370 WRITINGS OF JOSEPH HENRY. [1874 



RESEARCHES IN SOUND, IN RELATION TO FOG-SIGNALLING. 



(Report of the United States Light-House Board for 1874, pp. 83-87.) 

 INTRODUCTION. 



Fog. — Among the impediments to navigation none per- 

 haps is more to be dreaded than that which arises 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 pro- 

 duced when cold air is mingled with warm air saturated 

 with moisture. 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 mul- 

 tiplicity of reflecting surfaces obstruct the rays of light in 

 the same way that a piece of transparent glass when pounded 

 becomes almost entirely opaque and is seen by reflection as 

 a white mass. So greatly does a dense fog obstruct light 

 that the most intense artificial illumination, such as that 

 produced by the combustion of magnesium, by the burning 

 of oxygen and hydrogen in contact with lime, and that pro- 

 duced between the charcoal points of a powerful electrical 

 apparatus are entirely obscured at comparatively short dis- 

 tances. Even the light of the sun, which is far more intense 

 than that of any artificial illumination, 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 difli- 

 culty is that of employing powerful sounding instruments 

 which may be heard at a suSicient distance through the fog 

 to give timely warning of impending 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 im- 



