MUSICAL INSTRUMENTS 121 



For the disk records a different method is used. A plate 

 of zinc is covered with a thin coating of acid-proof fat. The 

 stylus is arranged to work from side to side and traces a line 

 through this fat. This is treated with acid, which etches 

 out the line followed by the stylus. A mold is made by 

 electrotyping, and from this many copies may be made on 

 ebonite plates. 



Although the use of the phonograph in the home as a 

 source of enjoyment is the most common use now made, it 

 is being used for other practical purposes, and will doubtless 

 be so used even more in the future, as its various applica- 

 tions are perfected. A few of these uses will be merely 

 mentioned. The dictaphone is coming to be commonly 

 used by the business man. He dictates his letters into the 

 mouthpiece of a recording instrument. This dictation is 

 later copied by a typist. In learning new languages, records 

 may be used to obtain the correct pronunciation. Mr. 

 Edison has recently perfected an invention called a telescribe 

 by means of which a telephone conversation may be recorded 

 on a phonographic cylinder. A phonographic clock has been 

 devised which calls off the hours and the quarter hours. 



Types of musical instruments. The more common musi- 

 cal instruments may be divided into two groups : the stringed 

 instruments, such as the piano and violin, in which the sound 

 is produced by vibrations of strings ; and the wind instru- 

 ments, like the cornet and flute, in which the sound is pro- 

 duced by vibrations of columns of air. There is also a third 

 group called the percussion instruments in which the sound 

 is produced by the vibration of a membrane or metal disks, 

 as the drum and cymbals. 



There are three types of stringed instruments : first, those 

 in which the strings are set in motion by hammers, as in the 

 piano ; second, those in which the strings are set in vibration 

 by bowing, as in the violin ; and third, those in which the 

 strings are set in vibration by the fingers, as in the guitar. 



