« PreviousContinue »
Cone. A solid figure A B C,
like a sugar-loaf, of
other line. When the axis is at right angles to the base, the figure is a right cone.
Conic sections. Lines formed by a plane cutting a cone, of which there are five. If a right cone with a circular base, be cut at right angles to Fig. 2.
the base by a plane passing through the apex, the section will be a triangle. If the cone be cut through both sides by a plane-parallel to the base, the section will be a circle. If the cone be cut slanting quite through both sides, the section will be an ellipse, в a ▲ b, fig. 2. If the cone be cut parallel to one of its sloping sides, the section will be a parabola, D A B, fig. 3 ; and if the section cut only one side of the cone, and be not parallel to the other, it will be a hyperbola, D A в, fig. 4. Configuration. The position of bodies with regard to one another.
Conjunction. A planet is said to be in conjunction when it has the same longitude with the sun. Constellations. Groups of stars to which the names
of men and animals have anciently been given. The whole starry firmament is divided into such groups.
Contrasted colours. See Accidental colours.
Converging. Tending to the same point.
Convex mirror. A polished curved surface which, being protuberant, reflects parallel rays of light, so as to make them diverge.
Cosine of an arc or angle. In fig. 5, A D is the cosine of the arc C в, and of the angle в A C.
Crystal. A chemical or mineral substance having a regular form.
Curves of double curvature. Lines curved in two directions, so that no two of their indefinitely small parts lie in the same plane, as a corkscrew or a curved line drawn obliquely on the side of a cylinder which has its own curvature, at the same time that it partakes of the curvature of the surface on which it is drawn.
Curves of the second order. The conic sections. In a circle, the relation of the part a D, fig. 5, of the diameter to the perpendicular D C, is the same for every point in the circumference. The two lines A D, D C are called co-ordinates. The relation of these co-ordinates to one another is different in different curves, but remains invariable in any one curve; and lines are said to be of the first or second order, according as this relation can be expressed by the simple lines themselves, or by their squares and products.
Declination. The angular distance of a celestial ob
ject from the celestial equator.
Density. The quantity of matter in a given bulk.
Diagonal. A line drawn from angle to angle of a four sided figure, as c D, fig. 10.
A straight line, E B, fig. 5, passing through the centre, and terminated both ways, by the sides or surface of a figure.
Diameter, apparent. The diameter of a body as seen from the earth.
Dicotyledonous plants. Such as have seeds containing two lobes.
Dip. The angle formed by the direction of the magnetic force of the earth with the plumb-line.
Dipping needle. An instrument for measuring the dip of a magnetized needle.
Direct motion of a celestial body. Motion from west to east, according to the order of the signs of the zodiac.
Disc. The apparent surface of a heavenly body. Disintegration. Mouldering down, separating into parts.
Distance, mean. See Mean distance.
Distance, true. See True distance.
Distance, Perihelion. See Perihelion distance.
Double refraction. The power which some sub
stances possess of refracting or transmitting a ray of light in two pencils instead of one. If s 1, fig. 13, be a ray of light falling upon a doubly refracting surface & g, it will be transmitted in two pencils I 0, 1 E, so that the luminous
point s will appear double, if viewed through the substance G g; whereas if & g were of glass or water, the ray s I would be transmitted in a single pencil, 1 o, and only one image of s would be seen.
Dynamics. The science of force and motion.
Ecliptic. The great circle traced in the starry heavens by the plane of the ecliptic.
Ecliptic, plane of. An imaginary plane passing through the earth's orbit, and extending to the starry heavens.
Elasticity. The property bodies possess of resum
ing their original form, when pressure is removed.
Elastic media. Atmospheric air, gas, ether, &c.,
which are highly compressible, and instantly resume their volume or bulk, when pressure is removed.
Electrics. Substances in which electricity may be
excited, but which are incapable of conducting it. Electric induction. The effect of electrified
bodies to produce an electric state opposite to their own, in all bodies near them capable of receiving it.
Electro-magnetism. The science which determines the reciprocal action of electricity and magnetism.
Electro-magnets. Cylinders which have all the properties of magnets when a stream of electricity is passing through them.