PHYSICAL ASTRONOMY. 319 



motion about that axis, because it brings with it, 

 into its new situation, more velocity than it can 

 retain, and must therefore communicate a part of 

 it to the general mass. The contrary happens 

 when a body recedes from the axis, and the quan- 

 tity of the acceleration or retardation thus produ- 

 ced, may be calculated, on the principle, that the 

 total momentum of the Earth must remain the 

 same, notwithstanding of any change that can arise 

 from the action of its parts on one another. 



From this it follows, that if any body change its dis- 

 tance from the axis, the momentum of rotation of 

 the whole Earth, is to the change in the momen- 

 tum of rotation of the body, as the velocity of the 

 diurnal rotation to the variation in that velocity, 

 arising from the motion of the body. 



As the first of these terms is incomparably greater 

 than the second, so must the third be than the 

 fourth. 



Hence, though the degradation of mountains, and the 

 carrying of matter by the rains from a higher to 

 a lower level, are effects that go on continually, 

 the amount can never be so great as to be sen- 

 sible. 



In some cases, even these small changes are imme- 

 diately compensated. The constant current in 

 the atmosphere, from the poles to the equator, is 

 counteracted, in the retardation it tends to pro- 

 duce, by the contrary current, which, in the supe- 

 rior 



