178 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK n. 



the bass of all brass instruments, and all the family of instruments 

 with keys, saxophones, so called after the maker who invented them, 

 or at least improved the manufacture of them. Another modification 

 is found in the trombone, a kind of sliding trumpet of ancient origin, 

 formed of two parts encased one in the other, which the performer 

 can draw out or in at will by a rectilinear movement of the right 

 hand. 



Lastly, a third method has been introduced in which the length 

 of the column is varied by the introduction of pistons, as in the 

 cornet d pistons, so well known in the present day in orchestras, and 

 especially military orchestras. The pistons are nothing but portions 

 of tubes, two or three in number, which are moved up and down in 

 cylindrical parts communicating with the tube of the instrument. 

 They are pierced laterally with apertures which correspond to 

 appendages intended to increase the length of the vibrating column. 

 According as the piston is lowered or raised, the apertures in question 

 are placed in front of those of the appendages or in contact with the 

 full portion ; the communication is open or closed, as shown in 

 Figs. 129 and 130, which represent a section of the cylinders holding 

 the pistons, and of the pistons themselves. The performer presses 

 sometimes on one, sometimes two, and sometimes on the three pistons. 

 The appendages are themselves composed of movable pieces which 

 can be lengthened or shortened to a certain degree. Lastly, the 

 portion of the tube of the instruments on which the mouthpiece is 

 h'xed, can be more or less lengthened, according to the music to be 

 played. In this way, the instruments can be tuned with all necessary 

 exactness. 



IV. BAGPIPES. 



All the wind-instruments we have already mentioned, whether the 

 mouth-pieces are flute-, reed-, or bell-shaped, receive the current of 

 air or wind which puts the column of air in the tube into vibration 

 from the mouth or lips of the performer directly. 



Before studying the organ, in which instrument the current is 

 produced mechanically by bellows, we ought to say a few words 

 about another kind of instrument in which the air which causes the 



