ADAMS— THE EINSTEIN THEORY. 177 



molecular, chemical forces — forces of which we have no direct sense, 

 but which nevertheless must be regarded as having a real existence. 



In an attempt to clear away the indeterminateness involved in 

 the conception of force as fundamental, and the complexity in- 

 herent in a multiplicity of forces, Hertz developed a system of me- 

 chanics in which the idea of force as one of the fundamental con- 

 cepts was banished. In this system of mechanics all forces are 

 the result of constraints arising from concealed or cyclic motions. 

 If we should experiment with a rapidly spinning wheel enclosed in 

 a box, not knowing what there was in the box, we should come to 

 the conclusion that the box was in a field of force quite different 

 from a simple gravitational field ; or in other words the potential 

 energy of the box would appear to be different from its potential 

 energy with the wheel at rest. But knowing of the wheel in rota- 

 tion, what would appear as potential energy arising from an external 

 field would really be kinetic energy of cyclic motion. So Hertz 

 attempted to interpret every force acting on a system as arising from 

 cyclic motions, with a single law governing the motion of the sys- 

 tem — the law of" the straightest path. There is a close relation 

 between Hertz's system of mechanics and Einstein's theory of gravi- 

 tation to which we shall return later. 



Let us now go back to the Newtonian view and regard force 

 as a fundamental concept. The force that we are most familiar 

 with is the force of gravity. Newton showed that not only the 

 motion of bodies falling to the earth, but the motion of the planets 

 about the sun could be accounted for by assuming that every particle 

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

 proportional to the product of the masses and inversely proportional 

 to the square of the distance between them. This was, of course, 

 no explanation of the force of gravity, and the idea that matter 

 could act upon matter at a distance was distasteful to Newton him- 

 self, as it has been almost universally ever since. However, up to 

 about the middle of the nineteenth century it was considered a suffi- 

 cient goal to attain in a variety of physical phenomena to account for 

 them by means of forces acting at a distance between elements of 

 the system. Particularly in the fields of electricity and magnetism 

 this goal seemed near attainment. There was, however, a very 



