234 HISTORY OF PHYSICAL ASTRONOMY. 



has accelerated Jupiter and retarded Saturn ever 

 since the restoration of astronomy. 



Thus the secular inequalities of the celestial 

 motions, like all the others, confirm the law of 

 universal gravitation. They are called "secular," 

 because ages are requisite to unfold their existence, 

 and because they are not obviously periodical. They 

 might, in some measure, be considered as extensions 

 of the Newtonian theory, for though Newton's law 

 accounts for such facts, he did not, so far as we 

 know, foresee such a result of it. But on the other 

 hand, they are exactly of the same nature as those 

 which he did foresee and calculate. And when we 

 call them secular, in opposition to periodical, it is 

 not that there is any real difference, for they, too, 

 have their cycle ; but it is that we have assumed 

 our mean motion without allowing for these long 

 inequalities. And thus, as Laplace observes on this 

 very occasion M , the lot of this great discovery of 

 gravitation is no less than this, that every apparent 

 exception becomes a proof, every difficulty a new 

 occasion of a triumph. And such, as he truly adds, 

 is the character of a true theory, of a real repre- 

 sentation of nature. 



It is impossible for us here to enumerate even 

 the principal objects which have thus filled the 

 triumphal march of the Newtonian theory from its 

 outset up to the present time. But among these 

 secular changes, we may mention the Diminution of 

 29 Syst. dn Monde. 8vo ii. 37- 



