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IIUTTONIAN THEORY. 



437 



Note xx. § ii8. 



. Inequalities in the Planetary Motions 



384. The affertion that, in the planetary mo 



<s» 



difcover 



mark, either of the com 



mination of 



fent ord 



refers to the late difcoveries of La Grange and 



La Place, which h 



ibuted fo much 



the perfed 



f phyfi 



aftronomy 



Fr 



thefe 



the principle of univerfal gravitation, 

 thematicians have demonftrated, that all the va- 

 riations in our fyllem are periodical ; that they 

 are confined within certain limits ; and confift 

 of alternate diminution and increafe. The or- 

 bits of the planets change not only their po- 

 fition, but even their magnitude and their form: 



the longer a 



is of each has a flow angular mo- 



tion ; ^nd, though its length remains fixed, the 

 lliorter axis increafes and diminilhes, fo that the 

 form of the orbit approaches to that of a circle, 

 and recedes from it by turns. In the fame man- 

 ner, the obliquity of the ecliptic, and the incli- 

 nation of the planetary orbits, are fubje6l to 

 change ; but the changes are fmall, and, being 



firfl in one diredion, and then in the opponte, 



Ee 3 



t h e V 



