EXPLANATION OF TERMS.

a

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

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.

А

.

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

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

each other.

Bissextile. Leap year, every fourth year.

a

Circumference. The boundary of a circle.
Civil duy. The time comprised between two conse-

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

coal.

Cobalt. A metal.

Cohesion. The force with which the parts of bodies