Technics of the Lights

The Technics of Foghorns

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Fog Signals.—The introduction of coast fog signals is of comparatively recent date. They were, until the middle of the 19th century, practically unknown except so far as a few isolated bells and guns were concerned. The increasing demands of navigation, and the application of steam power to the propulsion of ships resulting in an increase of their speed, drew attention to the necessity of providing suitable signals as aids to navigation during fog and mist. In times of fog the mariner can expect no certain assistance from even the most efficient system of coast lighting, since the beams of light from the most powerful electric lighthouse are frequently entirely dispersed and absorbed by the particles of moisture, forming a sea fog of even moderate density, at a distance of less than a 1 4 m. from the shore. The careful experiments and scientific research which have been devoted to the subject of coast fog-signalling have produced much that is useful and valuable to the mariner, but unfortunately the practical results so far have not been so satisfactory as might be desired, owing to (1) the very short range of the most powerful signals yet produced under certain unfavourable acoustic conditions of the atmosphere, (2) the difficulty experienced by the mariner in judging at any time how far the atmospheric conditions are against him in listening for the expected signal, and (3) the difficulty in locating the position of a sound signal by phonic observations. EB1911 - Lighthouse - Fig. 52.—Buoy Bell.jpg Fig. 52.—Buoy Bell. Bells and Gongs are the oldest and, generally speaking, the least efficient forms of fog signals. Under very favourable acoustic conditions the sounds are audible at considerable ranges. On the other hand, 2-ton bells have been inaudible at distances of a few hundred yards. The 1893 United States trials showed that a bell weighing 4000 ℔ struck by a 450 ℔ hammer was heard at a distance of 14 m. across a gentle breeze and at over 9 m. against a 10-knot breeze. Bells are frequently used for beacon and buoy signals, and in some cases at isolated rock and other stations where there is insufficient accommodation for sirens and horns, but their use is being gradually discontinued in this country for situations where a powerful signal is required. Gongs, usually of Chinese manufacture, were formerly in use on board English light-ships and are still used to some extent abroad. These are being superseded by more powerful sound instruments. Explosive Signals.—Guns were long used at many lighthouse and light-vessel stations in England, and are still in use in Ireland and at some foreign stations. These are being gradually displaced by other explosive or compressed air signals. No explosive signals are in use on the coasts of the United States. In 1878 sound rockets charged with gun-cotton were first used at Flamborough Head and were afterwards supplied to many other stations.[5] The nitrated gun-cotton or tonite signals now in general use are made up in 4 oz. charges. These are hung at the end of an iron jib or pole attached to the lighthouse lantern or other structure, and fired by means of a detonator and electric battery. The discharge may take place within 12 ft. of a structure without danger. The cartridges are stored for a considerable period without deterioration and with safety. This form of signal is now very generally adopted for rock and other stations in Great Britain, Canada, Newfoundland, northern Europe and other parts of the world. An example will be noticed in the illustration of the Bishop Rock lighthouse, attached to the lantern (fig. 13). Automatic hoisting and firing appliances are also in use. Whistles.—Whistles, whether sounded by air or steam, are not used in Great Britain, except in two instances of harbour signals under local control. It has been objected that their sound has too great a resemblance to steamers’ whistles, and they are wasteful of power. In the United States and Canada they are largely used. The whistle usually employed consists of a metallic dome or bell against which the high-pressure steam impinges. Rapid vibrations are set up both in the metal of the bell and in the internal air, producing a shrill note. The Courtenay buoy whistle, already referred to, is an American invention and finds favour in the United States, France, Germany and elsewhere. EB1911 - Lighthouse - Fig. 53.—St Catherine’s Double-noted Siren.jpg Fig. 53.—St Catherine’s Double-noted Siren. Reed-Horns.—These instruments in their original form were the invention of C. L. Daboll, an experimental horn of his manufacture being tried in 1851 by the United States Lighthouse Board. In 1862 the Trinity House adopted the instrument for seven land and light-vessel stations. For compressing air for the reed-horns as well as sirens, caloric, steam, gas and oil engines have been variously used, according to local circumstances. The reed-horn was improved by Professor Holmes, and many examples from his designs are now in use in England and America. At the Trinity House experiments with fog signals at St Catherine’s (1901) several types of reed-horn were experimented with. The Trinity House service horn uses a 15 ℔ pressure with a consumption of .67 cub. ft. per second and 397 vibrations. A small manual horn of the Trinity House type consumes .67 cub. ft. of air at 5 ℔ pressure. The trumpets of the latter are of brass. Sirens.