The machine gun emplacement should be carefully selected and constructed. The first consideration is a good field of fire; the second is good concealment. Usually it is placed on a flank of the battery. Situated on the flank of the battery, a greater field of fire is obtained for the gun. Moreover when firing against hostile airplanes, the position of the battery will not necessarily be disclosed. In case the battery is shelled, the position of the machine gun on the flank renders it less liable to be hit. The emplacement for a machine gun at a battery position differs from those generally built by the infantry in that it is designed primarily for anti-aircraft shooting. There can be no overhead cover, all protection being provided by well constructed concealment. Camouflage must be arranged for and so placed that it can be instantly thrown aside for firing. The plans for close defense of the battery should include the assignment of positions and sectors of fire for the automatic rifles. The methods of fire and the technical points involved, in firing against hostile airplanes, will be considered elsewhere. A sentinel equipped with a pair of field glasses must be on duty in the emplacement at all times. The members of the machine gun squad are detailed for this duty by roster. Types of Anti-Aircraft Mounts. The Ordnance Department furnishes no anti-aircraft mount, but improvised mounts can be easily made. BROWNING MACHINE GUN—MODEL 1917. The Browning Machine Gun, Model 1917, is water-cooled and is chambered for caliber .30 U. S. Standard Rifle Ammunition. The gun is classed as a heavy water-cooled gun, recoil operative and belt fed. The water jacket holds about 7 pints of water and is perfectly smooth on its interior. The steam escape tube is in the top of the water jacket, and is made up of two tubes. They slide one upon the other, the outer one being a little shorter than the inner one. This allows the outer tube to slide toward either end and uncover the highest steam escape hole. The force of recoil is utilized to perform the various mechanical operations of feeding in the cartridges, loading, cocking, firing the gun and ejecting the empty shells through the bottom. The several cams of the gun make each moving part of the gun have a positive motion. The feed belt is made of woven fabric and has no metal parts to cause feed jams and to add extra weight to the piece. Also the metallic disintegrating links are used with the aeroplane gun. The fibre belt usually contains from 250 to 500 rounds. The metallic belts can be made to contain any number of rounds desired.