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It

is, where the compass needle is horizontal. encircles the earth, but does not coincide with

the terrestrial equator.

Magnetic meridian.

The vertical plane passing

through the direction of the needle of the compass at any place.

Magnetic poles. Points of the earth where the intensity of the magnetic force is greatest.

Magnetic induction. The effect of magnets to excite magnetism in bodies near them. Magneto-electric induction. The effect of galvanic currents to produce magnetism in bodies near them capable of receiving it.

Major axis or greatest diameter of an ellipse. See Ellipse, A B, fig. 2.

Mass. The quantity of matter in a body. It is proportional to the density and volume conjointly. Mathematics. The science of number and quantity. Mean distance. The mean distance of a planet from the sun, or of a satellite from its planet, is equal to half the major axis of its orbit.

Mean longitude. See Longitude.

Mean motion. Equable motion in a circular orbit at the mean distance during the same time that the body accomplishes a revolution in its elliptical orbit.

Mean time. The time shown by clocks and watches well regulated.

Mechanics.

The science of the equilibrium and motion of bodies.

Meridian. A vertical plane passing through the poles of the earth.

Meteorites. Stones which fall from the heavens.

Mica. A certain mineral.

Minor axis. See Ellipse.

Minus. Less. The sign of Subtraction.

Molecules. The indefinitely small or ultimate particles of matter.

Momentum. Force measured by the mass and simple velocity conjointly.

Monocotyledonous plants. Such as have seeds of one lobe.

Moon's southing. The time when the moon comes to the meridian of any place, which happens about forty-eight minutes later each day.

Multiple systems of stars. Three or more stars revolving about their common centre of gravity.

Nebula. White misty appearance in the heavens like the milky way; some of them, when viewed with powerful telescopes, are found to be clusters of stars, others always retain the cloudy form. Nebulosity of comets. The coma or misty appearance which always surrounds their heads, and of which their whole mass is often composed.

Nickel. A metal.

Nodes. The two opposite points N and n, fig. 8, in

which the orbit NA n P of a planet or comet intersects the plane C N En of the ecliptic. Part, NA n, of the orbit lies above the plane of the ecliptic, and part, n P N, below it. The ascending node N is the point through which the body passes in rising above the plane of the ecliptic, and the descending node n is the point in which the body sinks below it. The nodes of a satellite's orbit are the points in which it intersects the plane of the orbit of its primary.

Nodes, line of. The intersection N n, fig. 8. of the plane of the orbit of a planet or comet with the plane of the ecliptic. It passes through S, the centre of the sun.

Nodal points. Points of a sonorous body which remain at rest during its vibrations.

Nodal lines. Lines of sonorous surfaces which remain at rest during their vibrations.

Non-electrics. Substances in which electricity cannot be sensibly excited by friction.

Nucleus of a comet. The part of its head which appears to be dense. Frequently they have

none.

Nucleus of the earth. The solid part.

Nutation. A variation in the obliquity of the ecliptic from the attraction of the sun and moon on the protuberant matter at the terrestrial equator.

Nutation of the lunar orbit. A variation in the inclination of the lunar orbit from the action of the matter at the earth's equator on the moon. It is the reaction of terrestrial nutation.

Oasis. A fertile spot in a desert.

Oblate spheroid. A solid like an orange, which may be formed by the rotation of an ellipse about its minor axis, and is therefore flattened at the poles.

Obliquity of the ecliptic. The angle formed by the plane of the terrestrial equator with the plane of the ecliptic.

Oscillation. A motion to and fro, like the pendulum of a clock.

Occultation. The eclipse of a star or planet by the moon or by another planet.

Opposition. A body is said to be in opposition when its longitude differs from that of the sun by 180°. Optics. The science of light and colours.

Optic axis of a crystal. A ray of light passing through a doubly refracting crystal, such as Iceland spar, is generally divided into two rays, but in certain directions it is transmitted in one ray only these directions are called the optic axes of a crystal.

Orbit. The track or path of a celestial body in the heavens.

Ordinary refraction. See Refraction.

Ordinary ray. See Refraction.

Parabola. One of the conic sections. It is the line

described by a cannon ball, and has two infinite branches, A B, A D, fig. 3. and there is a point r

within it called the focus, to which every point

in the curve bears a certain relation.

Parabolic elements. See Elements of an orbit.

Parallax. The angle under which we view an object; it therefore diminishes as the distance increases.

Parallax of a celestial object. The angle which the radius of the earth would be seen under, if viewed from that object.

Parallax, horizontal. The parallax of a celestial body when in the horizon. Parallax is then at its maximum; it decreases as the height of the body above the horizon increases.

Parallax, annual. The angle which the diameter of the earth's orbit would be seen under, if viewed from a celestial body, as a fixed star.

Parallactic motion. The motion of a body is said to be parallactic when the space described by it subtends or is seen under a sensible angle.

Parallelogram. A four-sided plane figure, A B, fig. 10. whose opposite sides are parallel: the dia

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meter is the straight line joining two of its opposite angles.

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