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
phenomena are one and the same thing, and are due to the same cause,
the difference between them being one of degree.

  The explanation of these and many other phenomena is this. When
the sun warms the earth the evaporation which takes place is
necessarily of two kinds, not of one only as some think. One kind is
rather of the nature of vapour, the other of the nature of a windy
exhalation. That which rises from the moisture contained in the
earth and on its surface is vapour, while that rising from the earth
itself, which is dry, is like smoke. Of these the windy exhalation,
being warm, rises above the moister vapour, which is heavy and sinks
below the other. Hence the world surrounding the earth is ordered as
follows. First below the circular motion comes the warm and dry
element, which we call fire, for there is no word fully adequate to
every state of the fumid evaporation: but we must use this terminology
since this element is the most inflammable of all bodies. Below this
comes air. We must think of what we just called fire as being spread
round the terrestrial sphere on the outside like a kind of fuel, so
that a little motion often makes it burst into flame just as smoke
does: for flame is the ebullition of a dry exhalation. So whenever the
circular motion stirs this stuff up in any way, it catches fire at the
point at which it is most inflammable. The result differs according to
the disposition and quantity of the combustible material. If this is
broad and long, we often see a flame burning as in a field of stubble:
if it burns lengthwise only, we see what are called 'torches' and
'goats' and shooting-stars. Now when the inflammable material is
longer than it is broad sometimes it seems to throw off sparks as it
burns. (This happens because matter catches fire at the sides in small
portions but continuously with the main body.) Then it is called a
'goat'. When this does not happen it is a 'torch'. But if the whole
length of the exhalation is scattered in small parts and in many
directions and in breadth and depth alike, we get what are called
shooting-stars.

  The cause of these shooting-stars is sometimes the motion which
ignites the exhalation. At other times the air is condensed by cold
and squeezes out and ejects the hot element; making their motion
look more like that of a thing thrown than like a running fire. For
the question might be raised whether the 'shooting' of a 'star' is the
same thing as when you put an exhalation below a lamp and it lights
the lower lamp from the flame above. For here too the flame passes
wonderfully quickly and looks like a thing thrown, and not as if one
thing after another caught fire. Or is a 'star' when it 'shoots' a
single body that is thrown? Apparently both cases occur: sometimes
it is like the flame from the lamp and sometimes bodies are
projected by being squeezed out (like fruit stones from one's fingers)
and so are seen to fall into the sea and on the dry land, both by
night and by day when the sky is clear. They are thrown downwards
because the condensation which propels them inclines downwards.
Thunderbolts fall downwards for the same reason: their origin is never
combustion but ejection under pressure, since naturally all heat tends
upwards.

  When the phenomenon is formed in the upper region it is due to the