Rotation. The motion of a body round an axis. Sauri or Suurians. 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 meridian. 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 colored 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 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 irregu lar 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. Subtend. To be opposite. In fig. 5, the arc c в subtends the angle C A B. Sulphate of lime. A mineral capable of being split into thin transparent plates; it consists of 327 of lime, 46.3 of sulphuric acid, and 21 of water. Synodic revolution of the moon. consecutive new or full moons. The time between two 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. watches. Time shown by ordinary clocks and Thermo-electric currents. Streams of electricity, excited by heat. Transit. place. The passage of a body across the meridian of a Transit of Venus and Mercury. The apparent passage 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. Undulation. A wave. Undulatory theory. The mechanical principles of the motion of waves. Vapor. Steam. Variation. A periodic inequality in the motion of the moon. Variation of the compass. The deviation of the compass needle from the true north. Vertical. The direction of the plumb-line. Vertical plane. A plane passing through the plumb-line, consequently at right angles to the horizon. Vesicles. Small hollow spheres of water. Vibration. A motion to and fro. Visual ray. A ray of light coming from any object to the eye. Volta-electric induction. The disposition of electric currents to produce similar currents in bodies near them capable of receiving them. INDEX. Caloric, rays independent of -from all substances, 187 Catalogue of stars by Herschel, Celestial bodies now seen, 130; Centrifugal force opposed to the Chimborazo, Mount, 47 Climate, 200, 211, et seq. tion of Encke's and other re- 39 Connection of the physical sci- Crystal, 165 Crystallization, 190, 239 Dew, phenomenon of, 187 Diaphanous bodies, 180 Distances in the universe calcu- Fresnel, M., 169, 171 Glass, musical vibrations of, 125; Dynamics, principles of, 67; the Gravity, variation in, 42 theory of, 316 Eclipses, comparison of, 32, 36 variation of the, 20 Elevation of the earth at the Eras and chronology, 82, 84 Faraday, Mr., 233, 252, 260 Gymnotus electricus, 239 304 Sir John, 117, 289, 298, Herschel, Miss Caroline, 306 Horizon, its density of atmos- Horoscopes, accordance with as- Humboldt, Von, 98, 106, 209, 216, 269 Ice, formation of, 68; thaw, 193 Indians, their lunar tables, 84 Interferences, laws of, 121, 125, Isothermal lines, the, 208 Ivory, Mr., investigations by 41, Jupiter, the revolution of, 18, 23; Fluids, properties of, 168, 172, Kepler, theory of, 10, 18, 54, 61 184, 192 Forests, influence of, 206 La Grange on Periodical Ine- |