Sights

SIGHTS. It is a matter of common experience that, owing to the curvature of trajectories, it is necessary in order to propel a missile to any considerable distance, to throw it upward as well as forward, or, to use the technical term, to give it elevation. It is also well known that the trajectories of artillery projectiles fired from rifled guns have in addition a slight curvature in the hori zontal plane known as drift. These considerations are discussed quantitatively in the article BALLISTICS and it is there shown how to calculate the elevation and drift correction necessary in any particular case. The process of pointing the gun in the proper direction and giving it the proper elevation is known as laying the gun. Sights are the mechanical devices employed to facilitate ac curate laying. Before going on to any detailed discussion of sights it will be convenient here to give definitions of the various tech nical terms employed in sighting:— Line of Fire—The horizontal direction of the target from the gun.

Line of Sight—The line joining gun and target.

Angle of Sight—The angle between the line of sight and the horizontal plane through the gun.

Axis of the Sight—The line joining the hind sight to the fore sight. In the case of telescopic sights, the sight axis is the optical axis of the telescope.

Elevation—The angle the axis of the gun is elevated above some line of reference ; in particular, quadrant elevation is the angle between the axis of the gun and the horizontal plane, while tangent elevation is the angle between the axis of the gun and the line of sight.

The process of laying is best considered under two distinct headings (a) laying for line, or direction (b) laying for elevation.

In the earliest days of smooth-bore cannon and short ranges, lay ing for line was achieved by looking along the top of the gun and laying for elevation was neglected, the range being so short that it was sufficiently accurate to lay the gun along the line of sight (known as point-blank fire). The earliest pattern of sight used in laying for elevation was the tangent scale and foresight, and this is still the arrangement used for the sighting of rifles and machine guns.

The principle can easily be seen from fig. 2. The clamp is loosened and the tangent bar raised by an amount varying with the desired range. To bring the notch on the tangent scale, the foresight and the target into line it is necessary to elevate the gun and the amount of elevation will depend on the amount the tan gent scale has been raised. Thus the graduations on the tangent scale can be marked with the number of yards corresponding to the range realized by that particular elevation. It will be seen that the fundamental principle is that of establishing an angle between the axis of the sight and the axis of the gun, then bringing the axis of the sight to point at the target thus elevating the gun above the line of sight. Since in this case the line of sight is taken as

the line of reference, laying is by tangent elevation.

Practically all modern sights are based on this principle, the chief developments being the use of a telescope mounted on a rocking bar (fig. 3) in lieu of the notch and foresight and the mounting of the sight as a whole on some non-recoiling part of the carriage instead of on the gun itself. This latter development, rendered possible by the introduction of mountings in which the gun recoils axially in a cradle, has led to increased rates of fire and also to the use of accurate optical and mechanical systems which could not have withstood the stresses experienced by any form of sight mounted on the gun itself.

Laying for Line.

A brief consideration of the factors affect ing laying for line will indicate the requirements to be fulfilled by any satisfactory sight. (i.) Obtaining the line of fire. This consists in pointing the gun in the direction of the target by it will be necessary to apply minor corrections, called deflec tions, to compensate for the above effects. This can be done by mounting the telescope so that it can be moved in a horizontal plane right or left of the line of fire. A typical arrangement is that shown in the rocking bar sight (fig. 3). (iii.) Correction for drift. The deviation due to drift (see BALLISTICS) necessi tates a correction varying with the range, and since it can be cal culated beforehand it should, if possible, be set on the sight auto matically. This can be done by means of a cam, or better, as follows : It can be shown that the angu lar deviation due to drift is, in all cases, approximately proportion al to the tangent of the angle of elevation. On this basis a correc tion can be applied with sufficient accuracy by tilting the sight and applying the elevation at a small angle to the vertical (fig. 4). (iv.) Correction for tilt of trunnion axis or want of level of mounting. If the axis about which the gun is elevated is not truly horizontal the act of elevating will also swing the gun to the right or left, necessitating in consequence a correction for line. This error can be corrected by giving a deflec tion correction the amount of which must be calculated from the measured value of the tilt, but in most modern equipments the correction is made automatically by means of what is known as the oscillating bracket. Briefly the principle is as follows :—The sight ing arrangements are mounted on a bracket which can be rotated ("cross-levelling") about an axis parallel to the axis of the gun— before laying for line the whole of the sighting arrangement is corrected for tilt by a cross-levelling screw and spirit bubble.