MATTER AND FORCE 65 



are now known to move. 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 plan- 

 etary 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 it which 

 renders his name and fame immortal. Starting from the 

 principle that 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 proved that the planetary motions must be what ob- 

 servation makes them to be. 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 con- 

 ception of which was necessary to Newton's intellectual 

 peace, is called the force of gravitation. 



Gravitation is a purely attractive force, but in elec- 

 tricity and magnetism, repulsion had been always seen 

 to accompany attraction. Electricity and magnetism are 

 double or polar forces. In the case of magnetism, expe- 

 rience soon pushed the mind beyond the bounds of ex- 

 perience, compelling it to conclude that the polarity of 

 the magnet was resident in its molecules. I hold a mag- 

 netized strip of steel by its centre, and find that one half 

 of the strip attracts, and the other half repels, the north 

 end of a magnetic needle. I break the strip in the mid- 

 dle, find that this half, which a moment ago attracted 

 throughout its entire length the north pole of a mag- 

 netic needle, is now divided into two new halves, one 

 of which wholly attracts, and the other of which wholly 



