilill farther in.proving tlle *Praxis of AlRrojlo.rsy; and thcfe beilln alfio foon fuc- cecded by the slttor.dcrfdl DiScoverics made by our Grcat Dletwton} as tO its Theory; the Sciencc, in both reUeEt$) had acquircd filch cxtraordinary Ad- ssanc;ncne, that iture Agcs Scemed to have Jietle roona left fbr making any great Improstelllents. But> in faEto \X7C find thc Caft to be very difirent; ir, as src advancc in thc mea15s of making more nice Inquirics7 ntw Points gencrally offer them6clstes,. tllat demand out Attcntion. The SubjeA of my prcScnt Lettcr- to your LordtMip, is a Proof of the Trtltlu of this Rcmark: fbr} as Coon as I had dirco vered thc Caufe, an{ fettled thc Laxvs of thc Aber rations of thc fixed Stars? arifing fronz the Motion of Light, eic xvl,crcof I gaze vn Accollnt ill N°* 406. of the rPhilothi! IrarMons; my Attcn- tiOll xvas again excitcd by another n Phtrowevon, %i2;. an annuai ChaIlre of Dcclination in fome of tilc fixcd Stars; wllich appcared to bc fcnfiS'ygrffltzr about tllat tirneX ttllal. a PreceElon of ti'M EqUinoctial

Poir.ts of S°'8 isl a Ycar would harc occalloncdb Thc Q>r.tity of the DiffUrcnce) tho} fnzall in itScl&>

was



3 was rendered perceptible, tllro' the Exadnefs of my lnflrument, even in the firPt Yeat of my C}bSersta- tions; but being then at a Lofs to guefs, from what -CauSe tllat ;,reater Change of Declination ploceededX I elldeavoured to allow for it in Sy ComputatiOnsx by makina ufe of the obfierqyed ant.ml Difference7 as ulelltioned in p. 652* of the fame EranfaSion.

from thvt time to the prefent, I have continued tO rnakc ObSerstations at tdX as Opportunity offered, With a View of difcovcring the Laws and Cavlfe of this 1Phaenomexon: For, by the Favour of nly very kind and worthy Friend SI6ltthew 7e- sordefQlV Efq, my Ixwfirunient has remained where it was firPr ereded; So that I havc been able, writh- OUt any Interruption, wyhich the Removal of it tO anotller Place would hasre occa(ioned, to proceed on with my intended Series of Obfervations, for the Space of twenty Years : a Term fomewhat exceedina tlle whole Period of tlle Changes, that happen ill thls sPhomenonX

MZ11c11 I {hall nlelltion tllefi^nall QMantity of the Deviation, which the Stars are fabjeEt to, from the CauSe that I llave been fo long fearching after; I as apprellenfive, that I may incur the Cenfure of fome PerfonsX for having fpent fo n1uch Time in the Pur- fuit of fuch a feeming Trifle: But the candid Lovers of Scierlce xvill, I 11ope, lllake due Allowance for that natural Ardour, nvitll whicll tlle Milld is urged on towards tlle Difcovery of Truths, in themfelves perhaps of fimvll Moment, x^?ere it not that tlley tend to illuRrate others of greater Ufes

The apparent Motions of the heavenly Bodies are fo colllplicated3, and affeEted by fucll a Variety of

A 2 Callfes;



[ 4 ] CSf;S) tat in tnany Cafes itis extremelydifficult to alEz,n to eacll itS due Sllarc of Influence; or diSinEtly eo point out, wvhat Part of tlle Molion is the Effk& of one Caufe, and+zvllat of another: And svllilPr the joint EffcEts of Al arc only attended to) g:eat Irre^ ¢,ularities and icnuins InconllItencies frcquently oct C6lt; xxherea<; nvhen ssrc are able to allot to eacll partiular Cau4^ its propcr Effed) Harmony and Uni form.ty ufiually enfue.

Suctl fecrr,ing Irregularilics bein? alb blenid with thc unasoidab'e Errors, wvhich Altronomical Obirvations nnllEt bo alnvays liabXe to} as sxrell from tlle Imperfc£tiol1 of our SenS<st as of the InAruments that sve make ufe of, hase ofren very much pcr- plcxXd rhoIEX xvllo hastc attempted tO lUlve thc EPh noge7za* and till lMeans are diScosered, xYhercby svc can feparate anct diffin;,uiCh thc partirBlar Pare of the whole Motiotz, thac is owinD tO each reI>eEtive Caufea it xx ill be jL11POS1blC, tQ l)e weil aCaired or the Truth cf any Solution. For theSe Reafons, ve enerally find, tluat the morc exaEt the Infiruments ale, that xve malzc uSe ot alud thc nnore reOular thv Serics of ObSersations is, that nc take; thc fooner xxre are ellabted to diScozter tht Cau of any nexy Pkaogeson. For xxJhcn u>e can bc well aircd of thc Limits)* xvherein the Errors of the Ob<ersra tions are containJd ; and llave rcduced then1 within xs narronv Bounds as poStble) by the Pcrfcflricn ot rhe InRrumer.ts ssrhicl1 xxre enploy; nvc necd not hcv xttate to aCcribe icil apparcnt Chan7esX as manif8fily exceed tllofe Limits, tO fbme other CauRs. Upon} the,2 N*co.l;ts it -is incunqbcnt upon thc praS i

AItron.onler.,r



[ 5 ] ARronomer, to Set out at firS with the lixan ination of thc Corrednefs of his Infiruments; and to be af- furcd that tlzcy are fufficiently exad;t for the Ufe he intends tO make of them : or at leaR he {hollld knowX xvithin what Limits their Errors are confined.

This Pradice has, in an eminent manner, been lately recommended by your Lordlhip s noble ExamZ pie; who havingX OUt of a fingular Regard for the Science of Aflronomy, eredred an ObServatory, and furnithed it svit-h as complere an HXpparatgs of laRru- ments, as our beA Arttfls could contrive; would no£ fully rely on their ExadtneSs, till thetr Diviftons had un- dergone the firiEtefc Re-examination: whereby they a.e probably now render'd as perfedc in thcir kind, as any extant, or as human Skill can at preSent produce.

The Lovers of this Science in gencralX cannot but ac}<nowledge their Obligations to your Lordlhip or this Account; but I find mySelf more particularly bound tO do it; fince, by mealls of your Lord(hip's moS accurate Obfervations, I have been enabled to fettle fome prinripal Elements; wYhich I could not at preSent otherwife have done, for want of an In- firument at the Royal ObServatory, proper for that Purpofc: For the large mgral 2gadrant, which is tllere fixed tO obServe Obj.eds lying Southward of tlle Zenith, hoxvever perfeA arl InArument it may be in it felf, is not alone fufficient tO determine, x^7ith proper ExaAneSsx cither ti4: Latitde of the ObServatory, or the Quantity of Rcfradtion corre- fpondinz, to different Altitlldes: For it bcing too heav) to be convenicntly removed; and the Room wherein it is placed7 bcing too fmall to admir of lti

bming



r - =S * ,

L b - '

izcing turned to the oppo;te Side of the NVallX xvhcrotl it tiosr llarl:,s; 1 cannot, by a^/aalObServa- tions of the cirWulmpolar StarsX fettle tllore nWceI5ary

Points; and rllerefoLe llaYe endeavourcd to do it, by collap>r t; tms oun with your LordShip's Obferva- t ions: allli un t i . ti lis VcfEct in tlle MIpparvtgs belong- il., tv £he Ro al ObSesatorat be renzoved, we muR be irzdebted tO @\ OWr Lorttffiip, for th: RnowlcdDe of Its true Situatlotl.

A NI;lltl intent UpOll t}ae Pllrfult of any 3tind of Ellosx ledbc, will always be agreeably entertained, with zinat can ipply the moPc proper means of atrainill3 it: Such, to the practical ARronomerX are exact and xêll-contriv'd InLtrunacnts; ;tnd I reflect with Plea- iurc on the Opportunities I have enoyed, of cultiva ting an =Ncqualntance and FriendShip nvith the Per fon, thata of all othcrs, has nzoll contributed to their Improvement, Tor I am fenfible) that if my osvn indeastours have, in any refpect, been effectual to the Adslansernent of A&ronorny7 it has prillcipally been onvint, to the Advice and AEance given me by ollr worthr Member Mr. George Graham; whoWe tJrcae Skill and Jud ,rnent in WIechanicks, joinvd with a completc and practical IEnowled^,e of the Ufics of A&rononlical InAraments, enable him to contrine alld execute them in the moli perfect manner.

The Gentlemcn of thc ROYRI HAaXeMY 05 Scie7sces} to xvhom nve arc SO highly ol)iiged for their exact AdMcali' Cment of tllC Quantity of a Degree under the Arctic Circle, kave already given the World very convincing Proofs of his Care and -Abilities in thofc ReEpects; and the particular Delineation, uthich they J;aave latcly publifhed7 of the feveral Patts

of



r n of tlle SeAor, svlz;ch he luade fol tllem, hath nonv rendered it llCedltfS, tO enter upon a11at minute DcScriptio 43f Zne at IzZasJZed; both being conAruded upon tllc fame PrinciplesX and differing in tlzeir component Parts, cllicfly on account of the diffierellt PurpoSes3 for whieh they were inte1zded;.