—The most powerful and efficient of all compressed air fog signals is the siren. The principle of this instrument may be briefly explained as follows:—It is well known that if the tympanic membrane is struck periodically and with sufficient rapidity by air impulses or waves a musical sound is produced. Robinson was the first to construct an instrument by which successive puffs of air under pressure were ejected from the mouth of a pipe. He obtained this effect by using a stop-cock revolving at high speed in such a manner that 720 pulsations per second were produced by the intermittent escape of air through the valves or ports, a smooth musical note being given. Cagniard de la Tour first gave such an instrument the name of siren, and constructed it in the form of an air chamber with perforated lid or cover, the perforations being successively closed and opened by means of a similarly perforated disk fitted to the cover and revolving at high speed. The perforations being cut at an angle, the disk was self-rotated by the oblique pressure of the air in escaping through the slots. H. W. Dove and Helmholtz introduced many improvements, and Brown of New York patented, about 1870, a steam siren with two disks having radial perforations or slots. The cylindrical form of the siren now generally adopted is due to Slight, who used two concentric cylinders, one revolving within the other, the sides being perforated with vertical slots. To him is also due the centrifugal governor largely used to regulate the speed of rotation of the siren. Over the siren mouth is placed a conical trumpet to collect and direct the sound in the desired direction. In the English service these trumpets are generally of considerable length and placed vertically, with bent top and bell mouth. Those at St Catherine’s are of cast-iron with copper bell mouth, and have a total axial length of 22 ft. They are 5 in. in diameter at the siren mouth, the bell mouth being 6 ft. in diameter. At St Catherine’s the sirens are two in number, 5 in. in diameter, being sounded simultaneously and in unison (fig. 53). Each siren is provided with ports for producing a high note as well as a low note, the two notes being sounded in quick succession once every minute. The trumpet mouths are separated by an angle of 120° between their axes. This double form has been adopted in certain instances where the angle desired to be covered by the sound is comparatively wide. In Scotland the cylindrical form is used generally, either automatically or motor driven. By the latter means the admission of air to the siren can be delayed until the cylinder is rotating at full speed, and a much sharper sound is produced than in the case of the automatic type. The Scottish trumpets are frequently constructed so that the greater portion of the length is horizontal. The Girdleness trumpet has an axial length of 16 ft., 11 ft. 6 in. being horizontal. The trumpet is capable of being rotated through an angle as well as dipped below the horizon. It is of cast-iron, no bell mouth is used, and the conical mouth is 4 ft. in diameter. In France the sirens are cylindrical and very similar to the English self-driven type. The trumpets have a short axial length, 4 ft. 6 in., and are of brass, with bent bell mouth. The Trinity House has in recent years reintroduced the use of disk sirens, with which experiments are still being carried out both in the United Kingdom and abroad. For light-vessels and rock stations where it is desired to distribute the sound equally in all directions the mushroom-head trumpet is occasionally used. The Casquets trumpet of this type is 22 ft. in length, of cast-iron, with a mushroom top 6 ft. in diameter. In cases where neither the mushroom trumpet nor the twin siren is used the single bent trumpet is arranged to rotate through a considerable angle. Table IV. gives particulars of a few typical sirens of the most recent form. Table IV. Station. Description. Vibrations per sec. Sounding Pressure in ℔ per sq. in. Cub. ft. of air used per sec. of blast reduced to atmospheric pressure. Remarks. High. Low. High. Low. St Catherine’s (Trinity House) Two 5-in. cylindrical, automatically driven sirens 295 182 25  32 16 The air consumption is for 2 sirens. Girdleness (N.L.C) 7-in. cylindrical siren, motor driven 234 100 30 130 26 Casquets (Trinity House) 7-in. disk siren, motor driven ..  98 25 .. 36  French pattern siren  6-in. cylindrical siren, automatically driven 326 .. 28  14 .. A uniform note of 326 vibrations per sec. has now been adopted generally in France. Since the first trial of the siren at the South Foreland in 1873 a very large number of these instruments have been established both at lighthouse stations and on board light-vessels. In all cases in Great Britain and France they are now supplied with air compressed by steam or other mechanical power. In the United States and some other countries steam, as well as compressed air, sirens are in use. Diaphones.—The diaphone is a modification of the siren, which has been largely used in Canada since 1903 in place of the siren. It is claimed that the instrument emits a note of more constant pitch than does the siren. The distinction between the two instruments is that in the siren a revolving drum or disk alternately opens and closes elongated air apertures, while in the diaphone a piston pulsating at high velocity serves to alternately cover and uncover air slots in a cylinder. The St Catherine’s Experiments.