gently, the number of
vibrations of the reed is increased or decreased and hence the pitch
is altered.

[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
in