LIBRA. One of the twelve signs of the zodiac. That into which the sun enters, at the autumnal equinox.

LIGHT. That principle, by the aid of which we are able to discern all visible objects. It is generally believed to be a substance emitted by luminous bodies, and, exciting vision by pa.s.sing into the eye.

LONGITUDE. Distance measured in degrees and minutes, either in an eastern, or a western direction, from any given point either on the equator, or on a parallel of lat.i.tude. Degrees of longitude may amount to 180, or half a circle. A degree of longitude measured upon the equator, is of the same length with a degree of lat.i.tude; but as the poles are approached, the degrees of longitude diminish in length, because the circles upon which they are measured, become less.

LUNAR. Relating to _Luna_, the moon.

LUNATION. The time in which the moon completes its circuit. A lunar month.

LUMINOUS BODIES. Those which emit light from their own substance; not shining by borrowed, or reflected light.

MACHINE. Any instrument, either simple or compound, by which any mechanical effect is produced. A needle, and a clock, are both machines.

MAGIC LANTHORN, OR LANTERN. An optical instrument, by which transparent pictures, painted upon gla.s.s, are magnified and exhibited on a white wall or screen, in a darkened room. The phantasmagoria, is a species of magic lanthorn.

MATHEMATICS. The science of numbers and of extension. Common arithmetic, is a lower branch of the mathematics. In its higher departments, it extends to every thing which is capable of being either numbered or measured.

MATTER. Substance. Every thing with which we become acquainted by the aid of the senses; every thing however large, or however minute, which has length, breadth, and thickness.

MECHANICS. That science which investigates the principles, upon which the action of every machine depends; and teaches their proper application in overcoming resistance, and in producing motion, in all the useful purposes to which they are applied.

MEDIUM. In optics, is any body which transmits light. Air, water, gla.s.s, and all other transparent bodies, are media. Medium also denotes that in which any body moves. Air is the medium which conveys sound, and which enables birds to fly.

MELODY. A succession of such single musical sounds, as form a simple air or tune.

MERCURY. That planet which is nearest to the sun. Quicksilver, a metal, which remains fluid at the common temperature of the atmosphere. It is capable of being rendered solid, by intense cold.

MERIDIAN. Midday. A meridian line, is one which extends directly from one pole of the earth to the other; crossing the equator at right angles. It is therefore half of a great circle. The hour of the day is the same at every place situated on the same meridian. Longitude is measured from any given meridian, to the opposite meridian. Places at the same distance in degrees, to the east or west of any meridian, have the same longitude.

MICROSCOPE. An optical instrument, by which minute objects, are magnified, so as to enable us to perceive and examine such as could not be seen by the naked eye.

MINERAL. Earths, stones, metals, salts, and in general all substances dug out of the earth, are denominated minerals.

MINUTE. In time, the sixtieth part of an hour. In length, the sixtieth part of a degree. A minute of time, is an unvarying period; but a minute in length varies in extent, with the degree of which it forms a part.

The degrees and minutes are equal in number, upon a common ring, upon the equator of the earth, or, on any circle of the heavens.

MIRRORS. Polished surfaces of metal, or of gla.s.s coated with metal, for the purpose of reflecting the rays of light, and the images of objects.

Common looking-gla.s.ses, are mirrors. Those used in reflecting telescopes, are made of metal.

MOBILITY. Capable of being moved from one place to another. This is accounted one of the essential properties of matter, because we cannot conceive of its existence without this capacity.

MOMENTUM. The force, or power, with which a body in motion acts upon any other body, or tends to preserve its own quant.i.ty of motion. The momentum of a body, is compounded of its quant.i.ty of matter, and its velocity. A body weighing one pound, moving with a velocity of two miles in a minute, will possess the same momentum with one weighing two pounds, moving with a velocity of one mile in a minute.

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...o...b..t, its whole ma.s.s is constantly occupying a new portion of s.p.a.ce.

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...o...b..t. When pa.s.sing 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 pa.s.sage 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 LAt.i.tUDE. See LAt.i.tUDE.

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...o...b..t.

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.

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.