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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 or heat. Accidental colours. If the eye has been dazzled by
looking steadily at a bright colour, 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 colour of red, and vice versa. Each tint has its accidental colour. When the real and accidental colours are of equal intensity, the one is said to be the complementary colour of the other, Aliquot parts. The parts into which a quantity is
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. Afinity or cohesive force. The force with which the
particles of bodies resist separation. Algæ. Sea weeds or marine plants.
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 po
larized 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. Antenne. 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. Apparent diameter. See Diameter. Apparent time. See Time. 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 continu
ally 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 A B, fig. 2.
Buse. In surveying, a base is a line measured on the
surface 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
Bissextile. Leap year, every fourth year.
Caloric. The material of heat; heat being the sen
sation. Centre of gravity. A point in a body, which, if sup
ported, the body will remain at rest. Capillary attraction. The attraction of tubes with a
very minute bore, such as thermometer tubes, which causes liquids to ascend and remain sus
pended 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.
cutive 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 do not produce light but destroy vegetable
colours. Chronometer. A watch which measures time more
accurately than those in common use. Coal measures. The strata which contain beds of
Cobalt. A metal.
Cohesion. The force with which the parts of bodies
resist any endeavour to separate them. Hardness, softness, tenacity, fluidity, and ductility,
are modifications of cohesion, Collecting wires, or Collectors. Wires for collecting
and conveying electricity. Complementary colours. See Accidental colours. Compression of a spheroid. Flattening at the poles.
It is equal to the difference between the greatest
and least diameters divided by the greatest. Concave mirror. A polished curved surface which,
being hollow, reflects parallel rays of light so as
to make them tend to meet. Concentric. Having the same centre. Conducior. A substance which conducts the electric