138 THE APPLICATIONS Ob 1 PHYSICAL FORCES. [BOOK n. 



ancient ones are composite : the sounds of the strings being too feeble 

 by themselves, they are strengthened by boxes in which the included 

 air and sides enter simultaneously into vibration. 



We shall divide them into three classes, according to the method 

 of vibration of the strings. In the first, we shall group the bow 

 instruments of the violin class. In the second, the instruments with 

 strings which are plucked or pulled, either by the fingers of the 

 performer, or by a point of wood or quill ; an example of this class is 

 the harp or guitar. Lastly, our third class will include instruments 

 with strings which are made to vibrate by the fall of a hammer; 

 these are instruments with keys, of the piano class. 



It is clear that another classification would be possible, that we 

 could distinguish between instruments with fixed lengths, each of 

 which only gives one note, and those with strings which can be 

 shortened at will by the performer, and therefore are susceptible of 

 variation either in a limited or indefinite manner. It would be 

 possible also to arrange them according to the nature of the sub- 

 stances of which they are composed, and of the tones they produce. 

 But these different points of view only affect indirectly the subject of 

 which we treat. We wish only to point out on what principles of 

 musical acoustics the construction of each type of instrument is 

 founded. 



II. THE VIOLIN. 



We will begin with the most perfect of musical instruments, the 

 violin. As in most stringed instruments, we have to study, first, two 

 principal parts, from a sound-producing point of view, one being the 

 system of strings from which the sound originates, the strings being 

 put directly into vibration by percussion produced either by pluck- 

 ing with the finger or by the friction of a bow ; the other part con- 

 sisting of a hollow box or chest on which the strings rest, and which 

 is intended to strengthen the sounds produced and to give them the 

 qualities of power, sweetness, and mellowness, and to impart to the 

 instrument its peculiar tone. 



The sides of the box and the mass of air contained in it contribute 

 in a certain measure to this result. We will describe these two parts 

 with special reference to the functions they have to perform. 



