THE PERCUSSION INSTRUMENTS 299 



which, on being pressed, vary the length of the tube and 

 alter the pitch accordingly. In the trombone, valves are re- 

 placed by a section which slides in and out and shortens or 

 lengthens the tube. 



274. The Percussion Instruments. The percussion instru- 

 ments, including kettledrums, bass drums, and cymbals, are 

 the least important of all the musical instruments; and are 

 usually of service merely in adding to the excitement and gen- 

 eral effect of an orchestra. 



In orchestral music the various instruments are grouped 

 somewhat as shown in Figure 192. 



