Both the sappers and working parties are divided into reliefs, usually of eight hours. The sappers of the tracing parties superintend the work of the first relief of the working party, but are relieved long enough before them (½ hour to 1 hour) for the second relief to become acquainted with the details of its sections before the second relief of the working party arrives. A similar arrangement is made for the third relief. EXECUTION OF PARALLELS AND APPROACHES. =17.= =Simple Trench.=--A trench made by excavating the earth and forming a parapet without revetment of any kind is known as a “_simple trench_,” or as “_simple trench-work_.” _Flying sap_ or _flying trench-work_.--When, in order to obtain cover more quickly, gabions are used to hold the earth first excavated, and subsequently to serve as a revetment to the interior slope of the parapet, the trench is known as a “_flying sap_” or “_flying trench-work_.” =18.= =Construction by Simple Trench.=--The first parallel and the distant approaches are usually constructed by the use of the simple trench, as follows, viz. (Pl. I, Figs. 9 and 10): The men having been posted along the tracing-tape at five-foot intervals, as previously described, and their positions verified by the engineer officers, the command “Commence work” is given. Each man marks the left and front of his task by a line dug with his pick, and, commencing at the left of his task, at once excavates a trench 3 feet long, 1½ feet deep, and 6½ feet wide, throwing the earth to the front, and making a parapet 1½ feet high, leaving a berm of 1½ feet. Then, commencing at 1½ feet from the front of his trench, he deepens it to 4 feet, making the parapet 3 feet high. When the task of a party is finished each man cleans off his pick and shovel, places them at the rear of the trench, and leaves them there for the use of the second relief. By excavating in this way, partial cover while at work and a defensible parapet are rapidly obtained, and, at the completion of the task, the parapet admits of a strong defence, and affords cover sufficient to allow the first relief to be withdrawn and the second to be posted without exposure. Special care must be taken during the work to make the men face toward the parapet while digging, in order to avoid striking their neighbors with the pick when raising it for a blow. The second relief widens the trench 4 feet; forms a bottom step 18 inches wide with such materials as are available; heightens the parapet to 4½ feet, and throws the rest of the earth to the front to thicken it (Pl. I, Fig. 8). The third relief widens the trench 2½ feet at the bottom and slopes off the reverse as steep as the earth will stand. The earth is used to thicken the parapet, additional shovels and shovellers being provided if found necessary. The approaches (Pl. VIII, Fig. 82) are extended in a similar manner; the tasks of the reliefs are marked on the sections.