ISAAC NEWTON 109 



gravitation was to see whether it agreed numerically with 

 the radius of the earth and of the moon's orbit, whereby 

 however, it remained to be proved that the force is to be 

 reckoned from the centre of the earth, that is to say, that 

 gravitation acts through the whole mass of the earth, even 

 from the most distant part of it, quite undiminished. 

 According to what was then known concerning the earth's 

 radius, the agreement was only approximate, and this led 

 Newton to lay the idea aside and devote himself entirely 

 to his optical and mathematical studies. 



Not until the year 1 679 did he again take up the mechanics 

 of the heavens, but he then continued to work at it with 

 the greatest energy. Apparently the occasion for this was 

 furnished by the expression by members of the Royal Society 

 of various ideas, which obviously were linked with Kepler's 

 laws and also with Huygens' laws of centrifugal force 

 (published in 1673), and had already led to the discussion of 

 a law of inverse squares.^ Newton then, after a new measure- 

 ment of the earth's meridian in 1682 had furnished a secure 

 basis for the above mentioned calculation with respect to the 

 moon, put forward in place of suppositions and disconnected 

 statements, his well-constructed Principia Mathematica 

 Philosophiae Naturalis, the rich contents of which we have 

 already admired. The work was finished at the end of 

 1684; it appeared in print in 1686. 



In the year 1685 an event of importance in Newton's life 

 occurred. King James II desired to bring Catholicism into 

 power in England, and for this purpose wished to have a 

 monk of no particular capabilities accepted as a graduate 



^ In particular Hooke, and Halley (the first eight years, the latter 

 thirteen years younger than Newton), and also Wren, may be mentioned 

 as likewise busily engaged in considering the origin of the mechanics <:,i 

 the heavens; there can be no doubt that Hooke was rather a hindrance 

 to Newton, whereas Halley was a help, inasmuch as he continually urged 

 Newton to publish the Principia, and was also a close friend in other 

 ways. Halley 's assistance, particularly as regards the comets, has already 

 been mentioned above. 



