[1190]. Guarding obstacles. It has been found necessary to keep a constant watch over obstacles after they have been placed.
[1191]. Listening posts. One of the best methods is to post one or more men in listening posts in or beyond the line of obstacles. These listening posts are rifle pits with over head cover, fully protected from fire from the rear as well as front, and loop holes for observation and fire. They are connected with the fire trenches by means of a covered communication or even tunnels in some cases and are provided with some form of prompt communication with the firing trenches by telephone, bell or other means. The communicating trench or tunnel is provided with a strong door which may be closed to prevent an enemy from securing access to the fire trench, in case the lookout is surprised. Pits with trap doors are also used to prevent an enemy from creeping up the tunnel to the fire trench.
These lookouts can give early warning of the approach of an enemy, either for the purpose of assault or cutting through the obstacles. In many instances they have detected mining operations of the enemy by hearing the blows of picks under ground.
[1192]. Automatic alarms. Many automatic alarms have been used to give warning of attack on the obstacles. These vary from the simple setting of a pistol or rifle, which is fired when the enemy attempts to cut through the entanglement, to intricate electrical alarms.
[1193]. Searchlights. Searchlights have been provided so that, the instant an alarm is given the obstacles are flooded by a brilliant light and the enemy exposed to fire.
CHAPTER XII
TRENCH AND MINE WARFARE[14]
[1194]. Asphyxiating gases. The asphyxiating gases employed may be divided into three general classes, viz:
Suffocating gases, the most common of which are carbonic and nitrogen.