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On The Heavens   



for air in sufficiently large quantity will contain a larger number of

triangles or solids or particles. It is, however, manifest that no

portion of air whatever moves downward. And the same reasoning applies

to lightness, if that is supposed to depend on superiority of quantity

of matter. But if, secondly, the kinds of matter are two, it will be

difficult to make the intermediate bodies behave as air and water

behave. Suppose, for example, that the two asserted are void and

plenum. Fire, then, as moving upward, will be void, earth, as moving

downward, plenum; and in air, it will be said, fire preponderates,

in water, earth. There will then be a quantity of water containing

more fire than a little air, and a large amount of air will contain

more earth than a little water: consequently we shall have to say that

air in a certain quantity moves downward more quickly than a little

water. But such a thing has never been observed anywhere. Necessarily,

then, as fire goes up because it has something, e.g. void, which other

things do not have, and earth goes downward because it has plenum,

so air goes to its own place above water because it has something

else, and water goes downward because of some special kind of body.

But if the two bodies are one matter, or two matters both present in

each, there will be a certain quantity of each at which water will

excel a little air in the upward movement and air excel water in the

downward movement, as we have already often said.



6



The shape of bodies will not account for their moving upward or

downward in general, though it will account for their moving faster or

slower. The reasons for this are not difficult to see. For the problem

thus raised is why a flat piece of iron or lead floats upon water,

while smaller and less heavy things, so long as they are round or

long-a needle, for instance-sink down; and sometimes a thing floats

because it is small, as with gold dust and the various earthy and

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