Sound: A Course of Eight Lectures

"...when moving directly away from a station, the sound is audible for a distance of a mile, is then lost for about the same distance, after which it is again distinctly heard for a long time. This phenomenon is common to all acoustic signals and has been observed at all the stations. At one of these, the signal is located on a bare rock twenty miles from the mainland, with no surrounding objects to interfere with the sound."

It is not necessary to assume that a "belt" of silence exists at a specific distance from the station. The passage of an acoustic cloud original: "acoustic cloud"; Tyndall’s term for invisible regions in the air—caused by variations in temperature or humidity—that reflect, scatter, or block sound waves. over the station itself would produce the observed effect.

Skipping over the records of many other valuable observations in General Duane’s report, I will highlight a few very important remarks that directly relate to the current topic.

"Based on three years of careful observation of the fog-signals on this coast," writes the General, "and from reports received from the captains and pilots of coastal vessels, I am convinced that, under certain atmospheric conditions, even the most powerful signals will occasionally be unreliable.¹

"It often happens that a signal which, under ordinary circumstances, would be audible at a distance of fifteen miles cannot be heard from a ship only one mile away. This is probably due to the reflection mentioned by Humboldt Alexander von Humboldt (1769–1859), a Prussian polymath who theorized that sound is reflected by non-uniform layers of air, such as those with varying temperatures..

"The temperature of the air over the land where the fog-signal is located being very different from that over..."

¹ If I had been aware of this report's existence earlier, I might have used General Duane's own words to express my views on the point mentioned here. See Chapter VII, pages 319–320.