Transparency and Opacity of the Atmosphere. 383 



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 Fore- 

 land into snowy weather ; but an observation of my own made on 

 December 29, in the Alps, during a heavy snowstorm, distinctly 

 negatives the statement of Derham. 



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

 to discover any thing conclusive as to its alleged power of deaden- 

 ing sound. 1 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 re- 

 port 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. 

 Beazeley'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. Dou- 

 glass 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, more- 

 over, has heard the Lundy-Island gun " at Hartland Point," a dis- 

 tance 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, there- 

 fore, it is alleged, as it is so often alleged, that the power of fogs 

 to deaden sound is well known, the disjunctive not is to be inserted 

 before the predicate. 



The real enemy to the transmission of sound through the atmo- 

 sphere has, I think, been clearly revealed by the foregoing inquiry. 

 That enemy has been proved to be not rain, nor hail, nor haze, nor 

 fog, nor snow — not water in fact in either a liquid or a solid form, 

 but water in a vaporous form, mingled with air so as to render it 

 acoustically turbid and flocculent. This acoustic turbidity often 

 occurs on days of surprising optical transparency. Any system of 

 measures, therefore, founded on the assumption that the optic and 

 acoustic transparency of the atmosphere go hand in hand must prove 

 delusive. 



There is but one solution of this difficulty : it is to make the 

 source of sound so powerful as to be able to endure loss by partial 

 reflection and still retain a sufficient residue for transmission. Of 

 all the instruments hitherto examined by us, the syren comes 

 nearest to the fulfilment of this condition ; and its establishment 

 upon our coast will, in my opinion, prove an incalculable boon to 

 the mariner. 



