174 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK n. 



III. WIND INSTRUMENTS WITH BELL-SHAPED OR HORN 

 MOUTHPIECES. 



In the musical instruments which remain for us to notice, the 

 mouthpiece is simply formed of a tube, widened out in a conical 

 form, or of a tube terminated by a hemispherical cavity which is 

 placed against the lips (Fig. 122). In these instruments it is the 

 vibration of the lips themselves which is communicated to the column 

 of air inclosed in the differently shaped tubes which constitute the 



sounding body of the instrument. These 

 vibrations can be produced more or less 

 rapidly according as the performer presses 

 the mouth against the aperture, and as the 

 current of air passing through the lips is 

 more or less narrowed. It requires great 

 practice to calculate the dimension of this 

 aperture, and the velocity and force of the 

 current exactly to the pitch of the notes 

 required in short, to make the lips vibrate 

 in unison with the fundamental note of the 

 instrument, or with its harmonics. This is called using the lips. 



FIG. 122. -Types of bell and 

 horn mouthpieces. 



FIG. 123. Cor d' harmonic. 



The most typical of wind instruments with horn mouthpieces, is 

 the horn itself, which is formed of a tube bent into a spiral form, 



