Accelerated Motion. Motion is said to be accelerated, when the velocity is continually increasing.

Accidental Properties. Those properties of bodies which are liable to change, as colour, form, &c.

Acute.—See Angle.

Air. An elastic fluid. The atmosphere which surrounds the earth, is generally understood by this term, but there are many kinds of air. The term is synonymous with Gas.

Air Pump. An instrument by which vessels may be exhausted of air.

Altitude. The height in degrees of the sun, or any heavenly body, above the horizon.Angle. The space contained between two lines inclined to each other, and which meet in a point. Angles are measured in degrees, upon a segment of a circle described by placing one leg of a pair of compasses on the angular point, and with the other, describing the segment between the two lines. If the segment be exactly 1-4th of a circle, it is called a right angle, and contains 90 deg. If more than 1-4th of a circle, it is an obtuse angle. If less, an acute angle. See plate 2.

Angle of Incidence, is the space contained between a ray which falls obliquely upon a body, and a line perpendicular to the surface of the body, at the point where the ray falls.

Angle of Reflection. The space contained between a reflected ray, and a line perpendicular to the reflecting point.

Angle of Vision, or visual angle. The space contained between lines drawn from the extreme parts of any object, and meeting in the eye.

Antarctic Circle. A circle extending round the south pole, at the distance of 23 1-2 degrees from it. The same as the south frigid zone.

Aphelion. That part of the orbit of a planet, in which its distance from the sun is the greatest.

Area. The surface enclosed between the lines which form the boundary of any figure, whether regular or irregular.

Aries. See Sign.

Asteroids. The name given to the four small planets, Ceres, Juno, Pallas, and Vesta.

Astronomy. The science which treats of the motion and other phenomena of the sun, the planets, the stars, and the other heavenly bodies.

Atmosphere. The air which surrounds the earth, extending to an unknown height. Wind is this air in motion.Attraction. A tendency in bodies to approach each other, and to exist in contact.

Attraction of Cohesion. That attraction which causes matter to remain in masses, preventing them from falling into powder. For this attraction to exist, the particles must be contiguous.

Attraction of Gravitation. By this attraction, masses of matter, placed at a distance, have a tendency to approach each other. Attraction is mutual between the sun and the planets.

Axis of the Earth, or of any of the planets. An imaginary line passing through their centres, and terminating at their poles; round this their diurnal revolutions are performed.

Axis of Motion. The imaginary line, around which all the parts of a body revolve, when it has a spinning motion.

Axis of a Lens, or Mirror. A line passing through the centre of a lens, or mirror, in a direction perpendicular to its surface.

Balloon. Any hollow globe. The term is generally applied to those which are made to ascend in the air.

Barometer. Commonly called a weather-glass. It has a glass tube, containing quicksilver, which by rising and falling, indicates any change in the pressure of the atmosphere, and thus frequently warns us of changes in the weather.

Body. The same as Matter. It may exist in the solid, liquid, or Æriform state; and includes every thing with which we become acquainted by the aid of the senses.

Burning-glass, or Mirror. A lens, or a mirror, by which the rays of light, and heat, are brought to a focus, so as to set bodies on fire.

Camera Obscura, a darkened room; or more frequently a box, admitting light by one opening, where a lens is placed; which, bringing the rays of light, from external objects, to a focus, presents a perfect picture of them, in miniature.

Capillary Tubes. Tubes, the bore of which is very small. Glass tubes are usually employed, to show the phenomenon of capillary attraction. Fluids in which they are immersed, rise in such tubes above the level of that in the containing vessel.

Centre of a Circle. A point, equally distant from every part of its circumference.

Centre of Gravity. That point within a body, to which all its particles tend, and around which they exactly balance each other. A system of bodies, as the planets, may have a common centre of gravity, around which they revolve in their orbits; whilst each, like the earth, has its particular centre of gravity within itself.

Centre of Motion. That point about which the parts of a revolving body move, which point is, itself, considered as in a state of rest.

Centre of Magnitude. The middle point of any body. Suppose a globe, one side of which is formed of lead, and the other of wood, the centres of magnitude and of gravity, would not be in the same points.

Central Forces. Those which either impel a body towards, or from, a centre of motion.

Centrifugal. That which gives a tendency to fly from a centre.

Centripetal. That which impels a body, towards a centre.

Circle. A figure; the periphery, or circumference of which, is every where equally distant, from the point, called its centre.

