66 THE REALITIES OF MODERN SCIENCE 



pass him if he walks toward the point from which the 

 train is coming. 



In trying to determine what are the objective realities 

 in the case of sound we have so far recognized that 

 one reality is matter. The other apparently involves 

 motion, but because motion is relative, it has seemed 

 preferable to look further. The other reality in sound 

 we shall now see to be energy, the ability to do work. 

 In order to show this we shall consider in more detail 

 the vibratory motion of a sounding string. 



The string, originally at rest, is set in vibration by 

 momentarily deforming it, as by picking it. Elas- 

 ticity is called into play; and when the stress is re- 

 moved the string flies back to its original position. 

 It passes through this position, bowing out in the 

 opposite direction. Elastic forces are now operative 

 just as they were in the case of the original deforma- 

 tion. The string, therefore, swings back through its 

 center of vibration and assumes a symmetrical form 

 on the other side. 



In producing the original deformation, work is done 

 against the elastic forces, and the stretched string 

 acquires an ability to do work. This principle was 

 early applied practically, in the bow of the ancients, 

 where the energy was used to impart motion to an 

 arrow. If the vibrating string is a source of sound, 

 the matter, to which motion is imparted, consists of 

 the adjacent molecules of air and also of the molecules 

 of the string itself. With each vibration of the string 

 some of the original energy is imparted to adjacent 

 molecules and transmitted by successive layers of 

 molecules away from the vibrating string. The 



