76 FRAGMENTS OF SCIENCE. 



intellectual nature was left unsatisfied by the mere act of 

 observation. He sought after the principle which ruled 

 the fact. Whether this anecdote be true or not, it illus- 

 trates how the ordinary operations of Nature, which most 

 people take for granted as perfectly plain and simple, are 

 often those which most puzzle the scientific man. To the 

 conception of the matter of the apple, Newton added that 

 of the force that moved it. The falling of the apple was 

 due to an attraction exerted mutually between it and the 

 earth. He applied the idea of this force to suns, and plan- 

 ets, and moons, and showed that all their motions were 

 necessary consequences of this attraction. 



Newton, you know, was preceded by a grand fellow 

 named John Kepler — a true working-man — who, by analyz- 

 ing the astronomical observations of his master, Tycho 

 Brahe, had actually found that the planets moved as they are 

 now known to move. As a matter of fact, Kepler knew as 

 much about the motion of the planets as Newton did ; in 

 fact, Kepler taught Newton and the world generally the 

 facts of planetary motion. But this was not enough. The 

 question arose — "Why should the facts be so ? This was 

 the great question for Newton, and it was the solution of 

 this question which renders his name and fame immortal. 

 He proved that the planetary motions were what observa- 

 tion made them to be, because every particle of matter in 

 the solar system attracts every other particle by a force 

 which varies as the inverse square of the distance between 

 the particles. He showed that the moon fell toward the 

 earth, and that the planets fell toward the sun, through the 

 operation of the same force that pulls an apple from its 

 tree. This all-pervading force, which forms the solder of 

 the material universe, and the conception of which was 

 necessary to Newton's intellectual peace, is called the force 

 of gravitation. 



All force may be ultimately reduced to a push or a pull in 