Circle, Great. On the globe, or earth, is one that divides it into two equal parts, or hemispheres. The equator, and meridian lines, are great circles.

Circle, Lesser. Those which divide the globe into unequal parts. The tropical, arctic and antarctic circles, and all parallels of latitude, are lesser circles.

Circumference. The boundary line of any surface, as that which surrounds the centre of a circle; the four sides of a square, &c.

Comets. Bodies which revolve round the sun, in very long ovals, approaching him very nearly in their perihelion, but in their aphelion, passing to a distance immeasurably great.

Cohesion. See Attraction.

Compressible. Capable of being forced into a smaller space.

Concave. Hollowed out; the inner surface of a watch-glass is concave, and may represent the form of a concave mirror, or lens.

Convex. Projecting, or bulging out, as the exterior surface of a watch-glass, which may represent the form of a convex mirror, or lens.

Cone. A body somewhat resembling a sugar-loaf; that is, having a round base, and sloping at the sides, until it terminates in a point.

Conjunction. When three of the heavenly bodies are in a straight or right line, if you take either of the extreme bodies, the other two are in conjunction with it; because a straight line drawn from it, might pass through the centres of both, and join them together. At the time of new moon, the moon and sun are in conjunction with the earth; and the moon and earth, are in conjunction with the sun.

Constellation, or Sign. A collection of stars. Astronomers have imagined pictures drawn in the heavens, so as to embrace a number of contiguous stars, and have named the group after the animal, or other article supposed to be drawn; an individual star is generally designated by its fancied location; as upon the ear of Leo, the Lion, &c.

Convergent Rays, are those which approach each other, so as eventually to meet in the same point.

Crystals. Bodies of a regular form, having flat surfaces, and well defined angles. Nitre, and other salts, are familiar examples. Many masses of matter, are composed of crystals too minute to be discerned without glasses.

Curvilinear, consisting of a line which is not straight, as a portion of a circle, of an oval, or any curved line.

Cylinder. A body in the form of a roller, having flat circular ends, and being of equal diameter throughout.

Degree. If a circle of any size be divided into 360 equal parts, each of these parts is called a degree. One quarter of a circle contains ninety degrees; one twelfth of a circle, thirty degrees. The actual length of a degree, must depend upon the size of the circle. A degree upon the equator, upon a meridian, or any great circle of the earth, is equal to 69¹/₂ miles.

Straight lines are sometimes divided into equal parts, called degrees; but these divisions are arbitrary, bearing no relationship to the degrees upon a circle.

Density. Closeness of texture. When two bodies are equal in bulk, that which weighs the most, has the greatest density.

Diagonal. A line drawn so as to connect two remote angles of a square, or other four-sided figure.Dilatation. The act of increasing in size. Bodies in general, dilate when heated, and contract by cooling.

Discord. When the vibrations of the air, produced by two musical tones, do not bear a certain ratio to each other, a jarring sound is produced, which is called discord.

Divergent Rays. Those which proceed from the same point, but are continually receding from each other.

Divisibility. Capability of being divided, or of having the parts separated from each other. This is called one of the essential properties of matter; because, however minute the particles may be, they must still contain as many halves, quarters, &c. as the largest mass of matter.

Echo. A sound reflected back, by some substance, so situated as to produce this effect.

Eclipse. The interruption of the light of the sun, or of some other heavenly body, by the intervention of an opaque body. The moon passing between the earth and the sun, causes an eclipse of the latter.

Ecliptic. A circle in the heavens. The apparent path of the sun, through the twelve signs of the zodiac. This is caused by the actual revolution of the earth, round the sun. It is called the ecliptic, because eclipses always happen in the direction of that line, from the earth.

Elasticity. That property of bodies, by which they resume their dimensions and form, when the force which changed them is removed. Air is eminently elastic. Two ivory balls, struck together, become flattened at the point of contact; but immediately resuming their form, they react upon each other.

Ellipsis. An oval. This figure differs from a circle, in being unequal in its diameters, and in having two centres, or points, called its foci. The orbits of the planets are all elliptical.

Equator. That imaginary line which divides the earth into northern and southern hemispheres, and which is equally distant from each pole.

Equilibrium. When two articles exactly balance each other, they are in equilibrium. They may, notwithstanding, be very unequal in weight, but they must be so situated, that, if set in motion, their momentums would be equal.

Equinox. The two periods of time at which the nights and days are every where of equal length. The vernal equinox is in March, when the sun enters the sign Aries; the autumnal equinox in September, when the sun enters Libra. At these periods, the sun is vertical at the equator.

