Text: In order correctly to understand the sense of hearing we must have acquaintance with the principal laws of acoustics involved. Sound travels through air at about the rate of 1050 ft. a second, in water at about four times this velocity, and in very elastic solid bodies eighteen times as rapidly. In passing from solids to water the velocity is diminished, and from solids to air still more so; the passage from water to air or from air to water, very difficult. Vibrations lose much of their intensity in passing from air to solids. The cases of passage therefore from the medium of least to that of greatest density; i.e., from the air to water, are the cases of greatest difficulty in the transmission of vibration. A dry stretched menbrane easily receives and transmits vibrations of the air; and such a membrane placed on the surface of water overcomes in a great degree the difficulty of the passage between air and water. This assistance is enhanced when the membrane is combined with some solid body. Any membrane conducts sounds well when only in water. Sounds, like light, are liable to be reflected whether traveling in water or air. Certain terms require explanation. Sounds are "communicated" when they are merely conveyed from one sounding body to another, and this can take place in a noise as well as a musical sound. Sounds are "excited" under two circumstances: when a body which is sounding and that to be excited have the same note and the vibration of one produces sympathetic vibration of the other, the bodies are mutually called "reciprocating", while of the vibration of one produces its harmonics in the other, the latter is said, with regard to the exciting body, to be "resonant". According to Helmholtz, "timbre" or "quality" depends on definite combinations or certain secondary sounds or harmonics with a primary or fundamental sound, and such combinations he calls "sound colours".

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