POSfTION OF irOUBtE STARS. ' Git 



thing farther on the fabje6t of this firft article of my claffifi- General obfer- 

 cation ; I may therefore immediately go to the fecond, which 7^^"' °"t^g* 

 treats of binary fidereal fyftems, or real double ftars. heavenly bodies. 



We have already (hewn the poffibility that two ftars, what- 

 foever be their relative magnitudes, may revolve, either in circles 

 or ellipfes, round their common centre of gravity; and that, 

 among the multitude of the ftars of the heavens, there ihould 

 be many fufficiently near each other to occafion this mutual 

 revolution, muft alfo appear highly probable. But neither of 

 thefe confi derations can be admitted in proof of the aftual ex- 

 iftence of fuch binary combinations. I (hall therefore now 

 proceed to give an account of a feries of obfervatipns on double 

 ftars, comprehending a period of about 25 years, which, if I am 

 not miftaken, will go to prove, that many of them are not 

 merely double in appearance, but muft be allowed to be real 

 binary combinations of two ftars, intimately held together by 

 the bond of mutual attradion. 



It will be neceflary to enter into a certain theory, by which 

 thefe obfervations ought to be examined, that we may find to 

 what caufe we ftiould attribute fuch changes in the pofition, or 

 diftance, of double ftars, as will be reported; and, in order to 

 make the required principles very clear, I ftiall give them in a 

 few (hort and numbered fentences, that they may be referred 

 to hereafter. 



In Plate. X. Fig 1. let us call the place of the fun, which Theory of ths 

 may alfo be taken for that of the obferver, O. In the centre of "^^^^""^ °^ '^ 

 an orbit or plane N F S P is a Geminorum ; and, if any other 

 ftar is to be examined, we have only to exchange the letter a 

 for that by which fuch double ftar is known. This letter is 

 always underftood to reprefent the largeft of the two ftars which 

 make up the double ftar; and a general expreflion for its fmaller 

 companion will be x. N, F, S, P, reprefent the pofitions of the 

 different parts of the heavens, with refpe6l to a, north, fol- 

 lowing, fouth, and preceding ; and the fmall: letters n, /, s, p, 

 ftand for the fame dire6lions with refpedl to O. xaP, is the 

 angle of pofition of the two ftars x and a, with the parallel 

 F P. 



As the motion of an obferver a fre6ls the relative filuation of 



objeds, we have three bodies to confider, in our inveftigatlon of 



the caufe of the changes which will be pointed out; the fun, 



the large ftar, and the fmall ftar, or, as we have ftiortly called 



f2 them. 



