134 VIBRATION OF MUSICAL STRINGS. SECT. XVII. 



vibrating, at every quarter revolution the sound will 

 scarcely be heard, while at the intermediate points it 

 will be strong and clear. This phenomenon arises 

 from the interference of the undulations of air coming 

 from the two branches of the fork. When the two 

 branches coincide, or when they are at equal distances 

 from the ear, the waves of air combine to reinforce each 

 other ; but at the quadrants, where the two branches 

 are at unequal distances from the ear, the lengths of the 

 waves differ by half an undulation, and consequently 

 destroy one another. 



SECTION XVII. 



Vibration of Musical String's Harmonic Sounds Nodes Vibration of Air 

 in Wind Instruments Vibration of Solids Vibrating Plates Bells- 

 Harmony Sounding Boards Forced Vibrations Resonance Speaking 

 Machines. 



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 elas- 

 ticity, the form, and the mode of aggregation which 

 unites the particles of the body. These oscillations are 

 communicated to the air, and on account 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 oscil- 

 lations 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 so- 

 norous 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 



