Fog Signals and Foghorns
When the Light Fails
A lighthouse is, at its most fundamental, a visual aid. It requires the observer to see it. Fog removes that possibility entirely, and the stretches of coast that are most dangerous to navigate are frequently the most prone to thick weather. The Newfoundland banks, where the cold Labrador Current meets warm Gulf Stream water, generate some of the densest and most persistent fog on the North Atlantic. The approach to San Francisco through the Golden Gate is fog-bound for a substantial portion of each year. The Baltic approaches to the major Swedish and Danish ports experience weeks of fog in spring and autumn. These are also stretches where shipping is densest and navigation most critical.
The response to this fundamental limitation of the visual lighthouse was the fog signal: an audible warning, sounded at intervals, identifying the station by its timing and character just as the nocturnal light identified it by its flash sequence. The development of fog signals ran roughly parallel to the development of lighthouse optics through the nineteenth century, driven by the same recognition that the maritime economy depended on safe coastal navigation and that the cost of wrecks — measured in ships, cargoes, and lives — could be partially offset by investment in aids to navigation.
From Bells to Guns
The earliest fog signals were bells. A large bell mounted on a lighthouse or beacon, struck by a mechanical hammer driven by clockwork or by hand, could be heard at modest distances in calm conditions. The deficiency was range: a bell of practical size produces a sound that carries well under a mile in anything approaching a breeze, and fog is frequently accompanied by wind. Bell buoys — anchored floats that rang as they rolled with the swell — were more widely deployed than lighthouse bells because they could be positioned on the shoals themselves, and the same wave action that made navigation dangerous helped to ring the warning.
Cannon were used as fog signals at some stations from the seventeenth century. A gun fired at intervals could be heard several miles away in thick weather, its sound carrying through fog more reliably than a bell. The disadvantages were the cost of powder, the labour involved in loading and firing, and the wear on the gun itself. Canadian lighthouse records document stations where keepers maintained fog guns for decades, firing at regular intervals through prolonged periods of thick weather; the physical and psychological demands were substantial.
The explosive fog signal, used at several British and American stations in the nineteenth century, extended the range of audible warning by detonating a small explosive charge suspended from a cable offshore. The explosive was detonated by electric current from the shore station at timed intervals. This system, called a reed explosive or maroon system, could be heard at ranges of five to seven miles in moderate conditions, substantially more than any mechanical fog signal of the same period.
The Siren and Diaphone
The first air-powered fog signals — sirens operating on compressed air — appeared in North America in the 1860s. The air siren uses a rotating disc with holes that interrupt an air stream at a regular frequency, generating a sustained tone. At sufficient volume, the siren could be heard at distances of several miles and was far less labour-intensive than any explosive system. The compressed air could be stored, generated by a steam engine or later by internal combustion, and the signal could be maintained automatically for hours.
The diaphone, developed by John Pell Northey in Canada around 1900, became the dominant fog signal technology of the twentieth century at major stations. The diaphone works on the same principle as the siren but uses a reciprocating piston rather than a rotating disc, producing a sound with a characteristic two-toned grunt — a sustained note ending in a brief lower-pitched 'groan'. This grunt was produced by the sudden collapse of the air column at the end of each blast and became so characteristic of major harbour approaches that the sound itself entered popular culture. The foghorn sound used in films and recordings is almost invariably a diaphone or an imitation of one.
Diaphones at major stations operated at powers of several kilowatts, producing sound pressure levels that were uncomfortable to be near and audible at distances of up to seven or eight nautical miles in calm conditions. The engine house required to run them — diesel generators, air compressors, storage receivers, control systems — was often the largest building at the lighthouse station. The keeper's duties during fog included monitoring the machinery, maintaining air pressure in the receivers, and recording the timing and duration of fog periods in the station log.
Character and Identification
Just as each lighthouse had a distinctive light character, each fog signal was assigned a character — a sequence of blasts and silences — that allowed a mariner to identify the station by ear alone, without any visual reference. A station might sound one blast every thirty seconds, or two blasts every twenty seconds, or a Morse character. The timing was precise enough to allow identification with a watch, and the light lists published by lighthouse authorities included fog signal characters alongside light characters.
In practice, identification by fog signal was less reliable than identification by light, for reasons that had nothing to do with the signal's quality. Sound in fog behaves unpredictably. Temperature inversions — layers of air at different temperatures that bend sound waves — can produce 'silent zones' in which a powerful fog signal is inaudible at a distance of half a mile, while the same signal is clearly heard five miles further away. A mariner might fail to hear a signal that was functioning perfectly and correctly timed, and conclude that he was at a safe distance from the station when in fact he was dangerously close.
This acoustic phenomenon — documented extensively in the nineteenth century and eventually explained by the physics of atmospheric sound propagation — was one of the most frustrating aspects of fog signal operation. Lighthouse engineers designed experiments to measure the propagation of sound from various signal types in different weather conditions. The results were inconsistent enough that no single technology could be declared reliably superior to another. The combination of a powerful signal, careful timing, and sailor's alertness remained the only practical approach.
The Radio Beacon and Radar
From the 1920s, radio beacons at lighthouse stations supplemented acoustic fog signals with an electromagnetic alternative. A radio beacon transmitting a continuous signal on a known frequency allowed a navigator with a radio direction finder to take a bearing regardless of visibility. The radio beacon did not depend on atmospheric acoustics; its signal travelled reliably over long distances and was not subject to the silent zone effect.
The combination of visual light, acoustic fog signal, and radio beacon at a major station — achieved by the mid-twentieth century at most important landfall lights — gave the navigator three independent methods of fixing position, each working in different conditions. In fog with a following wind, when the acoustic signal might not be audible on the vessel's bridge, the radio bearing was available. In radio silence, the acoustic signal provided directional information. In clear visibility, the light character confirmed position.
Radar, available to commercial shipping from the late 1940s and to smaller vessels by the 1970s, changed the relationship between fog and navigation more fundamentally than any previous development. A radar-equipped ship could navigate in fog without any acoustic or visual signal at all, using the radar picture to identify coastlines and other vessels. The fog signal became supplementary rather than primary.
The consequence was a progressive reduction in the number of operational fog signals from the 1980s onward. Trinity House decommissioned most of its fog signal installations in the 1980s and 1990s as radar became universal on commercial vessels. The US Coast Guard conducted a similar programme. The diaphone horns were silent, then removed. Some were preserved; others were scrapped. A few still operate at stations where small-craft traffic justifies the cost, or where heritage considerations have influenced the decision to maintain them.
The sound of a diaphone in fog — heard from a small boat approaching an unfamiliar harbour, the deep two-toned grunt emerging from grey weather that makes the land invisible — was for over a century one of the defining sounds of the sea. That experience is now rare enough to be remarkable. You can find the remaining active fog signal stations among the lighthouse records on Open the map, where the characteristics of operational signals are noted alongside the visual light data.