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us now proceed to determine the nature of sound and hearing. Sound is double—one actual and another potential; for we say that some substances, such as sponge and wool, are without sound; and that others, as brass, and bodies which are hard and smooth, have sound, because such objects are able to sound; are able, that is, to create actual sound by the action of the medium between the object and the hearing. Sound of the actual kind is the invariable result of something in relation to something and in something; for its producing cause is percussion. It is impossible, therefore, that sound should be produced when there is only one substance, as that which percusses must be distinct from that which is percussed; so that the sonorous object sounds by its relation to another object. But there can be no percussion without movement, and sound is not produced by the percussion of any kind of substance, as we have said, (since wool, however percussed, does not, while brass and smooth and hollow bodies—brass because it is smooth—do give out sound,) and hollow bodies create, by reflexion, many percussions after the