1. By the noise of the gas escaping from the cylinders.
  2. By the appearance of a cloud of any color over the enemy's trenches. If the attack takes place at night, the cloud will not be visible from a distance.
  3. By the smell of the gas in listening posts.
  1. Action To Be Taken In The Trenches On Gas Alarm:
    1. Respirators to be put on immediately by all ranks (a helmet, if no box respirator is available).
    2. Rouse all men in trenches, dug-outs and mine shafts, warn officers and artillery observation posts and all employed men.
    3. Artillery support to be called for by company commanders by means of prearranged signals.
    4. Warn battalion headquarters and troops in rear.
    5. All ranks stand to arms in the front trenches and elsewhere where the tactical situation demands.
    6. Blanket curtains at entrances to protected shelters to be let down and carefully fixed.
    7. Movement to cease except where necessary.

    1. All men in cellars or houses to be roused.
    2. The blanket curtains of protected collars, etc., to be let down and fixed in position.
    3. Box respirators to be put on immediately, the gas is apparent.
  3. Action To Be Taken In Billets And Back Areas:

H. Action During a Gas Attack:

(1) Protective Measures:

There should be as little moving about and talking as possible in the trenches. Men must be made to realize that with the gas now used by the enemy, observance of this may be essential for their safety.

When an attack is in progress, all bodies of troops or transport on the move should halt and all working parties cease work until the gas cloud has passed.

If a relief is going on, units should stand fast as far as possible until the gas cloud has passed.

Supports and parties bringing up bombs should only be moved up if the tactical situation demands it.

If troops in support or reserve lines of trenches remain in, or go into, dug-outs, they must continue to wear their anti-gas appliances.

Officers and N.C.O's must on no account remove or open up the masks of the box respirators or raise their helmets to give orders. The breathing tube may be removed from the mouth when it is necessary to speak, but it must be replaced.