306 POPULAR LECTURES AND ADDRESSES. 



speed 1000 feet per second. Sometimes we call it 

 a thousand musical feet per second, it saves trouble 

 in calculating the length of organ pipes ; the time 

 of vibration in an organ pipe is the time it takes 

 a vibration to run from one end to the other and 

 back. In an organ pipe 500 feet long the period 

 would be one per second ; in an organ pipe ten 

 feet long the period would be 50 per second ; 

 in an organ pipe twenty feet long the period would 

 be 25 per second at the same rate. Thus 25 per 

 second, and 50 per second of frequencies corre- 

 spond to the periods of organ pipes of 20 feet 

 and 10 feet. 



The period of vibration of an organ pipe, open 

 at both ends, is approximately the time it takes 

 sound to travel from one end to the other and 

 .back. You remember that the velocity in dry air 

 in a pipe 10 feet long is a little more than 50 

 periods per second ; going up to 256 periods per 

 second, the vibrations correspond to those of a 

 pipe two feet long. Let us take 512 periods per 

 second ; that corresponds to a pipe about a foot 

 long. In a flute, open at both ends, the holes are 



