[Illustration: Plate 24a.--Section of traversed type of fire trench showing entrance to squad dugout.] COMMUNICATIONS The fire trench is connected with the cover for supports by a system of zigzag trenches having the profile shown in Plate 23. The arrangement is shown in Plate 1. [Illustration: Plate 25.--Profile of trench mortar emplacement.] TRENCH MORTAR POSITIONS Somewhere between the first line fire trench and the cover for the supports is a line of emplacements for the trench mortars. Plate 25 shows a profile of the emplacement. The arrangement of the position is shown in Plate 1. [Illustration: Plate 25a.--Trench mortar.] These trench mortars are used to hurl charges of high explosives varying from 25 to 100 pounds into the enemy's lines. They have a range of from 300 to 1,800 yards. [Illustration: Plate 25b.--Improvised catapult.] COVER FOR SUPPORTS At a variable distance to the rear of the first line fire trench is located the cover for supports, which is organized much in the same manner as the first line system of trenches and affords a second position in the system to fall back to in case of necessity. These trenches are provided with overhead cover and numerous dugouts for the protection of the men. MACHINE GUNS At every available place throughout the defensive position machine guns are located, typical positions of which are shown in Plate 1. The typical types of cover are shown in Plates 26 and 27. Machine guns are a very potent factor in trench warfare. They are now being employed to a far greater extent than ever before, and the number is increasing on all the battle fronts as fast as they can be manufactured. The machine-gun positions are carefully concealed from the enemy, and fire is not opened until it is certain that it will be effective. [Illustration: Plate 26.--Profile of type of cover for machine gun.] [Illustration: Plate 27.--Type of cover for machine guns.] The selection of the sites for the emplacements should be made with a view to bringing a powerful enfilade or oblique fire on the attacking enemy at effective range, to provide a flanking fire for supporting troops, and to sweep gaps in the line of obstacles. Their fire should come as a surprise to the attacking party. In the construction of cover for machine guns the following points should be observed: 1. They must have a platform for the gun and gunner. This may be provided for in the construction of the emplacement or built up with sandbags. The platform should be 3 feet wide and 6-1/2 feet in length. 2. If head cover is provided, it should not differ in appearance from that constructed elsewhere in the trenches. The loopholes must be blinded with gunny sacks. 3. The front of the emplacement should be cut under to receive the leg of the tripod, thus bringing the gun up closer to the parapet and furnishing more cover for the gunner. 4. Splinter-proof shelters should be provided near at hand for the members of the gun detachment.