ALt.i.tUDE. The height in degrees of the sun, or any heavenly body, above the horizon.

ANGLE. The s.p.a.ce 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 compa.s.ses 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 s.p.a.ce 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 s.p.a.ce contained between a reflected ray, and a line perpendicular to the reflecting point.

ANGLE OF VISION, or visual angle. The s.p.a.ce 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 ma.s.ses, preventing them from falling into powder. For this attraction to exist, the particles must be contiguous.

ATTRACTION OF GRAVITATION. By this attraction, ma.s.ses 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 pa.s.sing 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 pa.s.sing 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-gla.s.s. It has a gla.s.s 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 aeriform state; and includes every thing with which we become acquainted by the aid of the senses.

BURNING-GLa.s.s, 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. Gla.s.s 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 circ.u.mference.

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 circ.u.mference 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 lat.i.tude, are lesser circles.

CIRc.u.mFERENCE. 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, pa.s.sing to a distance immeasurably great.

COHESION. See ATTRACTION.

COMPRESSIBLE. Capable of being forced into a smaller s.p.a.ce.

CONCAVE. Hollowed out; the inner surface of a watch-gla.s.s 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-gla.s.s, 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 pa.s.s 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 ma.s.ses of matter, are composed of crystals too minute to be discerned without gla.s.ses.

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 69-1/2 miles.