The angle of sight, or the elevation, is the vertical angle included between the line of sight and the plane containing the axis of the piece and a horizontal line at right angles to it.
The natural line of sight is the line of sight nearest to the axis of the piece. In guns without rear sights it is the right line through the highest point of the base-ring and swell of the muzzle or top of the front sight when there is one. It is sometimes called the [line of metal], as in mortars. For convenience and accuracy the natural line of sight is usually parallel to the axis of the piece. When special breech-sights are used, it passes through the zero of the scale, which in the [pendulum hausse] and other vibrating scales coincides with the axis of vibration. All other lines of sight are called [artificial lines of sight].
Point-blank, in small-arms, is the second point in which the natural line of sight (when horizontal) cuts the trajectory. In artillery, it is the point where the projectile first strikes the horizontal plane on which the gun stands, the axis of the piece being horizontal.
Pointing Guns and Howitzers.—In pointing old model guns and howitzers under ordinary angles of elevation, the piece is first directed toward the object, and then elevated to suit the distance. The accuracy of the aim depends: (1) On the fact that the object is situated in the plane of sight; (2) That the projectile moves in the plane of fire, and that the planes of sight and fire coincide, or are parallel and near to each other; and (3) On the accuracy of the elevation. The first of these conditions depends on the eye of the gunner, and the accuracy and delicacy of the sights; the errors under this head are of but little practical importance. When the trunnions of the piece are horizontal, and the sights are properly placed on the surface of the piece, the planes of sight and fire will coincide; but when the axis of the trunnions is inclined, and the natural line of sight is oblique to the axis of the bore, the planes are neither parallel nor coincident, but will intersect at a short distance from the muzzle, and the aim will be incorrect. If the natural line of sight be made parallel to the line of fire, by making the height of the front sight equal to the dispart of the piece, the planes of sight and fire will be parallel. Field-guns of the present day have special breech-sights or pendulum-sights. The zero of the scale and top of front sight are in a line parallel to the axis of the piece, and in pendulum-sights this zero coincides with the pivot at which the scale vibrates. Siege and sea-coast cannon are generally fired from fixed platforms, which renders the axis of the trunnions horizontal; they are, therefore, not furnished with pendulum-sights, but usually with breech-sights set in sockets at the breech. In the absence of a breech-sight the piece can be pointed with a natural line of sight so as to strike objects not situated at point-blank distance. Owing to the shape and size of the reinforce of sea-coast cannon, the natural line of sight is formed by affixing a front sight to the muzzle, or to a projection cast on the piece between the trunnions. Although the latter arrangement does not give quite so long a distance between the sights as is desirable, it permits the use of a shorter breech-sight, and the front sight does not interfere with the roof of the embrasure, when the piece is fired under high elevation.
Errors in Pointing.—When the platform or ground upon which the gun stands is not level there is an error in pointing (except when compensating sights such as the [pendulum hausse] are used), which varies in direction with the circumstances of the pointing and in amount with the elevation of the piece.
If the [natural line of sight] is pointed upon the object and the elevation then given by a gunner’s quadrant or other device, the shot will go to the side of the lower wheel. If the gun be depressed, it will go to the side of the upper wheel.
If the [tangent scale] or old pattern [breech-sight] is used with the ordinary fixed muzzle-sight, and it be placed on a chalk-mark just determined, the shot will fall on the side of the upper wheel. If the scale is placed on the permanent mark, the contrary will hold.
If a [socket] or fixed [breech-sight] is used, the shot will fall on the lower or upper side according as the gun is elevated or depressed.
Pointing Mortars.—In pointing mortars, the piece is first given the elevation, and then the direction necessary to attain the object. Mortars are generally fired from behind epaulements, which screen the object from the eye of the gunner. The elevation is first given by a gunner’s quadrant, and the direction is given by moving the mortar-bed with a handspike, so as to bring the line of metal into the plane of sight, which passes through the object and the centre of the platform. The plane of sight may be determined in several ways; the method prescribed is to plant two stakes, one on the crest of the epaulement, and the other a little in advance of the first, so that the two shall be in a line with the object, and the gunner standing in the middle of the rear edge of the platform; a cord is attached to the second stake and held so as to touch the first stake; a third stake is driven in a line with the cord, in rear of the platform, and a plummet is attached to this cord so as to fall a little in rear of the mortar. The cord and plummet determine the required plane of sight into which the line of metal of the mortar must be brought. With the 13-inch mortar mounted upon centre-pintle chassis, the plane of sight must be so determined as to pass through the pintle to obtain perfect accuracy. One of the best methods of pointing mortars so mounted, is to place on the crest of the parapet in line with the axis of the platform a goniometer, the alidade of which can be directed upon the object,—the angle is read from the vertical plane containing the axis of the platform. The traverse circle is similarly graduated from the axis of the platform. A pointer attached to the chassis enables the gunner to lay the mortar very nearly in the vertical plane passing through the object,—the error being the perpendicular distance from the pintle to the plane of sight. This is the method of Lieut. A. B. Dyer, 4th U. S. Artillery. Gen. Abbot of the U. S. Engineer Corps used a similar principle during the late war, 1861-65. The usual angle of fire of mortars is 45°, which corresponds nearly with the maximum range. The advantages of the angle of greatest range are: (1) Economy of powder; (2) Diminished recoil, and strain on the piece, bed, and platform; (3) More uniform ranges. When the distance is not great, and the object is to penetrate the roofs of magazines, buildings, etc., the force of fall may be increased by firing under an angle of 60°. The ranges obtained under an angle of 60° are about one-tenth less than those obtained with an angle of 45°. If the object be to produce effect by the bursting of the projectile, the penetration should be diminished by firing under an angle of 30°. When the object is not on a level with the piece, the angle of greatest range is considered in practice to be 45° increased or diminished by one-half the angle of elevation or depression of the object. The angle of fire being fixed at 45° for objects on the same level with the piece, the range is varied by varying the charge of powder. Stone-mortars are pointed in the same manner as common mortars; the angle of fire for stones is from 60° to 72°, in order that they may have great force in falling; the angle for grenades is about 33°, in order that their bursting effect may not be destroyed by their penetration into the earth.
Night-firing.—Cannon are pointed at night by means of certain marks, or measurements, on the carriage and platform, which are accurately determined during the day.