A SYSTEM OF PARTICLES. 249 



14. Let us proceed, in the next place, to examine some of the 

 results which we have above deduced, and to apply tiiem to the case of 

 sound. 



We suppose, in this application, that the two media are air and 

 vapour, and we have seen that, on the hypothesis of a repulsive energy 

 (7) in each, the velocity of transmission of the vibration of vapour 

 is equal to that of air, not only when the two media are united in 

 an uniform mixture, but even when they are not so, provided they are 

 so distributed, as to be in equilibrium when not under the influence 

 of an external disturbance. 



This would probably be the case during a day, in which the 

 fluctuations of the barometer were inconsiderable; on other occasions 

 we might expect a slight difference in the velocities, and thus might 

 be explained the circumstance, mentioned by Mr Herschel in the 

 Encyclopa?dia Metropolitana, of the double report of a gun. I should 

 scarcely, however, consider the above results consistent with the fact. 

 did it not appear probable, that such phenomena occur at a time, 

 when either the vapour is pa.^sing into water, or some other changj 

 is taking place which destroys the equilibrium of the mass. 



15. When the vapour has actually become water, if it act on air 

 by an attractive or repulsive force, its eff^ect on the transmission of sound 

 wHl disappear altogetlier: the formula for this case is that which we 

 have investigated (8) for a mixture with air of material particles, or 

 particles whose magnitude or inertia is such, that whilst they affect 

 the motion of air, themselves are not sensibly affected by its vibrations. 

 We found that such a system introduced no additional force to that 

 which the particles of air exert on each other. 



16. On examining the expression for the velocity, it is evident 

 that unless \ be supposed very great, we should expect to find the 

 velocity influenced by the length of the wave, so as to increase with 

 it. It is generally assumed that no such influence is sensible, but no 

 very conclusive experiments appear to have been undertaken for the 

 express purpose of ascertaining what the fact really is. That of 



