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CH. VIII.] is constantly giving out sound, just as a horn does; for the air within the ears is continually moving in some peculiar manner, and yet sound is foreign to that air and forms no part of its properties. It is on this account, however, that we speak of hearing by a void and something resonant, because we hear by the part which contains the air confined within it.

But is it that which percusses, or that which is percussed, which gives out sound ? Or do both contribute to its production, each in its own way? Now, sound is the motion of something which admits of being moved after the manner of bodies rebounding from smooth surfaces, whereon they may have been impelled. But every kind of body, whether percussing or percussed, does not, as has been said, give out sound ; as when a sharp point, for example, strikes a sharp point, there is no sound; but in order to produce sound, that which is percussed must be so smooth, that the mass of air upon its surface may rebound from, and be agitated over it. The distinctions among sonorous bodies are revealed in the actual sounds which they give forth; for as without light colours are not visible, so without sound the acute and grave are not audible. These terms (acute and grave) are derived from tangible properties, and employed, in a metaphorical sense, for sounds; for the acute moves the hearing quickly and sharply, the grave moves it slowly and dully; not, however, that