As mine xras originally defiglzed to takc onlar tllc iDiferenaes of the Zenitll DiAances of Stars, in the various SeaSons of the YcarX withollt any Viesr of difcovering their trge Places; I llad no Occafion tC3 knesr exadly, whae Point on; the Limb correEponded to the trge Zenith: and thcxeforc no Prosition was made in 1ny SeEtor, for the changing of ItS Situation for tllat Purpofe. Neither was it necenary that the D- wiFons or Points on the Arc thould be fct of, witl tlle utmoR Accuracy, EquidiRallt fror; eacll otller ) lJecaufe, nvllen I obServe any paSticular StarX the fame Spor or Poinx beinr firft lJifcEted by the Plumb-lille, and then the Screxv of tlle lKlicrometer turn'<{ until the Star appears upon the middle of tlle Wirc) tllat is fixcd in tlle comnzon Focus of tlle GlaIlses of tlle Telefcope; I can thereby colleEtX hc>Nv far the Star i5^ fro1n that-giszen Point at tlle Tlm-e of ObScl*vation: and afterwards, by co1nparing togetller tllc Ccvcr.*i Obfervations tllat are made of it, I anl ablc tc) <AiCco- ver nzhat appalent Changc 13S happell'd. The t1<n tity of the viSble silteration, in the PoStion Ot tilC Stars, being expreed by Restolutions alld Parts o* a Revoltltion, of thc SrXev of r;ze l\5.iCi'OIMi5.CL; I endeasZovlred tO deternaiwe, svitll !5lJCht (' i1ca th= true Angle anfarcrinF tllerero: an;t aStc: \1LiOUS Trialx, I theroll;,hly fatisficd 357fiv75v t0s-i et- ti) Etl*laliwy 0t'

t1Ac..



: 8 3 the Thrcads of tl-le Screx^, alld of the preclfe Ntum- bcr of Seconds col4LeppondillD eo them.

- But altho' thtSe Points could be fettled svith great

Cer.anty, I xvas neveLtllelefs obliged to make one

Suppofttion; sahicll perhaps to fome Perrons may

feem of toc) ^,leat Moment in the prefent Inquiry}

to be admitted without an cs7ident Proof from Fadts

and Experiments. For I /^Zgppofe, that the Line of

Collimation of my Telefcope has invariably prcierved

the fame DircEtion, with reEped to the Disrifiions up

on tlle Arc, during the svllole Courfe of my 0b-

fiervatioIls. And indeed it svas on account of the

Objedions, nahich miglat has?e been raifed againft

fuch a 'PoJ?alate, that I thought it necelEary, tO con-

tinue my Series of Obfervations for fo many Years,

before I publithed the Conclafions, xvhich I I\alL at

prefent endeanour to dravv from thcm.

\Nthoever compares the Refult of the feveral

Trials, that has!e been made by the Gentlemen of

the Scadee of Svzersces, for determining the Ze-

nith Poilat of their SeEtor, ftnce their Return from

the Nsorth; will, I prefume, allonv that mine is not

an unrealonable or precarious S<poftion: fince it

is evident, from their ObServations, tbat the Line of

Collimation of that Inltrument underu7ent no fenf-

ble Change in its Diredion, during the Space of

more than a whole Year; altho' it was feveral times

taken down, and fet up a:,ain in different and re-

mote Placcs; whereas mine hath always remained

fuEpendtd in the fame Place.

13ut bcEldes fuch a LRrong Argllment for the Pro-

bability of the Truth of my Suppoftion, I have the

Satisfaftion of findinD it aciually vexified by th£

ObServations



[ 9 ] ObScraratlons themfelves; which plainly prove, that at the End oftshefull Periodofthe Deviationsxvhich I am going to mention, the Stars are found to have the fame Pofitions by the Inflrumcnt, as they ought to hase, fuppofing tlle Line of Collimation to hanre continued unaltercd from the Time when I firll began to obferve.

I have already taken notice, in what manner this Phinomenon diScover'd itfelf-to me at the End of my firflc Year's ObServations, ttiz. by a greater ap^ parent Change of Declination in the Stars near the Equinodial Colure, than collld arife from a Pre- ceffilon of S°" in a-Year; the mean Quantity now uEually allowed by A0cronomers. But there appear- ing at the fame time, an E6eXt of a quite contrary Naturc, in fome Stars near the SolRitial Colure7 svhich feemvd to alter their Declination Aefs than a PreceElon of so8' required; I was thereby con vinced, that all the 2Phinosena, in the different Stare, could not be accounted for, merely by fuppofing, that I had affiumed a wrong Quantity for the Preceffilon of the Equinodial Pointso

At firrc, I had a Sufpicion, that Some of theSe fmall apparent Alterations in the Places ofthe Stars, might polElbly be occafioned by a Change, in the Materials, or in the Pofition of the Parts of my Sedor: But, upon confidering how firmly tile Arc, on which the Divifions or Points are made, is faRened to the Plate, wherein the Wire is fixed that lies in the Focus of the Object-GlaSs; I faw no Reafon to ap prehend, that any Change could have happened it the Pofition of that Wire and thofe Points. The SuEpenfion therefore of the Plummet being the moR likely CauSe, from whence I conceived any Uncer-

B taintt



t TO ] tainty could ariIê; and the Wire of which had been broken three or four times in the firR Year of mzr ObServatiorl$: I attempted to examine, whether Part of the 'forementiond apparent Motions migllt nor have been owing, to the different Plumblines that had bcen made tlCe of. In order to detcrmine this, I adjuRed a particular Point of the Arc to the-Plumbw line, with all the Exactnefs I could; and then taking offthe old Wire, I immediately hung on another, with which the fame Spot was again compared. I repeated the Experiment three or bttr times, and thereby fully fatisfied myfelfs that no iinfible Error collld ariSe from the Ufe of different Plumb-lines; lince the rarious AdjuPtments of the fame Point agreed with each other, within leEs than half a Second.

Having then) from fuch TriaIs, fufficient ReaNon to conclude, that thefe J*econd unexpedred Devia- tions of the Stars, wcre not owing to any Imperfedion of my Inrcrument; after I had Settled the Laws of the Aberrations atifillg from the Motion of Light, efPrt I judged it proper to continue my ObServatiorls of the fame Stalws; hoping that, by a regular and longer Series of tllem, carried on thro' feveral fucceeding Years, I migllt, at Iength) be enabled to diScover the real CauSe of fuch apparent InconfiRencies,

As I reStded chiefly at Wan/?ed, after my Seftor nas crc2cd tllere in the Year I727- eill the Begin- ning of May I73 2. when I removed from thence eo Oxford: 1 had, duting my Abode at AiV:anfed fre quene Opportunities of repeating my Olzfervations > alld thereby diScovered fo many Particlllars relating

to



t Il ]

to thefe EPhznofflena, that I began to guefs xvhat was the real CauEe of them.

1t appeared from my ObServations, that, duting this Interval of Time, fome of the Stars near the Sollitial Colure, had changed their Declinations 9't or xo" leJ"s7 than a Precefl lon Of S°" would have pro- duced; and, at the fame time, that, others near the Equinod;tial Colure, had altered theirs about the fame Quantity more, than a like PrecelElon would llave occaCioned: the North Pole of the Equater feeming to have approached the Stars, which come tQ the Meritian with the Sun, about the Vernal Equinox and the Winter SolPrice; and to have receded from thofes which come to the Meridian with the Sun,aboue the Autumnal Equinox and the S-ummer Sol{tice.

When I confider'd theSe Circumflcancesa and the Situation of the AScending Node of the Moon's Orbit, at the time when I firPt began my ObServa tions; 1 fafpe+Eted, that the Moos Adion upon the Equatorial Parts of the Earth might produce ehefc Effed:ts: For, if the Preceffilon of the Eqllinox be, according tO Sir I%aac lewtor's Principles, caufied by ehe Aftions of the Sun and Moon upon thoSe Parts; the Plane ofthe Moons Orbit being at onc time, above ten Degrees more inclined to the Plane of the Equator, than at another; it was reafonable tO conclude, that the Part of the whole annual Pre ceINton, which arifes from her Adion, wollld in difZ ferent Years be varied in its Quantity; whereas the Plane of the Ecliptic, wherein the Sun appears, keeping always nearly the fame Inclination to thc Equator, that Part of the PreceElon, which is ow- in;, to the Sunss Adcion, may be the fame evertr

B 2 Year:



[ I2 ]

Ycar: And from hence it would follow, tllat, aTtho the mean annual PreceElon, proceedirlg from- the joint Adrions of tlle Sun and Moon, wcre so"; yct the apparent annual PreceE1on might Sometimcs ex^ ceed, and fometimes fill {hort, of that mean Quan- tity, according to the various Situations of the Nodes of the Moonss Orbit,

In the Year x7X7. wllen my InArument was firl fet up, the Moon's AScending Node was ncar the Beginning of Xries; and confequently, hel Orbit was as much i-nclined to ttie Equator, as it can at any time be; and then the apparent annual PreceIElon was found, by my firR Yearss ObServationsX to be greater tharl tlle mean. whtcll proved, that the Stars near tlae Equirlodial ColureX wllofe DecIina- tDns are moll of all affcAed by tlle PreceElon, had changed thears, aboYX a tenth Part nzore tllan a Pre- cefl1on of 50" would have -cauSed. Ihe fucceeding Years ObScrvatiotus prostcd the fame Thing; and- in thrce or four Y¢ars time the Differencc be- came fo confiderable, as tQ leate no Room to fuS- ped, tllat it was owirlg to any ImperfeEtiorl, either of tlle Iallrumcnt or ObServations.

