Sound: A Course of Eight Lectures

...from the sea, the sound—as it passes from the former to the latter—is reflected at the boundary where they meet. This view is made more likely by the fact that when the sound is blocked in the direction of the sea, it has been observed to be much stronger inland.

"Experiments and observations lead to the conclusion that these irregularities in how sound travels and is directed from fog signals are mainly caused by a lack of uniformity in the surrounding atmosphere. Furthermore, snow, rain, fog, and the direction of the wind have much less influence than has generally been supposed."

General Duane's report is characterized throughout by its commitment to facts, rare wisdom, and cautious theorizing. In my opinion, the last three paragraphs quoted above represent the only approach to a true explanation for these phenomena found in the Washington Report The official reports of the United States Lighthouse Board, which conducted extensive research on coastal safety and acoustics.. At this point, however, the distinguished Chairman of the Lighthouse Board This refers to Joseph Henry (1797–1878), a famous American scientist and first Secretary of the Smithsonian, who disagreed with Tyndall's "acoustic cloud" theory. offers the following critique:

"In the preceding text, I differ entirely from General Duane’s opinion that the fading of powerful sounds is caused by the atmosphere's uneven density. The speed of sound original: "velocity of sound" is not affected at all by barometric pressure. However, if a difference in pressure is caused by heat or by the pressure of water vapor mixed into the air, a slight obstruction of sound may be observed. But we believe this effect is far too small to produce the results noted by General Duane and Dr. Tyndall. Instead, we will find a true and effective cause in the action of air currents moving above and below."