396 PHILOSOPHICAL TRANSACTIONS. [aNNO 1773. 



A like set of 8 days observations, from tlie 15th And again another set of 10 days observation, 



to the 29th of June, 1748, give, from the 1 Ith to the 29th of June, 176'6, give. 



For the mean 28" 10' 55.8" For the mean 28" 11' 10.5" 



Sun's parallax — 4. 1 Sun's parallax — 4..1 



28 10 51.7 28 11 6.4 



Nutation +6.1 Nutation +7.6 



28 10 57.8 28 11 14.0 



Error of the line of collimation +6.3 Error of the line of collimation +2.6 



Mean solstitial zenith dist., 1748 28 11 4.1 Mean solstitial zenith dist., 176628 11 16.6 



Then follow a series of 10 days observations of the zenith distances of Arctu- 

 rus, from May 12 to July 1, 1743; when these are corrected for refraction, 

 aberration, nutation, precession, the medium of all is as follows : viz. 



The mean 31° 7' 33.6" 



Error of the line of collimation +2 



Mean zenith distance of Arcturus, June 21, 17+3 31 7 35.6 



Mean zenith distance of the sun's centre, June 21, 1743 28 11 2.8 



Mean distance of Arcturus from the sun's centre, 1743 2 56 32.8 



In like manner, 5 days observations, from June 4 till Oct. Q, 1746, give for 



The mean 31° 8' 26.4" 



Error of the line of collimation + 6.3 



Mean zenith distance of Arcturus, June 21, 1746 31 8 32.7 



Mean zenith distance of the sun's centre, June 21, 1746 28 11 4.1 



Mean distance of Arcturus from the sun's centre, 1746 2 57 28.6 



Again, a like set of 4 days observations, from May 13, to June 23, when 



corrected, give for < 



The mean 31° 14' 50.3" 



Error of the line of collimation +2.6' 



Mean zenith distance of Arcturus, June 21, 1766 31 14 52.9 



Mean distance of the sun's centre, June 21, 1766 28 11 l6.6 



Mean distance of Arcturus from the sun's centre, 1766 3 3 36.3 



From the foregoiner observations, it appears Variat. in lOO years. 



1 L 1 .-.• 1 -.u J- . • 1743. . 28° 1 1' 2.8" 60" 



that the mean solstitial zenith distance in sum- 1745 28 11 4.1 62.5 



mer was as here annexed : and by comparing 1748. .28 11 4.1 69.4 



the 3 former with the latter, the variation of ' ' 



the obliquity of the ecliptic in 100 years is as is expressed in the last column of 

 the table. 



By comparing the distance of Arcturus from the sun's centre in 1743, with 

 the same distance as observed in 17 66 (an allowance being made for the proper 

 motion of the star during the interval, as also for its variation in declination 

 arising from the precession of the equinoxes), it appears that its distance is 17".3 

 less than it would have been, if the distance of the sun's centre from the equator 

 had remained unvaried. By that quantity therefore the obliquity of the ecliptic 

 has altered in 23 years ; which is at the rate of 75".2 in 100 years. By comparing, 

 in like manner, the distance in 1746, the obliquity of the ecliptic has diminished 

 1 5". 6 in 20 years, or 78" in 100 years. 



Distance in 1743 2° 56' 32.8" In 1746, 2« 57' 28.6" 



Motion of the star in decl. southward . . 7 20.8 6 23.3 



