of Solid Bodies by Internal Resistances. 165 



is a difference in the intensity of the reflection by the caoutchouc 

 sides of sound-vibrations of different heights. 



Helmholtz* explains in a similar way the softer tone of wooden 

 as compared with metal pipes, " inasmuch as the sides of the 

 former do not so well resist agitation by the sound-waves, by 

 which the higher-sound vibrations seem more easily destroyed 

 by friction." 



Hence the phenomena in the conduction of sound through 

 the air in caoutchouc tubes are to be referred to the phenomena 

 of the conduction of sound through solid caoutchouc ; and the 

 question now arises as to the explanation of the unequal enfee- 

 blement of sounds of different heights when conducted through 

 solids. 



Part II. 



When vibrations are entirely and alone sustained by elastic forces, 

 a diminution of the wave-length (vibrating portion), and there- 

 with an increase of the mean molecular displacement for the 

 same amplitude of maximum vibration, must be inseparably con- 

 nected with a diminution of the duration of vibration in one and 

 the same body. Hence the author, in order to separate these 

 two influences, has combined magnetic with elastic forces, and 

 has had recourse to torsion vibrations sufficiently prolonged to be 

 capable of definite measurement. If a magnet be suspended 

 by a thread, only by changing the directive force of the magnet 

 by another magnet suitably placed can we alter the duration 

 of the torsion vibrations of which this system is capable. The 

 thread, on the other hand, can be shortened, and the change 

 in the time of oscillation of the system thus produced can be 

 compensated by a suitable displacement of the external magnet. 



This idea was executed by means of a kind of torsion-balance. 

 To be able to alter at will the length of the threads, they were 

 fastened at the top to a rod which could be moved vertically in the 

 tube; at the other end was a scale-beam which had a groove to 

 hold the magnet, and was moreover provided with a silvered 

 vertical glass mirror. In this system, suitably protected against 

 disturbance, torsion vibrations could be produced by means of 

 external magnetic influences. Through a plane parallel glass 

 inserted in the vessel of the balance, the deflections of the beam 

 could be observed by means of a scale and telescope. Caout- 

 chouc threads and fine silk, glass, and metal threads were inves- 

 tigated. 



It was ascertained in this manner that within elongations of 

 6° and 2° from the position of equilibrium, to which limits the 

 observations were restricted, the series of deflections may be 



* Lehre von den Tonempfindungen, p. 153. 



