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Glossary of terms & abbreviations

Exterior Ballistics.-The theory of the motion of the projectile from the
gun to the target.

Interior Ballistics.-The theory of the motion of the projectile in the gun.

Gunnery.-The practical application of the theory of ballistics.

Axis of the Gun.-The straight line passing down the centre of the bore ;
the longitudinal axis.

Axis of the Trunnions.-The straight line passing through the centre of the
trunnions, at right angles to the axis of the gun ; the transverse axis
about which the gun is rotated when elevating. In machine guns the trunnions
are generally part of the mounting.

Calibre.-The nominal diameter of the bore in inches, measured across the
lands. Trajectory.-The curve described by the centre of gravity of the projectile
in flight.

Muzzle Velocity.-The velocity of the projectile at the muzzle of the gun
; it is sometimes called initial velocity.

Remaining Velocity.-The velocity of the projectile at any point of the trajectory.
Striking Velocity. The velocity of the projectile at the point of impact.

Terminal Velocity.-When the retardation due to air resistance is exactly
equal to the acceleration of gravity there is no force acting on the projectile.
The velocity is then constant, and is called " the terminal velocity."
If C is known, it can be found from the ballistic tables by noting the velocity
where the value of p is equal to the ballistic coefficient C = W/nd2.

Line of Sight.-The straight line passing through the sights and the target.
(The line G.T., Fig. 1.)

Vertical Plane of Sight.-The vertical plane containing the line of sight.

Lateral Plane of Sight.-The plane passing through the line of sight at right
angles to the vertical plane of sight.

Line r f Departure.-The straight line representing the direction of motion
of the projectile at the moment of leaving the gun. It is the tangent to
the trajectory at the muzzle. It coincides with the axis of the gun when
there is no jump, and when the gun recoils axially on a stationary mounting.

Plane of Departure.-The vertical plane containing the line of departure.
It is sometimes called " vertical plane of fire."

Jump.-The vertical angle between the axis of the gun before firing and the
line of departure. '"'hen the line of departure is above the axis of
the gun the jump is said to be positive ; when below it is negative. (The
angle J, Fig. 1.)

Elevation.-An angle measured in a vertical plane. It is positive when measured
upwards from the line of reference, and negative when measured downwards
; it is then sometimes called " depression."

367

Angle of Sight -The " elevation " of the line of sight with reference
to the horizontal plane. It is measured in the vertical plane of sight,
and is positive when the line of sight is directed above the horizontal
plane. (The angle S, Fig. 1.) When a negative angle of sight is given to
a gun it is described as " . . . degrees . . . minutes depression."

Angle of Departure (of the Projectile).-The " elevation " of the
line of departure, with reference to the horizontal plane. (The angle D,
Fig. 1; see note 1).

Angle of Projection (of the Proiectile).-The " elevation " of
the line of departure with reference to the lateral plane of sight. When
the angle of sight is zero, it is the same as the angle of departure. (The
angle P, Fig. 1; see note 1.)

Quadrant Elevation (of the Gun).-The " elevation " of the axis
of the gun immediately before firing with reference to the horizontal plane.
It differs from the angle of departure by the angle of jump only. (The angle
Q, Fig. 1; see note 1.)

Tangent Elevation (of the Gun).-The " elevation " of the axis
of the gun immediately before firing, with reference to the lateral plane
of sight. It differs from the angle of projection by the angle of jump only.
(The angle T, Fig. 1; see note 1.)

Rise.-The mean alteration (in minutes) of elevation to give 100 yards alteration
in range at any given distance.

Deflection.-A correction to direction. It is consequently an angle measured
at right angles to the vertical plane of sight, either in the lateral plane
of sight or in the horizontal plane (see note 2). It is applied in compensating
for :

(1) Lateral travel of gun or target during the time of flight. ' (2) Deviation
of the projectile due to cross wind.

(3) Drift, unless automatically allowed for by the sighting.

(4) Any other cause affecting direction. -

Point of Departure-The axis of the rifle barrel at the muzzle at the moment
when the projectile leaves it. (The point G, Fig. 1.)

