"A Manual of Signals: For The Use Of Signal Officers In The Field."
Washington, D.C., 1864.
Transient signals are so rapidly made, that they are repeated with difficulty. Permanent signals are repeated with precision. Where repetitions is to be habitual over long lines, some of the simple semaphores, once of common use, afford perhaps the most rapid and available means for communication. They have been improved by long experience to a degree which renders them almost perfect. For ordinary ranges, and for common military uses, any of the different manual codes, heretofore described, are always available. Permanent semaphores need not be used except for convenience, or when a long line is to be worked continuously with a feeble force.
When a message is to be repeated over a line of stations, either by transient or permanent signals, a warning signal is first given, in order that there may be proper attention at the intermediate stations before commencing the message. This warning may be a message as thus: "repeat to" (naming the town). This warning is sent, from station to station, until it reaches the station named; this station replies by a concerted signal of "ready, " and each immediate station, repeating this signal back to the first station, stands ready to repeat the message which the first station commences to forward, signal by signal, at once on receiving the ready signal. The intermediate stations repeating each signal letter and number as fast as they are received, as, for instance, was a message about to be sent from Washington to Frederick, the officer at Washington would first send over the signal line the warning "repeat to Frederick." This warning is repeated from station to station. On receiving it, the officer at Frederick, makes the ready signal, which signal is repeated back, from station to station, to Washington. Each station then stands ready to repeat the signal message which is to follow. On securing the ready signal, the officer at Washington sends forward the communication, each station repeating each letter and number in its turn as it receives them.
When a message is being thus repeated through a number of signal stations, the officer at each station will call the proper number for each letter, and pause as he receives them, to his flagman, who, placed facing from the sending station and towards the station next in line, makes each signal in its proper order. Each officer, after signalling from his station each letter and pause, waits until he sees it repeated at the next station before he signals another.
The advantage of permanent signals have become apparent, for a permanent signal may be kept in view until it is repeated with certain correctness. All signals made at the repeating station will appear to the observers at the sending station reversed. When permanent signals are used, each sending station keeps its signal in view until that signal has been repeated at the next station, when it resumes the position ready, and waits the next signal from the station of departure.
Long lines of signal stations, with a small military force at each, being thus each in communication with the other, may constitute picket lines of great length and importance for holding and keeping under observation lines of communication, rivers, or extensive tracts of country liable to incursion or to be ravaged by predatory bands of the enemy, each station, having the power of communicating with those on either side of it, has virtually thus the advantage of their support, and no one can be attacked without the enemy being exposed to the concentration of forces called for by signals from different stations. On river lines, where the protection of commerce is of importance, such stations afford at once shelter to the moving vessels, are able to warn them while at a distance of danger as of the location of the enemy upon the banks, or in case of attack to call to their assistance the vessels of war assigned to the duty of patrolling the stream. In the great river courses of this county, the advantage of picket lines of this description, guarding our rivers passing hostile territory, cannot be over estimated."