388 PHILOSOPHICAL TKANSACTIONS. [aNKO 1773. 



they are probably not affected with this latter error. We are at present con- 

 cerned with the difference of 2 zenith distances, and not with the absolute quan- 

 tity of them. The conclusions may indeed be affected with an error in the divi- 

 sions; and from the examination which Mr. H. has been able to make, he is of 

 opinion that the arc of Mr. Flamsteed's instrument was not of the proper quan- 

 tity ; and that, though the observations generally erred in defect, yet in some 

 parts they erred in excess. 



On Feb. 14, 1690, Mr. Flamsteed observed, that a small star, of the 7th 

 or 8th magnitude, whose place is not determined in the British catalogue, and 

 which star was named by him Infra Arcturum, preceded Arcturus 3* in time, or 

 3^3, when an allowance is made for the error of the plane of the instrument := 

 O' A1,".Q, and was 26' 30* to the south of Arcturus.* By a mean of 8 observa- 

 tions made at Oxford, on or near June the 10th, 1767, with the transit instru- 

 ment, and with a refracting telescope of 8 feet, furnished with a micrometer; 

 the difference of right ascension was l' 8''.75 of a degree, the star following 

 Arcturus; and by a mean of 3 observations, the extremes differing only 3", the 

 small star was 23' 55".0 to the south of Arcturus. 



The right ascension of Arcturus and the small star being nearly the same, the 

 change in declination ought to be so likewise. But, from the observed difference 

 in declination, the right ascension of the two stars must vary unequally, though 

 with a very small difference. Accordingly it appears from computation (in which 

 the annual precession is supposed = 50".35, the obliquity of the ecliptic at the 

 middle of the interval of the time = 23° 28' 30", and the right ascensions and 

 declinations of the two stars taken at a mean between the times of observation) 

 that the variation of Arcturus in right ascension was 3270*.6, and of the small 

 star 3277".6, in 77.287 years. Therefore the right ascension of Arcturus alters 

 less than that of the star; and consequently Arcturus should in 17 67 have fol- 

 lowed the star by 42".6. But the star was observed to follow Arcturus by 1' 

 8".75. The right ascension therefore of Arcturus has increased less than that of 

 the star, or Arcturus has moved westward l' 51 ".35 in 77-287 years; and has 

 gone southward 2' 35" in the same time, supposing the small star not to have 

 moved, which is highly probable. 



On the same day the difference of right ascension in time between the star n 

 Bootis and Arcturus was 21"" 32' of mean solar time, = 5° 24' 2".2, when a 

 proper allowance is made for the going of the clock, and for the error of the 

 plane of the instrument, and the difference of declination was 50' 45^.6, when 

 an allowance is made for refraction. On the 24th, 26th and 29th, of May, 

 and the Qth of June of the year 17 68, Mr. H. determined the difference in 



* This is the only observation of that star made by Mr. Flamsteed. — Orig. 



