Ellipse, 34 Kepler's discovery of, 34 Elliptical nebulae, 320 Emission theory of light, 122 Encke's comet, 293, 296 resisting medium, 228 Energy, 83, 84 what it is, 83 conservation of, 84 transformation of, 86 potential, 87 kinetic, 89, 164 radiant, 109 electrical, 179 and motion, 91 Engine, Carnot's heat, 116 Envelopes of comets, 298 Epicureans, 44 Equal areas, 36 times, 36 Equator, N. and S. Poles, 29 magnetic, 202 terrestrial, 29 Equipotential surfaces, 171 Equivalents, electro-chemical, 189, 333 Euler, 123 Exchanges, theory of, 105 Experience, 4 Experiments of Rumford, 98 of Michelson and Morley, 67, 227 of Lebedew, 302 of Nichols and Hull, 153 of Hertz, 165 of Faraday, 65, 285 of M. Faye, 302 of Joule, 114 Falling stone, 21, 29 Faraday, 45, 175, 220 on matter, 56 on Lines of Force, 168, 200, 203-5 on magnetic space, 208 on gravitation, 287 on electricity, 332 on induction, 176 Faye, M., 111, 302, 319 Faye's comet, 293, 296 Field, electric, 179 magnetic, 199 Fizeau, 149, 224 Fluids, 48 Food, 89 Forbes, 121 Force, 16, 19, 90 centrifugal, 9, 13, 236 centripetal, 9, 12, 282 lines of, 200 Forces, direction of the, 26 proportion of the, 26 Foucault, 149 Franklin, 106 Fresnel, 75, 131 Frictionless medium, 17, 131, 136, 224 Fundamental medium, 54 Galaxy, 325, 346 Gaseous nebulae, 313 Gases, condensation of, 47 liquefaction of, 47 kinetic theory of, 49 Glazebrook, Prof., on aether and gravitation, 22 Globular clusters of stars, 314 Gravitation, law of, 61, 65 universal, 24 intensity of, 27 law of proportion, 26 cause of, 1, 282 and binary stars, 25 Gravity, centre of, 325 Great Bear, 307 Grove, 83 Halley's comet, 296-7 Hamilton, Sir W. R., 247 Head of comets, 298 Heat is motion, 98 a mode of motion, 107 radiant, 109 dynamical equivalent of, 116 effects of, 107 nature of, 98 radiation of, 109 transformation of, 87 Heat engine, Carnot's, 116 of sun, 109 and matter, 100 and work, 114 refraction of, 121 Helmholtz, Von, 45, 86 Hercules, 257, 271, 307 Herschel, Sir J., 3, 4, 7, 226, 228, 281, 299, 301, 310, 321 Sir W., 109, 271-2, 313 Hertz, 138, 159, 182 Hicks, Prof., 13 Hodograph of planets, 247 Hot springs, 90 Huggins, 313-14 Huyghens, 54, 123, 125, 128 Hydrogen, 40, 42, 44, 60, 93, 314, 333 Hypotheses, 3, 338 Ice, 42 Identity of heat and light, 119 light and electricity, 156, 160, 165 aether and electricity, 331 Imponderable, 71 Impressed force, 20 Impressible aether, 78 Induction, electric, 174-5 magnetic, 199 Faraday's theory of, 175 Inertia of matter, 52 of aether, 330 Intensity of light, 28, 146 of heat, 28, 113 of electricity, 184 of gravity, 28 Iron, 42 expansion by heat, 101 contraction by cold, 102 Joule, 85, 114