THE REALITIES OF SCIENCE 65 



that a listener sways his head from side to side just in 

 time with the motion of the layer of air adjacent to 

 his eardrum. It was the presence of this air, moving 

 the drum periodically, which we saw to be the cause 

 of his auditory impressions. With reference to the 

 ground the motion of his eardrum will be made just 

 what it was when he was hearing the sound, but now 

 there will be no relative motion of the eardrum and 

 the skull. A relative motion of these is required to 

 produce an effect on his auditory nerves, so that he 

 now hears no sound although the conditions of the 

 source and of the intervening air are just what they 

 were before. 



Another illustration which shows both the subjec- 

 tivity of sound and the relativity of motion is a matter 

 of common observation. The pitch of an automobile 

 horn or of a locomotive whistle is higher if the machine 

 is approaching than when it is stationary. The prin- 

 ciple of this phenomenon, which occurs in light as 

 well as in sound, is known as Doppler's. Conversely, 

 if the whistle is receding from the listener the pitch 

 is lower. The vibration frequency of the horn does 

 not depend upon which direction the machine is going 

 with reference to the listener. Why should the pitch 

 be different? The number of pulses reaching the ear 

 each second is greater when there is a relative motion 

 of the listener toward the machine. This is most 

 evident if we think in terms of a train. In fact we 

 usually speak of a condensation and its succeeding 

 rarefaction as a "wave" and of the succession of these 

 as a "tram of waves." Any one who ever watched a 

 long freight train realizes that more cars per minute 



