490 RESEAECHES IN SOUND. 



PAET II.— EEMAEKS ON SOME ABNORMAL PHENOMENA OF 



SOUND.* 



The commuuicatiou which I propose to make this evening is brought 

 forward at this time especially on account of the presence of Dr. Tyn- 

 dall, he being connected with the light-house system of Great Britain, 

 while the facts I have to state are connected with the light-house service 

 of the United States, and must therefore be of interest to our distin- 

 guished visitor. The facts I have to present form part of a general re- 

 port to be published by the United States Light-House Board. 



The Light-House Board of the United States has from its first estab- 

 lishment aimed not only to furnish our sea-coast with all the aids to 

 navigation that have been suggested by the exi^erience of other couutries 

 and to adopt the latest imi^rovements, but also to enrich the light-house 

 service with the results of new investigations and new devices for the 

 improvement of its efficiency, or, in other words, to add its share to the 

 advance of a system which pertains to the wants of the highest civiliza- 

 tion. 



Among the obstructions to navigation none are more serious, especially 

 on the American coast, than those caused by fogs. 



Fog, as it is well known, is due to the mingling of warmer air sur- 

 charged with moisture with colder air, and nowhere on the surface of 

 the earth do more favorable conditions exist for producing fogs 

 than on both our Atlantic and Pacific coasts. On the Atlantic the 

 cold stream of water from the polar regions in its passage southward, 

 on account of the rotation of the earth, passes close along our eastern 

 coast from one extremity to the other, and parallel to this but opposite 

 in direction, for a considerable distance is the great current of warm 

 water known as the Gulf Stream. Above the latter the air is constantly 

 surcharged with moisture, and consequently whenever light winds blow 

 from the latter across the former, the vapor is condensed into fog, and 

 since in summer along our eastern coast the southerly wind prevails, 

 we have during July, August, and September, especially on the coast 

 of Maine, an almost continuous prevalence of fogs so dense that distant 

 vision is entirely obstructed. 



On the western coast the great current of the Pacific, after having 

 been cooled in the northern regions, in its passage southward gives rise 

 to cold and warm water in juxtaposition, or, in other words, a current of 

 the former through the latter, and hence whenever a wind blows across 

 the current of cold water, a fog is produced. 



From the foregoi-ug statement it is evident that among the aids to 

 na^dgation fog-signals are almost as important as light-houses. The 

 api^lication however of the science of acoustics to the former is far less 

 advanced than is that of optics to the latter. Indeed, attempts have 

 been made to apply lights of superior i^enetrating power, as the electric 



* Made before tlie ''Pliilosoijliical Society of Wasliington," December 11, 1872. 



