VOL. XLV.] PHILOSOPHICAL TKANSACTIONS. 423 



Newton's principles, caused by the actions of the sun and moon on those parts; the 

 plane of the moon's orbit being at one time, above 10° more inclined to the plane 

 of the equator, than at another ; it was reasonable to conclude that the part of 

 the whole annual precession, which arises from her action, would in different 

 years be varied in its quantity ; whereas the plane of the ecliptic, in which the 

 sun appears, keeping always the same inclination to the equator ; that part of 

 the precession, which is owing to the sun's action, may be the same ever}' year : 

 and hence it would follow, that though the mean annual precession, proceeding 

 from the joint actions of the sun and moon, were 50"; yet the apparent annual 

 precession might sometimes exceed, and sometimes fall short, of that mean 

 quantity, according to the various situations of the nodes of the moon's orbit. 



In the year 1727, when my instrument was first set up, the moon's ascending 

 node was near the beginning of Aries ; and consequently her orbit was as much 

 inclined to the equator as it can at any time be ; and then the apparent annual 

 precession was found, by my first year's observations, to be greater than the 

 mean : which proved that the stars near the equinoctial colure, whose declina- 

 tions are most of all affected by the precession, had changed theirs, above a 

 tenth part more than a precession of 50" would have caused. The succeeding 

 year's observations proved the same thing, and in 3 or 4 years' time the differ- 

 ence became so considerable, as to leave no room to suspect that it was owing to 

 any imperfection, either of the instrument or observations. 



But some of the stars which I had obser\'ed, that were near the solstitial co- 

 lure, having appeared to move, during the same time, in a manner contrary to 

 what they ought to have done by an increase in the precession ; and the devi- 

 ations in them being as rernarkable as in the others ; I perceived that something 

 more than a mere change in the quantity of the precession, would be requisite 

 to solve this part of the phenomenon. On comparing my observations of stars 

 near the solstitial colure, that were almost opposite to each other in right ascen- 

 sion, I found that they were equally affected by this cause ; for while y Draco- 

 nis appeared to have moved northward, the small star which is the 35 th Camelo- 

 pardali Hevelii in the British Catalogue, seemed to have gone as much towards 

 the south : which showed that this apparent motion, in both those stars, might 

 proceed from a nutation in the earth's axis ; whereas the comparison of my ob- 

 servations of the same stars formerly enabled me to draw a different conclusion, 

 with respect to the cause of the annual aberrations arising from the motion of 

 light. For the apparent alteration in y Draconis, from that cause, being as 

 great again as in the other small star, proved that that phenomenon did not pro- 

 ceed from a nutation of the earth's axis ; as on the contrary, this may. On 

 making the like comparison between the observations of other stars, that lie 

 nearly opposite in right ascension, whatever their situations were with respect to 



