Musical and harmonic sounds explained--Power of breaking glasses with the voice--Musical sounds from the vibration of a column of air--and of solid bodies--Kaleidophone--Singular acoustic figures produced on sand laid on vibrating plates of glass--and on stretched membranes--Vibration of flat rulers and cylinders of glass--Production of silence from two sounds--Production of darkness from two lights--Explanation of these singular effects--Acoustic automaton--Droz’s bleating sheep--Maillardet’s singing-bird--Vaucanson’s flute-player--His pipe and tabor-player--Baron Kempelen’s talking-engine--Kratzenstein’s speaking-machine--Mr. Willis’s researches 179 LETTER IX. Singular effects in nature depending on sound--Permanent character of speech--Influence of great elevations on the character of sounds, and on the powers of speech--Power of sound in throwing down buildings--Dog killed by sound--Sounds greatly changed under particular circumstances--Great audibility of sounds during the night explained--Sounds deadened in media of different densities--Illustrated in the case of a glass of champagne--and in that of new-fallen snow--Remarkable echoes--Reverberations of thunder--Subterranean noises--Remarkable one at the Solfaterra--Echo at the Menai suspension bridge--Temporary deafness produced in diving-bells--Inaudibility of particular sounds to particular ears--Vocal powers of the statue of Memnon--Sounds in granite rocks--Musical mountain of El-Nakous 212 LETTER X. Mechanical inventions of the ancients few in number--Ancient and modern feats of strength--Feats of Eckeberg particularly described--General explanation of them--Real feats of strength performed by Thomas Topham--Remarkable power of lifting heavy persons when the lungs are inflated--Belzoni’s feat of sustaining pyramids of men--Deception of walking along the ceiling in an inverted position--Pneumatic apparatus in the foot of the house-fly for enabling it to walk in opposition to gravity--Description of the analogous apparatus employed by the gecko lizard for the same purpose--Apparatus used by the Echineis remora, or sucking-fish 244 LETTER XI. Mechanical automata of the ancients--Moving tripods--Automata of Dædalus--Wooden pigeon of Archytas--Automatic clock of Charlemagne--Automata made by Turrianus for Charles V.--Camus’s automatic carriage made for Louis XIV.--Degenne’s mechanical peacock--Vaucanson’s duck which ate and digested its food--Du Moulin’s automata--Baron Kempelen’s automaton chess-player--Drawing and writing automata--Maillardet’s conjurer--Benefits derived from the passion for automata--Examples of wonderful machinery for useful purposes--Duncan’s tambouring machinery--Watt’s statue-turning machinery--Babbage’s calculating machinery 264 LETTER XII.