68 THE REALITIES OF MODERN SCIENCE 



motion through the unstressed position is a character- 

 istic of bodies which we describe by saying that they 

 have inertia. 



We know by experience that it takes work to set a 

 body into motion, to make a body which is already in 

 motion move in a different direction, or to stop it. 

 In deforming the string against the elastic forces we 

 give it the ability to do work, which it does in setting 

 its particles into motion toward their original positions. 

 When, however, these particles arrive at this position 

 they are in rapid motion and thus in turn have the 

 ability to do work. This they do against the elastic 

 force of the string, which becomes effective the moment 

 the latter passes through its unstressed position. A 

 deformation opposite to the original one is thus brought 

 about. It is also smaller, for the particles cannot do 

 quite as much work in assuming it, since some of the 

 original energy has been transmitted away by the air 

 and some dissipated in the frictional motions of the 

 particles of the string itself. For this reason the 

 vibration is damped. 



If a vibrating source is surrounded by some other 

 medium, as for example by water, similar phenomena 

 would occur, although the wave train would not travel 

 at the same speed and the source would damp down 

 more quickly. All phenomena, comprising a vibrating 

 source and an elastic medium, by the vibrations of the 

 molecules of which energy is transmitted away from 

 the source, should be classified as sound, although only 

 under special conditions can the vibrations be detected 

 by a human ear. 



The realities, for which we have searched in con- 



