250 HISTORY OF PHYSICAL ASTRONOMY. 



comes more difficult. Many excellent measures, 

 both of Degrees and of Pendulums, have been 

 made since those just mentioned. The results of 

 the Arcs 38 is an Ellipticity of 1 -298th ; of the Pen- 

 dulums, an Ellipticity of about 1 -285th. This dif- 

 ference is considerable, if compared with the quan- 

 tities themselves ; but does not throw a shadow of 

 doubt on the truth of the theory. Indeed, the ob- 

 servations 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 accuracy 

 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 pro- 

 tuberant matter of the spheroid produces the Pre- 

 cession of the equinoxes, and a Nutation of the 

 earth's axis. The Precession had been known from 

 the time of Hipparchus, and the existence of Nuta- 

 tion was foreseen by Newton; but the quantity is 

 so small, that it required consummate skill and 

 great labour in Bradley to detect it by astronomi- 

 cal observation. Being, however, so detected, its 

 amount, as well as that of the Precession, gives us 

 the means of determining the amount of Terrestrial 



38 Airy. Fig. Earth, p. 230. 



