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.