VOL. Lxm.] 



PHILOSOPHICAL TRANSACTIONS. 



391 



was observed to be 42' 37".5, by taking the mean of two observations on the 

 24th and 26th of May, 1768, differing only 4".7. According to computation, 

 the variation of » Bootis in declination during the interval of the two observa- 

 tions was 1359".3, and of Bootis 1256''.0; therefore the difference of variation 

 in declination was l' 43".3, by which the distance of the stars was increased. 

 The difference in declination therefore in 1768, if neither of the stars moved, 

 should have been 42' 4".l; but it was observed to be 33".4 greater, by which 

 quantity tlierefore n Bootis must have moved southward in 73 052 years. 



By reducing all the foregoing deductions to 78 years, Arcturus appears to have 

 moved. 



By the small star, Feb. 14, 1690. ..,..,..., 1' 52".3SO. . 



•I Bootis Feb. U, 1690. . .V. iV. ':■.'. ... 1 18 .541 . . 



« Bootis April 5, 169I. 

 II Bootis May 4, 16'91. 

 « Bootis May 27, I692. 



By r Bootis May ,7, 1(>92 1 



Westward. Southward. ^; • 



. . 2' 36".43 

 ..2 5 .S8 '*- 



32 .557 2 6 .75 



32 .906 2 1 1 .87 



.30 .752 2 12 .74 



36' .707 2 37 .07 rr- 



X Bootis Feb. 14, 1690 not observed 2 31- .69 



But the star » Bootis appears also to have moved southward. 



By >r Bootis Feb. 14, 1 690 0' 29".503 



» Bootis May 27, 1692 25 .049 



By f Bootis April b, l693 3+ .712 



By a mean 29 .755 



As Arcturus appears to have moved southward of n Bootis 2' 9".31, by taking 

 a mean of the 4 quantities resulting from the comparisons with that star; and as 

 n Bootis has also moved southward of some of the neighbouring small stars by 

 29".755 in the same time, Arcturus on the whole has moved 2' 39*.o6 to the 

 south, by the comparisons with n Bootis only; and therefore, by taking a mean 

 of all the results, Arcturus has altered its right ascension less than the neigh- 

 bouring stars by l' 33''''.97 in 78 years, in which time it has also moved 2' 36".81 

 to the south of the same stars. 



In order to see how far the motion of right ascension is to be depended on, 

 which is deduced from the above comparisons, Mr. H. selected and computed 

 the following observations, made at Shirburn castle with a transit instrument of 

 S-J feet, placed exactly in the plane of the meridian, and consequently more to 

 be relied on than those made with a mural instrument. By a mean of 5 obser- 

 vations, made on the 7th, 12th, 23d, 24th, and 31st of May, 1741, o. s., the 

 difference in right ascension between -n Bootis and Arcturus was 5" 22' 38".9, the 

 extremes differing only 4".4 of a degree. The difference in the variation of 

 right ascension to the end of May, 1768, is 20 ".5, by which the ascensional dif- 

 ference is diminished. It should therefore have been 5° 22' 1 8".4 ; but it was 

 observed to be 5° 2 1' A3". A. Therefore in 27 years Arcturus has moved west- 

 ward 35".0. 



On the 16th and 20th of May, 1744, the difference in right ascension be- 



