JOSEPH HENRY 141 



Lighthouse Board. This Board was organized by act of Con- 

 gress in 1852 to discharge all administrative duties relating to the 

 lighthouse establishment on the American coasts. The duties as- 

 signed to Professor Henry in this connection included experiments 

 of all kinds pertaining to lights and signals. The illuminating 

 power of various oils was made the subject of exact photometric 

 experiments, and large sums were thus saved to the Govern- 

 ment by the adoption of those illuminators which gave most light 

 in proportion to cost. The necessity of fog-signals led to what 

 are, for our present purpose, the most important researches in 

 this connection, namely, his investigations into the phenomena of 

 sound. Acoustics had always been one of his favorite subjects. 

 As early as 1856 he published a carefully prepared paper on the 

 acoustics of the public buildings, and he frequently criticized the 

 inattention of architects to this subject. His regular investiga- 

 tions of sound in connection with the Lighthouse Board were 

 commenced in 1865. It had long been known that the audibility 

 of sounds at considerable distances, and especially at sea, varies 

 in a manner which has seemed quite unaccountable. There were 

 numerous instances of a sound not becoming audible until the 

 hearer was immediately in its neighborhood, and others of its 

 being audible at extraordinary distances. Very often a sound was 

 audible at a great distance and was lost as the hearer approached 

 its source. The frequency of fogs on our eastern coasts and the 

 important part played by sound signals in warning vessels of 

 danger rendered it necessary to investigate the whole theory of 

 the subject, and experiment upon it on a large scale. 



One of the first conclusions reached related to the influence of 

 reflectors and of intervening obstacles. That a sound in the focus 

 of a parabolic reflector is thrown forward and intensified in the 

 manner of light has long been a well-known fact. The logical 

 consequence of this is that the sound is cut off behind such a reflec- 

 tor, so that at short distances it is many times louder in front of 

 the reflector than behind it. In the case of light, which moves in 

 right lines, it is well known that such an increased volume of light 

 thrown in one direction will go on indefinitely. But in the case of 



