When two or more guns are working together, the depth of the effective zone can be increased by ordering different elevations for each gun, both, however, using the same point of aim. The respective depth and width of effective zones may be taken as being:— ───────┬───────────────┬─────────────────────── Range. │ Mark VI. │ Mark VII. ───────┼───────┬───────┼───────┬─────────────── „ │Depth. │Width. │Depth. │ Width. ───────┼───────┼───────┼───────┼─────────────── Yards. │Yards. │ Feet. │Yards. │ Feet inches. 500│ 150│ 4│ 220│ 2 6 1,000│ 70│ 8│ 140│ 5 0 1,500│ 60│ 13│ 70│ 10 0 2,000│ 50│ 19│ 60│ 17 6 ───────┴───────┴───────┴───────┴─────────────── From this it is apparent that, allowing an error of 10 per cent. in ascertaining the range, the flatness of trajectory within 800 yards will insure the target being included in the effective zone, therefore with Mark VII ammunition combined sights should not be used at less than 800 yards. Bracketing. When only a section is available and the “combined sights” method does not give sufficient assurance of fire effect, “bracketing” should be used as follows:— The range estimated, probable error determined, and the limits between which it is necessary to search with fire, the maximum and minimum elevation will be allotted to right and left gun respectively, who fire a burst and then work inwards (one higher, one lower) by altering their respective elevations 50 yards each time, eventually passing each other. If observation of the fire can be obtained, correct elevation should be immediately ordered. With Mark VII ammunition this method would not be necessary at 1,000 yards, as with a 10 per cent. error the “combined sights” method would give a satisfactory result. Vertical Searching. The range to nearest point of objective is taken, sight adjusted and gun laid on this point. Sights are then set for the farthest point of the objective. This will bring the line of sight short of the aiming-point. A series of bursts are then fired, care being taken that each burst strikes just beyond the former one, but does not leave any gap between groups. Overhead Fire. Can be employed normally from or at a commanding position, or across a valley, but not when own troops and those of the enemy are on same plane. Range of target must be accurate. If 1,000 yards or under, angles of sight to enemy and own troops must not be less than 30 minutes. If range is between 1,000 and 1,500 yards, the angle must not be less than 60 mins. Overhead fire should not be used at 1,500 yards and over. The following method of sighting can be used.—