Page:Proceedings of the Royal Society of London Vol 1.djvu/147

Rh centre of gravity. Having started this hypothesis, he proceeds to give an account of a series of observations on double stars, comprehanding 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 nece» 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, 'yLeonis, eBootis, gHerculis, SSeIpentis, and 7 Virginis; reserving himself to treat of the others in a future paper.

His observations on the double star or, Geminorum commenced so long ago as the year 1779, and have been regularly continued to the present time. 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 progressive 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 examination, founded on geometrical and optical principles, whether the above-mentioned phaenomena 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.