Eut Ibme of tlle Stars7 which I had olferved, tllat xnere near the Solflitial Colurc) having sppeareJ to 1w10vC, during the fame time, in a manner contrary ro svhat they ouoht to have done, lty an IncreaSe itl rlue Preceffilon; and tlle Deviations in them being as remarkable as in the othersX I perccived that fome- tlling more, than a mere Change in the Quatitity of vlle Precefltlon, would be rcquifite to folve this Part of the (Phnomenon. Upon comparing my Obfer- sarions of Stan near ttze SolRitial C:olure, that were

almot



[ I3 1

almolt oppofte to eacil otllcr in lti(snllt\g>Cl-lliOtl, I found, tlat tllcy nvere equalls aSet',eei bt ilnis Catlfc; for nviilft > S)racoBts (iSpearect tO kAVe nzovcd NXortlN^tartl, tllC fmall St.lr, nvilicil is tile 3 sth CatneloWardali HeqJel. in thc Brit<Jh Catalo,uc, fccm'd tO liavc ,one as n uch tosârds tilc South: which thexY'd, that this apparcnt Motion, in both tilofe Stars, might procecd frola a Sutation in tile Eartll's Axis; nvhcreas thc Compari<oll of my Olzfcr- vations of tllc fame Stars, for7nerly endbied me to draw a diffcrene ConcluSion, Witll reEpeEt to thc CauSe of tlle annual Aberrations arirlng from the Motion of Lighe. For the apparent Alteration in > 1Oraronis, from that Caufe, being as great again as in the other Emall Star, proved, tllat tha; (Ph- nomenon did not proceed from a N:8tation of the Earth's Axis; as, on tlzc contrary, this may. UpOll making the like Comparifon between the Obrerva- tions of otllcr Stars, that lie nearly oppofite in Right Afcenfon, nvhatever their Situations were with le- fpeEt tO the Cardttlal Points of the Equator, it appeared, that their Change of Declinatiorl was ncarly equal but contrary; and fuch as a Nutationor Mo- tion of the Eartl<'s Axis would cffc&.

The Moon's Afcending lAlodc being got back towards the Beginning of Caprivora in the Year x732 the Stars near tlle EquinoEtial Colure appeared, about tllat tiRlC, tO changc their Declinattons no morc} than a PreceIIion of S°" rcquired; xvhilR fonne c)E thofe near-the SolQtitial Colure altered theirs abosc 218 in a Year lefs, than they ought. Soon after, I perceitted the annual Change of Declillation of ths orn,cr to bc dimgni{E.cd, fO as to bscome lefs thall

* s ,:);



[ I4 ]

go" of Precefllon wrould caufe; and it continued to diminilh till thc Year I 7 3 6. whcll tllc Moon's Atccndint, Nodc svas about tlle Beginning of IJibra, atld hcr Otbit 11ad the leaJ? Inclination to thc Equa- tcur. But by this timc, {ome of tlle Stars ncat tlle SolRitial Cc)lure had altcred their Dcctinations I8//

ZeJ>s, Ence tl e Ycar t727. than tllcy ought to have donc fronz a PreceElon of 50". For ^trDralsonis, xvhicll in tllofe ninc Years {hould have gone about 8 " nlore Southerly, was obrers ed irl I 7 3 6. tO appear

oJ more Northerly, than it did in tlle Ycar I727.

As this Appearance in > EDraronis, indicated a Di minution of thc Inclination of tlle Earth's Axis to the Plane of the Ecliptic; and as feveral ARrono- mcrs have fuppofed tZJat Inclination to diminilh re ,^,ularly; if this Phnozenon dcpended upon fuch a CauSe, axld amounted to I8/ in nine Years, the Obli- quity of the Ecliptic would, at that rate, alter a whole Minute in thirty Years; which is much faller than any ObServations, before made, would allow. I had Reafon tllerefore to think, tlwat fiotne EPart of this Motion at the lea0, if NOt the Mhole, was owing to the Moon's Adrion upon tIle Equatorial Patts of the Earth; which I corlceivcd, migllt cauSc a libratory Motion of the Eartll's Axis. But as I was unablc to udge, from only nine Ycars ObServations, whether

thz Axis nvould entirely recovcr the fame Pofition, that it llad in the Year I727. I found it necelEary to colltiIlue rny Obfersfations thro' a svhole Pcriod of the Moonvs N7ode ; .at thc End of which I had thz Satisfadion to fues that the Stars returned into the famc Pofitions again; as if thcre had been no Al- teration at all in the Inclination of tllc Earth's Axis

M hich



[ I5 3 which fillly convinced me, that I had gueSed rightly as to the CauSe of the (Phinomena. This CircumX Fance proves likewife, that if there be a gradual Diminution of the Obliquity of tne Ecliptic; it does not arife only from an Alteration in the Po fition of the Earth's Axis, but rather from fome Change in the Plane of the Ec]iptic itfxlf becallEe the Stars, at the End of the Period of the Moon's Nodes, appeared in the fame Places, svith reEped to the Equator, as they ought to have done, if the Earth's Axis had retairued the fame Inclination to an invariable Plane.

During the Courre of my ObServations, our in genious Secretary of the Royal Soriety, Mr. yohn Macbin, being employed in contidering the Theory of Gravity; and its ConSequences, with regard to th<: CeleRial Motions; I ac-quqinted him with tlle SPhenomena that I had obServed: and at the fame time mentioned7 twhat I fuEpeded to be the CauSe of thcm. He SOon after fent me a Table, contain- ing the Quantity of the annual PreceElon in the various Pofitions of the Moon's Nodes, as alSo the correEponding Nutations of the Earth's Axis; which was computed upon the S?ppoJ4t*on, that the mean annual PreceIElon -is fo"> arld that the Whole is governed by the Pole of the Moonss Orbit only: and therefore he imagined, that the Numbers in the Table would be too large; as itl Fad they were found to be. 13ut it appeareds that tlle Changes w}<ich I had obServed both in the annual Precef- fion and Nutation, kept the rame Law, as to in- creafing and decreafing, with the Numbers of his Table. Thofie wers calculatcd upon the Suppofitiona

that



[ I6 ] that tlle Pole of the Equator7 during a Period of the Moon's Nodes, moved lZound in the Periphery of a ltttle Circlet whoSe Center was 23° 29' dillant from tlle Polc of the Ecliptic ; having itbelf alSo an angular ^Motion of so" in a Ycar, about the fame Pole: trt Ntortll Pole of tlle Equator was conceined to be in that Part of the Emall Circle, which is Xrtheft from the North Pole of the Ecliptic, at the Xtlme wllen the Moonss Afcendtng Node is in the BegiruninO of Jrxes: and in the oppoFtte Point of its aYhen thW fame Node is in Libra.

Such a Hypothefis will account for an Accele- idtion and Retardation of the annual PreceIElon; as aIrO for a Nutation of the Earth's Axis: And if ehe I)iameter of tlle littlc Circle be fuppofed equal to I8J; which is tlle svllole qantity of the Nuta- tion, as colleded from my Obtervations of > i)ra- sonis: then all the SPhasnomena in the feveral Stars which I obEerved, 5rill be very nearly Solved by it. Let SPrepreSent tile

mban Place of the Pole oftheEquator, < < \

about which Point, / * * .

asaCenter, fuppofe /,. + ZAs >. \ tile true Polc tO 1.- : A P X sl ove in tlle RIrclc ;1. D . PA ,, r

nzeter is IS/'. LetE \ R ^* y lde the Polc of rhe \ EclipticX and ES) \ -t / lze equal to tlae naeart \ <

PiRance betw c e n . thc 1'oles of the thz Equator and Ecliptic; ancl fuppefe the true

po):



E I9 ]

Pole of the Equator to be at X, when the Moon's Afcending Node is in the Bcginnin, of Xries; alld at B, nvhers the Nz-ode g¢ts back to Capricrn } allc

at c, when tlle fame N5cyde is ill Lî6r: at x^thich time the North Pole of tlle Equator l3eil nearer the North Pole of the Ecliptic, by- tlle whole Di auleter of the little Circle SCequal tO I gtl ; the Obli- qllity of tbe Ecliptic will then bc fo mucll Zers tilAIl

it wasX wilen the Aloon's Afcellding N:ode nvas ils ifries. The Polnt tP is fuppofed to novc rounzi E, 5rith all equal retroDrade Motion, anfxveralzlc to the mean Precelllon arifing frol;n tlc jotnt Adions of the Sun and Moon: xvllile the true Pole of tllc Equator tnones round MP, ill tlle Circulzlfe rCllCC C9, Witil- a retrograde Motion likex^1ife, xn a Period of tIse Mooll's Nodes, or of eiglatcen Years, alld feven Motltlls. Ey tllis mealls, svhelu tlle Mooll's zNi*<:endillg Node is in Jries, alld the true Pole of the Equator at X, is moving ftota X to5&talds B: it xvill approach tlle Stars, that comc to tlue MeriRiarl XViti tlae Sunabout thc Vernal Equillox; and lecedc irotn tllofse tJlat conze ntitlo thc Sun near tlle Autunlnal EquiIlos faJfer tllall tlle mean Polc e does. So thara svliile tllC lMOOll'S N7ode goes back fiom Jries to (>dpricorsZ) tlle apparent PreceJlioll Evill fCCt5sl i-O

nlLlc1l greater tllall tlle mean; as tO CaU6¢ tllC Stars,

tllat lie in tllC Equsodial ColurcX to lzave aItctcd tlleilw IDeclinatwoll 9'lX ill alsout four \tcars alld cisrlwt Motltls, azore thatu the nzean Prereilloll nvould do aoad iil lile {-a11az tilile, t11c N5orth Polc of tllc Equa tor xîll Sccr tc) Iaaswe applvacilcd the Stars, tllat cona:

to tllc AtcIi.lia:z xîtll the Sun at <:)U1* \Arititel Sol fiiceX alzollt 9W!; and to 11avc r<edcd as znsh flolzz tlo5<X.5 tlaat c.ollaC \^ itll CIRC Stz1.l At th.C 5ullln'Ct-SOIfiiCCs

C rthUS



rOz Thus tive iPhtnoznena before recited arc m go

neral conrortzaable to tlwis H7-potllefls. But to bs more partictliar; 1ct

5' be the llace of a S StarX f S rlue CircIc / * ..... \ of Declinatiorl pa(l / ^ / .. +\

311g tllrO' itn reprc- . / * \ Cnting itS D;RanCc EDX . FO 9

fromttlellleallPoles ' t- Dw P/-- 1B

and 'PS its mean \ .. l. .f ^

Ri:,ht hfcenflonZ \ R c / Thea if OandS be \ £t f tlle : Points! wahcre \ tF /

tlle Circlc of [)e- CtiNati0 ClltS tllt

IttlC Circle SMO; the trge Po1¢ will be neareR that 5tar at 0) and farthe(t trom tt at R; the xmholc Dif- fcrcnceamealnting to I8!', or to thcDiametcr of tlle )ittlt Circlc. s the true Pole of the Equator is tuppofed to be at id) mthen the ^Moonss Alcending AYo12c is in Jrzes; and at Bt whtlu that Node gMtS

back to Compricorn s and t;ne ant,ular Motion of thz true: Pole about SP, is likcnviSc fuppoid etl.lal to that ot thc Moon S NodzX atOoUt a or tte Pole of the Eî ip tic: fillce, inthcCcC;abe:<> tlic truePoXeoftlle liclua- tc.r is go DeDrces betore the Moon S Afccaclillg biTode, it nnlutt be Io in all others.

