RESEARCHES IN SOUND. 



IXTEODUCTIOX.* 



Fog. 



Among the impedimeuts to navigation none perhaps are more to be 

 dreaded than those which arise from fogs, and consequently the nature 

 of this impediment and the means which may be devised for obviating 

 it are objects of great interest to the mariner. Fogs are in all cases 

 l^roduced when cold air is mingled with warm air saturated with moist- 

 ure. In this case the invisible vapor of the warmer air is condensed by 

 the cold into minute particles of liquid water, which, by their immense 

 number and multiplicity of reflecting surfaces, obstruct the rays of light 

 in the same way that a j)iece of transparent glass when pounded becomes 

 almost entirely opaque and is seen by reflection as a white mass. So 

 great!}' does a dense fog obstruct light, that the most intense artifi- 

 cial illumination, such as that produced by the combustion of magnesium, 

 by the burning of oxygen and hydrogen in contact with lime, and that 

 I)roduced between the charcoal points of a powerful electrical apparatus, 

 are entirely obscured at comparatively short distances. Even the light 

 of the sun, which is far more intense than that of any artificial illumina- 

 tion, is so diminished by a single mile of dense fog that the luminary 

 itself becomes invisible. Recourse must therefore be had to some other 

 means than that of light to enable the mariner to recognize his position 

 on approaching the coast when the land is obscured by fog. 



The only means at present known for obviating the difficulty is that 

 of employing powerful sounding instruments which may be heard at a 

 sufficient distance through the fog to give timely warning of iuq^ending 

 danger. Investigations therefore as to the nature of sound and its 

 applications to fog-signals become an important object to those in charge 

 of aids to navigation. Such investigations are of special importance in 

 connection with the light-house service of the United States. The 

 northeastern coast of the United States on the Atlantic, and the entire 

 western coa^t on the Pacific, included in our territory, are subject, 

 especially during the summer months, to dense fogs, which greatly im- 

 pede navigation, as well as endanger life and property. 



The origin of the fogs on our coast is readily exi)lained by reference 

 to a few simple principles of physical geography. In the Atlantic Ocean 

 there exists a current of warm water proceeding from the Gulf of Mex- 

 ico, between Cuba and Florida, which flows along our coast to the lati- 



* From the Report of the Light-House Boiud, for 1874. 



457 



