'g^C CAUSES or THE CHANG5S OF RELATIVE 



©bfcrvations admit that the folar velocity, in the direction we have caTctr- 



jr/eahT'th" ^^^^^' ^'^' produce an apparent retrogate motion in a, which, 



changes of rela- i" '^^l vcars will amount to 2",085 in right afcenfion. But the 



tive fituations in fame parallax will alfo occafion a change in declination, to- 



«car each other, wards the fouth preceding, of 3",701 ; and, as this will not 



agree with the obferved motion of a, we muft account for 



it by a proper motion of this ftar direftly towards the 



north. The real annual velocity required for this purpofe, 



muft be 1,3925. 



The apparent motion of x, by parallax, at the diftance we 

 have placed this ftar, will be 2'',832 towards the fouth pre- 

 ceding ; and, byadigning to it an annual proper motion of the 

 velocity I,3'354, in the direction of 73® 10' north preiSbding 

 its own parallel, thecf!e6l of the folar parallax and this proper 

 motion together, will have caufed the fraall ftar, in appear- 

 ance, to revolve round «, fo as to have produced all the 

 changes in the angle of pofition which my obfervations have 

 given ; and, at the fame time, ex. will have been feen to move 

 from its former place, at the annual rate of 0", 105 in right 

 afcenfion, and 0'^,12 in declination towards the north. 



In this manner, we may certainly account for the phenomena 

 of the changes which have taken place with the two ftars of a 

 Geminorum. But the complicated requi files of the motions 

 which have been expofed to our view, muft furely compel 

 every one who confiders them to acknowledge, that fucb a 

 combination of circumftanccs involves the hrgheft degree of 

 improbability in the accomphfliment of its conditions. On the 

 other hand, when a moft fimple and fatisfadory explanation of 

 the fame phenomena may be had by the etfeds of mutual at- 

 traction', which will fupport the moving bodies in a permanent 

 iyftem of revblution round a common centre of gravity, while 

 ' at ihe fame time tliey follow the direction of a proper motion 

 which this centre may have in fpace, it will hardly be poftible 

 to entertain a doubt to which hypothefis we ought to give the 

 preference. 



As I have now allowed, and even (hown, thepoflibility that 

 the phonoiwena of the double ftar Caftor may be explained by 

 proper motions, it will appear that, notwithftanding my fore- 

 going arguments in favour of binary fyftems, it was neceftary, 

 on a fornjer occafjon, to exprgfs myfeH in a conditional man- 

 ner,* 



I 



