After the return is completed the sharp angle in the trench is rounded off to allow gun-carriages, etc., to make the turn. Approaches are usually 4 feet deep, 9 to 12 feet wide at bottom, with slopes in front and rear as steep as the earth will stand, and have a rough parapet not less than 4½ feet high, separated from the trench by a berm of 18 inches, or more if necessary (Pl. VIII, Fig. 82). When drainage requires it, as it very frequently does, the bottom of the trench is sloped from front to rear about 6 inches; a ditch cut along the reverse slope, discharging into the drainage ditches of the vicinity, or into drainage pits excavated in rear of and outside the approaches. These may be lined with a gabion to prevent their sides falling in. TRACING AND CONSTRUCTING PARALLELS AND APPROACHES. =14.= =Tracing Parallels.=--Parallels are located by engineer officers after careful reconnoissance of the ground. Guiding points and lines are marked so as to be readily found in the dusk, but so that they cannot be seen by the defence. When completely screened from view important points are marked by posting sappers at them. When no other practicable method can be used, the directions are determined by the use of a pocket compass fastened to a straight measuring-rod. Tracing is begun as soon as the approaching darkness will conceal the parties from the defence, while close objects are still visible. The length of parallel traced by each officer should not exceed that occupied by one unit, usually a battalion, as a working party. (A battalion of 500 men will occupy 800 yards). The tracing party consists of one officer, one N. C. officer, and one sapper to each 50 yards of parallel, with one or two extra men. The officer is provided with a pocket compass and measuring-rod. The N. C. officer has a tracing-lantern and a mallet with muffled head. Each sapper carries a roll of tracing-tape, a tracing-picket, and a six-foot measuring-rod. The officer stations the first sapper at the initial point, and, taking one end of his tracing-tape, moves along the line of the parallel, accompanied by the rest of the sappers; the first sapper places his picket between his feet, and the N. C. officer drives it into the ground far enough to make it secure. The sapper drops the ball of tape on the ground and lets it run out through his hands until nearly out, when he checks it, and when it is all out places the loop over his picket and lies down to await the arrival of the working party. The N. C. officer, as soon as he has driven the first picket, follows on to the second, etc. The officer, having run out the tape of the first sapper, halts the second, takes the end of his tape, and proceeds as before until the parallel is traced, and a sapper is left at each 50 yards of its length. Each sapper is told the designation, by number and section, of the point he occupies.