SI 8 OUTLINES OF NATURAL PHILOSOPHY, 



As the annual precession is not always the same, the 

 length of the tropical year, in remote ages, has 

 been somewhat different from what it is at present. 

 In the age of HIPPARCHLTS, it was about 10" long- 

 er. The siderial year, as already observed, re- 

 mains invariable. 



Diurnal Rotation. 



32 C 2. The velocity of the Earth's rotation on 

 its axis, or the length of the day, is not affect- 

 ed by the action of the Sun or Moon, in such 

 a degree as can ever become sensible, even to 

 the nicest observation. 



Systeme du Monde, p. 271. The small inequalities so 

 produced, do not accumulate by time, but quickly 

 compensate one another. 



323. The motions of bodies near the surface 

 of the Earth, tend, in some cases, to alter the 

 velocity of the diurnal rotation ; but if these 

 motions are only such as we at present per- 

 ceive, their effects, like the preceding, must for 

 ever remain insensible. 



A body which, by descending from a height, or by 

 moving from the equator toward the poles, comes 

 nearer to the Earth's axis, tends to accelerate the 



motion 



