222 DYNAMICAL THE OK Y OF SOUND 



ponent of its velocity in the direction of the ray is alone 

 effective. Analogous effects are produced when the source is at 

 rest and the observer in motion. The principle is exemplified 

 in the apparent change of pitch of the whistle of a locomotive 

 as a train dashes through a station ; but its most striking and 

 fruitful applications are met with in the theory of radiation. 



