PRINCIPLES OP GUNNERY. 
27 
The intensity of effect is much greater, then, with rifled than with 
smooth-bore ordnance, and any required result can be attained with a 
much less expenditure of time and materiel. 
Accuracy office, combined with effect of shell, are the great objects to 
be attained by rifled ordnance. 
Accuracy of fire increases {cceteris paribus) with the velocity. The 
effect of shell increases with its weight, and its capacity for holding a 
larger bursting charge or a greater number of bullets. 
In order to secure the greatest accuracy of fire, it is necessary Projectile 
that the axis of the projectile should coincide at the muzzle with the centreda 
axis of the gun from which it is fired. The projectile is then said to 
be centred. 
The line of fire is the direction of motion of the projectile at thereof 
muzzle of the gun. This primary direction of motion is changed, 1 ’ 
during the flight of the projectile in the air, by various forces—viz., 
the force of gravity, wind, resistance of the air, &c.—which shape the 
path of the projectile in the air. This path is a curved line, and is Trajectory, 
called the trajectory. 
The line of fire is, then, a tangent to the trajectory at the muzzle. 
The plane of fire is the vertical plane passing through the line The plane 
of fire. _ offlre - 
The line of sight is the straight line passing through the notch Line of 
of the tangent sight, the top of the fore sight, and the object aimed at, “ ght * 
The angle of elevation of a gun is the angle which the axis of the Angie of 
gun makes with the line of sight,* and is measured by the number elevatlon * 
of degrees and minutes at which the tangent scale is set. 
The angle of departure is the angle which the line of fire makes Angie of 
with the line of sight. departure. 
The difference between the angle of departure and the angle of jump, 
elevation is sometimes called the jump. It varies with different guns, 
and with different charges, &c., in the same gun.f 
The angle of fire is the angle which the line of fire makes with the Angie of 
horizontal plane. fire ‘ 
The angle of descent is the angle which a tangent to the tra- Angie of 
jectory at the point of impact makes with the horizontal plane. 
descent. 
Suppose G to represent the muzzle of the gun, 0 the object aimed at 
* More accurately with the projection of the line of sight on a vertical plane through the axis 
of the. gun, on account of the inclination of the tangent scale to the left, to allow for derivation. 
(See p. 29.) 
f See "Proceedings, R.A. Institution, 1 ” Vol, VIII. p. 261, 
