PHILOSOPHICAL SOCIETY OF WASHINGTON, 345 



ing only 18 seconds. On a large block of soft India-rubber rest- 

 ing on the marble slab, the vibration was very rapidly extin- 

 guished, but without giving any sensible sound. This anomaly 

 required an explanation. By means of a compound wire of copper 

 and iron inserted into the piece of rubber, and having the extremi- 

 ties connected with a thermo-galvanometer it was found that in 

 this case the acoustic vibrations were converted into heat. Sheets 

 of India-rubber therefore are among the best absorbers and de- 

 stroyers of sound. A series of experiments was also made on the 

 reflection of sound, to determine the materials least and those best 

 adapted to this purpose. A resume of these researches, having 

 reference to the acoustic properties of public halls, was read before 

 the American Association in August, 1856. 



In 1865, as Chairman of the Committee of Experiments of the 

 U. S. Light-house Board, Henry commenced an extended series 

 of observations on the conduct and intensity of sound at a distance, 

 under varying meteorological conditions. Well aware that for the 

 practical purposes of giving increased security to navigation, the 

 experiments of the laboratory were of little value, he undertook a 

 number of experimental trips on board sailing vessels, and on 

 steamers, in order to make his observations under the actual con- 

 ditions of the required service. As many of his investigations 

 required intelligent co-operation, and sometimes at the distances 

 of many miles, he associated with him at different times, among 

 members of the Light-house Establishment, Commodore Powell, 

 Commodore Case, Admiral Trenchard, Commander Walker, Cap- 

 tain Upshur, General Poe, General Barnard, General Woodruff, 

 Mr. Lederle, and other engineers of different Light-house Dis- 

 tricts, and outside of the establishment. Dr. Welling and others. 



At the outset of his experiments, he found that sound reflectors, 

 which play so interesting a part in lecture-room exhibitions, were 

 practically worthless (of whatever available dimensions) for the 

 purpose of directing or concentrating powerful sounds to any con- 

 siderable distance. At the distance of a mile or two a large 

 steam whistle placed in the focus of a concave reflector 10 feet in 

 diameter could be heard very nearly as well directly behind the 

 reflector, as directly in front of it. In like manner the direction 

 of bell-mouths and of trumpet-mouths, was found to be of com- 

 paratively little importance at a distance ; showing the remarkable 

 tendency to diffusion, especially with very loud sounds. Most of 

 the observations made on ship-board were afterward repeated on 

 land ; and several weeks were occupied with these important re- 

 searches. 



"During this series of investigations an interesting fact was dis- 

 covered, namely, a sound moving against the wind, inaudible to 

 the ear on the deck of the schooner, was heard by ascending to the 

 mast-head. This remarkable fact at first suggested the idea that 



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