and tail planes are fitted and which contains the
engine and aviators seat.

Center of Pressure--Really a line of pressure along the under side of a
wing or aeroplane surface, on either side of which the pressures are
equal.

Center of Gravity--The center of weight, about which the vehicle
balances in all directions.

Chord--A straight line drawn between the ends of the arc of a circle or
other curve.

Dirigible (dir'-igihle)--Steerable or navigable; applied to balloons.

Derrick--A tower in which a falling weight is dropped in starting an
aeroplane.

Diagonal--A diagonal brace or stay in a framework.

Dihedral (di-he'dral)--Said of wing pairs inclined at an upward angle to
each other.

Elevator--A principal supplementary surface, usually of a miniature form
of the main planes. Used for purpose of altering the vertical direction
of machine.

Gap--The distance between two main planes in a biplane.

Gliding--Flying down a slant of air without power.

Gyroscopic Effect--The property of any rotating mass whereby it tends to
maintain its plane of rotation against disturbing forces.

Gauchissement (or warping)--Applied to the main planes and produces the
same ultimate effect as the use of ailerons.

Hangar (hang'ar)--A shed for housing balloons or aeroplanes, generally
the latter.

Horsepower--A rate of work equivalent to the lifting of 33,000 ft.-lb. a
minute.

Head Resistance--The resistance of a surface to movement through the
air; closely proportionate to its projected area.

Heavier-than-air--Applied to dynamic flying machines weighing more than
the air they displace.

Horizontal Rudder--A horizontally placed rudder for steering in vertical
directions.

Lift--The sustaining effect, expressed in units of weight of an
aeroplane or wing surface.

Monoplane--An aeroplane with one or more main surfaces in the same
horizontal plane.

Main Plane--Usually the largest or lowest supporting surface of a
multi-surfaced aeroplane.

Mast--A spar or strut used for the attachment of wire or other stays to
stiffen the wings or other parts of a structure.

Main Spars--Lateral spars upon which the main planes are built.

Main Landing Wheels--In an alighting gear, the wheels that take the
chief shock in landing.

Ornithopter--A dynamic flying machine of the heavier-than-air type, in
which sustension is provided by the effect of reciprocating wing
surfaces.

Pylon--A tower to mark the course in aerial rac