416 WRITINGS OF JOSEPH HENRY. [1874 



PAKT II.— ON SOME ABNORMAL PHENOMENA OF SOUND. 



(Bulletin Philosophical Society Washington ; vol. ii, Appendix, pp. 45-52.) 

 Read December 11, 1872. 



The communication which I propose to make this even- 

 ing is brought forward at this time especially on account of 

 the presence of Dr. Tyndall, 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 dis- 

 tinguished visitor. The facts I have to present form part of 

 a general report to be published by the United States Light- 

 House Board. 



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

 first establishment aimed not only to furnish our sea-coast 

 with all the aids to navigation that have been suggested by 

 the experience of other countries and to adopt the latest im- 

 provements, 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 advancement of a system which pertains to the 

 wants of the highest civilization. 



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 surcharged with moisture — with colder air, and 

 nowhere on the surface of the earth do more favorable con- 

 ditions 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 



