VII
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Some writers believe that the air is rent and separated into small parts with void spaces, as they suppose, between. They consider the easy flight of birds through it a proof that it has not a compact body, but has large empty spaces: fowls, great and small, pass through it without difficulty. But this is a mistake. For water also affords the same easy motion, and there is no doubt of its unity. When it receives bodies, it always retreats in the direction opposite to them. This the Stoics call displacement, in Greek it is peristasis,40 which takes place in air just as it does in water. For it literally stands round every body by which it is pressed. There is no need to assume an admixture of vacuum with the element. But more of this another time.