Since the head sapper has the most dangerous post, the second, third, and fourth sappers relieve him by alternately taking his place ; and the work advances about 8 or 10 feet per hour, according to the tenacity of the earth.
If the fire of the place should be considerably subdued by the action of the besiegers' batteries, the full and flying sap may be com bined in order to expedite the approaches. Thus a party of sappers advancing on their bands and knees, and rolling gabions before them, on arriving at convenient places set up the latter in line ; then two or three men, at 'certain intervals from each other, dig pits behind the gabions, in order to get cover, and afterwards work towards each other, making a small trench, and filling the gabions with earth. The trench may subsequently be made of the required width.
When the distance to be passed over to the work attached becomes very small, the angle formed by the adjacent zigzags becomes very small, or rather when 100 yards of the zig-zag does not carry the approaches so much as 32 yards in advance (those being the propor tionate rates at which the single and double sap can be executed), then the line of trench is carried on directly towards the place, sometimes by a simple trench with traverses at intervals, and sometimes by what is called the double sap. This is performed by two squads or demi
brigades, who work parallel to each other, each being covered in front by its own sap-roller, and there is a third roller in rear of the small interval between the others. A row of gabions is placed on the right of the trench executed by one squad, and on the left of that which is executed by the other; the distances between the rows of gabions is about twelve feet, and traverses are formed in the trench at intervals as the work advances. These project alternately from opposite sides; so as to leave a serpentine passage along the trench. Hence this is termed the serpentine sap. There is however another form of double sap, called Jebb's, or the double direct sap, which is new adopted in the English service. This is formed by three brigades of sappers with four sap rollers in front. The three brigades work parallel to one another, and drive three saps straight forward. Traverses are formed as the sap proceeds, projecting alternately from each side like teeth and over lapping one another. The intermediate space is then excavated, and a covered passage is thus obtained. The distance between the parapets on each side is 22 feet, or the breadth of eleven gabions.