Exhalations. All those articles which arise from the earth, and mixing with the atmosphere, form vapour.

Expansion. The same as dilatation, which see.

Extension. One of the essential properties of matter; that by which it occupies some space, to the exclusion of all other matter.

Figure. All matter must exist in some form, or shape; hence figure is deemed an essential property of matter.

Fluid. A form of matter, in which its particles readily flow, or slide, over each other. Airs, or gases, are called elastic fluids, because they are readily reduced to a smaller bulk by pressure. Liquids, are denominated non-elastic fluids, because they suffer but little diminution of bulk, by any mechanical force.

Focus. That point in which converging rays unite.

Force. That power which acts upon a body, either tending to create, or to stop motion.

Fountain. A jet, or stream of water, forced upwards by the weight of other water, by the elasticity of air, or some other mechanical pressure.

Friction. The rubbing of bodies together, by which their motion is retarded. Friction may be lessened, but cannot be destroyed.Frigid Zones. The spaces or areas, contained within the arctic and antarctic circles.

Fulcrum. A prop. The point or axis, by which a body is supported, and about which it is susceptible of motion.

Gas. Any kind of air; of these there are several. The atmosphere consists of two kinds, mixed, or combined with each other.

Geometry. That branch of the mathematics, which treats of lines, of surfaces, and of solids; and investigates their properties, and proportions.

Globe. A sphere, or ball. It has a point in its centre of magnitude, from which its surface is every where equally distant.

Gravity. That species of attraction which appears to be common to matter, existing in its particles, and giving to them, and of course to the masses which they compose, a tendency to approach each other. By gravity a stone falls to the earth, and by it the heavenly bodies tend towards each other.

Harmony. A combination of musical sounds, produced by vibrations which bear a certain ratio to each other; and which thence affect the mind agreeably, when heard at the same time. Sounds not so related, produce discord.

Hemisphere. Half a sphere or globe. A plane passing through the centre of a globe, will divide it into hemispheres.

Horizon. This is generally divided into sensible, and rational. The sensible horizon is that portion of the surface of the earth, to which our vision extends. Our rational horizon is that circle in the heavens which bounds our vision, when on the ocean, an extended plane, or any elevated situation. In the heavens our sensible, and our rational horizon are the same; its plane would divide the earth into hemispheres at 90 degrees from us; and a person standing on that part of the earth which is directly opposite to us, would, at the same moment, see in his horizon, the same heavenly bodies, which would be seen in ours.

Horizontal. Level; not inclined, or sloping. A perfectly round ball, placed upon a flat surface, which is placed horizontally, will remain at rest.

Hydraulics. That science which treats of water in motion, and the means of raising, conducting, and using it for moving machinery, or other purposes.

Hydrostatics. Treats of the weight, pressure, and equilibrium of fluids, when in a state of rest.

Hydrometer. An instrument used to ascertain the specific gravity of different fluids, which it does, by the depth to which it sinks when floating on them.

Image. The picture of any object which we perceive either by reflected or refracted light. All objects which are visible, become so by forming images on the retina.

Impenetrability. That property of matter, by which it excludes all other matter from occupying the same space with itself at the same time. If two particles could exist in the same space, so also might any greater number, and indeed all the matter in the universe, might be collected in a single point.

Incidence. The direction in which a body, or a ray of light, moves in its approach towards any substance, upon which it strikes.

Inclined Plane. One of the six mechanical powers. Any plane surface inclined to the horizon, may be so denominated.

Inertia. One of the inherent properties of matter. Want of power, or of any active principle within itself, by which it can change its own state, whether of motion, or of rest.

Inherent Properties. Those properties which are absolutely necessary to the existence of a body; called also essential properties. All others are denominated accidental. Colour is an accidental—extension, an essential property of matter.Latitude. Distance from the equator, in a direct line towards either pole. This distance is measured in degrees and minutes. The degree of latitude cannot exceed ninety, or one quarter of a circle. Places to the south of the equator, are in south latitude, and those to the north, in north latitude.

Latitude, Parallels of. Lines drawn upon the globe, parallel to the equator, are so called; every place situated on such a line, has the same latitude, because equally distant from the equator.

Lens. A glass, ground so that one or both surfaces form segments of a sphere, serving either to magnify, or diminish objects seen through them. Glasses used in spectacles are lenses.

