(_c_) Signalling: It is difficult to establish signal communication within the sector without being seen by the enemy. However, at night with flashlights it is possible, especially in hilly country, to communicate from front to rear within the position. The receiving post should be able to certify received messages. When practicable, communications should be established between the command post of the center of resistance and the sector command posts, and from the latter post to the artillery. This method of liaison is organized as a duplicating means or as a temporary substitute for the telephone. (_d_) Rockets: Signal rockets are used to communicate with the rear. Different forms and colors of rockets are used to signify different things. As we have seen, the most important use of the rocket is to signal for barrage fire. It is often necessary to relay a rocket signal from a second position in the rear to make absolutely sure that it reaches the artillery support. Signal rockets are also used to communicate with aëroplanes, but this means is usually only employed during an attack. (_e_) Runners: The liaison established with runners is best of all, from the point of view that it usually never fails. During heavy shelling it is the only sure means of communication. [Illustration: _Example of a Chain of Runners._] The ordinary messengers or orderlies are not used as runners. A chain of trained runners is used, located in a little post, shelters, or shell holes; Each post contains three runners, their distance being from 150 to 300 yards apart. Each runner is selected from the unit located near the next post in the chain, that is, the one to which he is to run. Often the divisional cavalry furnish these runners. The chain of runners is under the command of a N. C. O. who is responsible for its efficiency. He has an important task on his hands. He must assure himself that every runner knows, not only his two neighboring posts, but also the general direction of the chain. He must change disabled runners and fill gaps in his chain. It becomes his duty to instill into the runners the importance of their duty. (_f_) Carrier Pigeons: Carrier pigeons are the last means of communication. They have been found to give excellent service during heavy shelling or gas attacks. They do not fly very easily at night. Each command post of a center of resistance in the first line has a pigeon post. These consist of two reliefs of men, four pigeons, and the necessary equipment and food. The pigeons themselves have to be relieved every few days. The care given to the birds at the command posts must not be as good as that at the dovecot in the rear, so that when they are released they will return immediately to the latter place, from where the message is delivered to its proper destination. Instructions for Sentinel, Watcher, and Observer