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 