\Mhen tlle trut Pcxe is at 4 ie xrr1l be at tho Ame DiAaracc from tne Stars t}lt lie in the EqrlinoAial ColureX as tlle mean Pole ? is; for I negleEt at prefent thc Cafc of fuch Stars as are qvere ncalr the Pote of the Equator; and as the true Pole recedes back from X tnxards BX it wlll approach the Stars} that lie tn tllat -Part of the Colllre repreSentcd by Pr; a.lld recede from th{>fe? that lic irl e ; nor

indecct



[ 2I 3 indeed with an cqgowble Motion; but in the fiatio of tluc Size of tlac DiAallce of tl-e Moon's Ntod,e trel11 tile Bcîllni;lt, of i?it6.x. For if tlle *N'ode be I>uppol'cd to hane gOilC backxvalecls fr.l)ul Wries 302> or to thc Bcgillni1l r of CPilces; tilc Potllt, nvhiclu xcyrcints the Pla<c of tllC tlUC Po!c, xill itl tlac lwlean tinze, haste ozoscd in tlle littlc (Jitcic, tlaro sn Arc, as AO, of 3oc Sikcvvifc: al d nvoicl thel<- taorc in E5eci llasc approact-ld clle Stats tllat lic il; tlle Eguinodlial CJolurc Pt, ald hanc rccedcd frbll, t1.lozGe that lic )n i5, +>2; wlaicll is tlle Sine of 3sQ to the Ravis AP. For if a l)crpelldicular ta11 from- O upon 'PS, it nzzy bc collceircd as Part of a grcat CLircle, paffilng ttlro' the true l'olc ,alld any Star Iy-ing in -the Equi.noctial CGollur:* Stow the fatzze Proportioll, that 11olds in tllefc Starss will obeav1z likewife in all otllv-rs; ald troln latnce wc mas colleci a gelleral ltulc, for findin, how much nearer or fartller, any particuiar 5;tal iS, tO Ot trom, tlle medn Pole, in any givm Potition ofthe Moon's lstoue.

For, if from the Rie,sht-AJen.on of the Star, "=e bJ/rati the WiJtance of the Moon s ilJaending Woate fromArier-; tEentheRadi?stnill6eto tEe Siraeof the Remainder, as 9 , is to the NTumber of Seranafs, that tte Atar is nearer to, or farther from tte Erse, than tEe ItIean sPole. \07hen that Ivenzainder is lzfis tllan 80°, the Star is rsevrer tO tlle Truc, tl an tO tl.e lKlean Pole; ancl the contrary, xYhen it isgreater tllan 18G°4

This Motion of ttle trge Polc, about the mean ae tP, nzill alfo produce a Cllan*^,e in tlie Right Afseno ftons of the StArs, and in the Placcs of the Equinoc- tial Points; as ssell as in the Obliquits of the Eclip-

C 2 tiC



t22 ] tic: snd the Quantity of the Equations, in exther of thefe Cafzs, may be eafily computed for any ,iên Pofition of the Moon's S5odes. But as it nzay be needlcEs, to dwell longer on the Explication of tile Hypothefis; I fball nosxr proceed to Shew its Corre- fpondency with the Thenomena, relatIn: to the Al- terations of the Polar Diftances of fome of tlhe Stars ;xYhich I haxe obferved by laying before your Lolde fhip tlae Obfcrsrations themScives, together with tilc Computations that are neceSary; in c)rder to form a } it,ht l ildgment asotit the Caufe of thcft A ptrearan cesX

I have endeavoured to find the exaa Quantity of the meazx Preceffilon of tllc Equir.oEtial Points, by xotnparinC, my osvn Ohrervatlons made at Greev

vish, with thoSe of rycho Brahe and othcrs, which T judged to be mo(t p-roper for that PurpoSe. Eat as many of tize Stars, which I compared, ,ave a dif- ierent Q lantity ; I tEall affiume the mean RefLllt; .zilich rives a Precefllon of otle Degree in feventy-

tnc Years and an half: thls agreeing very well ike-

8Wi.ft NlI!tsl my ObServations that were takcn at tan- #ed. [lae >oumbcrs in the folioewing Tablcs, xyhic;h exprifs tne Chant,e of Declirlation in each Star atc s:ompl.tcd upon the Stlppofition, that tne mean Oo- 'i.nguiry of tlle Ecliptic was >3°. 2w'. 30", and tl.at -*tsontinucd the jaeneX during the whole Courfe of nr Obftrvation . Arld as the Moolzvs Afcending xtode xzras in the Beginnlng of dries about the 27th Day of ldarrD s727, I have rcduced the Place of cacll Star to that Time; by allowing thz proper *hange of Declinatiotl from that Day, to the Day t; each rtEpective Ob6crvation.

it b.ing alfo neceICary tO nzakc an Allos^tance fiot t.e XbErrAtioNtt of LiOht; I hase aain examined

nly



[ 23 ]

nny ObSerstat.ons, that were moR proper to deter- snine the Tranfverfe Axis of the Ellipfs, which cach Star Seems tO defcribe; and haxte found it to be nearePt to 40"; which Nunlber I therefore maJke - ufe of in the following Computations.

The Divifions or Points upon the Limb of my Sedcor are placed five Minutes of a Debree from each oeher; and are nunlbered fo, as tO {hew the Polar Dlfiances nearly; the trae Polar DiRance exceeding that, svhich is thewn by the InArument, about Ir. 3 S ' \Alhen I fir(t began to obServe, I generally made uSe of thvt Point on the Linlb, which was ncarelt to the Starss Polar DiRance, without regarding wl1ethnz it sras more Northerly, or morc Southerly tlzan the Star: but as it fiometimeS happened that the Oti- ginal Point, with which I at firR compared the Star, bccame, in Procefs of Time, pretty remote from it; I afterwards brought the Plunlmet to another Point, that was nearer tO it; and carefully examined, what N7umber of Rextolutions of the Screw of ehe Micro meter eir correEponded tO the I)illance between the diferent Points, that I had made ufe of by w hich means I was able to redttce all the ObServations of ehe famc Star to the fame Point, without fuppofng the feveral DiviSsons to be accurateXy 5 aEunder.

I have exprefficd the Diflance of cach Star from the Point of alic Arc, with which it was compared in Sevonds of a Dcgree and tenth (Parts of a Second, exaAly as it saras colleded from the ObEervations; altllot I an1 fenfible, that the Ob6crvations them- fcltes are liable to an Error of nzore s:han a whale Second; becauSe I meet .vith fome, that have been made wltl1in two or three Days of each other, tht dxffer 2tZ, evell whcn ehey are not marked as de feSite in any retpeEt. lt



E 243 It would be too tedioust to fet dewn the whole

Ntenlber of the Obfirvatiolls tlldt t have nzade; anel dlcrc^fore I thall gis;e only czetlgh of tllemX to placscr thcir (>otrelpondency mrith tllC Jforemelztiolzcd FIy- potllellis in tllc I(YCaa1 Ercars, whirein any wcre llsade of the Stars llcre recited. When feteral Ob- i^clvuitiolls hre beca- tzken of the fame S.tar, within n tcxzr Days of cah otllcr; I bave either fet do^tn ttlk m.can 2Wcp.ult, or that ObfiersZation whicll bxlt z¢rc< Willl it. I have likewiCe comnaonly cilofe tizolcs, tl.sat xyee rllade near rhe fame Seafoll of tlze Ycar} itl [llch Stars AS gase tne the Opportunity of natilg that (;hoice; particularly in > EZ)ravai3 xvlll:h svas generaily obServed about thc End of hb>Erfl or tlle- Beginning of September; 'Sat being t'c ufiual Tim.e) whcn I mtent tO #'a7xJ?ed Gn pur- pO6C tO obServe botll that, and aIfo fome of thc Stars in the gteat B r - But the \tWeathct provir cloudy at that Sealon in thc Yeat 174+, pr<ventvd nlv making a fingle ObSerszation, either of > S)rv- [nzs, or any other Star, wisi!e I was tllere; which s the CaulE of one Vacatlcy in a Scries Jof so fuc- cecding Y¢ars, wherein that particular Star had been obfetred. Such Stars, as tete either nu vifible in the -Dsy-tinze, towards the Beginning cS Septem6er or came at fucll Hours of the l!ight, as would have intonImoded rlae Fan^.i:y of the HouCe wherein tl£ lllnrumcnt is Sxed, werc but feldom claServed, aftcr 1 xsene tO rcfde ae Oxford: which is the Reafon, M 1ly tlC SericsofObServations ofthapt is CO ilnpcrfed, AS )alitinlCS tO lgave a Chaim for fevcral Years toge ticr BUt notwithRanding- tllis, I doubt not} but -ugen the xa-l-lote tlocy will be found fuldicient, tO

fatisfy



[ 25 3 fatisfie youw towdnr.ip of the genc,al Correfpolsdetlcy bctslJeen thc Hvptkvffi and tiac rP/.-}Zvomena, ir. tl-sz tXtCtal StarS ; ho; cster differcrt tllMi£' siuatiOsis ?re}

