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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
combustion of the exhalation。 When it takes place at a lower level
it is due to the ejection of the exhalation by the condensing and
cooling of the moister evaporation： for this latter as it condenses
and inclines downward contracts； and thrusts out the hot element and
causes it to be thrown downwards。 The motion is upwards or downwards
or sideways according to the way in which the evaporation lies； and
its disposition in respect of breadth and depth。 In most cases the
direction is sideways because two motions are involved； a compulsory
motion downwards and a natural motion upwards； and under these
circumstances an object always moves obliquely。 Hence the motion of
'shooting…stars' is generally oblique。
So the material cause of all these phenomena is the exhalation；
the efficient cause sometimes the upper motion； sometimes the
contraction and condensation of the air。 Further； all these things
happen below the moon。 This is shown by their apparent speed； which is
equal to that of things thrown by us； for it is because they are close
to us； that these latter seem far to exceed in speed the stars； the
sun； and the moon。
5
Sometimes on a fine night we see a variety of appearances that
form in the sky： 'chasms' for instance and 'trenches' and blood…red
colours。 These； too； have the same cause。 For we have seen that the
upper air condenses into an inflammable condition and that the
combustion sometimes takes on the appearance of a burning flame；
sometimes that of moving torches and stars。 So it is not surprising
that this same air when condensing should assume a variety of colours。
For a weak light shining through a dense air； and the air when it acts
as a mirror； will cause all kinds of colours to appear； but especially
crimson and purple。 For these colours generally appear when
fire…colour and white are combined by superposition。 Thus on a hot
day； or through a smoky； medium； the stars when they rise and set look
crimson。 The light will also create colours by reflection when the
mirror is such as to reflect colour only and not shape。
These appearances do not persist long； because the condensation of
the air is transient。
'Chasms' get their appearance of depth from light breaking out of
a dark blue or black mass of air。 When the process of condensation
goes further in such a case we often find 'torches' ejected。 When
the 'chasm' contracts it presents the appearance of a 'trench'。
In general； white in contrast with black creates a variety of
colours； like flame； for instance； through a medium of smoke。 But by
day the sun obscures them； and； with the exception of crimson； the
colours are not seen at night because they are dark。
These then must be taken to be the causes of 'shooting…stars' and
the phenomena of combustion and also of the other transient
appearances of this kind。
6
Let us go on to explain the nature of comets and the 'milky way'；
after a preliminary discussion of the views of others。
Anaxagoras and Democritus declare that comets are a conjunction of
the planets approaching one another and so appearing to touch one
another。
Some of the Italians called Pythagoreans say that the comet is one
of the planets； but that it appears at great intervals of time and
only rises a little