

THE REALITIES OF SCIENCE 63 



condensation which reaches it and allowed to move out 

 by each succeeding rarefaction. 



A musical note is seen to be the impression of a listener 

 whose eardrum is moved back and forth periodically. 

 Such a motion may be obtained from a vibrating body 

 through the medium * of the intervening air. The note 

 depends for its pitch upon the frequency. The " niter- 

 national pitch" A is a note of 435 vibrations per sec- 

 ond. When the frequency is above about 20,000 per 

 second many persons are incapable of hearing it. 

 Frequencies of 30,000 or more are inaudible to most 

 persons. These high notes may be produced by small or- 

 gan pipes, by whistles, by vibrating rods which are very 

 stiff and short, and by other means similar to those 

 used to produce the notes of music. The vibrations 

 from such bodies are transmitted to the ear through 

 the air hi the same way as are those of lower frequency. 

 In one case we say there is the sound of a musical note ; 

 in the other we hear nothing. The difference is in our 

 own brains, for sound is a subjective reality. 



What is the objective reality with which we have to 

 deal, which is present hi both cases, whether we hear 

 the sound or not ? The most obvious reali ty is matter, 

 both in the case of the vibrating body and in the ah* 

 which transmits these vibrations. Then there is the 

 motion of the vibrating body and of the surrounding 

 ah-. Shall we take motion 2 as the other reality? Be- 

 fore we decide let us see what we mean by motion. 



1 In elementary courses on physics it is usual to demonstrate the 

 function of the air by inclosing the source of sound in a chamber 

 which is then evacuated. 



2 "Motion" is here used in its root significance and with no con- 

 notation of "momentum." 