—Extensive trials were carried out during 1901 by the Trinity House at St Catherine’s lighthouse, Isle of Wight, with several types of sirens and reed-horns. Experiments were also made with different pattern of trumpets, including forms having elliptical sections, the long axis being placed vertically. The conclusions of the committee may be briefly summarized as follows: (1) When a large arc requires to be guarded two fixed trumpets suitably placed are more effective than one large trumpet capable of being rotated. (2) When the arc to be guarded is larger than that effectively covered by two trumpets, the mushroom-head trumpet is a satisfactory instrument for the purpose. (3) A siren rotated by a separate motor yields better results than when self-driven. (4) No advantage commensurate with the additional power required is obtained by the use of air at a higher pressure than 25 ℔ per sq. in. (5) The number of vibrations per second produced by the siren or reed should be in unison with the proper note of the associated trumpet. (6) When two notes of different pitch are employed the difference between these should, if possible, be an octave. (7) For calm weather a low note is more suitable than a high note, but when sounding against the wind and with a rough and noisy sea a high note has the greater range. (8) From causes which cannot be determined at the time or predicted beforehand, areas sometimes exist in which the sounds of fog signals may be greatly enfeebled or even lost altogether. This effect was more frequently observed during comparatively calm weather and at no great distance from the signal station. (It has often been observed that the sound of a signal may be entirely lost within a short distance of the source, while heard distinctly at a greater distance and at the same time.) (9) The siren was the most effective signal experimented with; the reed-horn, although inferior in power, is suitable for situations of secondary importance. (No explosive signals were under trial during the experiments.) (10) A fog signal, owing to the uncertainty attending its audibility, must be regarded only as an auxiliary aid to navigation which cannot at all times be relied upon. Submarine Bell Signals.—As early as 1841 J. D. Colladon conducted experiments on the lake of Geneva to test the suitability of water as a medium for transmission of sound signals and was able to convey distinctly audible sounds through water for a distance of over 21 m., but it was not until 1904 that any successful practical application of this means of signalling was made in connexion with light-vessels. There are at present (1910) over 120 submarine bells in service, principally in connexion with light-vessels, off the coasts of the United Kingdom, United States, Canada, Germany, France and other countries. These bells are struck by clappers actuated by pneumatic or electrical mechanism. Other submerged bells have been fitted to buoys and beacon structures, or placed on the sea bed; in the former case the bell is actuated by the motion of the buoy and in others by electric current, transmitted by cable from the shore. In some cases, when submarine bells are associated with gas buoys or beacons, the compressed gas is employed to actuate the bell striking mechanism. To take full advantage of the signals thus provided it is necessary for ships approaching them to be fitted with special receiving mechanism of telephonic character installed below the water line and in contact with the hull plating. The signals are audible by the aid of ear pieces similar to ordinary telephone receivers. Not only can the bell signals be heard at considerable distances—frequently over 10 m.—and in all conditions of weather, but the direction of the bell in reference to the moving ship can be determined within narrow limits. The system is likely to be widely extended and many merchant vessels and war ships have been fitted with signal receiving mechanism. The following table (V.) gives the total numbers of fog signals of each class in use on the 1st of January 1910 in certain countries. Table V. Sirens. Diaphone. Horns, Trumpets, &c. Whistles. Explosive Signals (tonite, &c.). Guns. Bells. Gongs. Submarine Bells. Totals. Power. Manual. England and Channel Islands 44 .. 27 31  2 15 .. 48 10 16 193 Scotland and Isle of Man 35 ..  6  2 ..  5 .. 16  3 ..  67 Ireland 12 ..  2  6 .. 11 3 11 ..  3  48 France 12 ..  7  1 ..  1 .. 25 ..  2  48 United States (excluding inland  lakes and rivers) 43 .. 35 15 59 .. .. 218   1 36 407 British North America (excluding  inland lakes and rivers)  6 66  5 79 16  8 .. 24 .. 11 215 When two kinds of signal are employed at any one station, one being subsidiary, the latter is omitted from the enumeration. Buoy and unattended beacon bells and whistles are also omitted, but local port and harbour signals not under the immediate jurisdiction of the various lighthouse boards are included, more especially in Great Britain. A foghorn give a sound signal to warn vessels of navigational hazards like rocky coastlines or of the presence of other vessels, in foggy conditions. The term 'fog horn' is most often used in relation to marine transport. When visual navigation aids such as lighthouses are obscured, foghorns provide an audible warning of rock outcrops, shoals, headlands, or other dangers to vesselping.

