S54 



Proceedings of Learned Socitlies. 



[Oct. I, 



which Jifi conGdrrs as a fcfjucl to his ac- 

 count (if the direction of tlie folar mo- 

 tion. With rcjrard to the proportional 

 rhll;ince of the iiars, Dr, II, obfcrvcs tliat 

 " neither tlic parallactic nor real ination 

 of a ftar can be afrertained till its rela- 

 tive dillauce is fixed upon. In attempt- 

 ing to do this, it will not be fatisfadtory 

 to divide the ilars into a few magnitudes, 

 and fup[)ofe t/ii-jh to rcprefent the relative 

 diftanccri we require. Th.ere are not 

 perhaps, anionsi ail tlie ftars of the hea- 

 vens, any two that are exactly at the fame 

 diitance from us; much lels can wc ad- 

 mit that the ftar.-i, which we call of the 

 Jirlt maguiLudo, arc equally diftant from 

 the i'an. And, indeed, if the brij^htnel's 

 of the ftars Is admitted as a criterion by 

 *hich we arc to arrange them, it i.s per- 

 fectly evident that all thofe of the lirft 

 magnitude muii diller us much in diilance 

 as they certainly do in Inltre , yet impcr- 

 fc6t as this may lie, it is at prel'ent the 

 only ru'c that v.e have to go by." The 

 relaf.'.e brightnefs of iix ftars may be ex- 



jirtUi (.' thus : Sirius Arcturus — 



<'a|..<.!l;i ^ Lyra Aldebaran ♦ I'ro- 



•"yi/U * 'i'licii the proportional dhiances 

 *ill be cxprclfed as follows : 



Sirius 1.00 Lyra 1.r>0 



Arciiirus 1.20 Aldcbaran 1.40 

 Capcila 1.'25 I'rocyon 1.40 

 The (HiVcrencc between Sirius and Arr- 

 tarus is lierc made very conl;dci"ablc ; 

 rjtit, accordiii£; to Dr. II., not more fo 

 tban the dili'ereuce in their brightnefs will 

 fully juftify. 



\\ e have next a table drawn out to 

 llicw that an inrrcafc or decreafe of the 

 folar i!ioti(in will h;s\ c a contrary ciTeCt 

 upon the required real motions of diiicr- 

 tiit ftars. Ijy this table, the real motion 

 of Arcturus ronijiared wrtli that of Alde- 

 haran, llicws that when the folar motion 

 is inc-eaicd from 1.0 to 1.5 and to 2", 

 tlic real motion of ArCturus will be i!,ra- 

 dually diminillicd from 1.57 to l.oO to 

 1.02, whik; tl'.at of Al.debaran undergoes 

 n contrary cl'.ur.Lre from 0.53 to O.UtJ and 

 to 1".10. 



I'rom thefe and otiicr confidcrations 

 f/e are told, tiiatthc motions of Arcturus 

 and Aldebartm being contrary to each 

 otlicr, may be made pcrtecdy equ:d by 

 fuppofing the fun's annuid motion to be 

 l",y5925 ; for then the real annual mo- 

 tion of Arcturus to-.varfln the parallactic 

 ' Centre is l".0ni, and that of Ahiebaran 

 towards the oppoiite jiart of the heavens, 

 in wliich tin' tuiar apex is j'laccd, will be 



l".O01 likewifc ; the frrft in a direction 

 55" 29' 39 fouth preceding, the latter 

 88° 16' 31" north following their rcfpec- 

 tive parallels : and a compofition of thefe 

 motions with the parallactic ones, arifing 

 from the folar motion, will produce thn 

 apparent motions of the ftars which have 

 been eftablilhed by obfcrvation. 



Dr. H. next proceeds to calculations, 

 for drawing figures that will reprelent the 

 obfcrved motions of the ftars : thefe arc 

 illul! rated with figures. He then goesou 

 to take a gcneraf view of the caufes of 

 tlie motions of celeftial bodies ; and he 

 obfervcs, that a motion of the ftars may 

 arife either from their mutual gravitation 

 towards each other, or from an original 

 projectile force imprelVed upon tliem, 

 Thefe caufes are known to act on all bo- 

 dies belonging to the folar fyllem, fo a^; 

 to give them u very particular appropiiatc 

 direction. 



As attraftion acts at all diftanccs, it i.s 

 to be conlidered whether the motions of 

 ftaj's can be accounted for by the nititual 

 gra\itatiQh of ftars towards each other, 

 or by a periodical binal revolution of 

 them about a conunon centre of gravity ; 

 or whether wc ought not to have recourfe 

 to iinne very diftant attradtive centre. 

 This (fays tlie doctor) may be decided 

 by calcuiation : — let tlie fun, for inftance, 

 and Sirius be two equal bodies placed in 

 the moft favtmrable fituation to permit a 

 nuitual approach by attraftion, then it 

 may be proved that the fpace over vvliieh, 

 one of them would pal's in a year, were 

 the matter of iioth collected iii the other 

 as an attractive centre, would be lels 

 than a five ihoufand millionth part of a, 

 fecond, fuppoiing that motion to be fecii 

 by an eye at the diftance of Sirius, and 

 admitting the parallax of the whole orbit 

 of the earth on this ftar to be one fecond. 

 This proves that mere attradtion* eaiir;ot 

 be the caufe of the obferved hdereal iw.r 

 tions. 



In the cafe of fuppofcd binal revolu- 

 tions of ftars about a conunon centre of 

 gravity, the united power of the con- 

 nected ftars, provided the mnfs of eith<>r 

 of them did not greatly exceed tliat of 

 the fun, would fall very fliort of the ai- 

 trartion re(piired. The ftar Arcturus, 

 wliich moves in an oppofile direcli(m to 

 the propofed lolar motion, were it con- 

 nected with the fun*, 'nind the proper pro- 

 jeffile motion could not deferibe an arch 

 of 1" about their common cctitrc in lets 

 than 102 years ; and thoiji;!) the n\ potite 

 motion of the fun by a [.•.ualluciie elTeCt 

 wonl<{ 



