SOUND. 119 



of the surface more and more distant from their 

 origin. In this instance, it is clearly not a portion 

 of the water which flows onwards ; but the dis- 

 turbance, the rise and fall of the surface which 

 makes the ring-formed waves, passes into wider 

 and wider circles, and thus the undulation is 

 transmitted from its starting-place, to points in 

 all directions on the surface of the fluid. 



The diffusion of these ring-formed undulations 

 from their centre resembles the diffusion of a 

 sound from the place where it is produced to the 

 points where it is heard. The disturbance, or 

 vibration, by which it is conveyed, travels at the 

 same rate in all directions, and the waves which 

 are propagated are hence of a circular form. 

 They differ, however, from those on the surface 

 of water ; for sound is communicated upwards 

 and downwards, and in all intermediate direc- 

 tions, as well as horizontally; hence the waves of 

 sound are spherical, the point where the sound 

 is produced being the centre of the sphere. 



This diffusion of vibrations in spherical shells 

 of successive condensation and rarefaction, will 

 easily be seen to be different from any local 

 motion of the air, as wind, and to be independent 

 of that. The circles on the surface of water will 

 spread on a river which is flowing, provided it 

 be smooth, as well as on a standing canal. 



Not only are such undulations propagated 

 almost undisturbed by any local motion of the 



