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, 463 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. Time, mean. Time shown by ordinary clocks and watches. 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. Undulation. A wave. Undulatory theory. The mechanical principles of the motion of waves. Vapour. 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 plumbline, 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 elec tric currents to produce similar currents in bodies near them capable of receiving them. INDEX. ABERRATION of light, 31 Acceleration of the moon, Action, compound and dis- Algol, the star, 388 Alps, the, 273 Amazons, river of the, 117 Andes, the, 134 Arabian science, 108 Arago, M., 135, 219, 266, 341 Astronomy, physical, 1; di- Astronomical tables, 68 and repulsion, elec- trical, 291, 354 capillary, 123, 125 Chimborazo, Mount, 56 Chladni, experiment of, 147 Climate, 257, 267, 271 Cold, 264, 266 Colours, prismatic, 172 et Comets, theory of, and de- Compass, the mariner's, 313, Compression, doctrine of, 90 Conductors, electrical, 294, Conjunction of planets, Connexion of the physical Crystallization, 235, 243, Curvature of the earth, 54 Day, the solar, 97 divisions of, 98, 107 Eras and chronology, 100, Ether, all-pervading, 359 Existence, grades of, 234 Faraday, Mr., 325, 336, 340, Figure of the earth, 48, 54, Fire, production of, 241 Forests, influence of, 264 Fresnel, M., 216, 218 Frost, hoar, 240 Galvanism, 300 et seq. 345 Galvanometer, 351 Glass, musical vibrations of, Sir John, 147, 195, 386, 391, 398 Isothermal lines, the, 267, Italy, the north of, 56 Jupiter, the revolution of, Kater, Captain, 106 La Grange on Periodical Language, its vocal articula- La Place, 22, 37, 73, 101, |