232 HISTORY OF PHYSICAL ASTRONOMY. 



In this manner, by selecting the best mean ele- 

 ments of the motions of the heavenly bodies, the 

 observed motions deviate from this mean in the 

 way the theory points out, and constantly return 

 to it. To this general rule, of the constant re- 

 turn to a mean, there are, however, some apparent 

 exceptions, of which we shall now speak. 



Sect. 3. Application of the Newtonian Theory to 

 Secular Inequalities. 



SECULAR Inequalities in the motions of the heavenly 

 bodies occur in consequence of changes in the ele- 

 ments of the solar system, which go on progressively 

 from age to age. The example of such changes 

 which was first studied by astronomers, was the 

 Acceleration of the Moon's Mean Motion, discovered 

 by Halley. The observed fact was, that the moon 

 now moves in a very small degree quicker than 

 she did in the earlier ages of the world. When 

 this was ascertained, the various hypotheses which 

 appeared likely to account for the fact were re- 

 duced to calculation. The resistance of the me- 

 dium in which the heavenly bodies move was the 

 most obvious of these hypotheses. Another, which 

 was for some time dwelt upon by Laplace, was 

 the successive transmission of gravity, that is, the 

 hypothesis that the gravity of the earth takes a 

 certain finite time to reach the moon. But none 

 of these suppositions gave satisfactory conclusions ; 



