1:20 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK n. 



plates, or of membranes ; 2nd, stringed instruments ; this numerous 

 family may be subdivided into three principal branches, the types 

 being the violin, harp, and piano ; 3rdly, wind instruments, which 

 are also divided, according as the mode of production of the sound 

 depends on the mouthpiece an orifice as in the flute, a mouthpiece 

 as in the horn, a reed as in the clarionet ; the flute, horn, and haut- 

 boy may serve as types of these three kinds, which are all contained 

 or imitated in the organ, that magnificent ensemble where all in- 

 strumental tones find expression, and which alone replaces a whole 

 orchestra. 



I. INSTRUMENTS BASED ON THE VIBRATIONS OF Eons OR 



PLATES. 



Plates and metallic rods of different forms vibrate when they are 

 rubbed transversely or longitudinally, or are struck in any one of 

 their points. Sounds are thus given out which are sometimes used 

 in musical bands. 



The triangle is an instrument of the last class, formed of a cylin- 

 drical rod of untempered steel, bent in the 

 figure of an equilateral triangle open at one 

 of its sides ; sometimes metal rings are added 

 strung on to the base. The performer strikes 

 one of the sides of the triangle with a steel rod. 

 From this percussion there - follows a series of 

 harmonic sounds the coexistence of which gives 

 to the instrument its sonorousness. This in- 

 strument contributes neither to harmony nor 

 7o.-The triangle. melody, but it accentuates the rhythm of a 



piece. 



The harmonica, with plates of glass, is an instrument formed of a 

 series of glass plates, the sizes of which are calculated in such a way 

 that when struck properly the successive sounds of the scale are 

 given out : the plates being of the same breadth and thickness, the 

 squares of their lengths are in inverse proportion to the number of 

 vibrations of the notes of the scale. The plates are supported by 

 horizontal strings in a strong wooden box. The strings support the 