-Xxith rerpe* tc the Cardillal Poin s of 1l>c Equator. As I nlad rnolc Obfelevations rjf > qDracanis

thall of asly other Star; and it bvinD likewiSe trery near the Zenittz of lEanAtec{; I nvili bcgin with thz Recital of Some of thctn. The I'oint upon tElc Li.m.b, u7ith x^7hich tl.is Star w7as compared, waw 38t. 25t from the Ntorth Pole of tlle Equatol^, according tO the NunwbcrR of the Arc of tay S'eAr. The StR Colunzn, in the follosv;tag Table, Ihews the Year and the Day of the LKlollth) nvllcn thc ObEer- at.ons xxtere made; tht ne^t givcs the Ssumber of Sesonds, that the Star was found tO be South of 3 8°. 2 S t : thz third contains the Altcrations of tlz: I>oIar Diflance, nZllich the meaN Prece«lon, at tbe rate of olwc Derce in 7l2 Years, would caufe in this Star, from the 27th Day of Marrh I7X7, tO

thc Day on which the Obfervatioll was takcn: the fourth ffiews the Aberrations of Liglt: the fifth,, the Equations arifing from the 'forernentioned Hy- pothefis: and the fixth gis?es the mean Di(lance of the Star from the Point sarith which it sras compalred,, f-ound, by colleding tlae Several Numbers, according to their Signsa in the 3d, 4th, and sth Columns, and applying them tO the Ob/erved l)iJ?anses con- tainXd in the Second.

If tlle ObScrs?atiolls had been perScEtly exad;t, and thz Scvcral Equations of thcir dse QMan.ity; thcn all the NsumDers in the laR Column would have been equal; but rtnce they diffcr a little from one another; if the mean of All be taken, and alld the lZxtremes are

D comparcd



[ 263 pared xvith it, sLre {hall find no greater DiScrence} than xiwhat may be fuppoSed to ariSc fro+n tlle Ctncer- tainty of tlle ObServctr¢ns the:rzzfielvc ; it no where amounting to more than 1"3. lDhe IiyDothcfis therefore feems} in this Star? to agree extrerncly svell xvith the ObSerstations herc fet dou-n; but as I had made abosze 300 of it; I took the Txolable of comparing each of them svith the Hypothefis: and altho' it might have been expeded) that, in fo large a Ntumbera fome great Errors would hasc oecurred; yet there are lrery fesr, qvix. only clescn, that differ from the mean of theSe SO much as 2t'; and not one-that differs fo mucll as 3". This fiurptifing A^,reement, therefore, irl fo long a Series of ObServations, talren in all the sZarious Seafons of the Ycar, as well as in the different Pofitions ofthe Moon's Nodes, feems to be a fuiiicient Proof of the Truth} both of this Hypo- thclis, and alSo of that nIvhich I formerly advanced7 rclatinO to the Aberrations of Light, fince the Polar DiRance in tllis Star may differ, in certain Circum- 0AncCs almoR a MinuteX siz. 56"lX if the CorF rcctions refulting fronz both thefe H>rpothefes arc neglected; whereas, svhen thofe Equations are rightly applied, the mean Place of the Star comes out the fame, as nearly, as can be reafonably ex- pectcd6

iOra-nas



o- yser;; *-StS--X Or E I SQULI]Of Prexf- Aizerra- Aluta- t4dian

> C)racoxis o / fion tion. ti.,xl :DiR. TX. 25

s -

l 27_

/t / // ' , f ar z7 September 3 7o+ - o4 + t9 2 8 9 8O.+

s728 March 18 to8.7 _ o X _ gbo - 8.6 8e 3 Scptember 6,i 70.2 - I*2 + I9.3 -g.l 80.2 729 March 6 to8#3 - I 6 I9 3 _ 7#4 80 0 , Septembr X§ 694 -- 2-1 -I9.3 _ 6.9 8O.2 ' I730 September 8 68.o _ 2N9 _ I9.3 -3'+ 80 S i 1 73 X Septeniber 8 66.o _ 3 .8 _ 1 9.3 _ 1 0 80 s s I732. September 6 64.g _ + 6 _ I9.3 + 2.0 8X.o t

I 73ts AUgtItt 29 60.8 _ 5*4 + 19.0 - 4.' 79 2 x734 Augufl 1 62.3 - 6.z + 16.9 _ 6. 79,^ t73S September I0 60.0 _ 7.I + 19.3 _ 7.9 SO.I I736 September 9 y.3 _ 8o + x9 3 _ 9O 796 _

_

I737 September 6 60 8 8.8 - I9.3 -8ei 79.8 738 September x3 62*o _ 9;6 _ I9.3 7.G 78.7 1:739 Seprember 2 66.6 _ IO.5 I9 2 4.7 SOo

I740 Seprember 5 70.8 _ II,3 _ I9.3 _ I.9 80.7

t74v September 21 7S 4 - I2.t ,_ I9.2 I I gI.4

_

, _ x742 September $ 76.7 - I2.9 - I9.3 -+° 79.r 1743 SReptember 2 8Is6 -I3*7 -I9.t -6.+ 806 X 745 September 3 86.3 - IS¢4 - I9.O _ 8.9 8I.2 x746 September I7 86.5 -16*2 _ 19i2 _ 8.7 80.8 7+7 September 2 86.s 1_ I7-0 _ 19.2 _ 7.6 80;7 .

I made about 2 jo ObServations of n Draasnis; which I find correrpond as well ^7ith tize Hypotlle- fis, as thofe of >; but fins:e the Pofitions of both tlle§C Starsa in reEpe& to tlle SolRitial (;olureX differ but little fiom eacll otheri ie will be needleSs to fer down rllc ObServations of ,8. I Rlall therefore proceed to lay bWforc yollr Lordlhip7 fotne Obrervations of a Xa11 Star that is almolt oppolite to > Oraro- D 2 FAS

r ^



[ 28 ] conis 1ll Riglat Arccul8on, licil,r, tile 3Stll Cgztnelo

rdali IXeqyel. ill the ISrit-if CstEllOg9tC i.K-ril t t*143z feed, indccd, has not gisVtcn tllw RigIst zXlnctnJCoia of tllis Stal*) btlt that bein:, llecc.Sary tf3 be 1e)+-)wl, in ordcr to conlputc tlle Cllalc 0t itS DCCli£sti'.911 arifillt, from rlle PreceElon of tllc Equillox; I com- palZcd tl.c Tilnt of its rfralart ovcr tllc McridianX witll that of Somc other Stars near the fame Pa- tallel; wllcreby I found, tllat its P<i,ht ASccullon was 85° 54.t1 at thc 13cginl-Ztn7 of tlle Ycal: I737.

Tllis ftulall Star sras conaparcd witll tlae lame DOillt of tlle lSimb of tlly SeEtol, as 3 Draconis; arld thc fccond Column, in the follosving Table, ffiews how marly Secands it sxtas found to bc South of that Point, at thz titne of cacll rcEpcdtiszc OlDfelvation. Tlle otiler Co'umns contain, as ill tllc ftoret,otng l abic, the Equations that arc necelEary to find N^llaE

3tS Wean DiRance kotn tlac fanae Poit-<t would laaac lzcn on tbc 27t11 Day of Marah I727, xvhich is cxhibited in tilC laR Coltlmu. Thc xvhole Numbcr of tnY Obswrvatiotls of this Star did nOt IBUCS1 CX cccd forty, tllt greatcLl PaLC OX xvhie:h wrerc madG b<rorc tIze Ntcar 173c); in femc of tlle folionving Years nonc wctc taken; and only a fingle ont in ,l9t othcr, cxcept ill I7 9. HoweverX tllcir Corre t;oo£zelency fcems fuicient to evillcc the Trucll of ti<c Hypothfis: for if the Mcan of tllefe, containd t1 tllc fable, be taken, llot oneX among tllc rcll of

4:*leO6;)Sersarior¢Sa .vill-differ fism it lzzOix tI<all 2-/tv

g5th



-

t rouch or 21e.c{- f?berra- >;icalf 35th Camglopard. O t fion. tion. .<utation. di{'- 2

ilatMelxt. > 8. 25 sOutt^.

ft I {J Xf v . /t

a727 O2ober 20 73.6 1+ o.g -6.7 + 8 9 76.7 8 Jlnuary IX 60.8 I.Z + 6.t 8*S 76*9

Nlarch I 57 8 I.+ + 9 4 X.7 77 X September 26 7S.a - 2. _ S.8 8.1 76.8

L 729 I#ebruary 26 S6 4 2sX- + 9 4 - 7 6 76 < t:730 March 3 S? 8 4 ,; 9.4 S 4 ?7o [73x February FI sg I S (; S;y + .o 762 X,g J;lnuary 3tI (S4.I S.7 82 . 2.9 78-t

8 Ve:C-mber 3o 6r.8 17.z ffi 3 6 S 7f) 8 -739 Fcbruary 4 T6*9 17*3 8 y 6.3 76.+ 1740 January 20 s6.o 18.6 7 ° -4*0 77 6 1747 February 7 32.t 28.s 9 4 + X.4 7g 6