Point of Arrival.-The second point of intersection of the trajectory with
the lateral plane of sight. (The point T, Fig. 1.)

Point of Graze.-The point of intersection of the trajectory with the horizontal
plane through the gun. (The point P, Fig. 1).

Drift.-The deviation of the projectile from the plane of departure due to
rotation. Vertex.-The highest point of the trajectory ; it is also called
" the culminating point."

. Angle of Descent.-The angle measured in a vertical plane, which a tangent
to the trajectory at the point of arrival makes with the lateral plane of
sight. (The angle P, Fig. 1; see note 3.)

Angle of Arrival.-The angle measured in a vertical plane, which a tangent
to the trajectory at the point of arrival makes with the horizontal plane.
(The angle a"), Fig. 1; see note 3).

Angle of Incidence.-The angle which the tangent to the trajectory at the
point of arrival makes with the normal to the surface struck. (The angle
i, Fig. 1).

Angle of Impact.-The angle which the tangent to the trajectory at the point
of arrival makes with the surface struck ; it is the complement of the angle
of incidence.

Vertical Fire.-Fire directed vertically upwards or downwards, that is, at
an angle of departure of 90°.

Mean Point of Impact.-A point which represents the mean position of the
points of II impact of a large number of rounds fired under the same conditions.

(28/456)z 2 A 3

368

Danger Space.-The horizontal distance, measured towards the gun, in which
a target would be hit by a given trajectory.

Time of Flight.-The time a projectile takes to reach the point of arrival,
reckoned from the moment it leaves the muzzle.

Range.-A distance measured along the line of sight.

Target Range.-The " range " to the target ; the distance between
gun and target measured along the line of sight.

Gun Range.-The " range " to the point of arrival ; the distance
between the gun and the point of arrival. (The length G T, Fig. 1.) It is
one of the functions of gunnery to make the gun range coincide with the
target range.

Notes. (1) The angles of departure and projection refer to the motion of
the projectile, not to the gun ; they may therefore be called " ballistic
" angles, and they depend upon ballistic theory. The angles of quadrant
elevation and tangent elevation refer to the gun, not to the projectile
; they may therefore be called " mechanical " angles. They are
fundamental to the method of laying the gun, the object being to ensure
that the projectile leaves the gun at the correct ballistic angle.

(2) Lateral deflection measured in a horizontal plane is sometimes called
" lateral deflection in azimuth." The azimuth of a point is the
bearing measured from true north. A difference in azimuth is therefore merely
a difference in bearing, and represents a lateral deflection measured in
the horizontal plane.

(3) At the point of graze the angle of descent is equal to the angle of
arrival.

The definitions above are from The Text Book of Ballistics and Gunnery,
prepared in the Military College of Science.

PHYSICAL DEFINITIONS

Fundamental Units.-The three indefinables in nature are space, time and
matter.

The British unit of length (space) generally employed in scientific work
is the foot, the third part of the Standard yard defined by Act of Parliament,
18 and 19 Vict., 1855, the length between the lines on two gold plugs at
62° F., on a bronze bar kept by the Warden of the Standards.

The unit of weight (matter) is similarly defined as the pound, the weight
of a piece of platinum marked " P.S. 1844 1 lb."

The unit of time is by universal consent the mean solar sexagesimal second.
All the astronomical observatories in the world agree as to its length.

Velocity (V) if constant is feet described in one second written as f/s.
; if variable it is measured at any instant by the feet which would have
been described in one second if the velocity had remained constant.

Acceleration (a) if constant is the number of f/s. added in one second written
as f/s.z ; retardation (- a) is the number of f/s. lost in one second. It
is negative acceleration.

Acceleration or retardation if variable is measured in the same way as variable
velocity.

The acceleration of gravity is the constant acceleration with which we are
most familiar. Denoting it as usual by g(= 32'2 f/s.2), then the velocity
of a body falling freely will grow g units of velocity (f/s.) per second.
In metric units g= 9•81 metres per second per second.

Force is that which produces change of motion either in amount or direction
in a body. As the field of force in which we live is that due to the attraction
of the earth, it is con

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