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. The sap-roller being placed along the line of the trench so as to cover the sapper in front, who is armed with a musket-proof headpiece and cuirass, this sapper commences the sap by placing a gabion on the line of the proposed trench and fills it with earth, working on his hands and knees. Having filled the first gabion, he pushes forward the sap-roller and places a second one next the first, stopping the open joint between the two with a stop-fagot. The second gabion being filled in the same manner as the first, others are successively established. When the first sapper has advanced a few feet, he is followed by a second, also in defensive armor, who increases the excavation and embankment; this sapper is then followed in the same way by a third and a fourth, after which the trench will be sufficiently advanced to be turned over to the ordinary workmen. The sap-fagots may be removed when the embankment becomes thick enough to resist grape. Fig. 52 represents a plan and section of a full-sap. When the direction of the trench is such that the men are exposed on both sides, it will be necessary to throw up an embankment both to the right and left. This operation is called the _double sap,_ and is executed by two parties of sappers, working side by side. In this sap it will be necessary to frequently change the direction of the trench, or to throw up traverses, in order to cover the men at a distance from the sap-roller. Wing-traverses, on the side of the trench which is least exposed, some times serve the same purpose as a double sap. _Mines_.--By _mining_, as a military term, we understand the operations resorted to for the demolition, with powder, of a military structure of any description. The term _mine_ is applied both to the excavation charged with powder for the purpose of producing an explosion, and to the communications which lead to this excavation. The place in which the charge of powder is lodged is called the _chamber_, the communication by which this place is reached the _gallery_, and the excavation made by the explosion is termed the _crater_.