PHENOMENON. Any natural appearance is properly so called; the term, however, is usually applied to extraordinary appearances, as eclipses, transits, &c. PISTON. That part of a pump, or other engine which is made to fit into a hollow cylinder, or barrel; and to move up and down in it, in order to raise water, or for any other purpose. PLANE. A perfectly flat surface. The plane of the orbit of a planet, is an imaginary flat surface, extending to every part of the orbit. PLANET. Those bodies which revolve round the sun, in orbits nearly circular. They are divided into _primary_, and _secondary_; these latter are also called satellites, or moons; they revolve round the primary planets, and accompany them in their courses round the sun. PLUMB-LINE. A string, or cord, by which a weight is suspended; it is used for the purpose of finding a line perpendicular to the horizon; the weight being always attracted towards the centre of the earth. PNEUMATICS. That branch of natural philosophy, which treats of the mechanical properties of the atmosphere, or of air in general. POLES. The extremities of the axis of motion either of our earth, or of any other revolving sphere. The poles of the earth have never been visited; the regions by which they are surrounded, being obstructed by impassable barriers of ice. POWER. That force which we apply to any mechanical instrument, to effect a given purpose, is denominated power, from whatever source it may be derived. We have the power of weights, of springs, of horses, of men, of steam, &c. PRISM. The instrument usually so called, is employed in optics to decompose the solar ray: it consists of a piece of solid glass, several inches in length, and having three flat sides; the ends are equal in size, and are of course triangular. PRECESSION OF THE EQUINOXES. Every equinox takes place a few seconds of a degree, before the earth arrives at that part of the ecliptic in which the preceding equinox occurred. This phenomenon is called the precession of the equinoxes. There is consequently a gradual change of the places of the signs of the zodiac: a fact, the discovery of which has thrown much light on ancient chronology. PROJECTION. That force by which motion is given to a body, by some power acting upon it, independently of gravity. PULLEY. One of the six mechanical powers. A wheel turning upon an axis, with a line passing over it. It is the moveable pulley only, which gives any mechanical advantage. PUMP. An hydraulic, or pneumatic instrument, for the purpose of raising water, or exhausting air. QUADRANT. A quarter of a circle. An instrument used to measure the elevation of a body in degrees above the horizon. QUADRATURES OF THE MOON. That period in which she appears in the form of a semicircle. She is then either in her first, or her last quarter; and exactly half way, between the places of new, and of full moon.