* 01 _

Tllc Olfcrvatiolls - of tilc forcgoint, Stars are tl}c moR ptopcr, to pros7e the Cllange of the Inclilaation of tllc Eartlas Axis to tlle Plallc of the Ecliptic; tllotc, wllicll fbllow, will fficw in 5vhat tnanncr the Stars, tllat lie ncar tlle E<luinoEtial Colure, arc afT- fc&ed, as wcll as othcrs} tbat are (liSrcntly fituated} vitll reEped to tllc Cardinal Poirlts of tlac Equator. Some of tllcfic Stars arc indeed naore remotc fronl the Zcnitlz) tllan I svould lzavc cheScn, if thcre lzad 1;cca otlltrs, of equal LuRtc) in naorc proper Po- fit ions; bccaufc Expericncc llas lorlg Sllce tau¢ht rne, that the ObScrvati<3ns of {uch StarsX as lie near

the- Zenith) do gcncrally agrce 15C1t willl one ano- tilcr) and arc rilcrcSolc tile fitttit to provc tilc

Truth of any Hy7poehtfs. 1 (hall begin witil thofc near thc Vernal Equinox. ct C4Ciofiez was compared Xt1) tlc Pcint nalkcd 34° ssJ; antl at fir(t

%,J$

w ^

29



-

[ 3o] xvas found-to be more Sovtherly, ltlt aft:^rvards trecane lnore Northerly tllAll tllat Point, as i£l tllC

folioxin? 1-2t')tC, tiC laR COIU11111 of NY}liCh «C\\tS iFS [z-)eSll Ditl&telCC 501gth of tllat Poillt oil tlac 27th Ot Msfarsh r727 The ObSclvatioll of thc 23d l:)ay of Ci^Derea2ber} ill twlle Year I738, diffcrs 3" fronz tl1c mean of thc otllers; as docs alfo anothcr, tllat xsas taken fivc DaYs after tllis; neither of wllich bc i1Jo marked as altlcertain I judged it proper to inScrt onc of them; alello' they give the meatl Placc of thz Star near 2 SCCQ11dS more l\lortherly thall ally other,

a SctiCS of above loo; all of whicll corrcfpondX witll tllc mevn of thefe herc recitcd, svitllin leSs tha a"; exceprilag two, that rive the Stars mean Di- iancc altnoR 3 '/ rtzore- Sostherly; but tlaciSe lall mclationed are nzarked as dubious; and indccd they appcar to havc bee2l bad, by comparing them witll 'esteral others, that utcre made near tlle farwc time} sronl which they differ almoR 2//.

bouth of

X C<ope. o t

s4 SS ,,

X j27 September 9 S5 ° t 7z8 (September 1 7 3 s.8 7<g june 8 3S7

't Vecember q N. 9 4 ,30 iune vii S. 3.8

December g N. Xo.8 t£732 January 8 N. 49 2

5> 3anuary 2II SLp.8

73 s J-aae I ) 62.X Xcombnr iI IOz 4

l tS L)ecern.ber 23 X 76. 3 t7X13 lune 169 t7e.7 F.brusty 2.78 33t g

Precel- Aberra- f1on. tion.

{f rz

+ 90 1+ 2.2 29 4 1+ 4.6 A3 8 -- I6.3 St*N tA 165 6+.o - 16.2

r 73-8 + 1tS.3 9S + I2,9

T T6.O + Io.o 8 ! E , } I

Ir 3 / t 16 2340 ' 1S2 z62.8 - 6. 397*t + 02

9 uta- tv1Cat tiotl. DiR.

wouth ., ..

2.4 68.6 S z 70.0 6 8 7e,0 7 7 (S b*+ 7o° S.S 68.1 8 9 68.o 79 69 S ° 69.9 37 68.2 7.2 6n 7 8.9 68.3 4 7 69.6

AlthoJ



[ 3I ] Altllo} I 11ase talxen 1zo ()bfcrstatiozz of z C.r47i

fillCc ttlc 22ti lvti) ol- 7471azz y 17+0; ytt, as tslis cStar is cry n<al tilC ZZllitil? S;lti a {;liciCllt TtitllF bcr xEtcIc Ila ade eal>OtIr ti< r i llaxs sx lictl tilc EquatiOll, rc iliti1R;, frolll llIv Hypotlacfixf nvas at itS lkIaxianam; 1 ju<rged it propcr to intclt fOlllc of tllem itl tllc llCXt rablc; tlle 1a(t Colulun of ^hiclu {hcnvs, llow lllUCil tllC Stal^'S enean l)iflslnct xZas Ssth of 3st'. 20'* 0 tllc 27th I)ay vf LIvrch 1727. Aulonr 11CJt 60 ObfCrVAtiollS l tUCCt xvith t\vo onlr, tilat diffct from tllc nzcall of tllcfc fo mucll as s"; and thofc diticr alnaoLt as nltlcll from tllc mcan of otllcts)

that svcre taken llcar thc famc titn: 0 that tllc Hypotllclis fccnls to corlc(posld, in gcnelal, xvitla the ObServations of tllis Star as well, as nvitll eithEr of the forcgoing.

nout[ ot I tvrtcel- | Aberra- iNuta- Mean T CPerJ4e. O Z fion. ration. tion. DiR.

>g. no South. I ,, I X, s . ,/ /, ,, _.

c7n7 Srptember IG 60.t 4 X + - 3 2 + 6.7 7l.0 r)ecomber ̂ 9 29.7 IT.9 + It.9 72 7I-7

8 December 21 22*S ^7*2 Iz 8 8 7 7I 2 . 8

t vo.9 . ectmrer 2 8. 9.2 42 l.I.s 9.o 7I.7

. . .

t7z1 JuT]udry 3 tS' 8.5 1 59.0 I2.8 8 3 7I-9 I752 JnlAry 8 22.D 74 8 * 12.7 6-7 72-2 E, 3 % January 2I tA 6 91.0 I I.7 + 4-3 72 + r738 December ) II.0 Ig34 I28 1_ go 70.2 [7+o January ^ l S , 2O02 1X.7 8 8.6 70.8

ziftcr the 1a1} recitcd ObferstationsX i. may pcr- Ilaps fecm nccdlcEs to add thoSc of z EPerlei, svhlch is farther from the Zenith; but howescr, as this Star lies sery ncarly at arl cqual Diflancc fronz tllc

E.qtlinO2ial



- - z

-

C 32 ] EquilloSial snd Solllitial Co!urcs, alld tlle Scrics of O'DLC1\tAtiOI1S of it is fonlewhat nlorc conzpl<e, rllan til^1t of T 'Pericii; l fAJ;l ;15!Crt OnC at ]ea!}, for can:l \tC.1r \\ 11CrCilL iC l.SaS l)C<I1 O!DS<rNXd; \N I1CrCbY it naay SppC3t, tllAt tlle i-tj'pOtllifiS l-olvcs tllc Phznomena cul iStAlS il' t;i.S SitUatiOll, SS CXaCtly as in otllers

for if a tAzran bc takelu of tllC N5unabcrs in tlle laR C80]l12ln c)ftlle tolioxnila,Table, wllicll cxprcSes thc ^Seva Dinancc-of l.llc Star SozBtD Of 4"LV. S' on ;11vrrh 27tll 1727, it Xvisl agree withill two Seconds, >itil.CscIy otle of so Oblcrtations, tllat llavi bcen Jnade or this Star.

Prccel- Aterra- iNutation fion. ration.

/} // ,,

+ toS + II 4 + 79 I4.3 - o.8 8,2 X 7.7 I I.4 g.5 3.8 + Io.6 8.8

372 97 8.9 S2^3 II.4 7 8 66.2 + II.4 + S 9

IOt o _ I I.+, 1.l

162.6 + I I .2 | 9]°

t77 4 IO 9 1- 8 2 275-4 6 6 ,+ 8.5

Mean DiR. KSt)uth. {/

to9.2 tog r

4 [O8.9

109 2 I I 0. l

1 I0.3 1O9.8

{°S S

to8 3 to8.o

| z Ter/Si

,27 D>cembWr tp 1728 April ,

Jllly S December I 5

1729 Decembr g r7 I lallWaty 3 1782J3nary 8 r7n July ll X / 3 8 Decernber 24 X 740 January 2 I 747 Februarv 27

Havill:, afready given Examples oSStars, Iying laear 1Dot-1w tllc SolLlices clnd tile Vernal Equilaox; I Shall ,oxY add tllc Obfcrsatiolls of one, tllat is not fa t1otn thc s#atzannal Equillox, ¢7>iz. n UrJ^Z- MIajoriz thc bii>llttft Star ill ttlat Part of tlle HcastensX whicll approachos ttic Zeluitll of tZangZe8( vitllin a Dc3rcc ;

and

bouth ot o ,

41. s ,,

_9 + 87 S 9+.6 6, 7

SX 4 ,8 6 26 8

,,

.). 2t 3

iNv So 3 7I 8 8s s



[ 33 ] tnd wlztch, by xcafon of its lullre and PoRtion, gave nze tlle -Opportllntty of rnaking my Serics of Obfiervations of It,- mwe conlple;tc tllarl of tnany -othcrs. Tbis Star was compared with thc Point xnarked 39°. I S* and was -Nogth of it as i-n the following Table; ̂ 7herein your Lordffiip will SeeX that tlae ObServatioll;s of tlle Years I740 and I74T giVC vlle Polar Diftallces 3' greaterX t-han the M@an oftllc other Years. Had there bectl only a Sngle OSbiSer- vation taken in either of thoSettears) Part of tlzis apparent Di-ffcrellce fnight thave been ftippoSed tO ariX from thcit Uncertainty; but as there utelXe s ObServat1ons taken within a M7eek, eithcr loefore or after tile 3d Day of 7z¢ne I740> which arret well sVitll each othcr) and thlce were made ;Nvithin 20 Days in Sitember I7+I WhiC*l likeV lt: COl'rt- Vfponded with -each other; I am illcltned to tllillk-, 2hat tlle 'firetnention'd DiScrences D1Ua be oxîng to fomctlling elCe} bcfides the E;^ror of tlle ObSer- vations. Tllis SPbanomenon thercfore may deferv: thc Collfideration of thore Gentlemen who haste employed tllcir Timc in making Computations relating tO the Q.uAntity of the -EfictsX whicll the Power of Grasity may} on varits OccStons, produce. For I fufpect, tllat the Pofitioll of the Moons Apogee, as wcll as of her Nodcs, has femc Relation to tlle apparent Motions of tlle Stars tlwat I aul nosg I;caking of.

