Illustration of difference between wave-velocity and wave-group velocity, 28 Inaudible sound, 140 Index of refraction, 54 Inductance, 214 Induction coil for wireless telegraphy, 267 Inefficiency of present methods of manufacturing light, 265 Inertia of the air, 111 Interference of air waves, 139 —— of electric rays, 248 —— of ripples and waves, 48 Irrotational motion, 59 K Kelvin, Lord, investigations on ship waves, 77 Kinetic energy, 25 Krakatoa, eruption of, 116 ——, sound produced by the eruption of, 116 L Laplace, calculation of, concerning sound-velocity, 69 Law connecting velocity and pressure in liquid motion, 67 Length of wave, definition of the term, 7 Light, velocity of, 187, 189 Liquid flow in constricted tube, 66 Lodge, Sir Oliver, coherer invented by, 211 Long wave, 7 Longitudinal waves, 7 Luminous efficiency, 264 —— rays, 254 M Magnetic force, 228 Major third in music, 161 Marconi coherer, 276 —— aerial wire, 266 ——, experiments with wireless telegraphy across the Atlantic by, 280 —— system of wireless telegraphy, 276, 279 Matter, 22 Maxwell’s electro-magnetic theory of light, 262 Mechanical explanations of electrical phenomena, 222 Methods of manufacturing light, 265 Minor third in music, 161 Model illustrating longitudinal wave, 114 —— —— nature of an air wave, 113 Models, illustrating wave-motion, 5, 6 Morse alphabet, 274 Motion, harmonic, 107 ——, irrotational, 59 —— of water in sea waves, 3 ——, periodic, 107 ——, rotational, 59 ——, vortex, 59 Movement of the air in the case of a sound wave, 112 Music, theory of, 159 Musical beats, 163 —— scale, notes of the, 160 —— tones and noises contrasted, 110 —— ——, sharp and flat, 161 N Natural period of vibration of a body, 148, 150 Node, 159 Non-conductor, electric, 197 Non-resistance to body moving through perfect fluid, 72 O Octave, 160 Open electric circuit, 217 —— organ-pipe, 171 Optical proof that a sounding body is in vibration, 105, 106 Organ-pipes, construction of, 169, 171 ——, distribution of air-pressure in, 172 ——, overtones of, 173 ——, relation between length of pipe and length of wave, 173, 174 Oscillations, electric, 185 —— of a stretched string, 154 Oscillator, Hertz, 207 Oscillatory electric sparks, photographs of, 202 Overtones, 156 P Pendulum, isochronism of the, 149 Perfect fluid, 59 Periodic motion, 107 —— time, 9 Phonograph, action of the, 142 Photographic study of the production of waves, 16 Photographs of ripples on a mercury surface, 51 Plane wave, 55 Potential energy, 25 Power required to propel ships, 90, 91 Prism for refracting a beam of sound, 136 Production of a sound wave, 111 Q Quality of sound, 115 R