Sound signals

The limitations of purely visual navigation very early led to the idea of supplementary audible warning in lighthouses. The first sound signals were explosive. At first cannon were used, and later explosive charges were attached to retractable booms above the lantern and detonated electrically. Sometimes the charges contained magnesium in order to provide an accompanying bright flare. Such signals could be heard up to four nautical miles away. Bells also were used, the striker being actuated by weight-driven clockwork or by a piston driven by compressed gas (usually carbon dioxide). Some bells were very large, weighing up to one ton.

Compressed air

About the beginning of the 20th century, compressed air fog signals, which sounded a series of blasts, were developed. The most widely used were the siren and the diaphone. The siren consisted of a slotted rotor revolving inside a slotted stator that was located at the throat of a horn. The diaphone worked on the same principle but used a slotted piston reciprocating in a cylinder with matching ports. The largest diaphones could be heard under good conditions up to eight nautical miles away. Operating pressures were at 2 to 3 bars (200 to 300 kilopascals), and a large diaphone could consume more than 50 cubic feet (approximately 1.5 cubic metres) of air per second. This required a large and powerful compressing plant, 50 horsepower or more, with associated air-storage tanks. A later compressed-air signal was the tyfon. Employing a metal diaphragm vibrated by differential air pressure, it was more compact and efficient than its predecessors.

Electricity

Modern fog signals are almost invariably electric. Like the tyfon, they employ a metal diaphragm, but in the electric signal they vibrate between the poles of an electromagnet that is energized by alternating current from an electronic power unit. Powers range from 25 watts to 4 kilowatts, with ranges from half a nautical mile to five nautical miles. Note frequencies lie between 300 and 750 hertz. Emitters can be stacked vertically, half a wavelength apart, in order to enhance the sound horizontally and reduce wasteful vertical dispersion.

Effective range

Propagation of sound in the open air is extremely accidentally, owing to the vagaries of atmospheric conditions. Wind direction, humidity, and turbulence all have an effect. Vertical wind and temperature gradients can bend the sound up or down; in the latter case it can be reflected off the sea, resulting in shadow zones of silence. The range of audibility of a sound signal is therefore extremely unpredictable. Also, it is difficult to determine with any precision the direction of a signal, especially from the bridge of a vessel in the fog.

