Masterpieces of Science 



blown continuously. To the engineer on the 

 footboard the whistle has a certain note; to the 

 listener who is standing still the whistle has a 

 somewhat shriller note, because the motion of 

 the engine towards him has the effect of shorten- 

 ing the sound waves, and shrillness increases 

 with the shortness of such waves with the great- 

 er number per second he hears. If all the engines 

 of the line have whistles exactly alike, a listener 

 with his eyes shut can easily tell whether it is 

 a freight-train that is advancing or an ordinary 

 express, or a "limited" running at fifty miles an 

 hour; the quicker the train, the shriller the sound 

 of its approaching whistle (Fig. 88) . Sir William 



FIG. 88. 



A, waves between two points at rest relatively to each other. 



B, waves between two points at a shortening distance apart. 



C, waves between two points at a lengthening distance apart. 



Huggins, the pioneer in applying this principle 

 to reading stellar motions, adopts a parallel 

 illustration: "To a swimmer striking out from 

 the shore, each wave is shorter, and the number 

 he goes through in a given time is greater than 

 would be the case if he stood still in the water. " 

 88 



