Variations from these sections are made when rendered necessary by the presence of rock or water in the soil (Pl. I, Fig. 11); when a wider trench is required for a tramway or for free communication; or, in special cases, when a narrower trench will answer the purpose and save work. Should a specially heavy fire make additional cover necessary, it may be obtained by deepening the ditch and thickening the parapet, leaving its crest at the same height. The sections above given have been found best for ordinary cases. =19.= =Construction by Flying Sap.=--The construction of the first parallel having indicated to the defence the front of attack, further operations will usually be subject to a more destructive small-arm and machine-gun fire. This will, as the siege advances, render the losses experienced in constructing a simple trench too extravagant, and a quicker method of obtaining cover must be used. This method is found in the flying sap (Pl. I, Figs. 8 and 13), which is executed as follows, viz.: The men are posted and the gabions placed as previously described. The engineer officer having marked the lines, the order “Commence work” is given. Each man marks with his pick the front and left of his task (which in this case is 4 × 6½ feet, leaving a berm of 1½ feet), and proceeds at once to dig on its left, filling first the left gabion, next the right, and then throwing the earth over and in front of the gabions. Each gabion, when it is half filled, is tipped outward until it has a slope of about 4 on 1. The filling is then completed. As each man of the first relief occupies only 4 feet of front (2 gabions), his task is four-fifths as great as it is in executing the simple trench. The second and third reliefs have the same tasks as in the simple trench. When the first relief finishes its task, every fifth workman (indicated by the sapper of the section) retains his pick and shovel and returns them to the depot when he marches past it. The others leave their tools for the use of the second and third reliefs. In good soil the gabions may be filled in from 7 to 15 minutes. The English sap-shield is designed for use when the fire is so severe that the flying sap with gabions becomes impracticable. Owing to its weight (80 lbs.) a man can carry but one; hence a carrying party equal to the working party assists in placing the shields and then retires. This gives to each workman a task of 3½ × 6½ feet. The shields are placed as shown in Pl. I, Fig. 6; the trench is executed as previously described, the earth being thrown over and beyond the shield. The shields are removed after the task of the first relief is finished.