Lever. One of the mechanical powers. An inflexible bar of wood or metal, supported by a fulcrum, or prop; and employed to increase the effect of a given power.

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 passing 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 latitude. 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 latitude; 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 glass, 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, glass, 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 glass coated with metal, for the purpose of reflecting the rays of light, and the images of objects. Common looking-glasses, 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 quantity of motion. The momentum of a body, is compounded of its quantity 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 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.

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.

Radiation. The passage of light or heat in rays, or straight lines; these being projected from every luminous, or heated point, in all directions.

Radius. The distance from the centre of a circle, to its circumference; or one half of its diameter. In the plural denominated radii.

Rainbow. An appearance in the atmosphere, occasioned by the decomposition of solar light, in its refraction, and reflection, in passing through drops of rain. The bow can be seen, only when the sun is near the horizon, when the back is turned towards it, and there is a shower in the opposite direction.

Ray. A single line of light, emitted in one direction, from any luminous point.

Reaction. Every body, whether in a state of motion, or at rest, tends to remain in such state, and resists the action of any other body upon it, with a force equal to that action. This resistance, is called its reaction.

Receiver. This name is applied to glass vessels of various kinds, appertaining to the air pump, and from which the air may be exhausted. They are made to contain, or receive, any article upon which an effect is to be produced, by taking off the pressure of the atmosphere.

Refraction, of the rays of light, is the bending of those rays, when they pass obliquely from one medium into another of different density. A stick held obliquely in water, appears bent or broken at the surface of the fluid.

Refrangibility. Capacity of being refracted. Light is decomposed by the prism, because its component parts are refrangible in different degrees, by the same refracting medium.

Repulsion. The reverse of attraction. A tendency in particles, or in masses of matter, to recede from each other. The matter of heat within a body, appears to counteract the attraction of its particles, so as to prevent absolute contact.

Retina. That part of the ball of the eye, upon which the images of visible objects are formed; and from which, the idea of such forms, is conveyed to the mind.

Revolution, of a planet; is either diurnal, or annual; the former, is its turning upon its own axis; the latter, is its passage in its orbit.

Satellites. Moons, secondary planets.

Segment of a circle. A portion, or part of a circle; called also, an arc of a circle.

Semi-diameter. Half the diameter. The semi-diameter of the earth, is the distance from its surface, to its centre.

Siderial. Belonging to the stars. A siderial day, is the time required for a star to reappear on a given meridian. A siderial year, the period in which the sun appears to have travelled round the ecliptic, so as to have arrived opposite to any particular star, from which his course was calculated.Signs, or Constellations. Collections, or groups, of stars. Those of the zodiac are twelve, corresponding with the twelve months in the year. In the centre of these the ecliptic is situated. The sun appears to pass in succession through these signs; entering the first degree of Aries, which is accounted the first sign, about the 21st of March.

Sky. That vast expanse, or space, in which the heavenly bodies are situated. Its blue appearance is supposed to arise from the particles of which the atmosphere is composed, possessing the property of reflecting the blue rays, in greatest abundance.

Solar. Appertaining to, or governed by, the sun: as the solar system, the solar year, solar eclipses.

Solid. Not fluid. Having its parts connected so as to form a mass. Solid bodies, are not absolutely so, all undoubtedly containing pores, or spaces void of matter.

Solstices. The middle of summer and the middle of winter; those two points in the orbit of the earth, in which its poles point most directly towards the sun.

Sonorous Bodies. Those bodies which are capable of being put into a state of vibration, so as to emit sounds.

Specific Gravity. The relative weight of bodies of different species, when the same bulk of each is taken. Water has been chosen as the standard for comparison. If we say that the specific gravity of a body is 6, we mean, that its weight is six times as great as that of a portion of water, exactly equal to it in bulk.

Spectrum. That appearance of differently coloured rays, which is produced by the refraction of the solar ray, by means of a prism, is called the prismatic spectrum; it exhibits most distinctly, and beautifully, all the colours seen in the rainbow.

Sphere. A globe, or ball.Spheroid. Spherical; a body approaching nearly to a sphere in its figure. The earth, is denominated an oblate spheroid; it not being an exact sphere, but flattened at the poles, so as to cause the polar diameter to be upwards of thirty miles less than the equatorial. Oblate, is the reverse of oblong, and means shorter in one direction, than in another.

Spring Tides. Those tides which occur at the time of new, or of full moon. The tides then rise to a greater height than at any other period.

Square. A figure having four sides of equal length, and its angles all right angles.

