Messages are sent by using the General Service Code and should always be as short as possible. Every time a letter can be omitted, the chance of error is reduced. A dot is made by a short flash of about ½ second duration. A dash is a longer flash of about two seconds duration. The interval between dot and dash is about ½ second duration. The interval between letters is about 2 seconds duration. The interval between words is about 4 seconds duration. In order that lamp signals may be easily read, it is necessary that the signals be not too rapid, 15 to 20 characters per minute should be taken as the upper limit. Successive letters must be well spaced. An interval of 2 seconds between letters will enable the receiving operator to call off each letter to his assistant as he receives it. In general, two men for each shift are necessary to operate a lamp station. At the sending station one man dictates the message letter by letter, and watches the receiving station for breaks. The other sends the message. At the receiving stations, one man receives the message and calls it off by letter to his helper who writes it down. To call a station, its call letter should be sent several times and at intervals the station calling should signal its own call letter. As soon as a station observes that it is being called, it will answer by signaling its call letter and the signal BR, “go ahead.” The message is then transmitted and the receiving station acknowledges receipt of each word. By one dot, if it has been understood. By the interrogation mark, if it has not been understood and repetition is desired. (While the interrogation is official, two dots are invariably used for this signal.) At the end of a message the sending station signals AR, meaning, “end of message.” The receiving station sends a dot if the message has been understood. Signaling by Means of Fireworks. The use of fireworks in modern battles for sending signals has been greatly developed and is now one of the most important means relied upon to send a few fundamental signals from the front line of the infantry to the supporting artillery within the division and between the ground and the airplanes. The Fireworks Code. As the signals that are made by fireworks are always of the most important character, it is essential that the system for their use be so perfectly worked out that there will be no chance of confusion. The smaller the number of signals to be sent by fireworks, the less chance there is of confusion. Classification of Fireworks. The fireworks now being used by the American Army are divided into the following classes: 1. Very Pistol cartridges. 2. VB cartridges (commonly called “Tromblons”). 3. Rockets. 4. Flares. The complete directions for firing these various fireworks are generally attached to the container or box in which they are packed. They are fully discussed in Annex 14, Translation of the 1917 “Instruction on Liaison for Troops of all Arms, A. E. F.”