[81] Mr. Sang, of Edinburgh, was, I believe, the first to treat this subject analytically. [82] This able paper was the starting-point of the experiments on sensitive flames, recorded in Chapters VI. and VII.; the researches of Thomas Young and Savart being the starting-point of the experiments on smoke-jets and water-jets.—J. T. [83] “Philosophical Magazine,” section 4, vol. xiii., p. 413, 1857. [84] “Philosophical Magazine,” section 4, vol. xiv., p. 1, _et seq._, July, 1857. [85] “Comptes Rendus” for August, 1853. Also “Philosophical Magazine,” section 4, vol. vii., p. 186, 1854. [86] “Philosophical Magazine,” section 4, vol. viii., p. 74, 1854. [87] “Proceedings of the Royal Institution,” January 15, 1875. [88] “Researches in Chemistry and Physics,” p. 484. [89] “Connaissance des Temps,” 1825, p. 370. [90] See Chapter VII., Part II. [91] The effect of the air of London is sometimes strikingly evident. [92] “Philosophical Transactions,” 1874, Part I., p. 208, and Chapter VII. of this volume. [93] Since this was written I have sent the sound through fifteen layers of calico, and echoed it back through the same layers, in strength sufficient to agitate the flame. Thirty layers were here crossed by the sound. The sound was subsequently found able to penetrate two hundred layers of cotton net; a single layer of wetted calico being competent to stop it. [94] The cut reached me too late for introduction at the proper place. INDEX A Acoustic clouds, echoes from, 325 —— reversibility, 461-469 —— transparency, great change of, 323 Air, process of the propagation of sound through the, 33 —— propagation of sound through air of varying density, 41 —— elasticity and density of air, 54 —— influence of temperature on the velocity of sound, 55 —— thermal changes produced by the sonorous wave, 60 —— ratio of specific heats at constant pressure and at constant volume deduced from velocities of sound, 62 —— mechanical equivalent of heat deduced from this ratio, 64 —— inference that atmospheric air possesses no sensible power to radiate heat, 66 —— velocity of sound in, 69 —— musical sounds produced by puffs of air, 89 —— other modes of throwing the air into a state of periodic motion, 91 —— reflection from heated air, 338 Albans, St., echo in the Abbey Church of, 50 Amplitude of the vibration of a sound-wave, 42 Arago, his report on the velocity of sound, 328 Atmosphere, reflection from atmospheric air, 335 Atmosphere, its effect on sound, 365 Auditory nerve, office of the, 32 —— manner in which sonorous motion is communicated to the, 33 B Bars, heated, musical sounds produced by, 87 —— examination of vibrating bars by polarized light, 209 Beats, theory of, 385 —— action of, on flame, 387 —— optical illustration of, 390 —— various illustrations of, 397 —— dissonance due to beats, 399, 428 Bell, experiments on a, placed _in vacuo_, 36-37