268 NATURE OF SOUND. 



rises and falls without approaching the shore. He may 

 thus be enabled to correct his erroneous idea of the onward 

 motion of the water. Again, he may stand beside a field 

 of ripening grain, and, as the breezes blow, he will see a 

 series of waves pass before him. But if he reflect and 

 observe carefully, he will see clearly that there is no move- 

 ment of matter from one side of the field to the other ; the 

 grain-ladened stalks merely bow and raise their heads. 

 Most persons are familiar with similar wave movements in 

 ropes, chains and carpets. Each material particle has 

 a motion, but that motion is vibratory, not progres- 

 sive. The only thing that has an onward movement 

 is the pulse or wave, which is only a form or change 

 in the relative positions of the particles of the un- 

 dulating substance. 



(a.) The motion of the wave must be clearly distinguished from 

 the motion of particles which constitute the wave. The wave may 

 travel to a great distance ; the journey of the individual particle is 

 very limited. 



417. Wave Period. When a medium is traversed 

 by a series of similar waves, each particle is in a state of 

 continued vibration. These vibrations are alike, they 

 being as truly isochronous ( 143) as those of the pen- 

 dulum. The time required for a complete vibra- 

 tion is called the period, and is the same for 

 all the particles. 



418. Wave Length. In such a series of similar 

 waves, measuring in the direction in which the waves are 

 travelling, the distance from any vibrating particle to 

 the next particle that is in the same relative posi- 

 tion or "phase" is called a wave length. In the case 



