Perpendicular. At right angles to a surface. This term is sometimes wrongly applied in referring to an object, particularly to an object which is vertical, meaning up and down. The blade of a square is perpendieular to the handle at all times, but the blade is vertical only when it points to the center of the earth. Pernicious. Bad; not having good features or possessing wrong attributes. Pendulum. A bar or body suspended at a point and adapted to swing to and fro. Perpetual. For all time; unending or unlimited time. Phenomena. Some peculiar happening, or event, or object. Pitch. In aviation this applies to the angle at which the blades of a propeller are cut. If a propeller is turned, and it moves forwardly in the exact path made by the angle, for one complete turn, the distance traveled by the propeller axially indicates the pitch in feet. Placement. When an object is located at any particular point, so that it is operative the location is called the placement. Plane. A flat surface for supporting a flying machine in the air. Plane of movement pertains to the imaginary surface described by a moving body. A bicycle wheel, for instance, when moving forwardly in a straight line, has a plane of movement which is vertical; but when the machine turns in a circle the upper end of the wheel is turned inwardly, and the plane of rhovement is at an angle. Pliant. Easily yielding; capable of being bent; liable to be put out of shape. Plurality. See multiplicity. More than one. Poise. Held in suspension; disposed in a particular way. Pontoon. Applied to a series of boats ranged side by side to support a walk laid thereon. In aviation it has reference to a float for supporting an aeroplane. Ponderous. Large; heavy; difficult to handle. Posterior. The rear end; the opposite of anterior. Principles. The very nature or essence of a thing; the source or cause from which a thing springs. Proportion. The relation that exists between different parts or things. Propounded. Questioned; stated; to state formally for consideration Proprietary. A right; the ownership of certain property. Primitive. The beginning or early times; long ago. Prelude. A statement or action which precedes the main feature to be presented. Proximity. Close to; near at hand. Prototype. That which is used as the sample from, which something is made or judged. Propeller. The piece of meebanism, with screw shaped blade, designed to be rapidly rotated in order to drive a vessel forwardly. It is claimed by some that the word Impeller would be the more proper term. Primarily. At the first; the commencement. Precedes. Goes ahead; forward of all. Propulsive. The force which gives motion to an object. Projected. Thrown forward; caused to fly through the air. Radially. Out from the center; projecting like the spokes of a wheel. Ratio. The relation of degree, number, amount; one with another. Reaction. A counterforce; acting against.