54 The Smithsonian Institution 



at night, and also the remarkable observations made by Arctic 

 explorers, notably Captain Parry, who was able to carry on a 

 conversation with a man a mile and a quarter away. The 

 clearness of sound " over water," which is so generally recog- 

 nized, is accounted for on the same principles. 



This interesting paper is a fitting prelude to the extended 

 and elaborate summary in the Report for 1878 of Henry's re- 

 searches in sound, conducted in the service of the United States 

 Lighthouse Board during the years 1865 to 1877. Henry's 

 long service as chairman of the Lighthouse Board consti- 

 tutes by no means the least important of his labors in the 

 interests of the general public. In this service he found 

 opportunity to utilize and apply his knowledge of physical 

 principles and the universally recognized high efficiency of 

 the Lighthouse establishment is due more to his intelligent 

 administration of its affairs than to any other single cause. 

 His researches in sound were among the most valuable of 

 his contributions toward the betterment of the service. As 

 every one knows, the presence of fog along the coast renders 

 the use of sound signals necessary; and for this purpose bells, 

 horns, trumpets, guns, etc., have long been in use. Many 

 curious and often contradictory phenomena have been noted, 

 especially in regard to variations in audibility under different 

 conditions, and the subject is one that has received much 

 attention among maritime nations. Generally under the direc- 

 tion of Professor Henry, the Lighthouse Board conducted an 

 extensive series of experiments during the years mentioned 

 above, the results of which were submitted as reports to the 

 Board and published by it, or, in some instances, read before 

 the Washington Philosophical Society, of which Henry was 

 President. On one of these occasions Professor Tyndall, 

 who was then in the country engaged in lecturing in the 

 principal cities, was present, being interested through his 



