VOL, 



LXIIl.] 



PHILOSOPHICAL TRANSACTIONS. 



397 



Computed distance in 1 766 3° 



Observed distance in 1766 3 



Variation of obliquity 



Tlie foregoing deductions prove, Mr. H. thinks, beyond all doubt, that the 

 obliquity has become less; but as the interval of time between the two terms of 

 comparison is so short, that the errors committed in observing, may bear a sen- 

 sible proportion to the small quantities just now found, and which perhaps are 

 somewhat too large; Mr. H. has recourse to Mr. Flamsteed's observations, and 

 compares them with observations made by himself, in the course of the last and 

 present years. For this purpose he reduced all the observations of the sun, made 

 in 1690, from May 26 to June 24, o. s. and also all the observations of Arctu- 

 rus, made in the same year, to their mean position at the summer solstice of that 

 year. The observations, together with his own made at Oxford, are as follow : 

 These observations of the zenith distances, both of the sun and Arcturus, when 

 corrected as the preceding, and the medium of all taken, give first, for the sun's 

 zenith distance at the solstice. 



The mean 58* 0' 54.?" 



Error of the line of coUimation .. — I 30 



27 59 24.2 



Sun's parallax —4.1 



27 59 20.1 



Nutation +9-5 



Mean sobtitial zen . dist. of the sun's 



centre, June II, I69O, o. s. 27 59 2.9.6 



And for the zenith distance of Arcturus, 

 The mean, January 1, 169O, o.s. 30° 39' 



Precession to June 1 1 , I69O 



Mean zenith distance of Arcturus, 



June 11, l6S0 30 39 



Mean solst. zen. dist. of the sun's 



centre, June 11, l6'90 27 59 



Mean distance of Arcturus in declin. 



from the sun's centre, 1690 . . . 2 40 12.8 



34" 

 + 8.4 



42.4 



29.6 



And for that of Arcturus the same year. 

 Mean zen. dist. of Arcturus, Jan. 1, 1772 31° 



Precession to Jvine 21, 1772 



Mean zen. dist. of Arcturus, June 21, 1772 31 



Error of the line of collimation 



True mean zenith dist. of Arcturus, June 2 1 , 



1772 31 



Mean zen. dist. of O's centre, June 21, 1*72 '.'8 

 Mean dist. of Arcturus in declination from 



0's centre, June 21, 1772 3 



Again, for the sun's zenith distance in 1771, 



The mean is 28° 17' 8.7' 



Sun's parallax —4.1 



28 17 4.6 

 Nutation — 6.8 



28 



Error of the line of collimation . . 



Mean solstitial zenith distance of 



the suns centre, 1 77 1 28 



And the same for 1772, gives, for 



The mean 28° 



Sun's parallax 



28 

 Nutation 



28 

 Error of the line of coUimation . . 

 Mean solstitial zenith distance of 



the sun's centre, 1772 28 



Mean dist. in June 169;) 

 Precession, &c. to June 



177 J 



Computed dist. in June 



1772 :. 



Observed dist. in June 



1772 



Diminution of obliquity 



in 82 years 



17 



16' 



+ 



17 

 17' 

 17 

 17 



57.8 

 4.8 



2.6 



13.4" 

 — 4.1 



9.3 

 -8.7 



0.6 

 + 4.8 



17 5.4 

 2° 40' 12.8" 



. +26 16.4 



3 6 29.2 



3 5 37.6 



5 37.6 



51.6 



From the foregoing observations it appears that, at the summer solstice of the 

 year 169O, Arcturus was 2° 40' 12".8 to the south of the sun's centre in decli- 

 nation: the motion of the star in declination, from that time to the summer 

 solstice of the year 1772, including its proper motion, is 26' l6*.4. Arcturus 

 therefore, in 1772, should have been 3° 6' 2g''.2 to the south of the sun's centre, 