My Scrics of ObSe-rsrations of Several Stars akund, of lAte Year¢, Witll 0 1nany atld long Interruprioll3; tllQt I cannot prerend tO ieteiomire this Point; but. pr-obably the Differcllces bcSore taken IlOtiCC of tt-t t1Xc Otbfierstations of : C+per, alld ioXcs otileU;

E tht



[ 34 ] tlsat I have found likcwiSe am-ong the Olzf¢rvation5 of orrer Stars, that are not here recited, nnay be oxaring to fuch a Caure; wllich, altho it thould not hae any large Share of Inflloence, nzay yet, in cer tain CircumXtances, diScover a Defect in a Hypothe- fis, that pays llo Regard at all to lt. But svllerlaer rllcCe lliSerences do arlfc fiô*l thz Caufie aIready iilitcd at; or uriletller thcy prc)ceed from any T)cs tc ct of tlle tiypotlleSs itSt l f in any ottler refS ect ; it s.ill Xlot bc ?cry materiwlillpointofPracticc; fince thzgt i-l)tpotilcfist Z5 it xras before laid down, appeals tf.8 iC ZfiI-cient to bolsZe all tlae fDsowena, to a.< i<!}'t't a I)<ree of ExacttlcEsX as xve can in gcncta

ivope or e.rpeE/; to lalallc Obfcrvations. Fol it I tal<c ri-$c myears of all the Nuxubers in tI1e laR Colunzil of zi-;c fcvlloxving Table for w Ur/8 AIajoris, and com- parc it svitll any cBnc of 16+ C)bfcrt7ations tlaat Brere tWkcn of it, tlwc lDiffcrence will not excecd tllrec Sccc>nds.

i bcsuth ot Precel- - Aberra- Nutation Mean [t tJr/eMajorzsg90 } fion. iion. SDoifihl

{/ /S // // { /

te7z7 O&ober il 1S3.3 -I0.2 $ l.0 -S.2 1389 7z8 January 24 r7<; 4 15 2 - 17 6 r s8 l37 8

July I /<150.8 23.9 + I7.8 6.9 t37.8 OEtober Il: .170.6 28.2 + 2.6 7vs [37 7

Z729 fianuary 16 196.6 33tI - I7.8 7.8 t37L9

July zl 70.4 434 + 178 84 1374

December z l89 6 68 7 - I6.7 8 9 I37 8

1/3 SCpttllStr- -lt zI8.t gI.9 + 94 8 + 37 2 £732 January lo 25o.7 87.7 _ 7 7 8o 137 3

April 1^ 2?8.7 92.3 - - o.8 7 7 rs7.9

1 724 J IlV 1 \ '-SS 7 3 [ 37 7

at UrAz



_ . . . nz - - _

| s t . v j

- Z

Aberra- tion.

JX

II.4 t.6 I7.2 I6.8

2.8

12.8

7-9 12.4

8.g

3.t

1> uration ean DiR. .South .

,{ //

+ I,2 t32.s 4. I 1 3/ 6 6.t 138. 7 9 l37 9

S 8,8 fAg s

X.g I+O 9

e 7.+ IX,lr 2

3-3 I32' *7

SS t387 7 8 I t9.

_ South of Precet: " UrJ Majorzs ° ' fion.

139 IS tt

r 73 S September t° 28 o. 8 [ 736 SeptembQr 8 ̂ }B9+ 7 ,737 July 3 303.cx [738 June 29 3 I930 { 7g 9 April 2 R4 8*9

.. 7+c) June 3o360.!

f 741 September 23 3 9o.9 * 74S vSeptember T +66.7 r 7+5 Scptember 20 +92.o t 747 vSeptemlrcr 2 So7.2

ro

IS4.6

> I72.8 I87-8 zoS.8 220 8 24I I 26S.0

3 3 7* I

3S(}.2 373*5

You may perceive, my lword, by infpeAin:, tElc Tablcs nvllicll e:ontain -tlle ObSelvations of U Ctz@zo- pea? and x Urf ]zajori.s; tlzat tlae grcatc(t Diffclellcvs tllat occur thcrcin tnasy be dim.ilai(Mcd, by fuppofiraD thz true POtc of the Equator ro tnove rourld tlle loint 23} in an ElExpJ4sX illftead of a Circle, For it tllc trallfvcrfc AxisX IJin^, ila the DircAion SC, lve 18", and thc Conju;,atc, as OB, bc about 16"j thc Equations, refuiting from luch an Hypotllcliss will make tllC NZumbcrs in the laI} Colllm1ls a^,ree with eac4 otller, ncarcr than as tlley llow Rand. Bllt fillce this nvould not entirely rcmove the Inequa- litiks, in alL tllc Pofitions of the MoonJs Nodes ; I ffiall rcfier the more accurate Vctermiliation of thz Locus of thc trge Pole to Theory; vnd at prefcnt olhly give tllC Equations br tlle Preceflion of thc Equinoctial I'oints, and ttze Ob;iquity of thc Eclip tiC, AS AsiO tllC rtal uantity of thc annuai PrcceS- fion} tO every stl. De:-ce of tlle Place ofthe Moon's Afcxnding B'odc, in thc folloNwinD Talulcs; juLt as

E 2 tllcg

r 35 3



-

[ 36 3 tTley refuIt fs;om. tlle Hyp9thefiS) as at fitR laid doXsW it appearinv,, from wlast has already been remarksd) that thcfc will bc i:uiciently cx.act for Pra<ticc ill a51 Caft.

_

I tie Eqbatie}n ot - 1 hcW Equationot the b the E.qutn¢e. Points * Obliqu ty of the Ecl!litick. 0 f

XS a 5lg t l 11 'Uhlt. XS a i5'}g- C t I Hdd Ifr<m r .0 v ,1 < Vl?l j\(1d trom w nig. VI ViI Vltt bUbiti o j // Xr XJ Q . o {/ {t ---

o oo xt 196 e c) s.o 78 45 zo

+ , . _ .

5 7o ;13o 2tS 2s s 9o 74 38 2s I I0 ze9 '+ S '1.2 20 I0 8.u 6 9 3.1 20 ls s8 6-s -Ig Is 1s s7 64 2.3 15

. . .. , . _

20 7 7 (7^3 2.2 10 - > 8 5 . 5 8 1.6 I0 z5 96 '8-S 22.5 5 , 25 8.2 S2 o-8 S gf) Ir t { 196 22.6 ° t° 78 tS °t O

. . h __

SU4Lt jig. * 1S Iff 3S !? aa(s IvS'i. \' 1V 111 Xsg Ad {'Big.X. X 1 1X |fr°mY rublt . ,;lg. Xl X IX fr<)m r

. ;

The Annual PreceSlon of the ,quinoEial Pc)in, s.

)'J ai

.rom X S.v;. O I IT ItI IV v _ . _ I . g // // // /{ 1/ /t O

o S2o s70 542 s°3 t44s 43? to s s7 9 S6.6 53 6 49.7 46 o R 43 4 2s IO S7 9 S6.2 St3 ° 49.0 45 S 1 43 2 1 20 r iS S7 7 SS 7 SZ 3 48 + 45 ° 43 ° IS 20 1-5 55.2 Sl 7 47 7 4T S t 42.8 IO 2S S 7 3 5 4. ? : 5 I ° 47 I 4+. X 42. 8 5 30 s7 ° 54 2 s° 3 46 s * 43 7 42.7 o

sig. X1 X IX VJll VIS V1 Ds n

| . trorn er



L ox J Sir I%ave iMetto in det{:ltairlillg tlse Qlantlty

ofttle anrlual Precefion from tlle TlleolAy of C,a- \'iti', upon Suppofttion that thC EqUatO1*ial iS tO tlle I)oXar Diamctcr of the Earth as 230 is to 229, tSds tlle Sun s Action fufficicnt to product a Preccfltvrs of 9'' only; and, collecting from the Tides t-llC

I'roportion betweetl the Sull's Forvc antl the hIoon s to bC GS I tO +, he fcttlcs tllc n1catz Ptec^0ion, re- fuLtia; from tilvir joint S\CtiOl]S) at 5°1t. BUt llnce tlle Di4-Z<>rcnce bctsvecn t3c Po]ar aI<d Equaterial I)iatnct<>r i found) bs t lle latc Obficrvat ions of thz Gen tlcrnen of t'ne Acadetnv of Ssiegres, to be greatcr thaa V Ilat Slr lfaac had conlputed it to be; tlle Preceflion) alifillg froLn the Sunts Action, murt l kesviSc be nrvatcr tlzAn svllat 11e has Rated it at, nearly in the nne Propo:-- tion. From whence it will follow) t'lat tI'c J\1oollts Force muQ Mcar a lefa Proportion tO thz Suns tJlan 4 to I; andpcr'laps th: SPha?nounana, x;?hicla Ihasc noxv becn gi-in;: an Account ot will fupply ttie bsrt qData for (CttlIIln tlziS Atattcr.

