in the relative Situation of double Stars , &c. 351 
resolved by a proper motion of x, let us suppose this star to be 
placed any where far beyond «, but so as to have been seen, in 
the year 1779, where the angle of position, 32 0 47' north pre- 
ceding, and the observed distance, near 2 diameters of the large 
star, required it. With a proper velocity, let it be in motion 
towards the place where it may now be seen at the same dis- 
tance from Castor, but under an angle of position only io° 53' 
north preceding. It may then be admitted, that a small decrease 
of the distance which would happen at the time when the angle 
of position was 21 0 50', could not have been perceived; so that 
the gradual change in the observed angle of position, as well as 
the equality of the distance of the two stars, will be sufficiently 
accounted for. But the admission of this hypothesis requires, 
that a. Geminorum and the solar system should be at rest ; and, 
by the observations of astronomers, which I shall soon have 
occasion to mention, neither of these conditions can be conceded. 
(b) If, according to No. 2, we admit the motion of a, we 
shall certainly be more consistent with the observations which 
astronomers have made on the proper motion of this star ; * and, 
as a motion of the solar system, which I shall have occasion to 
mention hereafter, has not been rigidly proved, it may, for the 
sake of argument, be set aside ; nor has a proper motion of the 
star x been any where ascertained. The retrograde annual 
proper motion of Castor, in right ascension, according to Dr. 
Maskelyne, is o",i05. This, in about 23^ years, during which 
* See Tob i je May eri Opera inedita. De motu jixarum proprio, page 80. Also 
Dr. Maskelyne’s first Volume of Observations. Explanation and Use of the Tables, 
page iv. Or Mr. Wollaston’s Astronomical Catalogue, end of the Preface. Like- 
wise Connoissance des Temps pour V Annie VI. page 203. Sur le Mouvement par- 
ticulier propre a dijferenles Eloiles; par Mons. De la Lande. 
Z Z 2 
