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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.

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 When all the sides are

equal and parallel.

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 IR, then the angle s IP 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

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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, extraordinary. 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 Saurians. 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 consecutive transits of the same star at the same


Sidereal year.

The time included between two consecutive 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


Species of plants. Plants of the same kind.

Sphere. A solid formed by the rotation of a semicircle about its diameter.

Spheroid of revolution, or Ellipsoid. A solid formed

by the revolution of an ellipse about one of its axes. 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 circular, like a thread wound about a round rod; or elliptical, like a thread winding about an oval stick.

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Subtend. To be opposite. In fig. 5, the arc CB 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 r t in fig. 2.

Tangential force. A force in the direction of the tangent.

Time, true. Time shown by a dial, or apparent


Time, mean. watches.

Time shown by ordinary clocks and

Thermo-electric currents. Streams of electricity, excited 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.

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