Part 92

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 aeriform 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.