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Passage at the perihelion. The passage of a body

through the point of its orbit that is nearest to

the sun. 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 mathema

tical reasoning. Pitch in music. The depth or shrillness of a note.

It 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 reflect

ing surface. Plane of refraction. The plane passing through the

incident and refracted rays of light s 1 and 1 0, 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 refrac

tion 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. Polarization, circular. The property which light

acquires, by transmission through quartz and certain liquids, of producing a succession of appearances which follow each other in a circular order, as the thickness of the medium is increased. This property is explained on the undulatory the ory by supposing the particles of the ether to vibrate in circles one after the other, the undulation going on in a circular helix like a cork

screw penetrating a cork. Polarization, elliplical. The property which light

acquires, by reflection at the surfaces of metals and in other ways, of producing appearances partly analogous to those of circular polarization. It is explained by supposing the undulation to

follow the course of an elliptical helix. Poles. The extremities of the axis about which a

body revolves. Poles of the earth. The extremities of the axis of

diurnal rotation. Poles, magnetic. Points in the earth where the in



tensity of the magnetic force is a maximum. Of these there are certainly three, probably four,

of which differ from the poles of rotation. Poles of a magnet. Points in a magnet where the

intensity of the magnetic force is a maximum; one of these attracts and another repels the same

pole of another magnet. Poles of maximum cold. Points in the surface of

the earth where the mean annual temperature is a maximum. There are several, but none of

them coincide with the poles of rotation. Precession of the equinoxes. A retrograde motion of

the equinoctial points in consequence of the action of the sun and moon upon the protube

rant matter at the earth's equator. Primary. In astronomy signifies the planet about

which a satellite revolves. Prism. A triangularly or polygonally shaped piece

of glass or other substance, like a three or more cornered stick, as fig. 11.

Fig. 11.



Prism, a doubly refracting. A prism made of a

doubly refracting substance, as Iceland spar.

Prismatic colours. The colours of the rainbow. Projected. Thrown; transferred by means of lines. Projection. A line or surface is said to be projected

upon a plane when parallel straight lines are drawn from every point of them to the plane. The projection of an orbit is therefore its daylight shadow, since the sun's rays are sensibly

parallel Prolate spheroid. A solid figure something like an

egg. See Ellipsoid. Pulse. A vibration. Pyramid. A solid bounded by a base having several

sides, and by a number of triangular planes whose summits meet in one point called the apex, as fig. 12.

Fig. 12.


Pyrometer. An instrument for measuring intense

degrees of heat.

Quadrant. Ninety degrees, the fourth part of a

circle. Quadrature. A celestial body is said to be in qua

drature when it is ninety degrees distant from

the sun.

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