PRINCIPLES OF GUNNERY. 
29 
ground plane are front, oblique, enfilade, and reverse fire. These terms 
are best explained by means of a diagram. 
B 
ABN represents the plan of work fired at, B the position of attacking 
batteries. Then BF, perpendicular to the face, BN, of the work, repre¬ 
sents the general direction of front fire; BO, oblique to BN, represents Front fire, 
the general direction of oblique fire; BF, parallel to the face BN, J? r b e lique 
represents the general direction of enfilade fire; and BR, perpendicular Enfilade 
to the reverse slope of BN, represents the general direction of reverse Reverse 
fire. flre * 
A combination of these terms allows of an accurate description of 
the. precise nature of artillery fire employed; e.g., direct front fire 
denotes that the fire is direct in the plane of fire, and front in plan, or 
in the ground plane. 
The deflection of a projectile is its deviation right or left from Deflection, 
the plane of fire at any point of its trajectory, and is measured at 
any given range by the perpendicular horizontal distance from that 
plane. 
The muzzle velocity of a projectile is its velocity at the muzzle of Muzzle 
the gun. . . . Vel ° City - 
Muzzle velocity is generally measured in feet per second (abbreviated 
thus—f.s”) 
The resistance of the air to a projectile in motion, is the force Resistance 
which tends to stop the projectile, and so continually diminishes its 
velocity. It is generally measured in lbs. avoirdupois. 
The remaining velocity of a projectile is its velocity at any given point Remaining 
of its trajectory. The remaining velocity of a projectile must in all ve 001 y * 
cases be less than the muzzle velocity, owing to the resistance of the 
air. 
Retardation is the rate at which the projectile loses velocity, owing Retarda- 
to the resistance of the air, tl0n ’ 
