2

Davy, 99, 107

Day and night, 4

D'Arrest's comet, 296

Deimos, 38

Democritus, 44

Density of matter, 51
of aether, 69
electric, 170
of earth, 242
of sun, 27
of planets, 242

Descartes, 221-2, 341

Dewar, Prof., 47

Diameter of earth, 29
polar, 29
equatorial, 29
of sun, 280
of planets, 29

Dielectric, 168, 175

Diffusion, 49

Dip, magnetic, 199

Distances of stars, 280

Diurnal variations, magnetic, 209

Dog star (Sirius), 306

Donati's comet, 298

Double stars, 308

Draco, 307

Dynamical equivalent of heat, 114
value of light, 150

Dynamics, laws of thermo-, 114-18

Dynamo, 87, 90

Ear, 120

Earth, mass of, 235, 242
a magnet, 196-8
size of, 29
rotation of, 219
orbit of, 276
periodic time of, 251
gravity of, 29
shape of, 29
distance from sun, 242, 251
velocity in orbit, 251

Eccentricity of orbits, 268, 276

Eclipse, 145, 148

Ecliptic, plane of, 277-9

Elasticity, 46
of matter, 51
of aether, 74

Electric current, 154
potential, 170
induction, 175
radiation, 182
density, 170
energy, 179
field, 166, 179
lines of force, 173

Electricity, 64, 162
two kinds of, 175
positive, 175
negative, 175
laws of, 184
theories of, 162

Electrons, 136, 163

Electro-static, 158, 164
kinetic, 158, 164
kinetic energy, 217
magnetism, 192-5
magnets, 199
magnetic theory of light, 229
chemical equivalents, 189, 333

Elements of matter, 47

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 gravitati