VOL. LXIII.] PHILOSOPHICAL TRANSACTIONS. 387 



in our hemisphere, the variation in the place of Arcturus is the most remarkable, 

 and such as cannot possibly be attributed to the uncertainty of observation. It 

 has accordingly been noticed by many astronomers: in particular, Dr. Halley 

 mentions it in N° 355 of the Phil. Trans: M. Cassini, in the Memoirs of the 

 Academy of Sciences for 1738, p. 231, has shown, that there is a variation of 

 5' in the latitude of that star, between his own time and that of Tycho, in an 

 interval of a century and a half; and M. le Monnier, in the Memoirs of the 

 Academy of Sciences for 1767, p. 417, proves, that the latitude of Arcturus 

 varies at the rate of 1" every year; and that the longitude decreases at the rate 

 of 60" in 1 00 years.* But as an inquiry both into the true <|uantity and into the 

 direction of this motion, has not hitherto been made public, Mr. H. proposes to 

 give some account of his own observations, made expressly with this view in the 

 years 1767 and 1768, with a transit instrument of 44 inches, and a moveable 

 mural quadrant of 33 inches, both constructed by Mr. Bird, and of the conclu- 

 sions resulting from a comparison between them and some observations made by 

 Mr. Flamsteed in 1 690. 



It may perhaps be objected, that the differences of right ascension, as deter- 

 mined by Mr. Flamsteed's mural instrument, are not to be depended on, from 

 the very nature of his instrument. Mr. Flamsteed was himself too good an 

 observer not to be aware of this, and accordingly, in the Prolegomena to the 

 3d volume of the Historia Coelestis, p. 132, he informs us in what manner he 

 determined the error of the plane at different distances from the zenith. By 

 distributing these errors in the best manner, Mr. H. is of opinion, that the error 

 of the plane of his instrument may be supposed to decrease uniformly at the rate 

 of half a second in time for every degree of zenith distance from 28° to 60°, the 

 error being 39' at the former, and 23" at the latter, by which quantity stars 

 passed the horary wire, in his instrument, before they came to the true meridian. 

 It should seem also, that the error continued nearly the same from 6o° to 75°, 

 being at the latter only 22"; but that it decreased irregularly from 75° to 85°, 

 viz. I" in time for each degree from 75° to 80°, and 0".4 for each degree from 

 80° to 85°. The mural arc was fixed on a stone pier, the southern part of which 

 was found to settle yearly, whence the error of the line of collimation to the 

 south necessarily became every successive year greater and greater. As Mr. 

 Flamsteed seems not to have had any method of adjusting his instrument by a 

 plumb-line, these errors must have been irregular at different seasons of the 

 same year, and were perhaps never truly determined. But as the observations 

 here referred to were made on the same day, and within the compass of an hour, 



* See also the Memoirs of the Academy of Sciences for 1769, p. 21. See also Astronomiae Fun- 

 damental by the Abbe delaCaille; who, in reducing his observations of Arcturus, supposes the 

 annual motion of declination in that star = \\j" , p l6y, and 187. — Orig, 



3d 2 



