Sound, production and propagation of, 32, 77 —— experiments on sounding bodies placed _in vacuo_, 36, 77 —— deadened by hydrogen, 38 —— action of hydrogen upon the voice, 40 —— propagation of sound through air of varying density, 40 —— amplitude of the vibration of a sound-wave, 42, 77 —— the action of sound compared with that of light and radiant heat, 45 —— reflection of, 45, 77 —— echoes, 48-50, 78 —— sounds reflected from the clouds, 49-50 —— refraction of sound, 51, 77 —— diffraction of sound, 53, 78 —— influence of density and elasticity on velocity, 54, 78 —— influence of temperature on velocity of sound, 55, 78 —— determination of velocity, 57, 78 —— Newton’s calculation, 58, 80 —— Laplace’s correction of Newton’s formula, 59, 80 —— thermal changes produced by the sonorous wave, 60, 80 —— velocity of sound in different gases, 69, 81 —— —— in liquids and solids, 70-73, 81 —— influence of molecular structure on the velocity of sound, 73, 81 —— velocity of sound transmitted through wood, 74, 81 —— diffraction of, 76 _note_, 78 —— physical distinction between noise and music, 82 —— musical sounds periodic, noise unperiodic, impulses, 83 —— —— produced by taps, 84 —— —— by puffs of air, 89 —— pitch and intensity o£ musical sounds, 90 —— vibrations of a tuning-fork, 91 —— M. Lissajous’s method of giving optical expression to the vibrations of a tuning-fork, 93 —— definition of the wave-length, 96 —— description of the siren, 97 —— determination of the rapidity of vibration, 101 —— and of the length of the corresponding sonorous wave, 102 —— various definitions of vibration and of sound-wave, 103 —— limits of range of hearing of the ear: highest and deepest tones, 106 —— double siren, 110 —— transmission of musical sounds through liquids and solids, 113-117 —— vibrations of strings, 120 —— the sonometer, or monochord, 121 —— influence of sound-boards, 123 —— laws of vibrating strings, 125 —— direct and reflected pulses, 129 —— stationary and progressive waves, 130 —— nodes and ventral segments, 130, 133 —— application of the results to the vibration of musical strings, 138 —— M. Melde’s experiments, 141, 427 —— longitudinal and transverse impulses, 144 —— laws of vibration thus demonstrated, 148, 162 —— harmonic tones of strings, 152, 163-164 —— definitions of timbre, or quality, of overtones and clang, 153, 164 —— relation of the point of string plucked to overtones, 155-156 —— vibrations of a rod fixed at both ends; its subdivisions and corresponding overtones, 165 —— —— of a rod fixed at one end, 166 —— Chladni’s tonometer, 168 —— Wheatstone’s kaleidophone, 170, 196 —— vibrations of rods free at both ends, 173, 197 —— nodes and overtones of a tuning-fork, 175-178, 197 —— —— rendered visible, 177-179, 197-198 —— vibrations of square plates, 184, 198 —— —— of disks and bells, 187-190, 198-199