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Quartz. Rock crystal; a siliceous mineral whose
primitive form is a rhomboid, fig. 14, but it is generally crystallized in six-sided prisms terminated by six-sided pyramids.
Radiation. An emission of rays.
of a spheroid to its equator. Radius, polar. A line drawn from the centre of a
spheroid to its pole. Radius of a sphere. Any straight line drawn from
the centre of a sphere to its circumference. Radius vector. The imaginary line joining the
centre of the sun and the centre of a planet or comet, or the centre of a planet and that of its
satellite, as s m, fig. 8. Ratio. A fraction expressing the relation which one
quantity bears to another. Proportion is the
equality of ratios. Rectangle. A four-sided plane figure, in which all
the angles are right angles, and its opposite sides equal and parallel. When all the sides are
equal, it is a square. Reflection. The bending back of rays of light or
sound from a surface. The angles made by the rays with a perpendicular to the surface, in coming and going, are equal. If the ray, s 1, (fig. 9) be reflected by a surface a B, in the direction i R, then the angle s i P is equal to RIP.
Refraction. The bending or breaking of a ray of
light in passing through media of different densities, as in going from air into water or glass, and the contrary. If g g (fig. 13.) be a re
O E fracting medium, as a piece of glass, then s i is
the incident, and I o the refracting ray. Refraction, ordinary. Light is said to suffer ordinary
refraction, when both the incident and refracted rays are in a plane at right angles to the refracting surface. This plane is called the plane of ordinary refraction, and the refracted ray is
named the ordinary ray. Refraction, extraordinury. Light is said to suffer
extraordinary refraction, when it is refracted in a different plane from that of ordinary refraction. The plane in question is called the plane of extraordinary refraction, and the ray so refracted is named the extraordinary ray.
In Iceland spar, and other doubly refracting substances, with one optic axis, the incident ray is split into two, one of which suffers ordinary, and the other extraordinary refraction, but in all doubly refracting substances, having two optic axes, both rays suffer extraordinary refraction.
Resulting force. The force resulting from the joint
effects of a number of forces. Retrograde motion of a celestial body. Its motion
from east to west, or contrary to the signs of the
zodiac. Revolution of a planet. Its motion round the sun. Revolution, sidereal. The consecutive returns of a
planet to the same star. Revolution, tropical. The consecutive returns of a
planet to the same tropic or equinox. Rhomb. A plane four-sided figure, whose opposite
sides are equal and parallel, but all its sides are
not equal, nor are its angles right angles. Rhomboid or rhombohedron. A solid formed by six
planes; the opposite planes being equal and similar rhombs parallel to one another, but all the planes are not necessarily equal nor similar, nor are its angles right angles (Fig. 14.)
Rotation. The motion of a body round an axis.
Sauri or Srurians. Reptiles of the lizard kind, as
crocodiles. Secular inequalities. Variations in the motions of
the heavenly bodies, requiring many ages for
their accomplishment. Sidereal day. The time included between two con
secutive transits of the same star at the same
meridian. Sidereal year. The time included between two con.
secutive returns of the sun to the same star. Sine. The perpendicular drawn from the extremity
of an arc to the diameter of a circle, C D, (fig. 5,)
is the sine of the arc C B. Solstices. The points in which the sun is farthest
from the equator. Solar spectrum. The coloured image of the sun
refracted through a prism. Space. The boundless region which contains all
creation. Species of plants. Plants of the same kind. Sphere. A solid formed by the rotation of a semi
circle about its diameter. Spheroid of revolution, or Ellipsoid. A solid formed by the revolution of an ellipse about one of its
The spheroid will be oblate or prolate, according as the revolution is performed about the minor or major axis of the ellipse. Spheroids
are sometimes irregular in their form, Spiral. A curve like a watch spring. It may be cir
cular, like a thread wound about a round rod; or elliptical, like a thread winding about an oval
stick. Stratum. A layer.
Subtend. To be opposite. In fig. 5, the arc c B
subtends the angle c A B. Sulphate of lime. A mineral capable of being split
into thin transparent plates : it consists of 32:7
of lime, 46.3 of sulphuric acid, and 21 of water. Synodic revolution of the moon. The time between
two consecutive new or full moons. Syzygies. The points in the moon's orbit where she
is new or full.
Tangent. A straight line touching a curve in one
point, as t t in fig. 2. Tangential force. A force in the direction of the
tangent. Time, true. Time shown by a dial, or apparent
time. Time, mean. Time shown by ordinary clocks and
watches. Thermo-electric currents. Streams of electricity, ex
cited by heat. Transit. The passage of a body across the meridian
of a place. Transit of Venus and Mercury. The apparent pas
sage of these planets across the sun's disc. Trigonometrical measurements. Mensuration of the
surface of the earth by a series of triangles. Tropical year. The period between the consecutive
returns of the sun to the same tropic or solstice. True distance. The actual distance of a body from the sun, or of a satellite from its planet.