266 KELVIN 



attained to Philadelphia or not ; I have had no experience 

 of it, but people tell me it is sometimes 59 or 60 degrees 

 in Philadelphia } and I believe them) in round numbers 

 let us call the 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 vibra- 

 tion 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 correspond 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 so arranged that the length 

 of the sound-wave is about one foot, for one of the chief 

 "open notes." Higher musical notes correspond to greater 

 and greater frequency of vibration, viz., 1,000, 2,000, 4,000, 

 vibrations per second ; 4,000 vibrations per second correspond 

 to a piccolo flute of exceedingly small length; it would 

 be but one and a half inches long. Think of a note from 

 a little dog-call, or other whistle, one and a half inches 

 long, open at both ends, or from a little key having a tube 

 three quarters of an inch long, closed at one end; you 

 will then have 4,000 vibrations per second. 



A wave length of sound is the distance traversed in 

 the period of vibration. I will illustrate what the vibra- 

 tions of sound are by this condensation travelling along 

 our picture on the screen. Alternate condensations and 

 rarefactions of the air are made continuously by a sound- 

 ing body. When I pass my hand vigorously in one direc- 

 tion, the air before it becomes dense, and the air on the 



