Sap (military)

Definition and stakes

Thomas Wilhelm,  A Military Dictionary and Gazetteer

“ From the nearness of the enemy’s works, running a sap is an extremely dangerous operation. When possible, therefore, it is carried on at night; in any case, the sappers are relieved at least every hour. When a sap is enlarged to the dimensions of a trench, it bears that name. When the fire of the enemy is slack, so that many gabions may be placed and filled at the same time, it is called a flying sap. If two parapets, one on each side of the trench, be formed, it is then called a double sap. ”
Source: Gutenberg

William Duane,  A military dictionary

“ SAP, (Sappe, Fr.) in sieges, is a trench, or an approach made under cover, ten or twelve feet broad, when the besiegers [615] come near the place, and the fire from the garrison grows so dangerous, that they are not able to approach uncovered.
There are several sorts of saps: the single, which has only a single parapet; the double, having one on each side; and the flying, made with gabions, &c. In all saps, traverses are left to cover the men.
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Source: Gutenberg

Portrait of H. W. Halleck H. W. Halleck,  Elements of Military Art and Science

“ The full-sap is employed when the works of the besiegers are within range of musketry, or when the grape fire of the besieged is so deadly that the flying sap can no longer be used. This is a difficult operation, and unless executed with great care and by well-instructed engineer troops, the construction of the trench will be attended with an immense loss of life. The work must be executed under cover of a sap-roller, which is a cylindrical mass of fascines, wool, or cotton, some two feet in diameter. On very smooth ground a ball-proof shelter on wheels might be used as a substitute. ”
Source: Gutenberg

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