=Trial Shots.=--A percussion-shell is fired with the elevation corresponding to the estimated range at a clearly visible and sharply defined fixed point of the target. If the first shot strikes short, or beyond the target, the range for the second is either increased or decreased, according to the case, so as to throw the shot beyond or short of the target. These corrections are generally made as follows: When firing at short ranges, 100 yards. " " " medium " 200 " " " " long " 400 " Such firing is pursued until two consecutive shots fall, one short of and the other beyond the target, which establishes "the long bracket, or fork." No round should be taken as a basis for correction if there is any doubt as to the reliability of the observations. A mean range is taken from those two which determined the long fork for the next shot, and then another shot is fired at a range obtained by taking the mean of the last range and the one of the long fork whose shot fell on the opposite side of the target. So proceed until the target is enclosed between two consecutive shots only 50 yards apart. This is called "the short fork, or bracket." Then a verifying group, generally a shot from each gun is fired with the elevation corresponding to the mean range of the short fork. If four out of six of the shots fall short, the range is established; and if shrapnel-fire is then resorted to the range is still further reduced by 50 yards. When the range has been measured, and not estimated, the short fork may be obtained from the first two shots. If the elevation has not been obtained as above indicated, at known ranges shrapnel-fire may commence, as if the range had already been determined by shell-fire; at unknown ranges the trial-shot firing with shrapnel is conducted as with shell; that is to say, the elevation is increased or reduced from shot to shot (by 100 yards at short range, 200 yards at medium range) until the target is enclosed between at least two accurately observed shots, one over and the other short. The difference is reduced as already described until the mean of the short fork is obtained. The large fork should be established by percussion-shrapnel, and then fire time-shrapnel. During the ranging platoon commanders indicate exactly the point to aim at, see that the sights are properly set, especially during the first rounds, observe similarly the time-fuze, and make any necessary correction for deflection; which deflection pertains to a particular gun, and is not carried to another. Bear in mind that each unit of the scale for deflection corresponds to 1/345 of the range for sights now used.[9] After the ranging has been completed, if a gun makes a constant error, the platoon commander may be permitted to change the elevation ± 25 yards.