ABE 



Ai\, Mr.' ASerncthy, in 1731, wrote a paper, in fuhrer- 

 TJcncv to this dcl'ifjn, with a view of Ihcwing the uiircaloii- 

 ablencfs and injulUce of iiU thofo laws, which upon account of 

 mcr* difference in religions opinions and fonns of worfhip, 

 excluded men of intcfrrity and nhihty from ferving their 

 countr)', and debarred them of thofe privileges and advan- 

 ta;»ej to which thev had a natural and jutl title as free-born 

 fubiefts. He particularly infilled that, confidtring the ftate 

 of Irclaod, it was a great error, in point of policy, to con- 

 tinue rellraints which weakened the protelbnt intereft, and 

 were prejudicial to the government. In 1733 the Irifh dif- 

 fcnters made an attempt for obtaining the repeal of the 

 obnoxious acl, and Mr. Abernet'iy again appeared from the 

 prcfs in favour of the fcheme ; but the defigii mifcarried. He 

 continued his labours in Wood-llrctt for ten years, and en- 

 joyed great fatisfaclion in the fociety and elleem of his 

 friends. From the ftrength of his conllitution, the vi- 

 gour of his fpirit, and the uniform temperance of his life, 

 there wa« reafon to hope, that his ufefulnels would have 

 been prolonged. But a fudden .-.ttack of the gout in the 

 head, to which diforder he had been fubjeft, frullrated the 

 expectations of his friends, and he died, December 1740, 

 in the 60th year of his age. For this event he was fully 

 prepared, and he met it with great compofure and firmnels of 

 mind, with a chearful acquiefence in the will, and a fixed 

 truft in the power and goodnefs of the Almighty. Mr. 

 Abeniethv was twice married ; firll, foon after his fettlement 

 at .Antrim, to a lady of excellent character, of whom he was 

 deprived in I 712; and again, after his removal to Dublin, 

 to another ladv, with whom he lived, in all the tendernefs 

 of conjugal affection, to his death. The moll celebrated 

 of Mr. Abernethy's writings were his two volumes of Dif- 

 courfcs on the Divine Attributes, which were much ad^ 

 miredat the time of their publication, and honourably recom- 

 mended by the late excellent Archblfhop Herring ; and are 

 ftill held in the higheft efteem by thofe who are difpofed to 

 approve the mod liberal or manly fentiments on the great fub- 

 jefts of natural religion. Four volumes of pofthumous Ser- 

 mons were likcwife publilhed, the two firft in 1 748, and 

 the others in 1757; to which is prefixed the Life of 

 the Author, written, as is generally underftood, by Dr. 

 Duchal. A volume, intitled, " .Scarce and valuable Trafts 

 and Sermons, &c." was publiflied in 1751. He alfo left 

 behind him a Diar^' of his Life, confiding of fix large 

 volumes in 410. of which the author of his Life has given 

 a large account, and from which he has made many valu- 

 able extracts, which bear ample teftimony to the fingular 

 excellence of his difpofition and character. Biog. Brit. 



ABERRATION, in ytjironomy, an apparent motion in 

 the celeftial bodies, occafioned by the progrefiive motion 

 of light, and the earth's annual motion in its orbit: the theory 

 of which is explained by Dr. Bradley : for an account of 

 whxh, fee Light and Star. 



This theory may be illuftrated and applied in the follow- 

 ing manner. If light be fuppofed to have a progreflive mo- 

 tion, the pofition of the telefcopc, through which any celef- 

 tial objeft is viewed, mud be different from that which it 

 would have been, if light had been inftantaneous; and there- 

 fore the place meafured in the heavens will be different from 

 thet rue place. Thus, if S' be a fixed liar (Astronomy, 

 platei. fig. I.), VF the diredion of the earth's motion, S'F 

 the direftion of a particle of light, entering the axis ac of a 

 telefcope at a, and moving through aY whilll the earth 

 moves from e to F, and if the telefcope be kept parallel to 

 itfelf, the light will defcend in the axis. For, let the axis 

 nm, Y'M, continue parallel to ac ; and if each motion be 

 eonfidered as uniform, that of the fpeftator, occafioned by 



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the earth's rotation, being difregarded, becaufe it is fo fmall 

