Sound: A Course of Eight Lectures

In 1867, the steam-siren original: steam-syren; an instrument that produces loud sound by blowing steam through a rotating disk, used as a powerful fog signal was mounted at Sandy Hook and examined by Professor Henry. He compared its performance with that of a Daboll trumpet A fog signal invented by Celadon Daboll that used a vibrating steel reed and a large trumpet to amplify sound. For this purpose, he used a stretched membrane covered with sand, placed at the narrow end of a tapering tube which concentrated the sound vibrations onto the membrane. The siren proved to be the most powerful. "At a distance of 50, the trumpet produced a clear movement of the sand, while the siren gave a similar result at a distance of 58." Professor Henry also varied the pitch of the siren and found that, when used with its trumpet, 400 impulses per second produced the maximum sound; meanwhile, the best result with the siren alone was obtained when the impulses were 360 a second.

Experiments were also conducted on the influence of pressure. These showed that when the pressure varied from 100 lbs. to 20 lbs., the distance reached by the sound (as determined by the vibrating membrane) varied only in the ratio of 61 to 51. Professor Henry also demonstrated that the sound of the fog-trumpet is independent of the material used to build it. Furthermore, he observed the decay of the sound as the angular distance from the center original: axis of the instrument was increased. Further observations were made by Professor Henry and his colleagues in August 1873, and in August and September 1874. In the brief but interesting account of these experiments, a hypothetical element appears which was absent from the records of the earlier observations.

It is quite evident from the information above that, regarding the question of fog signaling, the Lighthouse Board of Washington has not been idle. Added to this is the fact that their distinguished chairman provides his services free of charge.