Fog-signals, researches on the acoustic transparency of the atmosphere in relation to the question of, 305 —— station at South Foreland, 309 —— instruments and observations, 309 —— variations of range, 315-316 —— contradictory results, 317 —— solution of contradictions, 317-323 —— extraordinary case of acoustic opacity, 318 —— in fogs, 355 —— minimum range of, 371 —— its position, 370 —— disadvantages of the gun, 368 Foreland, South, fog-signal station at, 309 —— fog at, 354 G Gaines’s Farm, account of the battle of, 324 Gases, velocity of sound in, 69 Gun, range of, for fog-signals, 312-313 —— inferiority to the siren, 369 —— its disadvantages as a signal, 368 H Hail, doubt as to its power to obstruct sound, 342 Harmonic tones of strings, 152-154 Harmony, 410 —— notions of the Pythagoreans, 411 —— Euler’s theory, 419 —— conditions of harmony, 411 —— influence of overtones on harmony, 429 —— graphic representations of consonance and dissonance, 431 Harmonica, the glass, 176 Hawksbee, his experiment on sounding bodies placed _in vacuo_, 36 Hearing, mechanism of, 424 Heat, thermal changes in the air produced by the sonorous wave, 60 —— ratio of specific heats at constant pressure and at constant volume deduced from velocities of sound, 64 —— mechanical equivalent of heatdeduced from this ratio, 66 —— inference that atmospheric air possesses no sensible power to radiate heat, 68 —— musical sounds produced by heated bars, 87 Helmholtz, his theory of resultant tones, 405, 406 —— —— consonance, 414, 420 Herschel, Sir John, his article on “Sound” quoted, 50 —— his account of Arago’s observation on velocity of sound, 328 Hooke, Dr. Robert, his anticipation of the stethoscope, 75 —— his production of musical sounds by the teeth of a rotating wheel, 85 Horn, as an instrument for fog-signalling, 310-311 Hydrogen, action of, upon the voice, 40 —— deadening of sound by, 38 —— velocity of sound in, 55, 69 I Inflection of sound, 53 —— case of the Erith explosion, 53 Interference of sonorous waves, 381-382, 407 —— extinction of sound by sound, 383, 408 —— theory of beats, 385, 408 Intervals, optical illustration of, 440 J Joule’s equivalent, 67 Jungfrau, echoes of the, 49 K Kaleidophone, Wheatstone’s, formation of, 170, 196 Kundt, M., his experiments, 344 L Laplace, his correction of Newton’s formula for the velocity of sound, 58-59 Le Conte, Professor, his observation upon sensitive naked flames, 274-275 —— on the influence of musical sounds on the flame of a jet of coal-gas, 454-460 Lenses, refraction of sound by, 51 Light, analogy between sound and, 45-50 —— analogy of, 320