e speed by the workmen. Three rows of
fascines are usually placed on the top of the gabions to increase the
height. The most difficult part of the flying sap is executed by
engineer troops, and the trench is completed by the ordinary working
parties. Fig. 51 represents a section of this sap.

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_.--B