 as to produce no effecl, the fpaces dcfcribed in the fame time 

 will preferve the farce proportion ; but ^F and <jF being dc- 

 fcribed in the fame time, and as we have cY : aY : : en : av, 

 en and av will be detcribed in the fame time ; and therefore 

 when the telefcope is in the fituation nm, the particle of 

 light will be at v in the telefcope; and the cale being the 

 fame in ever)- moment of its defcent, the place meafured by 

 the telefcope at F is j' and the angle S'Fj-' is the aberration, 

 or the difference between the true place of the flar, and the 

 place meafured by the iuftrument. Hence it appears, that 

 if we take FS : F/ : : the velocity of light : the velocity of 

 the earth, join S/ and complete the parallelogram Yt^s, the 

 aberration will be equal to FS/, S will the true place of the 

 flar, and s tlie place meafured by the inllrument : and this 

 latter is the fame with the apparent place of the objedl, as it 

 would be feen by the naked eye. In order to prove this, let 

 it be confidered, that if a particle of light fall upon the eye 

 in motion, its relative motion with regard to the eye will be 

 the fame as if equal motions, in the fame diretlion, were im- 

 prefTed upon each at the moment of contact ; for it is well 

 known to be a principle in mechanics, that equal motions in 

 the fame diretlion, impreffed upon two bodies, will not affeft 

 their relative motions, and confequently the effcft of one upon 

 the other will not be altered. Let VF then be a tangent to 

 the earth's orbit at F, and reprefent the direction of the 

 earth's motion at F, and S' a ftar ; join S'F, and produce it to 

 G, and take FG : Fh : : the velocity of ligbt : the velocity 

 of the earth in its orbit ; complete the parallelogram FGHn, 

 and draw the diagonal FH. Since FG and nF reprefent the 

 motions of light and of the earth, if we conceive a motion 

 F« equal and oppofite to nY to be impreffed upon the eye 

 at F, and upon the particle of light, then the eye will be at 

 reft, and the particle of Hght, by two motions FG and Fa, 

 will defcribe the diagonal FH, which is its relative motion 

 with refpect to the eye itfelf. Hence it follows, that the 

 object appears in the direction HF, and confequently that 

 its apparent place differs from its true place by the angle 

 GFH = FS/. 



But by trigonometry, fine FS/ : fin. F/S wYt: FS : : the 

 velocity of the earth ; the velocity of light, and therefore 



^, ,, , , . ^ _, _ vel. of earth , . , , - 



the line or aberration = fin. r ta X — ; ?^.-r- ; and if thcfe 



vel. of hght 



velocities be confidered as conftant, the fine of aberration, or 

 the aberration itfelf, which never exceeds 20", varies as the 

 fine of F/S, and is therefore greatefl when F/S is a right 

 angle. Let j- then exprefs the fine of F/S, and we fhall 

 have I (radius) : s : : 20" : s x 20", the aberration. Hence 

 when Yt coincides with FS', or the earth is moving direftly 

 to or from a ilar, there is no aberration. And fince FS/ = 

 20" when F/S = 90°, we fhall have the velocity of the 

 earth : the velocity of light : : fin. 20" : radius : : i : 103 14. 

 It appears that the aberration S's' lies from the true place of 

 a ftar in a direction parallel to that of the earth's motion, 

 and towards the fame part. 



M. de Maupertuis, in his Elements of Geography, fami- 

 liarly illuftratcs the aberration by the direftion in which a gun 

 mud be pointed in order to fiioot a bird in its flight. In- 

 ftead of pointing ilraight to the bird, the fowler dircfts the 

 gun a little before it in the path of its flight ; and fo much 

 the more as the flight of the bird is more rapid with refpeCt 

 to that of the fhot. M. Clairaut too, in the Mem. of the 

 Acad. Scienc. for 1746, explains the aberration by fuppof- 

 ing drops of rain to fall quickly and rapidly after each other 

 from a cloud, under which a perfon moves with a ven' nar- 

 row tube ; in which cafe it is evident that the tube muft 

 have a certain inclination, in order to admit a drop which 

 2 enter* 



