156 VIBRATION OF MUSICAL STRINGS. [SBCT. xvn. 



SECTION XVII. 



Vibration of Musical Strings Harmonic Sounds Nodes Vibration of Air in 

 Wind-InstrumentsVibration of Solids Vibrating Plates Bells Har- 

 mony Sounding Boards Forced Vibrations Resonance Speaking Ma- 

 chines. 



WHEN the particles of elastic bodies are suddenly disturbed 

 by an impulse, they return to their natural position by a 

 series of isochronous vibrations, whose rapidity, force, and 

 permanency depend upon the elasticity, the form, and the 

 mode of aggregation which unites the particles of the body, 

 fhese oscillations are communicated to the air, and on ac- 

 count of its elasticity they excite alternate condensations and 

 dilatations in the strata of the fluid nearest to the vibrating 

 body : from thence they are propagated to a distance. A 

 string or wire stretched between two pins, when drawn aside 

 and suddenly let go will vibrate till its own rigidity and the 

 resistance of the air reduce it to rest. These oscillations , 

 may be rotatory, in every plane, or confined to one plane 

 according as the motion is communicated. In the piano- 

 forte, where the strings are struck by a hammer at one 

 extremity, the vibrations probably consist of a bulge running 

 to and fro from end to end. Different modes of vibration 

 may be obtained from the same sonorous body. Suppose a 

 vibrating string to give the lowest C of the piano-forte which 

 is the fundamental note of the string ; if it be lightly touched 

 exactly in the middle so as to retain that point at rest, each 

 half will then vibrate twice as fast as the whole, but in op- 

 posite directions ; the ventral or bulging segments will be 

 alternately above and below the natural position of the 

 string, and the resulting note will be the octave above C. 

 When a point at a third of the length of the string is kept 



