Gas projectiles can be used in all types of country. Woods, bushes, corn fields and clumps of buildings may hold the gas active for a considerable time. Two kinds of shell gases are used by the enemy, viz., lachrymators, which mainly affect the eyes, and poison gases, which may affect the eyes and are just as deadly as the gases used in the form of clouds. (1) TEAR, OR LACHRYMATORY SHELLS: These shells on explosion drive the liquid chemical which they contain into the air as a mist. They cause the eyes to water strongly and thereby gradually put men out of action. Their actual smell may be slight. Large concentrations of lachrymators begin to affect the lungs and cause sickness, coughing and general irritation. (2) POISON SHELLS: Besides the comparatively harmless lachrymators the enemy also uses projectiles which contain a gas, the action of which is very similar to that of phosgene. Because of their slight detonation, these shells are liable to be mistaken for blinds, but they emit large quantities of a gas which attacks the lungs strongly and is very dangerous, and even in slight cases may cause serious after effects. (3) SMOKE: The enemy may make use of smoke, either in the form of a cloud or emitted from shells and bombs. Smoke may be used with gas or between gas clouds; it may also be used alone to distract attention from a real discharge of gas, to cover the advance of infantry, or merely as a false gas attack. (4) MINE AND EXPLOSION GASES: The poisonous gases which occur in mines, and which are formed in large quantities when high explosive goes off in an enclosed space, _e.g._, from a direct hit in a shelter, or on the explosion of a charge in a mine, are not protected against by the ordinary anti-gas appliances. The chief of these gases is carbon monoxide. Protection against such gases will not be considered in these notes. Officers are held responsible that all the anti-gas appliances for protecting their men are maintained in perfect condition, and that all ranks under their command are thoroughly trained in the use of these appliances and in all other measures which may affect their safety against gas. Summary of Protective Measures: (a) Provision to each man of individual protective devices. (b) Arrangement for the inspection of those appliances and training in their use and instruction in all other measures of gas defense. (c) Provision of protected and gas-proof shelters. (d) Weather observations to determine periods when the conditions are favorable to a hostile gas attack. (e) Arrangement of signals and messages; for immediate warning of a gas attack. (f) Provision of appliances for clearing gas from trenches and shelters. C. Protection of Shelters: (1) METHODS OF PROTECTION: