Note.—In practice, it is well to fire two or three shot to determine the range experimentally, as it is affected by divers causes.
15. Should the line of metal be always directed in the vertical plane passing through the object?
No; as in practice there are circumstances (as, for instance, a strong wind blowing across the field of fire) which will cause a ball to deviate from this plane, it follows that to strike the object, in such a case, the line of metal must be directed to its right or left; the gunner judging of the distance by observing the striking of the shot.
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16. Is the line of metal a permanent line under all circumstances?
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.