No; in batteries for garrison and sea-coast defense, where the platforms are fixed, the line of metal may be considered as nearly permanent; but with siege guns, which are mounted on traveling carriages, the wheels of which are liable to vary in position from unevenness of ground, or unequal settling in newly constructed platforms, this line is constantly changing. It approximates the higher wheel in proportion to the difference of level between the wheels; and hence, to secure the greatest accuracy of fire, it must be frequently verified; the old marks, if not found correct, should be erased and new ones substituted. 17. When the notches or sights, which are sometimes made upon the base-ring and swell of the muzzle in field guns, for aiming the piece are used, how is the error of direction remedied when the wheels are not on the same level? The piece must be aimed more or less to that side which corresponds to the higher wheel, according to the inclination. 18. When the elevation or depression has once been ascertained for any given distance, how may the firing at that distance be facilitated? By noting some point on the elevating screw or quoin; adjusting some fixed measurement from a point on the stock to another point on the under side of the breech; or by a chalk mark drawn across the face of a trunnion and its corresponding cheek. [Sidenote: 55]] 19. When firing either within or beyond point-blank range, may remarkable points on the ground be taken advantage of, in order to furnish an object to aim at? Yes; some fixed object may often present itself which will serve as a point upon which to direct the line of metal. No means should be neglected that may tend to secure accuracy of aim; for the shot that is thrown away by carelessness in pointing, had better not be thrown at all. 20. How may precision of fire be secured at night? When a fixed object is to be fired at by night, the piece should be directed during the day, and two narrow and well-dressed strips of wood laid on the inside of the wheels, and two others outside of the trail of a siege carriage, and nailed or screwed to the platform. In case of a barbette carriage, the traverse-wheels should be chocked in the proper position. To preserve the elevation, measure the height of the elevating screw above its box, or take the measure between a point on the gun, and another on the stock; cut a stick to this length and adjust the gun on it at each fire. 21. Should night-firing with _guns_ be limited? Yes; it should be limited to a small number of rounds, as it consumes ammunition to little advantage. [Sidenote: 56]] PART II. SECTION II. POINTING MORTARS. 1. What is the rule for pointing mortars? First give the elevation, and then the direction. 2. How is the elevation given? By applying the quadrant to the face of the piece, and adjusting the quoin until the required number of degrees is indicated.