Sound: A Course of Eight Lectures

soundings." In the appendix, he records the various efforts made in the United States to establish fog-signals. He describes experiments with bells and the use of reflectors to amplify their sound. These reflectors, while effective at close range, were found to be useless at a distance. He corrects common errors concerning steam whistles, which some inventors mistakenly believed functioned like ringing bells. He cites the opinion of the Reverend Peter Ferguson, who believed that sound is heard better in fog than in clear air. This opinion is based on observations of the noise from locomotives; however, it should be noted that others conducting similar experiments have reached completely opposite conclusions.

Relying on the authority of Captain Keeney, he cites an incident where, "initially, the Captain was led to believe that fog significantly deadened sound, though later he reached the opposite conclusion." Professor Henry also describes an experiment conducted during a fog in Washington, D.C., in which he used "a small bell rung by clockwork—an apparatus taken from a moderator lamp A moderator lamp was a common 19th-century oil lamp that used a mechanical pump to regulate the flow of oil; the bell was a safety feature to warn the user when the oil supply was nearly empty., designed to warn the keepers when the oil supply ran out. He affirms that the result of the experiment contradicted the assumption that fog absorbs sound."

This conclusion is not based on comparative experiments but only on observations made during the fog. As Professor Henry adds, "the changes in atmospheric conditions—such as temperature and air movement—before the experiment could be repeated in clear weather made the results not entirely satisfactory."

This, I should note, is the only experiment regarding fog that I have found recorded in the appendix.