HEARING. 105 § 3. Varieties of Air-waves and their Equivalents in Consciousness. We have seen that air-waves, originally set in motion by the impact of one body upon another, impinge upon the ear, through the mechanism of the tympanum and the labyrinth, set up sympathetic vibrations in the elastic bodies which are attached to the terminals of the nerves, and thus communi¬ cate a stimulation to the fibres themselves. The impulse so imparted to the auditory centre is there subjectively cognized as Sound. We have next to enquire what are the various modes of aerial undulation, and what the corresponding varieties of Sounds. The first great distinction between air-waves is the one already drawn of those which are produced by a single im¬ pact and those which are produced by the continuous vibra¬ tion of an elastic body. The latter are periodic and regular, the former non-periodic and irregular. But if the ear is to be differentially excited by these different stimulants, and so to cognize them separately, it must have special organs for the perception of each ; and the appendages of these special organs must be set in sympathetic motion by one or the other kind of stimulant respectively. Now we find in ex¬ ternal nature that some bodies are readily set in motion by irregular and non-periodic waves, but that such bodies only continue in motion for comparatively short periods, instead of vibrating freely for a considerable time : while on the other hand certain other elastic bodies are not set in motion by non-periodic waves, but only answer to undulations whose