XII 

 NEWTON AND THE LAW OF GRAVITATION 



WE come now to the story of what is by common 

 consent the greatest of scientific achievements. 

 The law of universal gravitation is the most far-reach- 

 ing principle as yet discovered. It has application 

 equally to the minutest particle of matter and to the 

 most distant suns in the universe, yet it is amazing in 

 its very simplicity. As usually phrased, the law is 

 this : That every particle of matter in the universe at- 

 tracts every other particle with a force that varies di- 

 rectly with the mass of the particles and inversely as 

 the squares of their mutual distance. Newton did not 

 vault at once to the full expression of this law, though 

 he had formulated it fully before he gave the results 

 of his investigations to the world. We have now to 

 follow the steps by which he reached this culminating 

 achievement. 



At the very beginning we must understand that the 

 idea of universal gravitation was not absolutely original 

 with Newton. Away back in the old Greek days, as 

 we have seen, Anaxagoras conceived and clearly ex- 

 pressed the idea that the force which holds the heaven- 

 ly bodies in their orbits may be the same that operates 

 upon substances at the surface of the earth. With An- 

 axagoras this was scarcely more than a guess. After 

 his day the idea seems not to have been expressed by 



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