In order that a shell from a gun hit the target, the gun should fire at a certain angle of elevation depending on the range, the ballistic characteristics of the gun, and upon the relative level of the gun and target. It should be given such a direction to the right or left of the target as to offset the deviation of the projectile due to drift and wind. The sights of the gun provided means of determining when the axis of the gun had the predetermined direction.
The simplest sight was the one over the line of metal, which laid for direction only. The second was the tangent sight, mounted on a range arc centred on the axis of rotation in elevation, usually having a deflection scale to correct for drift and to lead a moving target. It laid for range and direction. The telescopic or panoramic sight was mounted on a range arc and laid for direction only, the unit in direct fire laying for range and direction and markedly improving the vision of the gunner.
††††††††††† Direct laying
When the piece was sighted, both in elevation and direction by sighting directly on the target, the method was known as direct laying.
The line of sight was fixed in two different ways. The first method was to use plain or open sights, the rear one of which had a peep, or notch, capable of adjustment in a vertical or horizontal direction. This rear sight was equipped with an arc reading in fractions of the range, or degrees, by which the necessary elevation could be set off. In some cases the rear sight was designed to automatically correct for drift; if not, the drift had to be set off on a scale provided for this purpose on the rear sight. The projectile followed the movement of the rear sight, going higher as the sight was raised, and to the right or left as the sight was moved to the right or left.
The second method for direct laying was to use a telescope with cross hairs which took the place of the open sights although its principle of operation was the same.
The angle of elevation of a gun must be measured in the vertical plane through the axis of the piece. Since frequently happened that a mobile piece should be fired under conditions in which the trunnion axis was not level, thereby throwing the sight plane out of the vertical, the sight arm should be revolved about an axis parallel to the axis of the gun until the sight arm was vertical. At the beginning of World War 1 most wheeled carriage had such a provision made on their sights.
††††††††††† Indirect laying
The gun was said to be laid indirectly when it was laid by means other than aiming directly through the sights at the targets. The fire from modern quick-firing field guns was so accurate and destructive that it was always necessary to establish field batteries in position out of the view of the enemy for the sake of protection. Consequently indirect sighting became the usual method of sighting guns.
The line of sight was fixed in four different ways:
- by an auxiliary mark, called aiming point, for line, and by clinometer for elevation;
- by direct vision for line, and by clinometer for elevation;
- by an aiming point both for line and for elevation;
- by aiming point for line, and by direct vision for elevation.
artillery indirect laying was normally used, even if the targed was visible
from the guns. During the war against
The Turks, trained in the German school and lacking in the latest mechanical improvements in equipment, seemed either to have been ignorant of indirect fire, or else to have deliberately ignored it. Rarely did they place their guns in masked positions for indirect fire, although often positions affording flash defilade could easily have been obtained. Sometimes, however, three guns of a battery were concealed, the other, used as a directing piece, being left in the open or placed so as to fire directly over the crest. This almost always led to its destruction, although the other guns, at some distance to the right or left escaped.
†††††††† The panorama sight
The panoramic sight afforded the means of aiming the gun in indirect laying by directing the line of sight on any object in view from the gun; at the same time it afforded the advantage of a telescopic sight in direct or indirect aiming. At the beginning of World War 1 the Goerz panorama sight was almost universally adopted.
This panoramic sight was a telescope so fitted with a rotating head, reflectors and prisms, that a magnified image of an object anywhere in view could be brought to the eye without change in the position of the observerís eye.
The panoramic sight was often mounted in connection with the range-sighting mechanism, but sometimes in order to divide the duties of laying for direction and elevation, the panoramic sight was mounted on a shank on the left side of the cradle and used in laying for direction, while the range quadrant for laying in elevation was placed on the right side of the cradle and used by another gunner.
In connection with the range quadrant a range level was provided, which was a special form of clinometer. It was used in setting off the angle of site, thereby correcting for difference in level of the gun or target. The range quadrant was graduated in degrees or in fractions of the range. In the case of howitzers, the different zones of fire were sometimes shown.
While the use of the range quadrant separated the duties of the gunners in aiming, it did not comply with the conditions for the independent line of sight. The sight and range quadrant being attached to the cradle, both moved in elevation with the gun. The independent line of sight permitted to move and set in elevation a gun without any change in position of the sight used for direction aiming.