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EXPLANATION OF TERMS.

Aberration.

An apparent annual motion in the fixed stars, occasioned by the velocity of light combined with the real velocity of the earth in its orbit.

Absorbent media. Substances either solid, liquid, or fluid, which imbibe the rays of light and heat.

Accidental colors. If the eye has been dazzled by looking steadily at a bright color, as, for example at a red wafer, upon turning it to a white object a bluish-green image of the wafer will appear. Bluish-green is therefore the accidental color of red, and vice versa. Each tint has its accidental color. When the real and accidental colors are of equal intensity, the one is said to be the complementary color of the other, because the two taken together make white light. Acceleration. A secular variation in the mean motion of the moon.

Aëriform. Having the form of air.

Aërolite. A meteoric stone.

Aërostatic expedition. Ascent in a balloon.

Affinity or cohesive force. The force with which the particles of bodies resist separation.

Alga. Sea weeds or marine plants.

Aliquot parts. The parts into which a quantity is divided when no remainder is left.

Altitude. The height of an object above the horizon.

Analysis. Mathematical reasoning conducted by means of abstract symbols.

Analyzing plate. A piece of glass or a slice of a crystal used for examining the properties of polarized light. Analytical formula or expression. A combination of symbols expressing a series of calculation, and including every particular case that can arise from a general law.

Angle of position of a double star. The angle which a line joining the two stars makes with one parallel to the meridian.

Angular velocity. The swiftness with which the particles of a revolving body move. It is proportional to the velocity of each particle divided by its distance from the axis or centre of rotation.

Annual equation. A periodical inequality in the motion of the moon going through its changes in a year. Annual parallax. See Parallax.

Antimony. A metal.

Antenna. The thread-like horns on the heads of insects.

Aphelion. The point in which a planet is at its greatest distance from the sun-the point a in fig. 8, S being

the sun.

Apogee. The point in which the sun or moon is farthest from the earth.

Apparent motion. The motion of the celestial bodies as viewed from the earth.

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Apsides. The extremities a and P fig. 8, of the major axis of an orbit, or the points in which a planet is at its greatest and least distances from S the sun; also

those in which a satellite is at its greatest and least distances from its planet.

Arc of the meridian. Part of a plane curve passing through the poles of the earth, and along its surface. Areas. Superficial extent. In astronomy, they are the spaces passed over by the radius vector of a celestial body. Arithmetical progression. A series of quantities or numbers continually increasing or diminishing by the same quantity; as, for example, the natural numbers 0, 1, 2, 3, 4, &c., which continually increase by one. Armature. A piece of soft iron connecting the poles of a horse-shoe magnet.

Astronomical or solar day. The time between two consecutive true noons or midnights.

Atmospheric refraction. See Refraction.

Aurora. A luminous appearance in the heavens, frequently seen in high northern and southern latitudes. Axis of rotation. The line real or imaginary, about which a body revolves. The imaginary line passing through both poles and the centre of the earth is the axis of the earth's rotation.

Axis of a prism. The line a b, fig. 11, passing through the centre of a prism parallel to its sides.

Axis of a telescope. An imaginary line passing through the centre of the tube.

Axis of an ellipse. See Ellipse, line ▲ в, fig. 2.

Base. In surveying, a base is a line measured on the sur

face of the earth, and assumed as an origin from whence the angular and linear distances of remote objects may be determined.

Binary system of stars. Two stars revolving about each other.

Bissextile. Leap year, every fourth year.

Caloric. The material of heat; heat being the sensation. Centre of gravity. A point in a body, which, if supported, the body will remain at rest.

Capillary attraction. The attraction of tubes with very minute bore, such as thermometer tubes, which causes liquids to ascend and remain suspended within them. Centrifugal force. The force with which a revolving body tends to fly from the centre of motion. The direction of this force is in the tangent to the path the body describes.

Circumference. The boundary of a circle.

Civil day. The time comprised between two consecutive returns of the sun to the same meridian.

Civil or tropical year. The time comprised between two consecutive returns of the sun to the same solstice or equinox.

Chemical rays. The rays of the solar spectrum which dɔ not produce light but destroy vegetable colors.

Chronometer. A watch which measures time more accurate than those in common use.

Coal measures. The strata which contain beds of coal. Cobalt. A metal.

Cohesion. The force with which the parts of bodies resist any endeavor to separate them. Hardness, softness, tenacity, fluidity and ductility, are modifications of cohesion.

Collecting wires or Collectors. Wires for collecting and conveying electricty.

Complementary colors. See Accidental colors.

Compression of a spheroid. Flattening at the poles. It is equal to the difference between the greatest and least diameters divided by the greatest.

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