IX. 



ON SOUND IN RELATION TO FOG-SIGNALS, FROM 

 INVESTIGATIONS UNDER THE DIRECTION OF 

 THE U. S. LIGHT HOUSE BOARD. 



By JOSEPH HENRY. 



(Read Decbmbek 11, 1872.) 



(Before reading this paper Prof. Henry, as President, made the follow- 

 ing preliminary remarks.) 



The Committee of Arrangements have this evening called an 

 extra meeting of the Society to embrace the opportunity to invite 

 a few friends to meet Professor Tyndall. They have' extended 

 this courtesy to him as a mark of the high appreciation which 

 the Society entertains of his scientific labors. As the worthy 

 successor of Faraday, we recognize him as among the first con- 

 tributors to the physical science of the day. Although this is an 

 extra meeting, the proceedings will not differ essentially from 

 those of ordinary meetings, but will consist in the presentation 

 of communications purporting to be additions to knowledge, and 

 in discussions regarding them. We trust that Doctor Tyndall, 

 and the other invited guests, will join in the discussion, and in 

 the communication of any facts, which the occasion may recall to 

 memory, pertaining to the subjects under consideration. 



The communication which I propose to make this evening is 

 brought forward at this time especially on account of the presence 

 of Doctor Tyndall, he being connected with the Light House 

 system of Great Britain, while the facts I have to state are con- 

 nected with the Light House service of the United States, and 

 must therefore be of interest to our distinguished visitor. The 

 facts I have to present form part of a general report to be pub- 

 lished by the U. S. Light House Board. 



The Light House Board of the United States lias 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 improvements, but also 

 to enrich the Light House service with the results of new investi- 

 gations 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 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 



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