rd, varies with
differences of camber, and it is found by means of wind-tunnel research.

=Newton's Laws of Motion=--1. If a body be at rest, it will remain at
rest; or, if in motion, it will move uniformly in a straight line until
acted upon by some force.

2. The rate of change of the quantity of motion (momentum) is
proportional to the force which causes it, and takes place in the
direction of the straight line in which the force acts. If a body be
acted upon by several forces, it will obey each as though the others
did not exist, and this whether the body be at rest or in motion.

3. To every action there is opposed an equal and opposite reaction.

=Ornithopter (or Orthopter)=--A flapping wing design of aircraft intended
to imitate the flight of a bird.

=Outrigger=--This term is usually applied to the framework connecting the
main surface with an elevator placed in advance of it. Sometimes applied
to the "tail-boom" framework connecting the tail-plane with the main
lifting surface. [20]

=Pancake, to=--To "stall."

=Plane=--This term is often applied to a lifting surface. Such application
is not quite correct, since "plane" indicates a flat surface, and the
lifting surfaces are always cambered.

=Propeller=--See "Air-Screw."

=Propeller, Tractor=--An air-screw mounted in front of the main lifting
surface.

=Propeller, Pusher=--An air-screw mounted behind the main lifting surface.

=Pusher=--An aeroplane of which the propeller is mounted behind the main
lifting surface.

=Pylon=--Any V-shaped construction from the point of which wires are
taken.

=Power=--Rate of working. [21]

=Power, Horse=--One horse-power represents a force sufficient to raise
33,000 lb. 1 foot in a minute.

=Power, Indicated Horse=--The I.H.P. of an engine is a measure of the
rate at which work is done by the pressure upon the piston or pistons,
as distinct from the rate at which the engine does work. The latter
is usually termed "brake horse-power," since it may be measured by an
absorption brake.

=Power, Margin of=--The available quantity of power above that necessary
to maintain horizontal flight at the optimum angle.

=Pitot Tube=--A form of air-speed indicator consisting of a tube with open
end facing the wind, which, combined with a static pressure or suction
tube, is used in conjunction with a gauge for measuring air pressures or
velocities. (_No. 1 in diagram._)

=Pitch, Propeller=--The distance a propeller adva