Sound / by John Tyndall.

  • Tyndall, John, 1820-1893.
Date:
1883
Catalogue details

Licence: Public Domain Mark

Credit: Sound / by John Tyndall. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    middle row a x to represent the particles of air in equili- brium under the pressure of the atmosphere, and suppose the particle a to be suddenly drawn to the right, so as to occupy the position a in the highest line of dots: a is immediately followed by. 6, b by c, c by d, d by e ; and thus the rarefaction is propagated backward towards x, reaching a point o in the line of particles by the time a has completed its motion to the right. Now, why does b follow a when a is drawn away from it ? Manifestly because the elastic force exerted between b and a is then less than that between b and c. In fact, b will be driven after a by a force equal to the difference of the two elasticities between a and b and between b and c. The same remark applies to the motion of c after b, to that of d after c : in fact, to the motion of each succeeding particle when it follows its predecessor. The greater the difference of elasticity on the two sides of any particle the more promptly will it follow its predecessor. And here observe what the cold of rarefaction accomplishes. In ad- dition to the diminution of the elastic force between a and b by the withdrawal of a to a greater distance, there is a further diminution due to the lowering of the tem- perature. The cold developed augments the difference^ of elastic force on which the propagation of the rarefaction depends. Thus we see that because the heat developed in the condensation augments the rapidity of propagation of the condensation, and because the cold developed in the rarefaction augments the rapidity of propagation of the rarefaction, the sonorous wave, which consists of a con- densation and a rarefaction, must have its velocity aug- mented by the heat and the cold which it develops during its own progress. ■ It is worth while fixing your attention here upon the fact that the distance a' o' to which the motion has been propagated while a is moving to the position a' may be