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centre of gravity. Having started this hypothesis, he proceeds to 

 give an account of a series of observations on double stars, compre- 

 hending a period of twenty-five years, which, in his opinion, will go 

 to prove, according to the above assumption, that many of them are 

 not merely double in appearance, but must be allowed to be real 

 binary combinations of two stars, intimately held together by the 

 bond of mutual attraction. 



In this inquiry, three bodies or points are to be exclusively attended 

 to : 1st, the largest of the two stars which make up the double star ; 

 2nd, the smaller of these two stars ; and 3rd, the place of the sun, 

 which at the great distance of the fixed stars may be taken for that 

 of the observer. It is obvious, that as this investigation must be 

 conducted in an hypothetical manner, it is necessary to consider the 

 appearances that would be produced by the motion of these three 

 bodies, either singly, by pairs, or collectively. This renders it neces- 

 sary to contemplate a variety of cases, and previously to lay down a 

 certain theory to which the observations may be referred upon the 

 supposition of either of those cases. For this purpose the author 

 has drawn up tables, showing the appearances as to distance and 

 angle of position that must result from the motions of either of those 

 three bodies, whether in right ascension or declination. 



As the number of double stars in which he has ascertained mani- 

 fest changes in their relative positions amount already to more than 

 fifty, our author thinks it advisable to confine himself in this paper 

 to only a few of those instances ; and, accordingly, we find here an 

 investigation of the changes of six double stars, viz. a Geminorum, 

 y Leonis, s Bootis, Herculis, $ Serpentis, and y Virginis ; reserving 

 himself to treat of the others in a future paper. 



His observations on the double star a Geminorum commenced so 

 long ago as the year 1779, and have been regularly continued to the 

 present tune. In this interval of twenty-three years and a half, the 

 distance of the two stars has not varied, being constantly about two 

 diameters of the large star ; but the angle of position has altered 

 considerably. In the year 1779 it measured 32 47' north preceding ; 

 and by the last observation in the present year it is now only 10 54'; 

 so that in the space of twenty-three years and a half it has manifestly 

 undergone a diminution of no less than 21 54' : and the interme- 

 diate observations leave no room to doubt that this change has been 

 the effect of a gradual and regular motion. 



A revolving star, it is evident, would at once explain such a pro- 

 gressive change in the angle of position, without an alteration of the 

 distance ; but this being a supposition of which we have not hitherto 

 any precedent, it ought certainly not to be admitted without the 

 fullest evidence. Accordingly, our author enters into a minute ex- 

 amination, founded on geometrical and optical principles, whether 

 the above-mentioned phsenomena cannot be satisfactorily explained 

 by other motions of the stars or of the sun, according to the several 

 hypotheses above indicated, with the addition of others in which the 

 two stars are not supposed to be at equal distances from the sun. 



