41)4 MEMORIAL OF JOSEPH 1IKNRY. 



gave even' facility ai the disposal of the Institution to < reneral Mver 



for the completi f the organization, and indeed turned over the 



whole practical part of the subject to him. 



Among the services of Professor Henry outside of the held of 

 pure science and of the administration of the Smithsonian Institu- 

 tion the first place is due to those rendered in connection with the 

 Light-House Board. This Board was organized by act of Congress 

 in ls,")2 to discharge all administrative duties relating to the light- 

 house establishment on the American coasts. The duties assigned 

 to Professor Henry in this connection included experiments of all 

 kind- 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 Government 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 investi- 

 gations into the phenomena of sound. Acoustics had always been 

 one of his favorite subjects. As early as 1856 he published a care- 

 fully prepared paper on the acoustics of public buildings, and he 

 frequently criticised the inattention of architects to this subject. 

 lli> regular investigations of sound in connection with the Light- 

 House Board were commenced in I860. It had long been known 

 that the audibility of sounds at considerable distances, and espeeiallv 

 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 neighborh 1, 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 

 it> 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. 



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 lias long been a well-known fact. The logical con- 

 sequence of this is that the sound is cut oil' behind such a reflector, 