Ns I apprelscnel, th-;t t112 Ob.er-vutions alrcady fct do.n wiil 13c jlld ,>d fuiciellt7 tO proN!e ir gTncra;

tln(: Trli a of thc Hy?othefis bttorc adsZanced; I fhall not trouble :rOllr Lord>,^lip NVitll tllC llecital of more, t{aat I rtlade of Stars lytin:, atteatcr IDillallces from tllw Zerll ll; thofe l.ot bcillO fo proper, forthe Rca.- fon 'soforc-im;cntiond) ta eftabli^n .he Pol-O5t t>.a I t^.cl chietSy ..n V1C\RI. But as it maopvrhaps Sc o-ffotnc C4fe to lutlrc .Afirolaomers) to kntsv xvhat xverc the tveaa IDiScrenccs of D¢clin<tion3 At S{ gil'Crl Tirne, l)ctxvc-ea fcome Stll*s, that lic rAC>riy op, ofitc to ollc anothct in RiDht XXfcenfion, and not fir from either of the Colres; I r!ll ftt dosvn the ltefillt of tlle Compa- nfen of a feX) tat diS¢r bO little .rs Dcc jinti(,n,

* tz r ts



17 38 ] that I could dctcrnzille rlue QMantity of that DiScre cr;c wich great Certainty.

By the snea7s of s4* Oofcrsations, that xvere made of M Cal7afe befots: tlL< End of the Year 1728, I coltcSt, aFter allov.ring for the PrcceIElon, Aberra- tion ad Utation as in the foregoing Tables; tllat t}.lC meav DiEance of tllJ.s Stax was 68".7 South of t4°. 55', on the 27Fh Day of ]<arrh I727. By a Joixe Comparifoll of 4o Obfxrva.iorls, taken of , EJr% lalajarts durtag the fatrje -Intcrval of Tinle, L fizd tilis Star wras, at tile fame tinze 39".6 South of 340. 4f1. 1 sarefully meaftll*cd, with the Scrcw of thz lMicrometcr the DiRancc between the Points, wzritIl svlllcil tize6e Stars were compared; and found t. c=m ta bc 9' sg" from each other, or one Second lc.s than they ought to havc been. Hcnce it fol- 1(3vsr<, tllat the enean Diffcretlce of Dcciinatioll be- tssreetl rhete t\vo Stars, was 1 o'. 2 8".1, ola thc 27t1 Day of Marah I 727.

lAy tlle mean of 6S Obfiervations t'nat nverc taken of s Cvope} befbre taC End of the Year I72^8s tlliS Star Vras 25tJ.8 7Oftll of 32°* 20tx on the 27tll Day of SIarrh 1727: and by the mevx of s2 Ob- lervationc, s Ur% Majoris svas 8;'t.6 South of 329v

30' at the fame tinze. T-he DiRance betsreen tllcSe Poznts xvas found to be 9'* ss ' 3 ; from sYhence it follows, tllat tlic mean Diffcrcnrc of Dcclinatio < bc- twccn thefv two Starc wxJas I lt 5 2''.7 on March 27th

t7 Bxl tllc mean of lco Obfcrvation¢, taken beforc

the End of ffie Year I7z8} tllc mean Diltancc of > 1Draconis was 79".8 Sollth of 38o. 2S' on March 2;rtis 17/27; and by tlae mean of 3s O'vl urnratsons,

thz



w -

39 J the 3sth Cametapard. Heq3el. was South of tlle fame Spot 76".4 So that the mean Polar DiRance of > C:Draronis was only 3"r4 ,reater than that of tlle 3stll Carxelopard. Hevel. but as tllc Equat(3n for the l\utation, tn both acrc StArs, NSAS thcn nt'2l tlle Mazz?ztn, and to be applied BJith contrary Si^,ns; the Sppareat Polar DiSance of > 7Oraconi.r X2as 21'.4 grcatcr, on etle 27ih Day of .ll!4rrh 727.

Ille Differcnccs of the Polar Di(lances of tlle Stars, as hcrc fct doutnX may be prefumedX tooth on account of thc Radsus of the Ia>0lument and the Ntunzbct of Olfcrs7atlonsa to bc very exadly deters minedX to the Time xvhen tlae Moon's AfcendinD Wwode vas at tlle IJegi£lnir:^, of JJries; a^d if a liJve (otnpariton be hercaficr made, of ObServations t;2ken of vlle Eamc Stars7 near the fimc PolStion of the Moons Ntodcs; future A(rronomers may be ctlabled} to fcttle thc tantity of the mcan Pree:eflion of tlae Equinox, lo far as it affeds the Derclinatiotl of ttllQfe: Stars, xvith ercat Certainty: alad tilcy nlay likesvite difcover, by means of tl5M Stars ncar the So3flitin.t Co'ureX from what Caufc t]at apparent thange ir the Obliquity oftlle Eciiptic reaBly procecds) if t5C mean Obliquity be found to diminllh aradua!1y.

Thevforementioned Points in<;!eed can be fcttled cnly on tl-le SuppoSition} that the vnglllar DiRances of thefc Stars do continue always thz famc^, or that tlley have no real ^10tion in thenllelses; but are at Itefi in AbColute Space A S;uppofition) s^laich tlloUgll urlly madc by A0ronomersX ncvcrthcleSs Scenns to bc foundcd on too unccrtairl Iînciples} tO bc admitted in all Cafe.s. for if a3udrment may bc formed, xvith Regard to rlais Matter, from t}ê Re-

rult



04°3 Gu4 of the Cotnparifon of out beR modern ObScr- sfatlotlsX with fL3ch as were formerly tnadc xvith any tolerable Degree of ExaRnefs; tlaere appcars tO haswc b :en a veal Chanoe in rlle I)o'ition of lonac of the Sxed Stars, 'itll ttfpect tO cach oFher; and fucla, as {;;crr3s illdependent of anXt N.8Otion in out own s.>r- tCtN, and can onlv bc .U-+erred to fome Motion in tiac Stars thcnzf<lvc;9. S=r,rJ?rss afrords a Eronfr Proof r

czf rni: fol if Its p-cftlt Vecl,ination be comparcd xVitll its Place, as dctermined cither by TyakXo or I larsJ?eed; ttrlt DiSercnce wii} be found to be much XatcrX than Etstl<t can be fufpectecl to ariCe fi-on

riwc UTncertaintxr of tileir Obfervations. It i5 rcatonalzle to exped;t, that otlzcr InRances of

tllC llke k.nd muR alfo OCCU14 among thc great N7um- bcr of the lifible Stars: becallfe their relative Po- filtiorls may be alter'd bv various nzeans. For if our OWll Solar SyRctn be conceived tO change its Placc NVibll refpect to AbSolute Space; this naiDht, in Pro- ce& of Time, occa(iors a3;< apparellt Challge in tlle an(ular DiRances of the fixed Stars; alld in fuch a CaCc) the Placcs of tl-<e llearelt Stars bein^, tnore af- fcctcdX tllan of thofe that are very remote; their rc- 3ativc PoStions nligllt feem to altcr; tho' tlle Sta:s thcmSelvcs were really immon?eable. And on the othcr Hand, if our own Syttem be at ReR, and any of the Stars really in M-otion, tllis tnight likc-wiSe vary tlleirapparent l'ofitions; and the more fo, the nearcr they are to us, or thn fwifter their Morions; are, or the more propcr th: DArcctiors of tlrc Motion is, tO be rendelcd pcrceptible by us. Sincc then thc AcZ lativc Places of the Stars may be chal13ed from fLlch a Varicty of tulcs, confi<ierillO tbat amazinJ D1-

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S 42 3 It s-VcLe to be sPiiEljd therefore, thdt fuch PerSons

as are provided with proper InRrumellts, sr¢uld a- tempt to detcrm!rl>, x-;ith -great Cae. tllc preint relaiive Pofitions of feserat or the Principal Stars, in Yarious Pal ts Oc tl,e Hlkas ens; eXpecially of thofe, tlaat are lealt affcctcd by Pxcfraction: that CaluSe having many tiR;CS fo unccrtain an InSuence on the Placcs of Objects, that are sery rennotc from tlle Zcnith; that wherever 1t is conccrned, the ConcluSions, de- duced fronz ObServations tllat are mgch affected by it, will alnvays rem.ain doubtful) and tOO precarious, in many Cales, to be relsed upon.

The A-dnanta^,es, arifinO from different Perfons attemptinz tO Setele the fame Points of ARronomy near the fame time, are fo much the greater; as a Con- Cllt rencc in tlae Refult, sxtould remove all Sufpicion of IncorreceneEs in tlle Inftruments made ufe of. For shich ReaSon, I efl[eem the curious Spparatgs at Ahirbarn CaJ/le, and the Obfernations there taken, vs a moft staluable Criterion) whercby I may judge of thc Accuracy of thote, that are made at the Royal fJbfiervatcry : and as a Lostel of Science I cannot but wi5, that our N7ation abounded svith more frequent Examples) of Perfons of like Rank and Ability nvith your Lordthip, equally deltrous of promotinc, This, as well as every other -Branch of S7atural ICnow- edre, that tends to the Honour and Eenefit of our

country. But svere the Patrons of Arts -and Sciences eser .fo

numerous, thz Subject of my preSent Letter is of fuch a Nature, as muR direct meX tO beg Leavc tO

addrefs it to the Earl of MacalesyEeld; not only as a moR compctent udge of it; but as the fale Per-

fon,



[ 43 ] fon, in this Natio£, tllat hath InfErllments propcr to cxaaninc intc) the Truth ofthc Facts hctc rclated. Alld it iS a particular Satisfaction to lmeX that after fo lont, an Attendancc upon tllcSc (Phnomena, I am allowed thc Honour of tranfmitting the Account ot theal to the Public, throw your LordfEip's Hands: as it gives me at the famc time an Opportunity of profeffilng the grateful SenSe I {hall ever retain, both of the fignal Favours whiclu I formerly receitired from the noble Earl yollr Father, and of the many reccnt Obligations conferr'd by yourrelf upon

MY LORD,

rogr Lordlbips

moJ? obedien

Fmble SerqJant, Greentvich, Dec. 3so

I 747*

Ja Bradley

lI,



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A Letter to the Right Honourable George Earl of Macclesfield concerning an Apparent Motion Observed...

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