NEWTON AND THE LAW OF GRAVITATION 



should be explained that the argument does not 

 amount to an actually indisputable demonstration. It 

 is at least possible that the coincidence between the 

 observed and computed motion of the moon may be a 

 mere coincidence and nothing more. This probability, 

 however, is so remote that Newton is fully justified in 

 disregarding it, and, as has been said, all subsequent 

 generations have accepted the computation as demon- 

 strative. 



Let us produce now Newton's further computations 

 as to the other planetary bodies, passing on to his final 

 conclusion that gravity is a universal force: 



''PROPOSITION V., THEOREM V. 



' ' That the circumjovial planets gravitate towards Jupiter; 

 the circumsaturnal towards Saturn; the circumsolar tow- 

 ards the sun; and by the forces of their gravity are drawn off 

 from rectilinear motions, and retained in curvilinear orbits. 



" For the revolutions of the circumjovial planets 

 about Jupiter, of the circumsaturnal about Saturn, 

 and of Mercury and Venus and the other circumsolar 

 planets about the sun, are appearances of the same 

 sort with the revolution of the moon about the earth; 

 and therefore, by Rule ii., must be owing to the same 

 sort of causes; especially since it has been demon- 

 strated that the forces upon which those revolutions 

 depend tend to the centres of Jupiter, of Saturn, and of 

 the sun ; and that those forces, in receding from Jupiter, 

 from Saturn, and from the sun, decrease in the same 

 proportion, and according to the same law, as the force 

 of gravity does in receding from the earth. 



243 



