370 
MINUTES OF PKOCEEDINGS OF 
powder-gas ; but in rifled guns, the shell can only move forward by at the 
same time following the right-handed twist of the rifled grooves: the 
common shell has thus imparted to it a motion of translation combined 
with one of rotation around its longer axis corresponding to the helical 
twist of the grooves. As long as the projectile is in the bore the motion of 
translation is in the direction of the axis of the piece and the velocities of 
the motions of translation and rotation increase in virtue of the continuous 
pressure of the powder-gas. At the instant the projectile leaves the bore 
it has attained a certain so-called initial velocity in tlie direction of the 
motion of translation, and at the same time a certain velocity of rotation 
round the longer axis. Outside the bore, gravity and the resistance of 
the air act on the projectile; hence in flying through the air it traces a 
curved concave to the ground at the same time diverging to the right, with 
reference to the axis of the piece; it is therefore a curve of double curvature. 
The common shell finally reaches the ground and bursts by the explosion 
of the bursting charge, whereupon the splinters of the shell proceed on in 
diverging paths, more or less curved. If the fuze or the bursting charge 
fail in effect, that is, if the shell is blind, the projectile may either lie where 
it falls, or proceed on its path in several bounds or ricochets generally very 
irregular in height and amplitude, until the velocity of translation becomes 
nil. The curve traced by the projectile outside the bore is termed the 
trajectory ; the angle between the axis of the piece and the horizon is 
termed the angle of elevation, or of depression, according as its axis is directed 
above or below the horizon; further, the angle between the longer axis of 
the projectile just emerged from the bore and the horizon is termed the 
angle of departure . The horizontal distance from the muzzle of the gun 
to the intersection of the trajectory with a horizontal line through the 
centre of the muzzle is called the horizontal range ; the angle between the 
tangent to the trajectory and the ground where the projectile falls is the 
angle of incidence ; the angle formed by the line joining the point aimed at 
to the centre of the muzzle and the horizontal line through the former point 
is called the ground angle; finally, the deflection to the right from the 
vertical plane through the axis of the piece is termed the drift; this 
deflection with rifled guns has a certain amount of regularity. 
The horizontal range, firing at low-angles, is dependent on the initial 
velocity of the projectile, on the action of gravity, on the resistance of the 
air, and on the angle of elevation of the piece. All other circumstances 
being alike, the horizontal range whether firing at low or high angles 
increases with the angle of elevation up to a certain limit; practically it is 
by the increase or decrease of the angle of elevation that the range is 
regulated to the distance of the object to be fired at. 
The drift, or deflection to the right of the vertical plane of the trajectories 
of projectiles fired from guns rifled with a right-hand twist, depends on the 
range and on the velocity of the motion of rotation; it increases with a 
certain amount of regularity with the increase of both those factors. 
The velocity of the motion of rotation depends on the initial velocity, 
which again is determined by the charge of powder, the weight of the 
projectile, and on the pitch of the rifling. The following table gives the 
initial velocity, the velocity of rotation, the drift and time of flight of 
common shells with low-angle charges : — 
