NEWTON. /* v 197 



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through which the moon falls to the earth, and the space through whicfi 

 it ought to fall if his conjecture were correct Newton made the calcula- 

 tion, and was disappointed to find that the result did not verify his specu- 

 lation, for the force of gravity appeared by one-sixth too great for the 

 required effect on the moon. Now, the measurements of trie eajrth's 

 magnitude which were available at that time were extremely imperfect, 

 and Newton had estimated the diameter of the earth from the then com- 

 monly received measurement, which made a degree of latitude equal 

 to 60 British miles, whereas the real length is about 69-^ miles. The 

 difference between the moon's velocity and that calculated upon this 

 datum might have appeared small to another person, but to Newton it 

 seemed conclusive against his hypothesis. He imagined, however, 

 that some yet unknown cause, analogous perhaps to the vortexes of 

 Descartes, modified the effects of gravity. He did not therefore wholly 

 give up his notion, but he resolved with characteristic patience to let 

 it remain undisclosed to the world until study and reflection should 

 bring some knowledge of the cause that could so modify a law indicated 

 by such strong analogies. 



Newton therefore refrained from publishing anything upon this sub- 

 ject until ten years or more afterwards, when he heard of the more 

 accurate measurement of a degree of the meridian which had been 

 then recently made by Picard in France. He then recommenced his 

 former calculations of 1665 with new data. Observing as he advanced 

 the manifest tendency of these numbers to produce the expected re- 

 sult, his nervous excitement became so great that he was unable to 

 proceed with the calculation, and asked one of his friends to complete 

 it for him. This time there was no doubt about the conformity of the 

 computed with the observed result ; and no sooner had Newton re- 

 cognized the truth of his speculation, than he began to pursue it to its 

 consequences with a boldness and vigour which had before never been 

 shown in science. The one conclusive result he had obtained gave 

 him the clue to the whole mechanism of the heavens. The analogy 

 which was so manifest in the systems of satellites round Jupiter and 

 Saturn was extended to the system of planets revolving round the sun. 

 The greatest and most general of all physical laws was finally deduced 

 by Newton, namely, the Law of Gravitation, which affirms that every 

 particle of matter in the universe attracts every other particle with a force 

 which is inversely proportional to the squares of the distances between 

 them. ^Newton demonstrated that as a consequence of this law spheres 

 attracting each other in their individual particles would have the same 

 effect upon each other as */ the resultant force resided in the centre 

 alone. As the bodies of the solar system are very nearly spherical, 

 they will therefore act upon one another and upon bodies near their 

 surfaces as if their centres were so many centres of attraction. This 

 consequence of the gravitating energy of the particles of which the 

 earth is made up is so often erroneously conceived as a force actually 



