Concerning symphony, he shows how sounds harmonize. A quick sound is acute, a slow is grave. Therefore acute sounds move the senses the quicker; and these dying and grave sounds supervening, what arises from the contemperation of one with the other causes pleasure to the ear, which we call harmony. And by what has been said, it may easily be understood that air is the instrument of these things. For sound is the stroke upon the sense of the hearer, caused by the air; and the air strikes as it is struck by the thing moving,—if violent, acutely,—if languid, softly. The violent stroke comes quick to the ear; then the circumambient air receiving a slower, it affects and carries the sense along with it.