To take full advantage of the signals thus provided it is neces sary for ships approaching them to be fitted with special receiv ing microphones 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 they be heard at considerable distances and in all conditions of weather, but their direction in reference to the moving ship can be determined approximately. When they are established in conjunction with a wireless-beacon fog-signal and when the acoustic and radio-signals are made to synchronize, as at the Nantucket Shoals light-vessel (U.S.A.), they provide in com bination a valuable means of determining distance by observing the interval of time between the reception of the two signals (see below).
continuous transmission from light stations, provide a means by which a navigator may obtain a direct bearing on the transmitting station. They are of three types : ( ) a stationary wireless beacon transmitter from which a characteristic signal is sent out in all directions, so that it can be picked up by a ship fitted with a direction-finder or wireless-compass', (2) a rotating short wave directional wireless-beam transmitter, which sends out a different characteristic signal on all points of the compass as it revolves ; these signals are received on a special aerial, and as the wireless beam is directional and as each signal represents a definite bearing, the particular signal heard gives the bearing of the ship from the station; (3) a rotating wireless-loop transmitter, which is revolved at a pre-arranged constant speed and sends out on north and east zero points a characteristic signal, followed dur ing the remainder of its revolution by a continuous dash. The bearing in this case is taken by means of a stop-watch, in con junction with an ordinary wireless receiver, and is obtained by noting the time which elapses between the hearing of one or other of the zero-point signals and the total extinction of the continuous dash. As the loop makes so many revolutions in a definite period, the determination of the bearing is merely a matter of computation between the speed of rotation and the time recorded by the stop-watch.
(I) Wireless-beacon.—Each system has its particular merits, but the first, that is, the beacon station, is probably the most effective of the three. It is comparatively cheap to install and easy to maintain, while the system is applicable either on shore or on board a light-vessel. Moreover, this form of wireless fog-signal necessitates a ship being fitted with a direction-finder which enables a bearing to be obtained not only of the beacon station but also of other ships at sea during fog; or, as has fre quently been proved with advantage, of a ship in distress. When the wireless-signal is combined with a submarine-signal and made to synchronize with it, distance from, as well as direction of, the station can be obtained.
A beacon-station of this type was established at Round island in the Scilly islands in 1927. In the beacon, a simple valve transmitter of I kw. power generating interrupted continuous
waves is employed working on an open twin L aerial. The wave length is i,000 metres. Good bearings are obtainable from it at a distance of over soo m. The whole equipment is automatically controlled, even to the replacement of transmitting valves in case of failure, by a master-clock which sends out signals every half hour by Greenwich mean time. During foggy weather the time interval is reduced. Little attention is required and it has been found practicable to run the station without addition to the lighthouse staff. Similar signals were established in 1928 at 12 other British stations, including two light-vessels and others were in course of erection. Wireless fog-signals of the same general type have been installed by other lighthouse services, notably those of the United States (where 57 stations were in operation in 1928), Holland, France, Sweden and Canada. An experimental station was established in France in 1912 but the first permanent wireless fog-signals were put into service in the United States in 1921 on light-vessels in the approaches to New York harbour.
(2) Rotating-beam.--A wireless fog-signal in the class of the second type has been established at Inchkeith in the Firth of Forth (1922). It works on a 6.2-metre wave-length and though the receiving apparatus is simple and inexpensive, the trans mitter in its present state of development has disadvantages in that the range is small, and a large rotating structure is re quired for producing the beam. In addition, as the bearings must be given relative to a fixed zero, the system is inapplicable for use on a light-vessel.
(3) Rotating-loop.—The third system, which was developed by the Royal Air Force, possesses an advantage in that an or dinary receiving apparatus and stop-watch only are required for taking bearings, but the power employed is over ten times that of the beacon-transmitter for less range, while the cost is considerably greater. The cost of maintenance too is high, as a careful watch must be kept on the timing of the rotation of the loop and the large power-plant requires constant attention. The transmitter is delicate and must be free from local inter ference if accuracy of bearing is to be assured; so that sites for the Board of Trade, which controls, all finances.
On Jan. 1, 1928, the lights and fog-signals in service under the control of the several authorities in the British Isles were as follows :— this type of station have to be carefully selected. Like the sec ond system, it cannot be applied on a lightship because of the necessity of having fixed zero-points. Loop stations have been established experimentally at Gosport and Farnborough and further trials of the system are being carried out at Orfordness.
Wireless fog-signalling and position-finding are (1929) still in the state of active experiment and development.' They are undoubtedly destined to be one of the most important naviga tional aids in the future. The desideratum appears to be a system, applicable to stations both ashore and afloat, which employs a wireless signal whose bearing is quickly and directly ascertain able by the navigator himself and which, when combined with submarine acoustic-signals, can be used for ascertaining distance.