226 Mr. Wheatstone on the 



of producing powerful sounds "without accessory means ; but 

 the sound of vibrating bodies of smaller dimensions, such as 

 insulated strings, or tuning-forks, are scarcely audible at a 

 moderate distance from the ear ; but the sounds of the latter 

 are capable of considerable augmentation when communicated 

 to surfaces, as when they are placed to a table, or to the 

 sounding-board of a musical instrument. 



There are several circumstances which influence the inten- 

 sity of the resonance of a sounding-board. The principal of 

 these is the plane in which the vibrations of the sounding body 

 are made with respect to the reciprocating surface. Thus, its 

 vibrations may be so communicated as to be perpendicular, or 

 normal to the surface, in which case the sound is the most 

 greatly augmented ; or they may be tangential to, or in the 

 same plane with the surface, when the sound is the most feeble. 

 The first of these cases may be illustrated by placing a vibrat- 

 ing tuning-fork perpendicularly to the surface of a flat board ; 

 and the second, by placing it perpendicularly to one of the edges 

 of the board. In intermediate positions viz. when the vibra- 

 tions are communicated obliquely to the surface the sound 

 will be found to have intermediate degrees of intensity. 



These facts, which the extensive investigations of Savart 

 place in full evidence, being understood, the peculiarities of 

 the sounding-boards of various musical instruments admit of 

 easy explanation. 



The sounding-board of the piano-forte is better disposed 

 than that of any other stringed instrument, as the planes of the 

 vibrations of the strings are, on account of the direction in 

 which they are struck by the hammers, always perpendicular 

 to its surface. The difference of intensity when a string 

 vibrates in this way, and when it vibrates parallel to the sur- 

 face, is very obvious, and may be easily tried by striking it 

 with the finger in these two directions *. There is no other in- 

 strument now in use, in which the strings make their vibrations 

 perpendicular to the sounding-board. 



* It sometimes happens, when the impulse is oblique to the direction in which 

 the string presses on the bridge, that its plane of vibration assumes a rotatory 

 motion; the periodical changes of intensity thus occasioned, produce an effect 

 similar to that of the beating of imperfect unisons. This phenomenon is generally 

 erroneously attributed by tuners to a faulty string. 



