[Illustration:  FIG. 189—­1, clarinet; 2, oboe; 3, flute.]

In the oboe (Fig. 189, 2) the vibrating air column is set into motion by means of two thin pieces of wood or metal placed in the mouthpiece of the tube.  Variations in pitch are produced as in the clarinet by means of stops and varied breathing.  In the flute, the air is set into motion by direct blowing from the mouth, as is done, for instance, when we blow into a bottle or key.

The sound given out by organ pipes is due to air blown across a sharp edge at the opening of a narrow tube.  The air forced across the sharp edge is thrown into vibration and communicates its vibration to the air within the organ pipe.  For different pitches, pipes of different lengths are used:  for very low pitches long, closed pipes are used; for very high pitches short, open pipes are used.  The mechanism of the organ is such that pressing a key allows the air to rush into the communicating pipe and a sound is produced characteristic of the length of the pipe.

[Illustration:  FIG. 190.—­1, horn; 2, trumpet; 3, trombone.]

[Illustration:  FIG. 191.—­1, kettledrum; 2, bass drum; 3, cymbals.]

[Illustration:  FIG. 192.—­The seating arrangement of the Philadelphia orchestra.]

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 pressure of the musician; if he breathes fast, the pitch rises; if he breathes slowly, the pitch falls.  All of these instruments, however, except the trombone possess some valves which, on being pressed, vary the length of the tube and alter the pitch accordingly.  In the trombone, valves are replaced by a section which slides in and out and shortens or lengthens the tube.

274.  The Percussion Instruments.  The percussion instruments, 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 general effect of an orchestra.

In orchestral music the various instruments are grouped somewhat as shown in Figure 192.

CHAPTER XXIX

SPEAKING AND HEARING

[Illustration:  FIG. 193.—­The vibration of the vocal cords produces the sound of the human voice.]

275.  Speech.  The human voice is the most perfect of musical instruments.  Within the throat, two elastic bands are attached to the windpipe at the place commonly called Adam’s apple; these flexible bands have received the name of vocal cords, since by their vibration all speech is produced.  In ordinary breathing, the cords are loose and are separated by a wide opening through which air enters and leaves the lungs.  When we wish to speak, muscular effort stretches the cords, draws them closer together, and reduces the opening between them to a