394 PHILOSOPHICAL TRANSACTIONS. [aNNO 1773. 



varying from the effect of the nutation of the earth's axis ; as the limb of the 

 sun was sometimes approaching to, and sometimes receding from, the star, by a 

 kind of libratory motion, from the effect of the nutation ; and also as the obli- 

 quity of the ecliptic itself was, in all probability, continually diminishing ; from 

 a combination, and as it were involution of these motions, no certain conclusion 

 could be drawn, since, in the space of a few years, the apparent obliquity may 

 be the same, and yet the mean obliquity may have diminished, or perhaps, in 

 the space of a few years, the obliquity may appear to have increased, when it 

 may really have become less. Whereas, by reducing the observations to their 

 mean position, and by assigning to each known cause its proper and allowed 

 effect, a regularity and uniformity must necessarily take place, as far at least as 

 is consistent with the unavoidable errors in observing. 



M. Cassini, in the Memoirs of the Academy of Sciences for 1767, acquaints 

 us, that, in 1748, the apparent distance of Arcturus from the upper limb of the 

 sun, at the time of the solstice, was the same as in 1766. 



In 1748, distance of Arcturus from the sun's solstitial limb 3° 13' 36" 40'" 



Altitude of Arcturus 6l 41 17 



Therefore the apparent solstitial altitude .64 55 13 40 



In 1766, distance of Arcturus from the sun's solstitial limb 3 19 32 



Altitude of Arcturus 61 35 42 



Therefore the apparent solstitial altitude 64 55 14 



The same astronomer has, in the Memoirs for 1759, p. 325, communicated 

 the following conclusions. 



Dist. of the star from the sun's limb. Reduction. Solstitial distance 



1763. June 14 3" 7' 29" + U' 1" 3" 18' 30" 



16 + 8 13 3 18 29 



40 + 2 48 3 18 28 



1 + 19 22 3 18 23 



,3.. 2 2 54 55 + 2i 33 3 18 28 



~ " 18 + 28 8 3 18 26 



Mean. .3 18 27 



Mr. Le Monnier, in the Memoirs for 1762, p. 269, has published the 

 annexed distances of Arcturus from the limb of the sun, reduced to the solstitial 

 point, with a view to obtain differences in the apparent obliquity of the ecliptic: 

 and, from the observations made with the gnomon of St. Sulpice, and com- 

 municated by Mr. Le Monnier, 1 738. . 3° lO' 1 5" 

 in the same volume, it should 1740. .311 5 

 seem that that astronomer is of 1742. . 3 1 1 48 



opinion, that the obliquity of 1763. .3 18 40 with the mural 5-feet quad, 

 the ecliptic has no other varia- . . 3 18 35 with the large mural instru. 



tion than what the nutation of the earth's axis will occasion; and that therefore 

 we must either abandon the absolute diminution of the ecliptic, or at least 

 suppose it extremely small, . since, in the space of 1 8 years, it has not produced 

 a sensible alteration. 



