chap, xxxiii.] THE ORGAN PIPE. 445 



sounds will thus be evident. This will be referred to 

 immediately. 



The sounds produced by an organ-pipe are due to 

 resonance. Such a pipe is shown in Fig. 193. It 

 consists of a tube, open or closed at the upper end, as 

 the case may be. The lower end terminates in a 

 part of a construction similar to what may be seen, 

 in an ordinary whistle, and called the embouchure. 

 Air is blown in by a narrow pipe, and enters a small 

 chamber separated from the body of the pipe by a 

 wedge of wood except for a narrow slit i through which 

 the compressed air passes. The air, issuing in a thin 

 stream, strikes against a sharp edge projecting from the 

 wall of the pipe inside of a rectangular window bo in 

 the lower part of the pipe, through which the broken 

 air passes to the outside. The air is thus broken up 

 into pulses, and the pipe is tuned to take up some 

 one pulse and vibrate in harmony with it. Thus by 

 altering the length of the tube, everything else 

 remaining the same, the pitch of the note of the pipe 

 will be altered ; it resounds now to a shorter or longer 

 pulse than before. 



That the air in the organ-pipe is thrown into 

 vibration may be shown in various ways. If a light 

 ring, covered with membrane on which sand is strewn, 

 be lowered into an open sounding organ pipe, the 

 pattering sound of the sand on the membrane will 

 indicate the agitation of the air in the pipe. If the 

 pipe be sounding its fundamental note a node will be 

 found at the centre and ventral segments at each end. 

 "When the membrane is lowered to the middle the 

 noise of the agitated sand ceases, to be again resumed 

 if the membrane be raised or lowered. The same 

 thing may be shown in a still more remarkable way. 

 A small circular opening is made 'in the side of the 

 organ-pipe ; over this is fixed a small wooden box M 

 (Fig. 193), with an indiarubber floor, the indiarubber 



