When recourse to any method of sight signals can not be had on account of weather conditions or lack of suitable apparatus, sound signals may often be advantageously used. The commoner means of furnishing sound signals are the horn and the whistle, though many other kinds of apparatus are practicable. The necessary elements of any system can be indicated by one short, two shorts, and a long blast. The advantage of this system of signaling is that it can be used in any kind of weather, both in daytime and at night. On the other hand, sound signals are generally more difficult to read than sight signals and tend to disclose the presence of stations to hostile forces.
The object of this chapter has been to describe only the standard visual signaling equipment issued and generally utilized. Besides the methods detailed, there are many others which may be successfully employed by the ingenious signalman when the necessity for them arise. The use of any means of transmitting signals whatever is justifiable when for any reason the regular apparatus is not available. Special conventional scout signals are given in paragraph 82, Field Service Regulations.
In the field many instances will occur where it will be necessary to transmit information rapidly without recourse to the authorized equipment. This will be especially true of outposts, detached stations, patrols, and other small bodies of troops, and it will devolve upon individual commanders to improvise methods of signaling best suited to the occasion and the conveniences at hand.
AMERICAN CONTINENTAL ARMY AND Letters-- MORSE. MORSE. NAVY.
Numerals-- AMERICAN CONTINENTAL ARMY AND MORSE. MORSE. NAVY.
? Interrogation -- · · -- · · · -- -- · ·
The following abbreviations, conventional signals, and code calls are authorized in visual signaling:
International Code use ICU (Navy) telegraph dictionary use TDU (Navy) geographical list use GLU (Navy) general signal use GSU Navy list use NLU Vessel's numbers use VNU Cipher "A" use[a] CAU Cipher "B" use[a] CBU Cipher "C" use[a] CCU
[a] These calls are for preconcerted use in or with the navy.
Although the use of but one alphabet is authorized in visual signaling in the U. S. Army, emergencies may arise where it may be imperative to use either the Army and Navy, the Continental Morse, or the American Morse alphabet. Instructions for the use of either alphabet under such conditions are given.