and gun, when laid, lie in parallel vertical planes, so that neglecting drift, etc., correct direction is obtained.
_ Means to compensate automatically for drift of the projectile by giving appropriate deflection to the gun are usually introduced into the sight. As drift increases with the range, so also does the neces- sary deflection; but in some instances, a fixed deflection (repre- senting an average correction) is put on to the sight system. The correction is effected by arranging the sight, when setting for eleva- tion, to move in a plane inclined slightly to the vertical and inter- secting the vertical plane in a line parallel to the axis of the gun; thus the greater the elevation on the sight, the greater the auto- matic deflection imparted. This method of compensation allows of a convenient combination with the reciprocating sight, by mount- ing a transverse bubble in such a manner as to ensure that, when central, the body of the sight is tilted to the left at the required angle ; but the open, telescopic or panoramic sights are mounted on the rocking portion so as to have true verticality when the bubble is cross-levelled. Thus the cross-levelling gear neutralizes any want of level, and permits drift to be corrected for by movement of the sight in a tilted plane when setting for elevation; but the range- scale must be graduated to register elevation in a vertical plane.
In the system of independent line of sight in use with certain light field carriages the two parts of which the quadrant angle is com- posed the angle of sight and the tangent angle are applied to the gun independently and by different men. This system necessi- tates an intermediate component carrying the sight, and two ele- vating systems, the lower of which elevates the intermediate com- ponent, sight, cradle and gun for angle of sight, and the upper ele- vates the cradle and gun for tangent angle. Thus, conjointly, the two elevating gears give the quadrant angle. A sight clinometer is fitted to the sight to register the angle of sight for indirect laying, whilst a range-indicator, operated by the upper elevating gear, registers the tangent angle. Once the sight has been aligned on the target, the tangent angle may be altered to correct the range with- out disturbing the layer for direction. Such a system is useful when firing at moving targets, the direction and elevation having to be altered simultaneously, but it has a disadvantage in that it is diffi- cult to apply the reciprocating principle when the gun has to be moved independently of the_ sight. Drift is usually corrected for by inclining the trunnion axis. The latest method of securing this principle, though not truly " independent," combines the reciprocating sight with an automatic drift correction.
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The rocking-bar sight is mounted on a pivot which is always parallel to the gun axis, and thus can be rocked laterally under the control of cross-levelling gear to maintain verticality, which is indicated by a transverse bubble. An acorn-pillar, or front sight, and notched leaf, or rear sight, are fixed at the extremities of a sight-bar which is mounted on a vertical pivot on top of a rocking-bar; the rocking-bar is pivoted transversely at the front of a sight-carrier attached to a support on the cradle. The sight is set for elevation by means of a worm and arc-pinion drive in mesh with an arc on the rocking-bar, the range or tangent-angle being registered on a drum fixed to the pinion-spindle. The pivot of the rocking-bar is inclined so as to give an approximate drift correction at all ranges. To give deflection, the rear of the sight-bar is fixed to a nut mounted on an endless screw carried at the extremity of the rocking-bar; the nut traverses across a degree scale fixed to the rocking-bar. Bearings for a telescope are fixed to the sight-bar.
A sight clinometer is usually fitted to the rocking-bar so that, if necessary, the sight can be set first for angle of sight and subse- quently for tangent angle. In effect, the clinometer is an adjust- able spirit-level. A bubble is mounted in a sliding segment having worm-teeth cut on a curved base; the segment is moved relatively to a carrier by a screw mounted in the latter and in mesh with the teeth. Degrees of elevation or depression are registered on the face, while the screw is fitted with micrometer head to read minutes.
The employment of muzzle-velocity correctors for use in con- junction with the range drum is becoming general. The corrector (not to be confused with the " corrector " employed for varying the height of burst of time fuzes) also provides means for determining the muzzle velocity from firing at a known range, allowances being made for standard conditions; sights so fitted are known as cali- brating sights. The corrector, which may be attached to the oscil- lating bracket or to some part of the range-gear in independent-line sights, fits over the face of the range-drum and is graduated in rang- ing muzzle velocities; a knife-edge reader, hinged at one end, has the other end attached to a nut mounted on a screw in the corrector and engraved with an arrow for reading the muzzle-velocity scale. The knife-edge reads the yard-scale graduations while fine adjust- ment along the muzzle- velocity scale is ensured by the nut-and-screw arrangement. The yard-scale is engraved upon a spiral designed in conjunction with the muzzle-velocity scale to give the correct tan- gent angle for the muzzle velocity used.
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FIG. 2
With certain carriages it is inconvenient to mount the sight on either the trunnion or the cradle. In such cases the sight is mounted upon a pivot on the intermediate carriage, the sight-carrier being constrained, by means of parallel link-motion gear, to make the same angular movement as the gun. For the sight to be effective, the parallelogram must be correct, and adjustments may be required for the lengths of the link and the arm. In certain designs the independent line of sight is obtained without the use of an intermediate component, and combined independent-line and reciprocating sights are also in use.
The automatic sight is discussed and an example illustrated at 25.64; here, therefore, it is sufficient to recall its basic principle, which is, that for a gun mounted on a level platform at a fixed height above mean sea-level, there is only one angle of depression of sight and one quadrant angle for any given range. Thus, if the sight be pivoted transversely to the cradle and made with an arm projecting downwards from its front end, there is, for a given quadrant angle, one position only for the lower extremity of this arm; if the arm be caused to engage with a suitably cut cam fixed to the intermediate carriage, any quadrant angle on the gun will impart the correct sight-depression to the sight.
A distinctive feature in connexion with all anti-aircraft sights is the necessity for an automatic reduction in the tangent angle for a given range as the angle of sight and quadrant angle increase. Consequent on the varying range, height and speed of aerial targets, large corrections have to be applied to compensate for the vertical and lateral angular movements of the target ; these corrections inter-
