=Dugout Blankets.= Dugout protection is intended to prevent entrance of any gases, lethal, lachrymatory or irritant, into the enclosed space. This has been most efficiently accomplished by means of curtains hung upon wooden frames and fitting closely against all edges of the opening to be closed. These curtains have usually been of heavy material and have generally been spoken of as dugout blankets. Since they were designed to exclude all toxic gases, they had to be devised upon general mechanical principles rather than upon principles of chemical action with specific gases. Permeability to air has not been considered a necessity, it being held that sufficient ventilation is secured by means of the air entering through the soil. For large dugouts and extended use large air filters were designed to draw pure air into the dugout with a fan. The qualities aimed at, to which both fabric and treatment should contribute, are the following: (_a_) Impermeability to gas. (_b_) Flexibility, especially at low temperatures. (_c_) Non-inflammability. (_d_) Freedom from stickiness and from tendency to lose material by drainage under action of gravity. (_e_) Mechanical strength. (_f_) Simplicity of manufacture and treatment. (_g_) Low cost. Army blankets, both those for men and those for horses, proved suitable materials for curtains, but the scarcity of wool made it desirable to select an all cotton fabric. A large number of oils were studied as impregnating agents. The most satisfactory mixture consisted of 85 per cent of a heavy steam refined cylinder oil and 15 per cent of linseed oil. This is taken up to the extent of about 300 per cent increase in weight of the blanket during impregnation. It becomes oxidized to some extent upon the surface of the blanket, which becomes less oily than the soft, central core. The finished blanket possessed the following properties: It resists penetration of 400-600 p.p.m. of chloropicrin for 8 hours (dugout test) and mustard gas for 100-400 minutes (machine test). It is sufficiently flexible after standing for 2 hours at 18° F. to unroll of its own weight, and may be unrolled by applying a slight force at 6° F.; it is not ignited by lighted matches and shows but little loss by drainage. Two types of machines were designed for impregnation, one for use on large scale behind the line, and a field apparatus for use at the front. CHAPTER XVI SCREENING SMOKES The intelligent use of screening smokes in modern infantry tactics offers innumerable advantages through concealment and deception. It confers upon daylight operations many of the advantages which were gained by conducting operations at night with few of the disadvantages of the latter.