NEWTON AND THE LAW OF GRAVITATION 



parts of planet A, and its gravity towards any one 

 part will be to the gravity towards the whole as 

 the matter of the part to the matter of the whole. 

 Q.E.D. 



"Hence it would appear that the force of the whole 

 must arise from the force of the component parts." 



Newton closes this remarkable Book iii. with the 

 following words: 



" Hitherto we have explained the phenomena of the 

 heavens and of our sea by the power of gravity, but 

 have not yet assigned the cause of this power. This is 

 certain, that it must proceed from a cause that pene- 

 trates to the very centre of the sun and planets, 

 without suffering the least diminution of its force ; that 

 operates not according to the quantity of the surfaces 

 of the particles upon which it acts (as mechanical 

 causes used to do), but according to the quantity of 

 solid matter which they contain, and propagates its 

 virtue on all sides to immense distances, decreasing 

 always in the duplicate proportions of the distances. 

 Gravitation towards the sun is made up out of the 

 gravitations towards the several particles of which 

 the body of the sun is composed ; and in receding from 

 the sun decreases accurately in the duplicate proportion 

 of the distances as far as the orb of Saturn, as evidently 

 appears from the quiescence of the aphelions of the 

 planets ; nay, and even to the remotest aphelions of the 

 comets, if those aphelions are also quiescent. But 

 hitherto I have not been able to discover the cause of 

 those properties of gravity from phenomena, and I 



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