Page:Popular Science Monthly Volume 88.djvu/603

 The Undependable Fog-Horn

���A victim of the freakish fog-signal. The British freighter "Chalcas," feeling her way past Point Wilson, at the entrance of Puget Sound, and guided by the blasts of the siren, suddenly ceased to hear the fog-horn. Before it could be picked up again, the steamer was wrecked and a loss of seventy-five thousand dollars resulted. The siren had kept blowing, but the steamer had entered one of the "zones of silence," and had ceased to hear

��THE caprices of fog-horns present a less serious problem to the navi- gator than they did before the days of submarine signals, but as the use of the latter device is by no means univer- sal, the erratic behavior of aerial signals is still responsible for many marine dis- asters.

Whether the signal be a siren, trumpet, whistle or bell, its range of audibility is subject to remarkable fluctuations. A signal under certain circumstances audi- ble at a distance of ten miles, will on occasions be entirely includible at a distance of two; or, again, there will be certain zones or regions within a mile or two of the signal where no sound can be heard, while the signal is distinctly heard at much greater distances. These "zones of silence" have often been de- scribed, but never fully explained. More- over, many misleading statements are current in regard to them.

That fog has a blanketing effect upon sound was beliexed until dispro\ed by the classic experiments of T\ndall at the South F\)reland and elsewhere in Kng- land in the 'seventies of the last century. Tyndall pro\ed that, in general, sound carries farther in a fog than in clear weather. In the same series of experi- ments this physicist developed an hy- pothesis to account for zones of silence and aerial echoes. This explanation la>s particular stress upon a supposed "floc-

��culent" condition of the atmosphere, 7. e., the pressure of streams of air of mutually different temperatures and humidities, giving rise to invisible "acoustic clouds." Tyndall's hypothesis is, however, not now accepted in its en- tirety.

About the time of these experiments, similar inxestigations were carried out in America by General Duane and Pro- fessor Joseph Henry. One result of the American experiments for which the in- \estigators themselves were not responsi- ble, was the currency given to the idea that a "zone of silence" surrounding the source of sound is a more or less uniform and permanent phenomenon. Except under special conditions of topograph)-, this is not the case.

A typical case of acoustic fluctuations is shown by one of the accompan\ing diagrams. On the night of November 6, 1880, the steamer Rhode Island, xalued with her cargo at $1,000,000. was lost on Bonnet Point, in Narragansett Bay. This wreck occurred onl\- a little more than a mile from the fog-signal at Bea\er Tail Point — a Daboll trumpet — which was in full blast at the time, and, under ordinary circumstances, could be heard at a distance of six to eight miles. The conditions of audibility in this region were subsec|ucntl\- inxestigated b\' Com- mander (now Rear-Ad miral) Chadwick, U. S. N. His observations were made from a sailboat, in clear weather. (It

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