=25. Driving the Sap.=--The sap is driven as follows, viz.: Nos. 1 and 2 dig a ditch 4' 6" deep, 1' 6" wide at bottom, and 3' or more at top; the berm side has a slope of 3/1, and the reverse is vertical, or as nearly so as the earth will stand. They leave no berm, as they need all the cover they can get. Nos. 3 and 4 widen this trench 2 feet and form a berm of 1' 6" by digging away the foot of the parapet and throwing the earth upon its top and exterior slope. The head of their work is kept at 9' in rear of the head of the sap. The side parapet made by Nos. 1 and 2 is about 2' 6" high and bullet-proof (about 2' 6" to 3' thick) at 18 inches above the ground. The head parapet is made up of about 60 sand-bags, from ½ to 2⅔ filled. It joins the side parapet and extends across the head of the sap. It is about 2' 6" high. As the sap is driven forward the head parapet is advanced by throwing the rear sand-bags over to the front by hand or by the use of the sand-bag fork. In excavating the trench No. 1 kneels down, undermines, and digs down enough earth to advance his trench about 9 inches. He is replaced by No. 2, who shovels this earth upon the side parapet toward the head of the sap. No. 1 then resumes his place and throws the sand-bags at the head of the trench over the parapet until he has uncovered about a foot in advance. He uses a sand-bag fork when necessary. The trench is advanced 9 inches more in the same manner. No. 2, besides throwing out the earth dug by No. 1, trims up the slopes and gives the trench its proper width and depth. Nos. 1 and 2 change places when they have advanced the head of the sap 1' 6", and are relieved as before stated when 3 feet is gained. Nos. 3 and 4 work together upon their task. In shovelling the earth upon the parapet they throw it somewhat toward the front and regulate its height with the scraper. The rate of advance is usually from 2 to 4 feet per hour. _Widening the Sap._--The sap is widened by working parties, usually of infantry, who work at about, but not less than, 25 feet in rear of the head of the sap. In an approach their task is equal in volume to that of the sappers, and can be finished in one relief. In a parallel when steps are to be provided, a second relief makes the steps, drainage ditches, drainage pits, etc. _A change of direction_ in a full sap (Pl. II, Fig. 20) is made by No. 1 turning in the new direction and working through the old side parapet; No 2 throwing the earth over the old head parapet. The sand-bags of the old are gradually removed and used for a new head parapet, 20 or 30 additional sand-bags being ready for use if needed before the others can be safely removed. Nos. 3 and 4 follow on as before. A _return_ can be driven back when desired by another detachment of sappers. No head parapet will be needed, but the side parapet will be kept a little in advance of the head of the sap.