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.