PRINCIPLES OP GUNNERY. 
31 
distance to fall through in the descending* part of the trajectory, there 
becomes, owing to the resistance of the air (which increases very 
rapidly as the projectile is acquiring velocity under the action of 
gravity), a limit to the descending vertical velocity which it acquires; 
so that the descending vertical velocity ceases to increase, and becomes 
practically uniform. 
This limiting vertical velocity is acquired when the resistance of the 
air to the descending projectile in a vertical direction has become equal 
to the weight of the projectile; and it is called the terminal velocity Terminal 
— i.e. } the maximum descending vertical velocity which a projectile 0 f velocifcy * 
given weight could acquire on being let fall from any great height. 
The heavier the projectile, the greater the height it must fall through 
to attain to its terminal velocity. 
The calibre of a rifled gun is the diameter of the bore generally calibre, 
measured across the lands. 
In the Armstrong breech-loading system the projectile is made to fit windage, 
tightly into the grooves, ensuring perfect centring; but in all muzzle¬ 
loading systems there must be sufficient play between the dimensions of 
the projectile and those of the bore of the gun to allow of easy loading. 
This play—or windage as it is called—is the difference between 
the sectional area of the gun through its grooves and that of the body 
of the projectile through its studs : and is measured practically by the 
actual linear difference between, the calibre of the gun and the diameter 
of the body of the projectile,* which is called ivindage over body ;fi also 
by the linear difference between the diameter through the grooves of 
the gun and the diameter through the studs of the projectile, which is 
called windage over studs; and also by the linear difference between the 
width of the groove in the gun and the width of the stud on the pro¬ 
jectile, which is, called the play of the stud. 
Clearance is the linear distance between the body of the pro- Clearance, 
jectile and the bore of the gun. The amount of clearance before the 
projectile moves is the difference between the depth of the groove 
in the gun and the projection of the stud on the projectile ; but when 
the projectile is in motion and centred in the bore, the clearance all 
round the projectile will be half the windage over body. 
It is very important to reduce the windage as much as possible con¬ 
sistently with ease and rapidity of loading, in order to obtain accuracy 
of fire j but, on the other hand, it is most important that the projectile 
should not jam in loading. Sufficient windagej should therefore be 
given to render a jam improbable under ordinary conditions of service. 
All projectiles are gauged before issue into the service, by passing 
over them hollow cylinder gauges smaller than the calibre of the gun ; 
and all guns are gauged by a solid, cylindrical, studded plug, larger 
than the projectile, so as to ensure sufficient limits for easy loading. 
By this system of gauging, all manufacturing errors liable to cause a 
jam are detected and rectified. 
* See “Treatise on Ammunition,” p. 140. 
f With projectiles havirg a copper cup attached to their base, to give the necessary rotation 
without studs, the Avindage is simply measured by the linear difference between the calibre of the 
gun and the diameter of the projectile. 
J See “ Treatise on Ammunition,” p. 210. 
