That
is why we say that we hear with what is empty and echoes, viz. because
what we hear with is a chamber which contains a bounded mass of air.
Which is it that 'sounds', the striking body or the struck? Is not
the answer 'it is both, but each in a different way'? Sound is a
movement of what can rebound from a smooth surface when struck against
it. As we have explained' not everything sounds when it strikes or
is struck, e.g. if one needle is struck against another, neither emits
any sound. In order, therefore, that sound may be generated, what is
struck must be smooth, to enable the air to rebound and be shaken
off from it in one piece.
The distinctions between different sounding bodies show themselves
only in actual sound; as without the help of light colours remain
invisible, so without the help of actual sound the distinctions
between acute and grave sounds remain inaudible. Acute and grave are
here metaphors, transferred from their proper sphere, viz. that of
touch, where they mean respectively (a) what moves the sense much in a
short time, (b) what moves the sense little in a long time. Not that
what is sharp really moves fast, and what is grave, slowly, but that
the difference in the qualities of the one and the other movement is
due to their respective speeds. There seems to be a sort of
parallelism between what is acute or grave to hearing and what is
sharp or blunt to touch; what is sharp as it were stabs, while what is
blunt pushes, the one producing its effect in a short, the other in
a long time, so that the one is quick, the other slow.
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