Laying by tangent elevation necessitates mechanical devices for pointing the sights used for laying for line direct at the target, swinging the mounting as a whole in the horizontal plane. If the target is visible from the gun, this can be done by direct laying through a telescope mounted with its axis parallel to that of the gun. If the target is not visible from the gun, it will be necessary to get the gun in the line of fire by turning it through a calculated angle from the line to some object visible from the gun. A tele scope movable about a vertical axis on which is mounted a hori zontal circular disc graduated in degrees solves the fundamental part of the problem and is the basis of the panoramic or dial sight, described in the next section (fig. 6). (ii.) Corrections for wind, travel of target, etc. Having got the gun in the line of fire thus giving the correct angle of sight, and for elevating the gun through a further angle equal to the desired tangent elevation.
An arrangement of this nature permits of alterations in range without throwing the sights off the target. For this reason it was introduced into the British service for light field artillery, where one of the essentials is rapidity of fire—one layer can lay on the target continuously while the other puts on correction to tangent elevation. It should be noted that with the independent line of
sight the axis of the sight bracket is not parallel to the axis of the of correcting for tilt of mounting need be incorporated. (2) Econ omy of weight and space is not so important as in mobile mount ings. (3) Each gun being at a constant (except for rise and fall of tide) height above sea level, the angle of sight at once deter mines the range (see description of Depression Range Finders in article RANGE-FINDERS). (4) Direct laying for line can almost always be used.
Laying for line being direct a telescope mounted as in fig. 3 solves the problem, and also permits of the application of deflec tion for wind, travel of target, etc. Laying for elevation is usually by quadrant elevation and a geared elevation indicator graduated in yards. This constitutes the normal method of conducting fire with the medium and heavy natures. An alternative method is that by which the gun is given quadrant elevation by elevation indicator and is directed for line by giving it a bearing from true north. This method of control can be conducted from a distant gun (since the sights are mounted on the intermediate carriage, not on the cradle) so that corrections for drift cannot be made by tilt of sight, nor can want of level of mounting be corrected for by cross-levelling the sight. In guns fitted with the independent line of sight mounting, drift is usually corrected for by tilting the trun nion axis. The foregoing are the general principles and methods adopted in the design of sights. Sights for Mobile Artillery.—The almost universal practice is to provide a dial or panoramic sight for laying for line; the principle of the arrangement can be seen from fig. 6.
With light artillery a common practice in laying for eleva tion is to use a mounting with the independent line of sight, drift being corrected for by tilt of trunnions. In the latest British equipments the two gears on the carriage are not entirely inde pendent and the mounting is known as the "semi-independent line of sight" mounting. Cross levelling gear is provided and a means of correcting for drift is incorporated in the range gear. With medium and heavy mobile artillery oscillating sights of the rocking bar type are used. The arrangement of such a sight is shown diagrammatically in fig. 7.