Many modern navies have now adopted a system of underwater signalling in which sound impulses are communicated to the water by means of an electrically-driven oscillator and received by a microphone in the hull of other ships, who read off the signal in ordinary head telephones. Telegraphic speeds up to 20 words can be reached thus, though the range of transmission does not usually exceed 12 miles except under very good conditions.
Up to the end of the last century, the limitations of range in herent in the foregoing systems of signalling rendered long distance communication with ships impossible. Once at sea and over the horizon, it was only possible to direct their movements or issue orders by cable messages sent to their known ports of call or the coast signal stations with which they would speak on their voyage. Conversely, a ship out of sight of land was equally im potent to report her position or notify distress or other casualty.
It is hardly within the scope of this article to do more than in dicate a few of the ways in which wireless telegraphy (including also its younger cousin, wireless telephony) serves the mariner. So far as navigation is concerned, powerful shore stations send out daily time signals correct to a fraction of a second, direc tion finding stations (if the ship herself has not a direction find ing receiver on board) furnish her with her bearing from them, while a distress message in any of the seven seas will bring per haps half a dozen vessels rushing to her aid. By means of wireless, passengers in ocean-going liners get their daily newspaper and bankers are enabled to pursue their financial operations on the high seas. Shipowners can divert their vessels in search of profit able cargoes, and admiralties control their fleets with a precision undreamed of in the old naval wars.
signal codes of the ancients are believed to have been elaborate. Generally some kind of flag was used. Shields were also displayed in a preconcerted manner, as at the battle of Marathon, and some have imagined that the reflected rays of the sun were flashed from them as with the modern heliograph. In the middle ages flags, banners and lanterns were used to distinguish particular squad rons, and as marks of rank, as they are at present, also to call officers to the admiral, and to report sighting the enemy and get ting into danger. The invention of cannon made an important addition to the means of signalling. In the instructions issued by Don Martin de Padilla in 5597 the use of guns, lights and fires is mentioned. The introduction of the square rig permitted a further addition, that of letting fall a sail a certain number of times. Before the middle of the 17th century only a few stated orders and reports could be made known by signalling. Flags were used by day, and lights, occasionally with guns, at night. The signification then, and for a long time after, depended upon the position in which the light or flag was displayed. Orders, indeed, were as often as possible communicated by hailing or even by means of boats. As the size of ships increased the inconvenience of both plans became intolerable. Some attribute the first attempt at a regular code to Admiral Sir William Penn (1621-167o), but the credit of it is usually given to James II. when duke of York. Notwithstanding the attention paid to the subject by Paul Hoste and others, signals continued strangely imperfect till late in the 18th century. Towards 1780 Admiral Kempenfelt devised a plan of flag-signalling which was the parent of that now in use. Instead of indicating differences of meaning by varying the posi tion of a solitary flag, he combined distinct flags in pairs. About the beginning of the 19th century Sir Home Popham improved a method of conveying messages by flags proposed by R. Hall Gower (1767-1833), and greatly increased a ship's power of com municating with others. The number of night and fog signals that could be shown was still very restricted. In 1867 an innova tion of prodigious importance was made by the adoption in the British navy of Vice-Admiral (then Captain) Philip Colomb's flashing system, on which he had been at work since 1858.