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 first, owing to the medium within them having been set in motion and being unable to make its escape. Sound is audible in air, and so it is in water, although less distinctly; but neither air nor water is the efficient cause of sound, as for it there must be percussion of solid bodies against each other and against the air, and this is effected whenever the air, having been percussed, remains, is not, that is, dispersed. Thus, if the air be struck sharply and forcibly it gives out sound; for the motion of that which percusses should anticipate the dispersion of the air, as if any one were striking a rapidly moving heap or cloud of sand.

An echo is produced whenever the external air has been more than once repelled by the air contained within a vessel, by the sides of which that air is precluded from being dispersed, just as a ball rebounds. It seems as though an echo ought to be a constant occurrence, although it may not be audible, since that happens to sound which happens to light, and light is continually undergoing reflexion (for, otherwise, as light could not be everywhere, darkness would prevail beyond the spot illumined by the sun), but yet it is not everywhere reflected, as it is from water or brass or any other smooth body, so as to form a shadow whereby we are able to distinguish the light itself.

A Void is rightly said to be the sovereign cause of hearing;—for the air seems to be a void, and the