VOL. XXXV.] PHILOSOPHICAL TRANSACTIONS, 309 



article is not now so much to be wondered at, since it will appear very probable, 

 that the instruments commonly used by them were liable to greater errors than 

 many times that parallax will amount to. 



The success then of this experiment evidently depending very much on the 

 accurateness of the instrument, that was principally to be attended to. 



Mr. Molyneux's apparatus was completed, and fitted for observing, about the 

 end of November 1725, and on December 3 following, the bright star in the 

 head of Draco, marked y by Bayer, was for the first time observed, as it passed 

 near the zenith, and its situation carefully taken with the instrument. The 

 like observations were made on the 5th, 11th, and 12th days of the same 

 month, and there appearing no material difFerence in the place of the star, a 

 further repetition of them at this season seemed needless, it being a part of the 

 year when no sensible alteration of parallax in this star could soon be expected. 

 It was chiefly therefore curiosity that tempted Mr. Bradley, being then at Kew, 

 where the instrument was fixed, to prepare for observing the star on Dec. 17, 

 when having adjusted the instrument as usual, he perceived that it passed a little 

 more southerly this day than when it was observed before. Not suspecting any 

 other cause of this appearance, they first concluded that it was owing to the 

 uncertainty of the observations, and that either this or the foregoing were not 

 so exact as they had before supposed; for which reason they purposed to repeat 

 the observation again, in order to determine from whence this difFerence pro- 

 ceeded; and on doing it on Dec. 20, Mr. B. found that the star passed still 

 more southerly than in the former observations. This sensible alteration the 

 more surprised them, as it was the contrary way from what it would have been, 

 had it proceeded from an annual parallax of the star; but being now pretty 

 well satisfied that it could not be entirely owing to the want of exactness in the 

 observations; and having no notion of any thing else, that could cause such an 

 apparent motion as this in the star, they began to think that some change in 

 the materials, &c. of the instrument itself, might have occasioned it. Under 

 these apprehensions they remained some time, but being at length fully con- 

 vinced, by several trials, of the great exactness of the instrument, and finding 

 by the gradual increase of the star's distance from the pole, that there must be 

 some regular cause that produced it, they took care to examine nicely, at the 

 time of each observation, how much it was: and about the beginning of March 

 1726, the star was found to be 20' more southerly than at the time of the first 

 observation. It now indeed seemed to have arrived at its utmost liuiit south- 

 ward, because in several trials made about this time, no sensible difFerence was 

 observed in its situation. By the middle of April it appeared to be returning 

 back again towards the north ; and about the beginning of June, it passed at 



