434 PHILOSOPHICAL TRANSACTIONS. [aNNO 1747-8. 



the cardinal points of the equator, it appeared that their change of declination 

 was nearly equal, but contrary ; and such as a nutation or motion of the earth's 

 axis would effect. 



The moon's ascending node being got back towards the beginning of Capri- 

 corn in the year 1732, the stars near the equinoctial colure appeared about that 

 time to change their declinations no more than a precession of 30' required ; 

 while some of those near the solstitial colure altered theirs above 1" in a year 

 less than they ought. Soon after, I perceived the annual change of declination 

 of the former to be diminished, so as to become less than 50* of precession 

 would cause; and it continued to diminish till the year 1736, when the moon's 

 ascending node was about the beginning of Libra, and her orbit had the least 

 inclination to the equator. But by this time, some of the stars near the solstitial 

 colure had altered their declinations 18" less, since the year 1727, than they 

 ought to have done from a precession of 50'. For y Draconis, which in those 

 9 years should have gone about 8' more southerly, was observed in 1736 to ap- 

 pear 10" more northerly, than it did in the year 1727. 



As this appearance in y Draconis, indicated a diminution of the inclination of 

 the earth's axis to the plane of the ecliptic ; and as several astronomers have sup- 

 posed that inclination to diminish regularly; if this phenomenon depended on 

 such a cause, and amounted to 18" in Q years, the obliquity of the ecliptic would, 

 at that rate, alter a whole minute in 30 years; which is much faster than any 

 observations, before made, would allow. I had reason therefore to think that 

 some part of this motion at the least, if not the whole, was owing to the moon's 

 action on the equatorial parts of the earth; which I conceived might cause a 

 libratory motion of the earth's axis. But as I was unable to judge, from only 9 

 years observations, whether the axis would entirely recover the same position, that 

 it had in the year 1727, I found it necessary to continue my observations through 

 a whole period of the moon's nodes; at the end of which I had the satisfaction 

 to see that the stars returned into the same positions again ; as if there had been 

 no alteration at all in the inclination of the earth's axis: which fully convinced 

 me that I had guessed rightly as to the cause of the phenomena. This circum- 

 stance proves likewise that, if there be a gradual diminutionof the obliquity of the 

 ecliptic, it does not' arise only from an alteration in the position of the earth's 

 axis, but rather from some change in the plane of the ecliptic itself: because the 

 stars, at the end of the period of the moon's nodes, appeared in the same places, 

 with respect to the equator, as they ought to have done, if the earth's axis had 

 retained the same inclination to an invariable plane. 



During the course of my observations, our ingenious secretary of the R. S., 

 Mr. John Machin, being employed in considering the theory of gravity, and its 

 consequences with regard to the celestial motions, I acquainted him with the 



