1823.] Mr, Wheaistone on Sound, 87 



the nodes ai*e reproduced periodically at equal distances ; in 

 this we observe an analogous disposition with regard to light. 

 I had intended to include in this paper all the analogical facts 

 I have observed illustratory of the identity of the causes of these 

 two principal objects of sensation, but want of time, and the 

 danger of delay, now the subject is occupying so much the 

 attention of the scientific world, has induced me hastily to col- 

 lect the present experiments, and to defer the others for a future 

 opportunity. 



The thicknesses of conductors materially influence the power 

 of transmission, and there is a Hmit of thickness, differing for 

 the different degrees of tune, beyond which the vibrations will 

 not be transmitted. The vibrations of acute sounds can be 

 transmitted through smaller wires than those of grave sounds : a 

 proof of this is easy ; attach a tuning fork to one end of a very 

 small wire, and apply the other end to the ear, or a sounding 

 board ; on striking the fork rather hard, two co-existing sounds 

 will be produced, that which is more acute will be distinctly 

 heard, but the other will not be transmitted. If the vibrations of 

 a tuning fork be conducted through a piece of brass wire of the 

 size and thickness of a large needle, the sound, imperfectly 

 transmitted, will become more audible by the pressure of the 

 fingers on the conducting wire ; but if a steel wire of the same 

 length and thickness be employed, the sound will be unaltered 

 by any pressure, because steel has a greater specific elasticity 

 than brass. 



Polarization of Sound. 



Hitherto I have only considered the vibrations in their recti- 

 lineal transmission ; I shall now demonstrate, that they are pecu- 

 liarly affected, when they pass through conductors bent in 

 different angles. I connected a tuning fork with one extremity 

 of a straightconductingrod, the other end of which communicated 

 with a sounding board ; on causing the tuning fork to sound, 

 the vibrations were powerfully transmitted, as might be expected 

 from what has already been explained ; but on gradually bend- 

 ing the rod, the sound progressively decreased, and was scarcely 

 perceptible when the ang^e became a right one; as the angle 

 was made more acute, the phenomena were produced in an 

 inverted order ; the intensity gradually increased as it had before 

 diminished, and when the tv/o parts were nearly parallel, it 

 became as powerful as in the rectilineal transmission. By mul- 

 tiplying the right angles in a rod, the transmission of the vibra- 

 tions may be completely stopped. 



To produce these phenomena, however, it is necessary that 

 the axis of the oscillations of the tuning fork should be perpen- 

 dicular to the plane of the moveable angle, for if they be parallel 

 with it, they will be still considerably transmitted'. The follow- 

 ing experiment will prove this : I placed a tuning fork perpendi- 



