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.