Definitions

—The axis of the piece is the centre line of the bore. The line of fire is the axis of the piece prolonged. The plane of fire is a vertical plane through the line of fire. The line of sight is the right line from the eye to the object to be hit, pa ing through the front and rear sights. The plane of sight is a vertical plane through the line of sight. 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 pa es through the zero of the scale, which in the pendulum hau e 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.