180 Mr. Wheatstone on Resonance 



original sounding body ; or, in other words, the resonance and the 

 original sound have always been unisonant. The experiment I 

 shall now bring forward will show that this is not universally 

 the case, and that there are other phenomena of resonance which 

 have never, hitherto, been investigated in their theory, or in their 

 practical applications. I took a tube, closed at one end by a 

 moveable piston, and placed before its open end the branch of a 

 vibrating tuning-fork of the ordinary pitch, C ; the length of the 

 column of air was six inches ; on diminishing the length of the 

 column to three inches, the sound of the tuning-fork was no longer 

 reciprocated, but its octave above (the sound of the column when 

 it is directly excited) was produced. By employing a graver 

 tuning-fork and tubes of very small diameter, and successively ad- 

 justing the lengths of the columns of air so as to be one half, one 

 third, one fourth, one fifth, &c. of the column reciprocating the 

 fundamental sound, the octave, twelfth, double octave, seven- 

 teenth, &c. to that sound will be produced. The relative numbers 

 of the vibrations of these sounds, considering the vibrations of the 

 fork as unity, are 1, 2, 3, 4, 5, &c. It therefore is evident, from 

 experiment, that a column of air may vibrate by reciprocation, not 

 only Avith another body whose vibrations are isochronous with its 

 own, but also wheji the number of its own vibrations are any mul- 

 tiple of those of the original sounding body. >' 



The converse of this law does not hold ; for when the number 

 of vibrations of a column of air are any sub-multiple of those of the 

 original sounding body, there is no resonance. To prove this with 

 regard to the octave, let the length of the column of air unison to 

 the sound of a fork be doubled, and not the slightest trace of the octave 

 below (i. e. the real sound of the column) will be perceptible : this 

 negative experiment must be tried with a closed tube which is in- 

 capable of producing a harmonic octave ; an open tube would 

 resound unisonantly to the fork by its subdivision. 



§ 7. On the law experimentally established in the preceding 

 paragraph depends the explanation of the production of sounds by 

 the guimbarde or Jew's harp. This simple instrument consists of 

 an elastic steel tongue, riveted at one end to a frame of brass or 

 iron, the form of which is represented in the annexed figure ; the 

 free extremity of the tongue is bent outwards to a right angle, so 

 as to allow the finger easily to strike it when the iustrument i^ 

 placed to the mouth, and firmly supported by the pressure of the 

 parallel extremities of the frame against the teeth. 



