It is easy to tell a gas shell when it lights as it comes down, the same as a “dud” shell; that is, one which does not explode, the outer casing of the shell simply collapsing. The liquid soaks into the ground, and men should be warned against standing over this ground and inhaling any of the fumes, which are very slight and rather hard to notice but very powerful and with very quick action. When a man thinks he has inhaled any of this gas he should at once be made to lie down, not undergo any exercise whatever, and as soon as possible have him carried out on a stretcher to the dressing station. _Tear shells_, which are used in great profusion during an attack, are for the moment blinding in their effect, causing smarting of the eyes and a great amount of watering. This effect is only for a minute, and the men must be impressed with the fact that if they continue moving forward instead of sitting down and rubbing their eyes, it will pass off almost at once. These shells are also greatly used against the artillery during heavy bombardments. Adequate protection is furnished in the shape of goggles to fit over the eyes, as the gas has no other effect whatever. As in other branches of military art, the best means of learning defense is to have a thorough knowledge of attack. Thus, direction of wind must always be noted, and if favorable for an enemy attack, special observers must be placed to give warning and surprise guarded against in every way. Sentries are specially placed in the trench, and often in listening posts, to get early warnings of an impending attack. If a sentry at a listening post discovers that a gas attack is being made, he at once warns the sentry at the end of his cord or wire, giving a pre-arranged signal. This sentry passes the alarm on a Strombon horn, which is something similar to a Klaxton, and will automatically give a warning which can be heard for 3 or 4 miles, and which lasts about a minute. When a horn is not in use, generally shell cases are hung in the trenches. These are beat on by the sentry who is warned and taken up all along the line. It is then the duty of that sentry not for the instant to put on his gas helmet, but to proceed along the front line waking all the occupants of dugouts, etc., who may be sleeping in the area guarded by him. Every man without exception stands to in his trench with his helmet on and will not reënter dugouts until first given permission by officer or N. C. O. These attacks are generally carried out, when possible, just before dawn or during the middle of the night, and the only warning given before the actual gas reaches a trench, is a slight hissing sound which is made and can be easily heard as the gas escapes from the cylinder. Great care must be taken when the wind is favorable for an attack that this sound be listened for.