The distance from the muzzle to the first graze when the axis of the piece is parallel to the plane upon which the carriage stands.
6. Explain by a figure, the position of, and relations existing between the line of sight, the line of fire or axis of the piece, and the trajectory, and also what the point-blank range is.
64]
Fig. 1
ABcF, the line passing through the highest points of the base ring and swell of the muzzle, or the muzzle-band, is called, the natural line of sight. EPcG, is the axis of the piece or line of fire; the curved line, PgD, described by the projectile, is called the trajectory, and is entirely below the line of fire, in consequence of the action of the force of gravity giving the projectile a downward tendency. The point D is called the point-blank, and its distance from the mouth of the piece, the point-blank range.
7. Mention some of the causes which vary the point-blank range.
The form of the cannon; the weight or force of the charge; the diameter and weight of the projectile; and the inclination of the line of sight to the horizon.
8. Why has the form of the cannon an influence on the point-blank range?
Because as the difference between the diameter of the breech and muzzle become greater, the angle of sight, BcP = GcF ([see fig].) increases, and the point-blank D is removed farther off; on the contrary, as the diameters approach to an equality, the point-blank approaches the piece. Within a certain angle, or when there is no angle of sight, as is the case with some old howitzers in which the line of sight is parallel to the axis of the bore, there will be no point-blank, as the trajectory will be constantly below the line of sight.