A legitimate ruse, of which the enemy is fond and which should be guarded against, consists in their firing blank cartridges to mask an advance of their men. It is clear that men do not advance while bullets are fired from their own trenches. Nor does one fire in retaliation until the enemy's fire ceases. A little attention will be sufficient to spoil this plan as, when only blank cartridges are fired, no bullets will whiz by. As long as the enemy fires blank cartridges, withhold your own fire, be on guard against the appearance of patrols and be ready to receive them when they draw near.
49.—His use of flares.
In case the enemy sends up flares, patrols should lie flat and motionless till after the volley which often follows. The sentinels in the fire trench should note spot where flare was sent up and abstain from firing unless enemy is sighted out of his trenches.
50.—His machine guns.
If a machine gun opens fire from the trench opposite, try to locate it through the light and sound at night, through sound and actual sight during the day.
Fire a converging volley of two rounds in its direction, and repeat, but not over six rounds if unsuccessful.
At the same time let trench mortars fire bombs in the same direction.
51.—His field and trench artillery fire.
If enemy's artillery fires upon trenches (Cf. [par. 69] ff.)