280 Effects produced by Fog and Vapour on the Intensity of Sound. 



the waves passed from the rarer to the denser portion, or from 

 the denser to the rarer. In an irregular, or flocculent, admix- 

 ture of air and vapour of water such an abrupt change of density 

 would not be possible ; but there might be changes of density- 

 more or less rapid from point to point without change of elasti- 

 city ; and, so far as these changes tend to be abrupt, partial re- 

 flection of the waves might ensue. It is also to be considered that 

 the reflections would always take place in the directions of most 

 rapid change of density, whether the density be increasing or 

 decreasing ; so that in a flocculent admixture there might be 

 reflections in various directions, and the sound might be dis- 

 persed or absorbed in a manner analogous to the absorption of 

 light on its entrance into an opaque or partially transparent me- 

 dium. Thus air charged with vapour in an invisible state, 

 although optically transparent, might be in various degrees im- 

 pervious to sound. That this is actually the case was clearly 

 demonstrated by the experiments. This view is confirmed by 

 the fact, observed on October 8, that the intensity of the sound 

 was increased by a heavy fall of rain, the effect of which would 

 be, as is remarked by Dr. Tyndall, to remove to a certain amount 

 by condensation and precipitation the invisible vapour to which 

 the diminution of the intensity of sound is mainly attributable. 



Thus theoretical reasons can be given for the experimental 

 results that fog and haze do not diminish, and may even increase, 

 the audibility of sounds, and that loss of sound is chiefly due to 

 certain conditions of the invisible vapour mixed with the air. 

 Theory also indicates that neither by fog and haze, nor by that 

 condition of invisible vapour which diminishes or absorbs sound, 

 is the rate of propagation sensibly altered. But I know of no 

 apriori means of determining whether the amount of vapour in 

 regular admixture with the air may not have some influence 

 on the velocity of sound ; and this, I think, is a point which is 

 open to further inquiry by experimental means. 



The greater distance to which sounds are audible to leeward 

 than to windward of the place of excitation, is very little owing 

 to the transmission being accelerated by the wind in the former 

 case and retarded by it in the other. It is chiefly due to the 

 circumstance that in those vibratory movements of the air to 

 which the loudness of the sound is principally due there is an 

 excess of condensation above rarefaction, and the consequent 

 excess of the velocity corresponding to the condensation is in 

 the same direction as the current of the wind on the leeward 

 side and opposed to it on the windward side. As the action 

 on the ear is in each case proportional to the square of the 

 compound velocity, the result will be an excess of intensity on 

 the leeward side. 



