Tracing-tape is usually a white tape, about 1½ inches wide, in lengths of 150 feet, marked at equal intervals, usually 5 feet, by short pieces of tape sewed to it. A loop of strong cord is fastened to each end. For convenience in use it is ordinarily rolled into a ball.
Tracing-pickets are about 18 inches long and one inch in diameter. To make them visible in a dim light the bark is removed from them. Ordinary pickets are usually 3½ or 4 feet long, 1½ to 1¾ inches in diameter, sharpened to a triangular point.
The tracing-lantern ([Pl. I], Fig. 5) is a dark lantern with a reflector arranged to throw a light vertically downward.
The other tools, materials, and appliances above mentioned, not of the ordinary commercial patterns, are described in Field Fortifications and Military Mining, q.v.
CHAPTER III.
TRENCHES, APPROACHES, PARALLELS, SAPS, SPLINTER PROOFS, AND PASSAGE OF THE DITCH.
11. Trenches.—A military trench consists of a ditch and embankment affording cover from direct fire. Trenches are used for approaches (or boyaux), parallels, and communications with magazines, etc.
12. Parallels are trenches which take their name from the fact that they usually are located on lines approximately parallel to the general front of attack. In a regular siege at least three and frequently a greater number of parallels are used. The exterior one, which is first made, is known as the first parallel, the next one as the second parallel, and so on. They are used to cover the part of the besieging force known as “the guard of the trenches,” which protects the men making the approaches, etc., and also as “places of arms” for assembling troops for assault or for other purposes.
The trench of a parallel is usually 10 feet wide at the bottom and 4 feet deep, finished on the reverse with a slope and on the front with two steps and a berm, with treads of 18 inches and rises of 15-18 inches ([Pl. I], Figs. 7 to 13).
The parapet of the parallel should not be higher than 4 feet 6 inches. Its upper surface, particularly in the second and third parallels, should be made approximately plain with a scraper, and its interior slope should be finished and if necessary revetted, so as to afford a good infantry-fire. To allow the troops to move out to the front in line, portions of the interior slope should be cut into steps of not more than about 18 inches rise, and be revetted with fascines or other materials ([Pl. I], Figs. 12 and 13). These portions should be 25 or more yards long and near the approaches. If a general assault is to be made, the parallels must be similarly arranged for the necessary length of front.