used for indicating the
attitude of a body relative to the horizontal.

Instability--An inherent tendency of a body, which, if the body is
disturbed, causes it to move into a position as far as possible away
from its first position.

Instability, Neutral--An inherent tendency of a body to remain in the
position given it by the force of a disturbance, with no tendency to
move farther or to return to its first position.

Inertia--The inherent resistance to displacement of a body as distinct
from resistance the result of an external force.

Joy-Stick--See "Control Lever."

Keel-Surface--Everything to be seen when viewing an aeroplane from the
side of it.

King-Post--A bracing strut; in an aeroplane, usually passing through a
surface and attached to the main spar, and from the end or ends of which
wires are taken to spar, surface, or other part of the construction in
order to prevent distortion. When used in connection with a controlling
surface, it usually performs the additional function of a lever, control
cables connecting its ends with the pilot's control lever.

Lift--The vertical component of the reaction produced by the action
of driving through the air a surface inclined upwards and towards its
direction of motion.

Lift, Margin of--The height an aeroplane can gain in a given time and
starting from a given altitude.

Lift-Drift Ratio--The proportion of lift to drift.

Loading--The weight carried by an aerofoil. Usually expressed in pounds
per square foot of superficial area.

Longeron--The term usually applied to any long spar running length-ways
of a fuselage.

Mass--The mass of a body is a measure of the quantity of material in it.

Momentum--The product of the mass and velocity of a body is known as
"momentum."

Monoplane--An aeroplane of which the main lifting surface consists of
one surface or one pair of wings.

Multiplane--An aeroplane of which the main lifting surface consists of
numerous surfaces or pairs of wings mounted one above the other.

Montant--Fuselage strut.

Nacelle--That part of an aeroplane containing the engine and pilot and
passenger, and to which the tail plane is not fixed.

Neutral Lift Line--A line taken through a surface in a forward direction
relative to its direction of motion, and starting from its trailing
edge. If the attitude of the surface is such as to make the said line
coincident with the direction of motion, it results in no lift, the
reaction then co