Sound: A Course of Eight Lectures

...the reasoning of Professor Stokes, which suggests that a sound wave moving against the wind is tilted upward. An upper wind, blowing in the opposite direction, is proposed for the purpose of tilting that already lifted sound wave back downward. Professor Henry does not explain how the sound wave crosses back through the resisting lower air current, nor does he provide any clear idea of the conditions under which it could be shown that the sound would actually reach the observer.

This, as far as I know, is the only theoretical insight offered by the Washington Report regarding the conflicting results that have, until now, made experiments on fog signals so confusing. I fear it is a "fool’s fire" original: "ignis fatuus"; a Latin term for a deceptive light seen over marshes, used here to mean a misleading or delusional idea. rather than a safe, guiding light. Professor Henry, however, boldly applies this hypothesis to a variety of instances. But he dwells with particular emphasis on a case of "non-reciprocity" A situation where sound can be heard in one direction but not the other between two points. which he considers absolutely fatal to my views regarding the atmospheric "flocculence" Tyndall’s term for "clumpy" or uneven air density caused by patches of heat or moisture, which he believed reflected and scattered sound.. The observation was made on board the steamer City of Richmond during a thick fog on a night in 1872. "The vessel was approaching Whitehead from the southwest when, at a distance of about six miles from the station, the fog signal—which is a 10-inch steam whistle—was distinctly heard. It continued to be heard with increasing loudness until the ship was within about three miles, when the sound suddenly stopped. It was not heard again until the vessel approached within a quarter of a mile of the station, even though conclusive evidence from the lighthouse keeper showed that the signal had been sounding the entire time."

But while the 10-inch shore signal thus failed to make itself heard at sea, a 6-inch whistle on board the steamer was heard on shore. Professor Henry thus turns...