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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,
39

Action, compound and dis-
turbed, of the spheres, 12
Air, elasticity of, 90
Airy, Professor, 218, 225
Algæ, 279

Algol, the star, 388

Alps, the, 273

Amazons, river of the, 117

Andes, the, 134

Arabian science, 108

Arago, M., 135, 219, 266, 341
Arcs of the meridian, mea-
surement of, 54, 56
Asia, north-west basin of,
133

Astronomy, physical, 1; di-
visions of the science of,
68, 96

Astronomical tables, 68
Atmosphere, the, 123, 129,
130, 137, 163, 296
Attraction, its power uni-
versal, 1

and repulsion, elec-

trical, 291, 354

capillary, 123, 125

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Chimborazo, Mount, 56
China, observations made
in, 104

Chladni, experiment of, 147
Christian era, the, 100
Chronology of the visible
heavens, 389

Climate, 257, 267, 271
Coal, 83

Cold, 264, 266

Colours, prismatic, 172 et
seq.; of the stars, 393
Columbus and Lerius, 280,
314

Comets, theory of, and de-
scription of Encke's and
other remarkable comets,
67, 359 to 383

Compass, the mariner's, 313,
330

Compression, doctrine of,

90

Conductors, electrical, 294,
298

Conjunction of planets,
dates of, 46

Connexion of the physical
sciences, conclusion, 413
Constellations, motion and
revolution of, 391
Crystal, 212

Crystallization, 235, 243,
308

Curvature of the earth, 54

Day, the solar, 97

divisions of, 98, 107
Dew, phenomenon of, 240
Diameters of the sun and
planets, 67

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Eras and chronology, 100,
103

Ether, all-pervading, 359
Ethereal medium, the, 192,
217
Eudoxus, 105

Existence, grades of, 234

Faraday, Mr., 325, 336, 340,
356

Figure of the earth, 48, 54,
106

Fire, production of, 241
Flora, reign of, 273, 277
Fluids, properties of, 24,
219, 235, 249

Forests, influence of, 264

Fresnel, M., 216, 218

Frost, hoar, 240

Galvanism, 300 et seq. 345

Galvanometer, 351
Geology, 83

Glass, musical vibrations of,
157; prismatic phenome-
na, 172, 188; refraction
and polarization of light
by, 203, 216; imperme-
able to rays of solar heat,
229, 232
Gravitation, 410
Gravity, variation in, 50
Gymnotus electricus, 357
Heat, the earth's, 82; solar
radiation, 84, 228, 238;
of the atmosphere, 132;
theory, 244
Herbarium, Tartar, 275
Herschel, Sir William, 79,
228, 236, 384, 392, 397

Sir John, 147, 195,

386, 391, 398

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Isothermal lines, the, 267,
269

Italy, the north of, 56
Ivory, Mr., investigations
by, 49, 56

Jupiter, the revolution of,
17, 24; satellites of, 38,
79; magnitude of, 67;
density, 96

Kater, Captain, 106
Kepler, theory of, 7, 18, 64,
74, 362

La Grange on Periodical
Inequalities, 21

Language, its vocal articula-
tions imitated by instru-
ments, 162; primitive
tongues, 284

La Place, 22, 37, 73, 101,
103, 115, 135

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