Contents 1 Description 2 History 2.1 Early fog signals 2.2 Mechanization 2.3 Diaphone 2.4 Obsolescence 3 Railway fog signals 4 See also 5 References 6 External links Description All foghorns use a vibrating column of air to create an audible tone, but the method of setting up this vibration differs. Some horns, like the Daboll trumpet, used vibrating plates or metal reeds, a similar principle to a modern electric car horn. Others used air forced through holes in a revolving cylinder or disk, in the same manner as a siren. Semi-automatic operation of foghorns was achieved by using a clockwork mechanism (or "coder") to sequentially open the valves admitting air to the horns; each horn was given its own timing characteristics to help mariners identify them.[1] History Early fog signals An early form of fog signal. The fog bell at Fort Point Light Station, Maine. Audible fog signals have been used in one form or another for hundreds of years, initially simply bells or gongs struck manually. At some lighthouses, a small cannon was let off periodically to warn away vessels, but this had the obvious disadvantage of having to be fired manually throughout the whole period the fog persisted (which could be for several days). Lighthouse windows and lighting apparatus were susceptible to damage depending on the proximity of the explosion.[2] One incident of lax handling of explosives nearby resulted in a concussion that propelled the lighthouse keeper at Fort Amherst, who was seated, to the other end of the room.[3] In the United States, whistles were also used where a source of steam power was available, though Trinity House, the British lighthouse authority, did not employ them, preferring an explosive signal. Throughout the 19th century efforts were made to automate the signalling process. Trinity House eventually developed a system (the "Signal, Fog, Mk I") for firing a gun-cotton charge electrically. However, the charge had to be manually replaced after each signal. At Portland Bill, for example, which had a five-minute interval between fog-signals, this meant the horns had to be lowered, the two new charges inserted, and the horns raised again every five minutes during foggy periods. Clockwork systems were also developed for striking bells.[4] Captain James William Newton claimed to have been the inventor of the fog signalling technique using loud and low notes.[5] Mechanization Foghorns near Lizard Point, Cornwall. This installation uses a siren to produce sound. Another Trinity House fog siren installation on Flat Holm, now restored by the Flat Holm Project Bearing mechanism of Sumburgh lighthouse Foghorn (Shetland) The first automated steam-powered foghorn was invented by Robert Foulis, a Scotsman who emigrated to Saint John, New Brunswick, Canada. Foulis is said to have heard his daughter playing the piano in the distance on a foggy night, and noticed the low notes were more audible than the higher notes: he then designed a device to produce a low-frequency sound, as well as a code system for use with it. Foulis repeatedly presented his concept to the Commissioners of Light Houses for the Bay of Fundy for installation on Partridge Island. While the Commissioners initially rejected Foulis's plan, one commissioner eventually encouraged Foulis to submit detailed plans to the Commission. For reasons unknown, the plans were given to another Canadian engineer, T. T. Vernon Smith, who officially submitted them to the Commissioners as his own. The foghorn was constructed at Partridge Island in 1859 as the Vernon-Smith horn. After protest by Foulis and a legislative inquiry, Foulis was credited as the true inventor, but he never patented or profited from his invention.[6] The development of fog signal technology continued apace at the end of the 19th century.[7] During the same period an inventor, Celadon Leeds Daboll, developed a coal-powered foghorn called the Daboll trumpet for the American lighthouse service, though it was not universally adopted.[8] A few Daboll trumpets remained in use until the mid-20th century. In the United Kingdom, experiments to develop more effective foghorns were carried out by John Tyndall and Lord Rayleigh, amongst others. The latter's ongoing research for Trinity House culminated in a design for a siren with a large trumpet designed to achieve maximum sound propagation (see reference for details of the Trials of Fog Signals[9]), installed in Trevose Head Lighthouse, Cornwall in 1913. One of the first automated fog bells was the Stevens Automatic Bell Striker.[10] Some later fog bells were placed under water, particularly in especially dangerous areas, so that their sound (which would be a predictable code, such as the number "23") would be carried further and reverberate through the vessel's hull. For example, this technique was used at White Shoal Light (Michigan).[11][12] This was an earlier precursor to RACON. Diaphone Main article: Diaphone From the early 20th century an improved device called the diaphone, originally invented as an organ stop by Robert Hope-Jones,[9] and developed as a fog signal by John Northey of Toronto, became the standard foghorn apparatus for new installations.[where?] Diaphones were powered by compressed air and could emit extremely powerful low-frequency notes. In 1982, the Dutch broadcaster VPRO aired a live foghorn concert on national Radar, relaying the sound of the foghorns in Emden, Calais, Nieuwpoort, Scheveningen, Den Helder, Lelystad, Urk, Marken and Kornwerderzand, mixed with studio music by sound artist Alvin Curran. [13] Obsolescence Foghorn on Ailsa Craig, where the fog signal was discontinued in 1966. Since automation of lighthouses became common in the 1960s and 1970s, most older foghorn installations have been removed to avoid the need to run the complex machinery associated with them, and have been replaced with electrically powered diaphragm or compressed air horns. Activation is completely automated: a laser or photo beam is shot out to sea, and if the beam reflects back to the source (i.e. the laser beam is visible due to the fog), the sensor sends a signal to activate the foghorn. In many cases, modern navigational aids have rendered large, long-range foghorns completely unnecessary, according to the International Association of Lighthouse Authorities.[14]