In numbers; the product of a number multiplied into itself; thus, the square of 3 is 9, and the square of 8 is 64.

Star. The fixed stars are so called, because they retain their relative situations; while the planets, by revolving in their orbits, appear to wander amongst the fixed stars.

Subtend. This term is applied to the measurement of an angle; when the lines by which it is bounded recede but little from each other, they are said to subtend; that is, to be contained under, a small angle.

Superficies. The surface of any figure. Space extended in length and width.

System. The mutual connexion, and dependance of things, upon each other. The solar, or Copernican system, includes the sun, the planets, with their moons, and the comets.

Tangent. A straight line touching the circumference of a circle; but which would not cut off any portion of it, were it extended beyond the touching point, in both directions.

Telescope. An instrument by which distant objects may be distinctly seen; the images of objects being brought near to the eye, and greatly magnified.Temperate Zones. Those portions of the surface of the earth situated between 23¹/₂ and 66¹/₂ degrees of latitude. Within these boundaries, the sun is never vertical; nor does he ever remain, during a whole day, below the horizon.

Thermometer. An instrument for measuring the temperature of the atmosphere, or of other bodies.Torrid Zone. That portion of the earth which extends 23¹/₂ degrees on each side of the equator, to the tropical circles; within this limit, the sun is vertical, twice in the year.

Transit. Mercury or Venus, are said to transit the sun, when they pass between the earth and that luminary. They then appear like dark spots, upon the face of the sun.

Transparent. Allowing the rays of light to pass freely through. The reverse of opaque. Glass, water, air, &c. are transparent bodies.

Tropics. Two circles on the globe on either hemisphere, at the distance of 23¹/₂ degrees from the equator. Beyond these circles, the sun is never vertical: and the countries within them, are denominated tropical.

Twilight. That portion of the morning or evening, in which the light of the sun is perceptible, although he is below the horizon.

Vacuum. Space void of matter. Such is supposed to be the space in which the planets revolve. We are said to produce a vacuum, when we exhaust the air from a receiver.

Valve. A part of a pump, and of some other instruments, which opens to admit the passage of a fluid in one direction, but closes when pressed in the opposite direction, so as to prevent the return of the fluid; a pair of bellows is furnished with a valve.

Vapour. Exhalations from fluid or solid substances, generally mixing with the atmosphere. The most abundant, is that from water.

Vertical. Exactly over our heads: ninety degrees above our horizon.

Vibration. The alternate motion of a body, forwards and backwards; swinging, as a pendulum.

Visual. Belonging to vision; as the visual angle, or that angle formed by the rays of light which enter the eye, from the extremities of any object.

Undulation. A vibratory, or wave-like motion communicated to fluids. Sound, is said to be propagated by the undulatory, or vibratory motion of the air.

Wedge. One of the mechanical powers; the form of the wedge is well known. It is of extensive use; serving to rend bodies of great strength, and to raise enormous weights.

Wheel and Axle. One of the mechanical powers, used under various modifications. Cranes for raising weights, the wheels and pinions of clocks and watches, windlasses, &c. are all applications of this power.

Zodiac. A broad belt in the heavens, extending nearly eight degrees on each side of the ecliptic; the planes of the orbits of all the planets are included within this space. This belt is divided into twelve parts or signs, each containing 30 degrees.

These signs are:

Aries; the Ram.
Taurus; the Bull.
Gemini; the Twins.
Cancer; the Crab.
Leo; the Lion.
Virgo; the Virgin.
Libra; the Scales.
Scorpio; the Scorpion.
Sagittarius; the Archer.
Capricornus; the Goat.
Aquarius; the Waterer.
Pisces; the Fishes.

The first six are called northern signs; because the sun is in them, during that half of the year, in which he is vertical to the north of the equator; the last six, are called southern signs; because, during his journey among them, he is vertical to the south of the equator.

The sun enters Aries, at the time of the vernal equinox; Cancer, at the summer solstice; Libra, at the autumnal equinox; and Capricornus, at the winter solstice.

The sun is said to enter a sign, when the earth in going round in its orbit, enters the opposite sign. Thus, when the sun appears in the first degree of Libra, it is in consequence of the earth having arrived opposite to the first degree of Aries. A line then drawn from the earth, and passing through the centre of the sun, would, if extended to the fixed stars, touch the first degree of Libra.

Zone. The earth is divided into zones, or belts. See Frigid, Temperate, and Torrid Zones.