ON THE ORIGIN OF THE PLANETARY SYSTEM. 149 



and all their satellites, in consequence of the mutual 

 action of each upon each, and which astronomers caH 

 disturbances disturbance, that is to say, of the simpler 

 elliptical motions about the sun, which each one would 

 produce if the others were absent that all these 

 could be theoretically predicted from Newton's law, 

 and be accurately compared with what actually takes 

 place in the heavens. The development of this theory 

 of planetary motion in detail was, as has been said, 

 the merit of Laplace. The agreement between this 

 theory, which was developed from the simple law of 

 gravitation, and the extremely complicated and mani- 

 fold phenomena which follow therefrom, was so com- 

 plete and so accurate, as had never previously been 

 attained in any other branch of human knowledge. 

 Emboldened by this agreement, the next step was to 

 conclude that where slight defects were still constantly 

 found, unknown causes must be at work. Thus, from 

 BessePs calculation of the discrepancy between the 

 actual and the calculated motion of Uranus, it was 

 inferred that there must be another planet. The 

 position of this planet was calculated by Leverrier and 

 Adams, and thus Neptune, the most distant of all 

 known at that time, was discovered. 



But it was not merely in the region of the attrac- 

 tion of our sun that the law of gravitation was found 

 to hold. With regard to the fixed stars, it was found 