FOG SIGNALS The function of a fog signal in the system of aids to navigation is to warn of danger and to provide the mariner with an audible means of approximating his position relative to the fog signal when the station, or any visual signal which it displays, is obscured from view by atmospheric conditions. Fog signals depend upon the transmission of sound through air. As aids to navigation, they have certain inherent defects that should be considered. Sound travels through the air in a variable and frequently unpredictable manner. It has been established that: fog signals are heard at greatly varying distances and that the distance at which a fog signal can be heard may vary with the bearing of the signal and may be different on different occasions; under certain conditions of atmosphere, when a fog signal has a combination of high and low tones, it is not unusual for one of the tones to be inaudible. In the case of sirens, which produce a varying tone, portions of the blast may not be heard; there are occasionally areas close to the signal in which it is wholly inaudible. This is particularly true when the fog signal is screened by intervening land or other obstructions; fog may exist a short distance from a station and not be observable from it, so that the signal may not be in operation; even though a fog signal may not be heard from the deck or bridge of a ship when the engines are in motion, it may be heard when the ship is stopped, or from a quiet position. Sometimes it may be heard from aloft though not on deck; the intensity of the sound emitted by a fog signal may be greater at a distance than in immediate proximity. All these considerations point to the necessity for the utmost caution when navigating near land in fog. Particular attention should be given to placing lookouts in positions in which the noises in the ship are least likely to interfere with hearing a fog signal. Fog signals are valuable as warnings, but the mariner should not place implicit reliance upon them in navigating his vessel. They should be considered solely as warning devices. Among the devices in common use as fog signals are: Radarbeacons which broadcast simple dot-and-dash combinations by means of a transmitter emitting modulated continuous waves; Diaphones which produce sound by means of a slotted reciprocating piston actuated by compressed air. Blasts may consist of two tones of different pitch, in which case the first part of the blast is high and the last of a low pitch. These alternate pitch signals are called “two-tone;” Diaphragm horns which produce sound by means of a diaphragm vibrated by compressed air, steam, or electricity. Duplex or triplex horn units of differing pitch produce a chime signal; Nautophones, electrically operated instruments, each comprising a vibrating diaphragm, fitted with a horn, which emits a high note similar in power and tone to that of the reed; Reed horns which produce sound by means of a steel reed vibrator by compressed air; Sirens which produce sound by means of either a disk or a cup-shaped rotor actuated by compressed air or electricity; Whistles which produce sound by compressed air emitted through a circumferential slot into a cylindrical bell chamber; Bells which are sounded by means of a hammer actuated by hand, wave motion, by a descending weight, compressed gas, or electricity; Guns and explosive signals which are produced by firing of explosive charges, the former being discharged from a gun, and the latter being exploded in midair; Fog Detector Lights—certain light stations, in addition to the main light, are equipped with fog detector lights for automatic detection of fog. These lights sweep back and forth through an area over which the fog watch is necessary, showing a powerful bluish-white flash of about 1 second in duration. The interval between successive flashes will vary with the position of the vessel within the sector. At the limits of the sector the duration of the flash may be considerably longer than 1 second. Fog detector lights operate continuously. Standby fog signals are sounded at some of the light and fog signal stations when the main fog signal is inoperative. Some of these standby fog signals are of a different type and characteristic than the main fog signal. Radarbeacons, RACONs, RAMARKs, and Radar direction- finders are mentioned in the List of Lights, but for detailed information, including the synchronization of Radar signals and sound signals for distance finding, the navigator should consult Pub. 117, Radar Navigational Aids. Note—use Chart No. 1 for the complete list of symbols and abbreviations commonly used in presenting the essential characteristics of lights, fog signals, and Radar aids found on charts. Fog Signals The function of a fog signal in the system of aids to navigation is to warn of danger, and to provide the mariner with a practical means of determinating his position with relation to the fog signal at such times as the station or any visual signal which it displays is obscured from view by fog, snow, rain, smoke, or thick weather. Among the devices