1874.] Transmission of Sound by the Atmosphere. 67 



This, I say, expresses the opinion generally entertained, " clear still 

 air" being regarded as the best vehicle for sound. We hare not, as 

 stated above, experimented in really dense fogs ; but the experiments 

 actually made entirely destroy the notion that clear weather is necessarily 

 better for the transmission of sound than thick weather. Some of our 

 days of densest acoustic opacity have been marvellously clear optically, 

 while some of our days of thick haze have shown themselves highly 

 favourable to the transmission of sound. Were the physical cause of the 

 sound-waste that above assigned, did that waste arise in any material 

 degree from reflection at the limiting surfaces of the particles of haze, 

 this result would be inexplicable. 



Again, Derharn, as quoted by Sir John Herschel, says that " falling 

 rain tends powerfully to obstruct sound.' 1 * We have had repeated reversals 

 of this conclusion. Some of our observations have been made on days 

 when rain and hail descended with a perfectly tropical fury ; and in no 

 single case did the rain deaden the sound ; in every case, indeed, it had 

 precisely the opposite effect. 



But falling snow, according to Derham, offers a more serious obstacle 

 than any other meteorological agent to the transmission of sound. We 

 have not extended our observations at the South Foreland into snowy 

 weather ; but an observation of my own made on December 29th, in the 

 Alps, during a heavy snow-storm, distinctly negatives the statement of 

 Derham. 



Reverting to the case of fog, I am unable in modern observations to 

 discover any thing conclusive as to its alleged power of deadening sound. 

 I had the pleasure of listening to a very interesting lecture on fog-signals 

 delivered by Mr. Beazeley before the United-Service Institution ; and I 

 have carefully perused the printed report of that lecture, and of a paper 

 previously communicated by Mr. Beazeley to the Institution of Civil 

 Engineers. But in neither of these painstaking compilations can I find 

 any adequate evidence of the alleged power of fogs to deaden sound. 



Indeed during the discussion which followed the reading of Mr. Bease- 

 ley's paper, an important observation in an opposite sense was mentioned 

 by Mr. Douglass, to whose ability and accuracy as an observer I am able 

 to bear the strongest testimony. Mr. Douglass stated that he had found 

 in his experience but little difference in the travelling of sound in foggy 

 or in clear weather. He had distinctly heard in a fog, at the Smalls rock 

 in the Bristol Channel, guns fired at Milford Haven, 25 miles away. Mr. 

 Beazeley, moreover, has heard the Lundy-Island gun " at Hartland Point," 

 a distance of 10 miles, during dense fog. Mr. Beazeley's conclusion, 

 indeed, accurately expresses the state of our knowledge when he wrote. 

 In winding up his paper, he admitted " that the subject appeared to be 

 very little known, and that the more it was looked into the more 

 apparent became the fact that the evidence as to the effect of fog upon 

 sound is extremely conflicting." When, therefore, it is alleged, as it is 



TOL. XXII. G 



