The Codes beat known in England are those of the Admiralty, 1808, and again in 1816 and 1826, and the modifications which form the present Admiralty Code ; Lynn'a Code, 1818; Squire's, 1820; Raper'e, 1823 ; Phillipps's, 1836 ; Rohde's, 1836; Walker's, 1841 ; Eardley Wilmot's, 1851 ; Roger's (American), 1854; Reynold's (French) 1855; Marryatt's (last edit.), 1856 ; Board of Trade (2nd edit.), 1859. In all these the general principle Is the name as above explained. A certain number of flags and pendants of different patterns have to each its own name, some being expressed by numerals, others by letters, while a third set are used for specific purposes : one called the Interrogatory, asks a question; another signifies an affirmation, another a negation, and so ou. In the Admiralty, this system is carried to a considerable extent, and requires about fifty flags or pendants; while there is a general system-book, by which orders are given for evolutions in the fleet, and much routine information. There is also a second telegra phic book, including common words in English, a list of ships in the Royal Navy, with the names of the flags by which each is distin guished, and other useful information. There is also a system of night signals, the same as the general signal book, but instead of flags, lamps, with or without blue lights or guns, convey the signals. There is also a system of fog-signals made by the firing of guns, the variations being marked by the intervals between the reports. In addition to all this, each commander is furnished with private and secret signals, which are only used to ascertain whether a ship of war is a foreigner or not ; for if a foreigner the proper return signal will not be known.
A general code adapted to merchant vessels was invented by Captain Marryatt, and continued in use up to the year 1857, when the Board of Trade published their Commercial Code of Signals for all Nations,' which soon came into general use. It includes only 18 flags or pendants. which were named after the consonants, and were so arranged as to show the distinguishing flag of every British merchant ship, a list of such ships as are registered being published every year by the Board of Trade, with an official number to each. Each ship always retains its name and number, although it may change its port. The signals provided for were 20,000, and 4 flags the greatest number for any signal. The system was modified by a committee appointed by the Board of Trade in 1855, and as now in operation provides by means of 18 flags and 3 pendants for upwards of 70,000 signals : the flags and pendants are given under FLAO. The signals are arranged in classes for easy reference. We may lastly refer to a system of boat signals arranged by Captain Wilmot.
Sir John Barrow, in the article before alluded to, observes that a telegraph employed for public purposes should be possessed of power, certainty, simplicity, celerity, and secrecy. It should have sufficient power to express, by distinct positions or combinations of moveable parts, any possible order or information, either by letters, words, or sentences. Its certainty will depend upon all its parts being clearly defined, wholly within the field of the telescope, and so distinct that there shall be no risk of mistaking one signal for another ; whence the importance of simplicity becomes obvious. In order to decide the
question as to distinctness, the shutter-telegraph at Nunhead, near New Cross, was left standing for some time on the same hill as the semaphore ; and the result of the trial for a whole winter Was, "that the semaphore was frequently distinctly visible when the boarded telegraph was so much enveloped in mist and fog that the particular boards shut or thrown open could not be distinguished ;" and that the number of days in the course of the winter upon which the semaphore was visible exceeded those upon' which the shutters could be seen by fully one-third.
Any means of telegraphic communication which depends upon the deciphering of signals exhibited at a distant station is necessarily de pendent upon contingencies of weather; but many plans have been contrived for effecting the object in such a manner as to be inde pendent both of light and of the state of the atmosphere. For com munication between the different parts of a house this object may be effected by a mechanical connection, by chains or wires, between two dials with revolving indexes or pointers, in such a way that when one pointer is directed to a particular letter or word inscribed upon the dial to which it is attached, the other may exhibit a similar movement. The attention of the servant is engaged previously by ringing a bell ; and when the required signal has been made, a spring returns both indexes to their original position. Speaking-pipes. or tubes to convey the voice from one place to another, are also available for short dis tances, but their range is too limited for application on an extended scale. One of the early schemes of this character depends upon the principle of water finding its level ; but, independently of the difficulty which might arise from the friction of water in a very long pipe of small diameter, such as would be required to connect the vertical tubes in which the observations would be made, such a plan involves the necessity of having all the communicating stations at or near the same level. Other hydraulic telegraphs depend upon the comparative incom pressibility of water or other liquids ; it being proposed to lay down small pipes of any required length, and to indicate different signals by pressing more or less upon the surface of the fluid contained in them, which would transmit the motion to the opposite end of the pipe, where it might be pointed out upon a dial, or in any other con venient manner. Mr. Valiance described such a method of telegraphic communication in a pamphlet, published in 1825, of which Hebert gives some account (' Engineer's and Mechanic's Encyclopredia ') ; and some similar schemes have been more recently propounded. Air con fined in small pipes has also been tried to a limited extent as a pneu matic telegraph ; but in this, as well as in the hydraulic system, the risk of leakage is a scrioua disadvantage.