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that is generated from it ought to hold good between the total
amount of air and the total amount of water。 Nor does it make any
difference if any one denies that the elements originate from one
another； but asserts that they are equal in power。 For on this view it
is certain amounts of each that are equal in power； just as would be
the case if they actually originated from one another。
So it is clear that neither air nor fire alone fills the
intermediate space。
It remains to explain； after a preliminary discussion of
difficulties； the relation of the two elements air and fire to the
position of the first element； and the reason why the stars in the
upper region impart heat to the earth and its neighbourhood。 Let us
first treat of the air； as we proposed； and then go on to these
questions。
Since water is generated from air； and air from water； why are
clouds not formed in the upper air？ They ought to form there the more；
the further from the earth and the colder that region is。 For it is
neither appreciably near to the heat of the stars； nor to the rays
relected from the earth。 It is these that dissolve any formation by
their heat and so prevent clouds from forming near the earth。 For
clouds gather at the point where the reflected rays disperse in the
infinity of space and are lost。 To explain this we must suppose either
that it is not all air which water is generated； or； if it is produced
from all air alike； that what immediately surrounds the earth is not
mere air； but a sort of vapour； and that its vaporous nature is the
reason why it condenses back to water again。 But if the whole of
that vast region is vapour； the amount of air and of water will be
disproportionately great。 For the spaces left by the heavenly bodies
must be filled by some element。 This cannot be fire； for then all
the rest would have been dried up。 Consequently； what fills it must be
air and the water that surrounds the whole earth…vapour being water
dissolved。
After this exposition of the difficulties involved； let us go on
to lay down the truth； with a view at once to what follows and to what
has already been said。 The upper region as far as the moon we affirm
to consist of a body distinct both from fire and from air； but varying
degree of purity and in kind； especially towards its limit on the side
of the air； and of the world surrounding the earth。 Now the circular
motion of the first element and of the bodies it contains dissolves；
and inflames by its motion； whatever part of the lower world is
nearest to it； and so generates heat。 From another point of view we
may look at the motion as follows。 The body that lies below the
circular motion of the heavens is； in a sort； matter； and is
potentially hot； cold； dry； moist； and possessed of whatever other
qualities are derived from these。 But it actually acquires or
retains one of these in virtue of motion or rest； the cause and
principle of which has already been explained。 So at the centre and
round it we get earth and water； the heaviest and coldest elements； by
themselves； round them and contiguous with them； air and what we
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
it。
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。
We see that motion is able to dissolve and inflame the air；
indeed； moving bodies are often actually found to melt。 Now the
sun's motion alone is sufficient to account for the origin of
terrestrial warmth and heat。 For a motion that is to have this
effect must be rapid and near； and that of the stars is rapid but
distant； while that of the moon is near but slow； whereas the sun's
motion combines both conditions in a sufficient degree。 That most heat
should be generated where the sun is present is easy to understand
if we consider the analogy of terrestrial phenomena； for here； too； it
is the air that is nearest to a thing in rapid motion which is
heated most。 This is just what we should expect； as it is the
nearest air that is most dissolved by the motion of a solid body。
This then is one reason why heat reaches our world。 Another is
that the fire surrounding the air is often scattered by the motion
of the heavens and driven downwards in spite of itself。
Shooting…stars further suffix to prove that the celestial sphere
is not hot or fiery： for they do not occur in that upper region but
below： yet the more and the faster a thing moves； the more apt it is
to take fire。 Besides； the sun； which most of all the stars is
considered to be hot； is really white and not fiery in colour。
4
Having determined these principles let us explain the cause of the
appearance in the sky of burning flames and of shooting…stars； and
of 'torches'； and 'goats'； as some people call them。 All these