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.
Radius, equatorial. A line drawn from the centre

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

Fig. 13.

R

I

G

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.)

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

a

.

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.

axes.

a

а

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