MUSIC AND SOUND INSTRUMENTS 427 



of different diameters having the same tension. This last may be observed 

 by attaching two strings about 2 inches apart to a board and allowing the 

 end of the string to run over a bridge on the edge of the table. Attach the 

 same weight to each string. 



Vibrating Instruments. The bell, cymbals, drum, tambourine, 

 and xylophone are examples of vibrating instruments. These 

 vibrators are supported usually at the nodes so that vibrations take 

 place freely about the support. 



Electro-pneumatic Pipe Organ Key Action. Figs. 319-320. The key held in 

 position by the center rail pin and guided by the front rail pin being de- 

 pressed, draws the flat wire springs into contact through the platinum points, 

 thus closing the circuit. The magnet draws the leather-covered iron disc from 

 the vent, thus permitting the collapse of the pneumatic. 



By means of the flat wire clip and the round wire stem the valve above is 

 opened and the freed air pressure (pneumatic wind) raises the leather membranes 

 which in turn open the pipe wind valves. If the stop action (not shown here) 

 has admitted the pipe wind to the chest the pipe speaks. 



The wind supply is automatically controlled by a regulator composed of a 

 weight resting upon the reservoir and a lever valve or a rheostat lever connected 

 by a pliable, galvanized wire rope. 



It readily will be observed that a large part of the mechanism is made from 

 round or flat wire, ranging in size from the music steel spring wire 10 4 00 inch 

 in diameter to the bellows feeder shafting having a diameter of from inch 

 to 3 inches. Wire is even more in evidence in the stop and combination actions, 

 making altogether a total of nearly 500 miles of round and flat wire in a modern 

 three-manual organ; and the progressive pipe-organ builder daily finds further 

 substitution of wire for wood and castings efficient and economical. 



Human Voice. The human voice is produced by a simple reed 

 instrument located at the top of the trachea. The place is marked 

 by the Adam's apple. Thin projecting membranes called vocal 

 cords are situated on each side of a slit. These are very loose when 

 a person is breathing, but when one wishes to speak muscular action 

 takes place, bringing the vocal cords close together so that the air 

 from the lungs when forced against them causes them to vibrate. 

 The pitch of the voice depends upon the muscular tension of the 

 cords, while the intensity depends upon the force of air and the 

 position of the mouth and throat. The mouth, throat, and nose 



