

PROPAGATION OF SOUND. 327 



transmission of the motion which constitutes sound 

 must, however, be distinguished from the progressive 

 motion of the air itself, produced by various other 

 causes ; just as the advance of waves on the surface of 

 water is distinct from the onward flow of the water. A 

 small body floating upon the wavy surface of water is 

 lifted up and down by the waves, but it has little or no 

 movement backwards or forwards. Each wave as it 

 glides onward under the floating body, moves it with 

 its front a short distance forward in the direction of its 

 motion ; but as soon as the crest of the wave has passed 

 beneath it, the body returns again upon the back of the 

 wave into its former position, the displacement forwards 

 being precisely equal to the subsequent displacement 

 backwards. Whenever a floating body has a continuous 

 progressive motion, this is due to some other cause than 

 the action of the waves, such as the pressure of the wind, 

 or to the actual flow of the water in which the body 

 floats. 



Careful experiments have shown that, when a uniform 

 series of waves follow each other along the surface of 

 water, the particles of the liquid which are disturbed by 

 them move in elliptical or circular paths, and that hence 

 bach particle returns again to the point from which it 

 Parted, while the onward motion of the whole wave is 

 ilue to the fact that each liquid particle commences its 

 notion somewhat later than the preceding one. 



Fig. 208 is intended to illustrate the formation of 

 .vaves by the circular motion of the individual particles 

 )f water. For the sake of clearness, only a few particles 

 ire represented in the figure, those which are shown 

 5eing so far apart that each one begins to move when the 



