NAVAL GUN SIGHTS AND GUNLAYING Sights fitted with telescopes are attached to the cradle or con nected with the trunnions of the slide. Setting the range on the sight gives it an angle of depression with the gun's axis. The movement of the gun necessary to aline the sight on the target gives elevation to the gun corresponding to the range on the sight.
It will be appreciated that change in the direction or bearing of the target may entail continuous training to keep on the object, and that change in range may entail constant alterations in the elevation of the gun. Such movements are, however, usually small, and readily adjusted by the responsible members of the gun's crew. A more difficult task in keeping the gun continuously laid is caused by the oscillatory motion of the ship, which, since it is of a harmonic and composite character, is very difficult to compensate. This motion is, as far as possible, discounted by means of "director firing" in which a definite position in the roll can be selected for discharging the gun. In this method of firing, an indicator is operated at the gun by an electrically connected master sight in the control position. This is followed by the action
of the gun-layer, who keeps a pointer attached to the gun in coincidence with the director indicator, thus giving the gun an elevation corresponding to the setting of the master sight.
The setting of this sight involves the rangefinder range, and the "spotting" correction obtained from observation of the fall of the shot, which may be assisted by aircraft observation. This spotting correction is inevitable as the actual muzzle velocity of the gun may differ from that for which the sights are graduated owing to wear of the gun and to the temperature of the charge. Also the retardation of the projectile may differ from the normal on account of its form and weight and because of the density of the atmosphere. Such effects are as far as possible allowed for by calculation in the initial setting of the sight ; but there remains a margin of error, which in most cases is chiefly due to the discrepancy between the rangefinder range and the true range.