Vapour in the Atmosphere on the Intensity of Sound. 279 



from a great depth. The experiments of Dr. Tyndall showed 

 that this is actually the case in a foggy or hazy atmosphere j and 

 accordingly this fact, so contrary to previous expectation, may be 

 considered to be accounted for by the present theory. 



Taking now into consideration the velocities U and W, it may 

 be proved (as is shown in the discussions above cited) that these 

 velocities are such that just as much fluid crosses any plane per- 

 pendicular to the direction of the propagation of the incident 

 waves as would have crossed if the globule had been absent. As 

 this is true with respect to each globule, it follows from the prin- 

 ciple of the coexistence of steady motions, that the mean effect 

 of the reaction of all the globules is simply to augment the velo- 

 city in the incident waves by contracting the space in which the 

 fluid moves, just as the rate of the flowing of a river is increased 

 by contraction of its channel. By this means an impetus 

 would be given to the sound-waves ; and it might thus happen 

 that, in the absence of countervailing causes, sound would be 

 audible in a hazy atmosphere at unusually great distances. This 

 in fact appears to have occurred in the experiments of July 1 

 and October 30. 



During a fall of rain or snow, the drops or the flakes might 

 by their size produce a sensible amount of reflection, and so di- 

 minish the intensity of the sound ; but, on the other hand, by 

 their occupation of space they would, in the manner explained 

 above in relation to the globules of vapour, give impulses to the 

 vibrations ; so that upon the whole (as appears from experiment 

 to be the case) rain or snow falling might have little effect on the 

 intensity of sounds. 



It is to be observed that the theory points to no change of the 

 rate of propagation of sounds by the action of visible vapour, but 

 exclusively to an alteration of their intensity. 



Let us now suppose vapour in an invisible state to be mixed 

 with the air. The vapour is connected with the air only by a 

 kind of mechanical suspension ; yet there is reason from expe- 

 riment to conclude that the elasticity of the compound differs 

 very little from that of the air itself. So long, therefore, as the 

 mixture is not irregular, there would appear to be no reason for 

 the production of reflected sound. But, to use Dr. TyndalFs 

 terms, an irregular or " flocculent " admixture would render the 

 air " acoustically opaque/' This effect may be conceived to take 

 place as follows. 



If it were possible that two portions of an elastic fluid 

 of different densities and the same elasticity could be in jux- 

 taposition on opposite sides of the same plane, in any case 

 of the propagation of waves from one portion to the other 

 reflection would take place at the plane of separation, whether 



