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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, N An, 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 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 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 inclina

tion 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.


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 colors.

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 arę called the optic axes of a crystal.


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 F within it cailed 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 diameter is

Fig. 10.



the straight line joining two of its opposite angles. Passage at the perihelion. The passage of a body through the point of its orbit that is nearest to the


Penumbra. The shadow or imperfect darkness which precedes and follows an eclipse.

Perigee. The points in which the sun and moon are nearest to the earth.

Perihelion. The point P fig. 8. of an orbit which is nearest to the sun.

Perihelion distance. The shortest distance of a planet or comet from the sun, P s, fig. 8.

Periodic inequality. An irregularity in the motion of a celestial body requiring a comparatively short time for its accomplishment.

Periodic time.

The time in which a planet or comet performs a revolution round the sun, or a satellite about its primary.

Perturbations. Irregularities in the motions of bodies from some disturbing cause.

Phanerogamous plants. Such as have apparent flowers and seeds.

Phases of the moon. The periodic changes in the enlightened part of her disc from a crescent to a circle, depending upon her position with regard to the sun and earth.

Phases of an undulation. Alternate changes in the surface or density of a fluid. The fluid particles in the tops or in the hollows of a series of waves are in the same phases, because their displacement and motion are equal and in the same direction: whereas the fluid particles in the tops of a series of waves are in different phases from those in the hollows, because the displacement and motion of the first are equal, but opposite to those of the second. For example: in waves of water, the particles in the tops have arrived at their greatest elevation, and are beginning to sink down, whereas those in the hollows have reached their greatest depression, and are beginning to rise up.

Phenomena. Appearances.

Physical. Belonging to material nature.

Physico-Mathematical sciences. Sciences in which natural phenomena are explained by mathematical reasoning.


Pitch in Music. The depth or shrillness of a note. depends upon the number of vibrations the sonorous body makes in a second. The more rapid the vibration the higher the pitch.

Plane. Length and breadth without thickness.

Plane of reflection. The plane passing through the incident and reflected rays of light or sound as s I, I R, fig. 9. It is perpendicular to the reflecting surface.

Plane of refraction. The plane passing through the incident and refracted rays of light s 1 and 1 o, fig. 13. it is perpendicular to the refracting surface.

Plane of polarization. The plane passing through the incident and polarized ray. It is at right angles to the plane of reflection, but deviates from the plane of ordinary refraction

Plus. More; the sign of addition.

Polarity. The tendency of magnetized bodies to point to the magnetic poles of the earth.

Polarized light. Light which by reflection or refraction at a certain angle, or by refraction in certain crystals, has acquired the property of exhibiting opposite effects in planes at right angles to each other. This property is explained on the undulatory theory by supposing the particles of the ether to vibrate in one plane.

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