THE REALITIES OF SCIENCE 67 



spherical layers are successively larger and consist of 

 a successively greater number of molecules. Since 

 a larger number must be set in motion, the motion 

 imparted to each molecule is smaller the farther it 

 is away from the vibrating source. This decrease 

 means a decrease in the possible motion of the ear- 

 drum and hence in the intensity of the sound as a 

 listener assumes positions farther away from the source. 

 At any instant, however, the energy imparted by the 

 previous vibrations of the string is associated with all 

 the molecules between the string and that most dis- 

 tant spherical surface, which the first pulse has just 

 reached. 



This is why the vibrations gradually decrease and 

 finally cease. The work done in producing the orig- 

 inal deformation is gradually passed on by the string 

 to the ah- molecules. Since the string in each swing 

 must push ahead of it the air molecules, it will not 

 swing quite as far as if there were no work to be done 

 in moving the air. The amplitude of the vibration then 

 damps down just as in the case of the swinging 

 pendulum. In fact, the string and the pendulum are 

 but two illustrations of a type known as simple har- 

 monic motion. 



We passed over the question of why the string, when 

 released from the initial deforming stress, should fly 

 through its center of rest. As an elastic body it should 

 return to this position, for each molecule of the string 

 is urged toward its unstressed position. In this position, 

 of course, the restoring force of elasticity is zero and 

 does not affect the motion which has been imparted 

 to the string. The fact that the latter continues in 



