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commonly call fire. It is not really fire, for fire is an excess of

heat and a sort of ebullition; but in reality, of what we call air,

the part surrounding the earth is moist and warm, because it

contains both vapour and a dry exhalation from the earth. But the next

part, above that, is warm and dry. For vapour is naturally moist and

cold, but the exhalation warm and dry; and vapour is potentially

like water, the exhalation potentially like fire. So we must take

the reason why clouds are not formed in the upper region to be this:

that it is filled not with mere air but rather with a sort of fire.

However, it may well be that the formation of clouds in that upper

region is also prevented by the circular motion. For the air round the

earth is necessarily all of it in motion, except that which is cut off

inside the circumference which makes the earth a complete sphere. In

the case of winds it is actually observable that they originate in

marshy districts of the earth; and they do not seem to blow above

the level of the highest mountains. It is the revolution of the heaven

which carries the air with it and causes its circular motion, fire

being continuous with the upper element and air with fire. Thus its

motion is a second reason why that air is not condensed into water.

But whenever a particle of air grows heavy, the warmth in it is

squeezed out into the upper region and it sinks, and other particles

in turn are carried up together with the fiery exhalation. Thus the

one region is always full of air and the other of fire, and each of

them is perpetually in a state of change.

So much to explain why clouds are not formed and why the air is

not condensed into water, and what account must be given of the

space between the stars and the earth, and what is the body that fills


As for the heat derived from the sun, the right place for a

special and scientific account of it is in the treatise about sense,

since heat is an affection of sense, but we may now explain how it can

be produced by the heavenly bodies which are not themselves hot.

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