288 SOUND 



pipe and a sound is produced characteristic of the length of 

 the pipe. 



In the brass wind instruments, such as horn, trombone, and 

 trumpet, the lips of the player vibrate and excite the air within. 

 Varying pitches are obtained partly by the varying wind pres- 

 sure of the musician ; if he breathes fast, the pitch rises ; if he 

 breathes slowly, the pitch falls. All of these instruments, how- 

 ever, except the trombone possess some valves which, on being 

 pressed, vary the length of the tube and alter the pitch accord- 

 ingly. In the trombone, valves are replaced by a section which 

 slides in and out and shortens or lengthens the tube. 



