4. The gas used must be heavier than the air, so that it will sift into the enemy’s trenches as it passes them. It is impracticable to decide upon any definite hour for launching the gas attack. Everything depends upon the direction and velocity of the wind. If an hour has been tentatively designated and the wind changes, the attack will have to be postponed. When an assault follows the gas attack the men should wear the smoke helmets for at least 30 minutes after the dissemination has ceased; in fact they must not be removed until the order to do so is given by the officer commanding the attack. You will appreciate that the enemy’s machine gunners may have better protection than the men in the bays of the trenches. [Sidenote: Shell and Grenade Method] In this method the gas dissemination is effected by means of shells or bombs being fired into the enemy’s trenches containing the desired substances which are released and give off irritant fumes on explosion. The grenades used weigh about 1 pound. They are similar in appearance to the ordinary tin can grenade. Their effect in a trench will continue for 20 to 30 minutes. In the attack a large number should be concentrated in a particular area to produce a large volume of gas. They are thrown by hand, trench mortar or catapult. [Sidenote: Defense] Surprise must be guarded against in every possible way. The direction of the wind must be continually watched, and when its velocity and direction are specially favorable the protective measures must be kept ready for instant use and special observers posted. Previous to an attack the enemy may remain comparatively quiet for several days. Noises like the moving of sheet iron may be heard. Preparations may be observed along the position. When the attack starts a hissing noise is heard; this latter is one of the indications that may be evident at night. [Sidenote: Helmets] Each man on duty in the trenches is provided with two smoke helmets, specially devised and constructed so as to absorb the gas and neutralize its effect, and which if properly cared for and used will provide complete protection from any substance likely to be used by the enemy. They are fitted with a valve tube through which to breathe and with goggles to see through. There are certain rules prescribed for their care and use. 1. They must not be removed from the protective covering except for actual use against an attack. 2. When the helmet has been used once it should be replaced by a new one. [Sidenote: Dummy Helmets] Dummy gas helmets will be provided in each organization by which the men may be practiced in putting them on. The men must be thoroughly drilled in the methods to be employed. The following directions accompany the helmets issued to the British Army. When our helmets are issued it is probable that each will be accompanied by a complete set of rules for its use and full instructions for the method of getting into it and for its care and preservation.