92 



SOUND 



may be of different sorts ; in the spring (Fig. 67) the 

 vibration of each particle is parallel to the direction of 

 the wave motion itself, while Fig. 68 shows each particle 



FIG. 68 



vibrating (ab) at right angles to the direction of the 

 wave, like ripples on water. A wave length is the dis- 

 tance from one particle to the next one which is in the 

 same state of vibration, as bd (Fig. 69). The rate of 

 vibration is the number of vibrations which pass a given 

 point in one second. 



103. Definition of Sound. We are familiar with wave 

 motion in water ; any disturbance the wind, a pebble 

 thrown into it, a moving boat or animal is enough to 

 cause ripples, even if slight, so that a body of water is 

 rarely free from waves. In just the same way the air 

 is constantly vibrating. Any slight disturbance sets up 

 wave motion in the elastic atmosphere, and because 

 there are so many more disturbances in air than in 

 water, there are also many more sorts of waves all the 

 time. Of course we cannot see these waves, and we can 



b d 



FIG. 69 



feel only the greater ones. How then can they be dis 

 covered? Nature has given us an ear for that purpose ; 

 we hear these waves in the air, and we call the sensation 



