MOTION. A continued and successive change of place, either of a whole body, or of the particles of which a body is composed; the earth in revolving upon its axis only, would not change its place as a body, but all the particles of which it is composed, would revolve round a common axis of motion. In revolving in its orbit, its whole mass is constantly occupying a new portion of space. NATURAL PHILOSOPHY. That science which enquires into the laws which govern all the natural bodies in the universe, in all their changes of place, or of state. NEAP TIDES. Those tides which occur when the moon is in her quadratures, or half way between new, and full moon; at these periods the tides are the lowest. NODES. Those points in the orbit of the moon, or of a planet, where it crosses the ecliptic or plane of the earth's orbit. When passing to the north of the ecliptic, it is called the ascending node; when to the south of it, the descending node. OBLATE. See SPHEROID. OCTAGON. A figure with eight sides, and consequently with eight angles. OPAQUE. Not transparent; refusing a passage to the rays of light. OPTICS. That branch of science which treats of light, and vision. It is generally divided into two parts. _Catoptrics_, which treats of the reflection of light, and _Dioptrics_, which treats of its refraction. ORBIT. The line in which a primary planet moves in its revolution round the sun; or a secondary planet, in its revolution round its primary. These orbits are all elliptical, or oval. PARABOLA. A particular kind of curve; that which a body describes in rising and in falling, when thrown upwards, in any direction not perpendicular to the horizon. PARALLELOGRAM. A figure with four sides, having those which are opposite, parallel to each other. A square, an oblong square, and the figure usually called a diamond, are Parallelograms. PARALLEL LINES. All lines, whether straight or curved, which are every where at an equal distance from each other, are parallel lines. PARALLEL OF LATITUDE. See LATITUDE. PERIHELION. That part of the orbit of a planet, in which it approaches the sun most nearly. PENDULUM. A body suspended by a rod, or line, so that it may vibrate, or oscillate, backwards and forwards. Pendulums of the same length, perform their vibrations in the same time, whatever may be their weight, and whether the arc of vibration, be long or short. PERCUSSION. The striking of bodies against each other. The force of this, depends upon the momentum of the striking body. PERIOD. The time required for the revolution of one of the heavenly bodies in its orbit. PERPENDICULAR. Making an angle of 90 degrees with the horizon. When two lines which meet, make an angle of 90 degrees, they are perpendicular to each other. PHASES. The various appearances of the disc, or face of the moon, and of the planets; that portion of them which we see illuminated by the rays of the sun.