MODERN AIDS TO NAVIGATION Directional Wireless.—Determining the position of a ship at sea by means of directional wireless telegraphy (see WIRELESS) is of inestimable value to ships in thick weather, when approaching the land. The bearing of the shore station from the ship can be obtained either by means of a directional receiving apparatus carried in the ship, or by the system whereby the bearing of the ship is determined by one or more directional receiving sets on shore and the result being communicated to the ship.
The method of receiving bearings from ashore has the advantage that no additional equipment outside of the ship's ordinary wire less equipment is needed. However, it is subject to the disad vantage that the navigator himself cannot take and check the bearings. Furthermore, the shore stations can give a bearing to only one ship at a time which prevents a constant, repeated check of the ship's position. On the other hand the method of carrying the directional receiving set in the ship, once considered an inter esting novelty, has been widely accepted as a necessary aid to navi gation. The advantages of this method can be readily appreciated. For the navigator can repeatedly check his bearings upon the shore station with no wait except for the momentary pause in the code signals characteristic of that particular shore station. A knowledge of telegraphy is not required to receive the code signals, and the operation of the ship's wireless direction finding equipment is practically simple.
Either method of course is dependent upon the establishment of wireless direction finding (D/F) stations at points of im portance to navigators.
The accuracy of the bearings is affected somewhat when the direction runs approximately parallel to the coast-line. Also bear ings sometimes have been found to be unreliable at night. Wire- , less waves travel along the arc of a great circle as the shortest dis tance between any two points on the surface of the earth. It follows, therefore, that the true bearing of the ship from the station, or vice-versa, must be corrected for convergency to obtain the mean mercatorial bearing which is required if a chart on Mer cator's projection is in use. A simple formula given in text-books enables this to be done. On a chart constructed on the gnomonic projection D/F bearings can be laid down without any c3rrection for convergency, since great circles appear as straight lines on this projection. If, however, a compass "rose" is used it is necessary to have one on the chart for each D/F station, to compensate for the angular alteration of the projection at that station.
time recorder capable of reading to one-thousandth part of a sec ond is connected to the microphones and enables the depth to be deduced. Sound travels through sea water at an average velocity of 4,800ft, per second. Consequently, by the acoustic method, very great depths can be reached in a brief space of time and the depths nominally required for navigational purposes will be reached in a small fraction of a second.
The highest practical development of the echo sounding prin ciple is a device named the Fathometer. (See FATHOMETER.) The Leader System.—The Leader system assists vessels to enter or leave harbour or to pass through narrow channels in thick weather. A submarine electric cable is laid along the channel, the shore end being connected to a station producing an alternating current. A simple and inexpensive receiving apparatus on board the ship enables the signals to be heard in telephones, or other form of amplifier, placed at or near the position from which the ship is usually conned. Two sets of coils are fitted, one on either side of the ship and so arranged that they can each be connected up to the receiving telephones. By connecting first one set of coils and then the other, to the receiving telephones, it is an easy matter, when within range, to determine whether the cable is to port or starboard. With practice, the distance of the cable can be esti mated with comparative accuracy, because the signals become stronger as the cable is approached, the maximum intensity of the signals occurring when the ship is close to, but not vertically over, the cable. The same cable enables one ship to enter and one to leave harbour at the same time, provided, of course, the navigable channel is of sufficient width ; it is a simple matter for each ship to steer to keep the cable a reasonable distance away on her port side, so that the two ships pass safely port to port. If space per mits, it is possible to lay two cables, one for entering and one for leaving, the signals produced in each cable being distinctive by their note and character. This system has not come into general use owing presumably to the somewhat heavy cost.
H. Raper, R.N., The Practice of Navigation (1840, 21st ed. 1920) ; W. R. Martin, R,N., Navigation and Nautical Astronomy (1888) ; Admiralty Manual of Navigation, 1921; British Admiralty, Signal Dept., Technical Notes on the Leader Cable System 0920 ; L. H. Walter, Directive Wireless Telegraphy (1921) ; E. J. Willis, The Mathematics of Navigation (1951) ; Rev. W. Hall, R.N., Modern Navigation; R. Keen, Direction and Position Finding by Wireless (1922) ; J. W. Norrie, A Complete Set of Nautical Tables (1922) ; Capt. K. Macdonald, Macdonald's Tables for Correcting Wireless Bearings from Latitude 5 deg. to 70 deg. North or South (5922); Captain Lecky, Wrinkles in Practical Navigation (13th ed., 1927) ; L. M. Berkeley, Great Circle Sailing (1924) and North Star Navigation (1925). (J. E. T. H.)