SEQUEL TO THE EPOCH OF NEWTON. 455 



is considerable, if compared with the quantities themselves ; but does 

 not throw a shadow of doubt on the truth of the theory. Indeed, the 

 observations of each kind exhibit irregularities which we may easily 

 account for, by ascribing them to the unknown distribution of the 

 denser portions of the earth ; but which preclude the extreme of ac- 

 curacy and certainty in our result. 



But the near agreement of the determination, from Degrees and 

 from Pendulums, is not the only coincidence by which the doctrine is 

 confirmed. We can trace the effect of the earth's Oblateness in certain 

 minute apparent motions of the stars ; for the attraction of the sun and 

 moon on the protuberant matter of the spheroid produces the Preces- 

 sion of the equinoxes, and a Nutation of the earth's axis. The Preces- 

 sion had been known from the time of Hipparchus, and the existence 

 of Nutation was foreseen by Newton ; but the quantity is so small, that 

 it required consummate skill and great labor in Bradley to detect it 

 by astronomical observation. Being, however, so detected, its amount, 

 as well as that of the Precession, gives us the means of determining the 

 amount of Terrestrial Ellipticity, by which the effect is produced. But 

 it is found, upon calculation, that we cannot obtain this determination 

 without assuming some law of density in the homogeneous strata of 

 which we suppose the earth to consist. 42 The density will certainly 

 increase in proceeding towards the centre, and there is a simple and 

 probable law of this increase, which will give 1 -300th. for the Ellipti- 

 city, from the amount of two lunar Inequalities (one in latitude and 

 one in longitude), which are produced by the earth's oblateness. 

 Nearly the same result follows from the quantity of Nutation. Thus 

 every thing tends to convince us that the ellipticity cannot deviate 

 much from this fraction. 



[2d Ed.] [I 'ought not to omit another class of phenomena in which 

 the effects of the Earth's Oblateness, acting according to the law of 

 universal gravitation, have manifested themselves; I speak of the 

 Moon's Motion, as affected by the Earth's Ellipticity. In this case, as 

 in most others, observation anticipated theory. Mason had inferred 

 from lunar observations a certain Inequality in Longitude, depending 

 upon the distance of the Moon's Node from the Equinox. Doubts 

 .vere entertained by astronomers whether this inequality really exist- 

 2d ; but Laplace showed that such an inequality would arise from the 

 oblate form of the earth ; and that its magnitude might serve to de- 



42 Airy, Fig. Earth, p. 235. 



