WATER-WAVES AND SOUND-WAVES. 169 



which the sound passes being the air. A sounding body in the middle 

 of a room, for instance, must send out shells of sound as it were, in all 

 directions, because people above, below, and all round it, would hear the 

 sound. Replace the stone by a tuning-fork. To one prong of this 

 fasten a mirror, and on this mirror throw a powerful beam of light. 

 When this tuning-fork is bowed, and a sound is heard, the light thrown 

 by the attached mirror shows the fork to be vibrating, and when the 

 tuning-fork is moved we get an appearance on the screen which re- 

 minds us of the rope ; or we may use the fork as shown in Fig. 3, and 

 obtain a wavy record on a blackened cylinder. 



Experiment shows that we have at one time a sphere of compression 

 that is to say, the air is packed closely together ; and, again, a sphere 



aa. a 



liiii! 



h : >i,:X 



IPlllf 



illllilflM 



null i 



I j 



1 



Fig. 5. Propagation of Sound- Waves along a Cylinder. 



of rarefaction, when the particles of air are torn farther apart than they 

 are in the other position. The state of things, then, that travels in 

 the case of sound, is a state of compression and rarefaction of the air. 

 Hence, the particle of air travels differently from the particle of water ; 

 it moves backward and forward in a straight line in the direction in 

 which the sound is propagated. 



4 



6 



7 



10 



11 



12 



Fig. 6. Sound-Waves. Particles of air, a, b, e, d, e, are in position 1 at rest. The remaining 

 positions show how they are situated at successive instants, wheD a continuous series of im- 

 pulses reaches them from the left. In position 2, e. p., only one particle has begun its oscilla- 

 tion ; in position 3, only two ; while, in position 6, all are in motion. 



