on the principles of Hydrodynamics. 365 



The numerical value of this quantity is less by only 3*17 feet 

 than the mean result (lOSQ'^S feet) of a large number of de- 

 terminations of the velocity of sound by direct experiment. 



The course of this investigation has shown that the velocity 

 of propagation (a) resulting from the hypothesis of plane- 

 waves, is not the correct value deducible from hydrodynamical 

 principles. The only true value given by hydrodynamics is that 

 I have exhibited above. The difference between the observed 

 velocity of sound and the value a, has been attributed, accord- 

 ing to a well-known theory, to the effect of the development 

 of heat accompanying sudden compressions of the air, and of 

 absorption of heat accompanying sudden dilatations. The 

 fact of such development and absorption is established by ex- 

 periments made on air in enclosed spaces. The walls of the 

 enclosure, by preventing the immediate escape of the deve- 

 loped heat, and the immediate restitution of the absorbed 

 heat, allow of ascertaining the fact by the thermometer. The 

 same development and absorption of heat must accompany the 

 condensations and rarefactions of aerial vibrations. But the 

 absence of enclosing walls makes an essential difference be- 

 tween this case and that of the experiment just mentioned. It 

 may not unreasonably be supposed that the thermometric 

 effect in the experiment is wholly due to reflexions at the 

 walls of the enclosure, by which the developed heat or cold is 

 made to traverse the enclosed air numberless times in an in- 

 appreciable interval, and thus produces a sudden change of 

 temperature. In a similar manner, when heat is radiating 

 from the earth's surface into a clear sky, as soon as a cloud 

 comes over, the air between the cloud and the earth, beco- 

 ming in some degree enclosed, and being traversed by the heat 

 reflected from the one to the other, has its temperature im- 

 mediately raised. As the heat developed or absorbed by 

 aerial vibrations cannot be supposed to produce a difference of 

 temperature by any analogous operation, a particular hypo- 

 thesis is required to account for an analogous effect in this 

 case. It is assumed that the air possesses the property of de- 

 taining the heat or cold set free by sudden compression or 

 dilatation, and that as the development or absorption is greater 

 the greater the condensation or rarefaction, there is always by 

 this detention an excess of temperature in the denser of two 

 contiguous elements of the vibrating air, and consequently an 

 effective accelerative force from the denser to the rarer por- 

 tion, which produces an apparent increase of the elasticity of 

 the medium. 



Respecting this theory, it must be said that it cannot be 

 considered as fully established, unless the property which it 



