162 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK n. 



The case varies in form, according to the general arrangement 

 of the instrument, which may be horizontal or vertical. This 

 arrangement having nothing essential about it, we will confine our- 

 selves to that which is prefered by piano players for the most 

 favourable development of sound, and we will describe the one 

 called a grand piano. In this the case has the form of a long- 

 triangle, similar to a harp, lying horizontally. The sounding case is 

 of wood, generally oak, with a thin lining of fir, formed of several 

 pieces glued and joined together ; the sounding-board is for the same 

 purpose in the piano as the upper plate, also made of fir, in the violin. 



It receives the first impression of 

 the vibrations of the strings, arid 

 through its fibres these are commu- 

 nicated to the case of the piano, but 



I,- . .m^maLmm^mammmMmmm particularly to the mass of air con- 

 |3j tained in it. 



Above the sounding-board and 

 parallel with its plane, the strings 

 are stretched on an iron frame-work 

 strengthened with bars of the same 

 metal, which give firmness and pre- 

 vent the frame giving under the 

 tension of the strings. These are of 

 metal, their length and thickness 

 being regulated according to the 

 pitch and volume of the note re- 

 quired. Each note is represented by 



a double string for the low, and by a triple string for the middle and 

 upper notes. 



All the wires are of steel ; but the lower ones have copper or 

 silver wire wound round them. These combinations are conformable 

 to the laws of the longitudinal vibrations of strings which teach us 

 that the number of these vibrations, that is to say, the pitch of the 

 note given by a string, is inversely proportional to its length, 

 diameter, and tension. 



The instrument is constructed in such a way that one of its 

 elements, the tension of each string, is left to the free will of the 

 tuner. By using an iron instrument or key, the tuner stretches each 



FK;. li:i Piano : arrangement of keys 

 and hammers. 