that were commonly used as fog signals are:15 Diaphones which produce sound by means of a slotted reciprocating piston actuated by compressed air. Blasts may consist of two tones of different pitch, in which case the first part of the blast is high and the last of a low pitch. These alternate pitch signals are called "tow-tone". Diaphragm horns which produce sound by means of a diaphragm vibrated by compressed air, steam, or electricity. Duplex and triplex horn units of differing pitch produce a chime signal. Reed horns which produce sound by means of a steel read vibrated by compressed air. Sirens which produce sound by means of either a disk or a cup-shaped rotor actuated by compressed air or electricity. Whistles which produce sound by compressed air emitted through a circumferential slot into a cylindrical bell chamber. Bells which are sounded by means of a hammer actuated by hand, by a descending weight, compressed gas, or electricity. Radiobeacons which broadcast simple dot-and-dash combinations by means of transmitter emitting modulated continuous waves. Marker radiobeacons are of low power for local use only. They operate continuously, and they typically transmit a series of ½ second dashes for part of a 15 or 30 second period followed by a silent period to the complete the cycle. NOTE: It is believed that the first official radiobeacon was operating in 1921. A 1923 light list was the earliest available light list that contained radiobeacon information. The 1920 light lists did not include radiobeacon information. 1921 or 1922 light lists were not available to determine if they included radiobeacon information. An article in the Winter 1998 issue of "The Keeper's Log" discussing the retirement of George R. Putnam, the Commissioner of Lighthouses, states "the addition of radiobeacons to lighthouses and lightships since 1921, and with other uses of radio bearings is an advance of first importance in safeguarding navigation". Identification Fog signals are distinguished by their characteristics as specified for each aid. The characteristic of a fog signal is described by the device used to create the sound, such as a diaphone, siren, bell, etc. The signal characteristic is the phase relationship of the recurring sound emissions. Fog signals on fixed stations and lightships produce a specific number of blasts and silent periods each minute, when operating, to provide positive identification. Fog signals on buoys are generally actuated by motion of the sea and, therefore, do not emit regular signal characteristics, and when the sea is calm, may emit no sound signals.16 Fog Signal Considerations25 Fog signals depend upon the transmission of sound through air. As aids to navigation, they have certain inherent defects that should be considered. Sound travels through the air in a variable and frequently unpredictable manner. It has been clearly established that: (a) Fog signals are heard at greatly varying distances and that the distance at which a fog signal can be heard may vary with the bearing of the signal and may be different on occasion. (b) Under certain conditions of atmosphere, when a fog signal has a combination high and low tone, it is not unusual for one of the tones to be inaudible. In the case of sirens, which produce a varying tone, portions of the blast may not be heard. (c) There are occasionally areas close to the signal, in which it is wholly inaudible. This is particularly true when the fog signal is screened by intervening land or other obstruction, or on a high cliff. (d) A fog may exist a short distance from a station and not be observable from it, so that the signal may not be in operation. (e) Some fog signals cannot be started at a moment's notice. (f) Even though a fog signal may not be heard from the deck or bridge of a ship when the engines are in motion, it may be heard when the ship is stopped, or from a quiet position. Sometimes it may be heard from aloft though not on deck. (g) The intensity of the sound emitted by a fog signal may be greater at a distance than in the immediate proximity. All these considerations point to the necessity for the utmost caution when navigating near land in a fog. Mariners are therefore warned that fog signals can never be implicitly relied upon, and that the practice of taking soundings of the depth of water should never be neglected. Particular attention should be given to placing lookouts in positions in which the noises in the ship are least likely to interfere with hearing a fog singal. Fog signals are valuable as warnings but the mariner should not place implicit reliance upon them in navigating his vessel. They should be considered solely as warning devices. Emergency fog signals. Emergency fog signals are sounded at some of the light and fog signal stations when the main and stand-by fog signal is inoperative. Some of these emergency fog signals are of a different type and characteristic that the main fog signal. The characteristic of the emergency fog signals are listed in column (7) of this publication (i.e., the light list). NOTE: While a light list may specify an emergency fog signal, this information was not extracted from the light list and, thus, is not included on this website. The mariner must not assume: (a) That he is out of ordinary hearing distance because he fails to hear the fog signal. (b) That, because he hears a fog signal faintly, he is at a great distance from it. (c) That he is near to it because he hears the sound plainly. (d) That the distance from and the intensity of the sound on any one occasion is a guide to him for any future occasion. (e) That the fog signal is not sounding because he does not hear it, even when close proximity.