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dry and warm evaporation。 Now frost and cold prevail over this
principle and quench it at its birth： that they do prevail is clear or
there could be no snow or northerly rain； since these occur when the
cold does prevail。
So the whirlwind originates in the failure of an incipient hurricane
to escape from its cloud： it is due to the resistance which
generates the eddy； and it consists in the spiral which descends to
the earth and drags with it the cloud which it cannot shake off。 It
moves things by its wind in the direction in which it is blowing in
a straight line； and whirls round by its circular motion and
forcibly snatches up whatever it meets。
When the cloud burns as it is drawn downwards； that is； when the
exhalation becomes rarer； it is called a fire…wind； for its fire
colours the neighbouring air and inflames it。
When there is a great quantity of exhalation and it is rare and is
squeezed out in the cloud itself we get a thunderbolt。 If the
exhalation is exceedingly rare this rareness prevents the
thunderbolt from scorching and the poets call it 'bright'： if the
rareness is less it does scorch and they call it 'smoky'。 The former
moves rapidly because of its rareness； and because of its rapidity
passes through an object before setting fire to it or dwelling on it
so as to blacken it： the slower one does blacken the object； but
passes through it before it can actually burn it。 Further； resisting
substances are affected； unresisting ones are not。 For instance； it
has happened that the bronze of a shield has been melted while the
woodwork remained intact because its texture was so loose that the
exhalation filtered through without affecting it。 So it has passed
through clothes； too； without burning them； and has merely reduced
them to shreds。
Such evidence is enough by itself to show that the exhalation is
at work in all these cases； but we sometimes get direct evidence as
well； as in the case of the conflagration of the temple at Ephesus
which we lately witnessed。 There independent sheets of flame left
the main fire and were carried bodily in many directions。 Now that
smoke is exhalation and that smoke burns is certain； and has been
stated in another place before； but when the flame moves bodily；
then we have ocular proof that smoke is exhalation。 On this occasion
what is seen in small fires appeared on a much larger scale because of
the quantity of matter that was burning。 The beams which were the
source of the exhalation split； and a quantity of it rushed in a
body from the place from which it issued forth and went up in a blaze：
so that the flame was actually seen moving through the air away and
falling on the houses。 For we must recognize that exhalation
accompanies and precedes thunderbolts though it is colourless and so
invisible。 Hence； where the thunderbolt is going to strike； the object
moves before it is struck； showing that the exhalation leads the way
and falls on the object first。 Thunder； too； splits things not by
its noise but because the exhalation that strikes the object and
that which makes the noise are ejected simultaneously。 This exhalation
splits the thing it strikes but does not scorch it at all。
We have now explained thunder and lightning and hurricane； and
further firewinds； whirlwinds； and thunderbolts； and shown that they
are all of them forms of the same thing and wherein they all differ。
2
Let us now explain the nature and cause of halo； rainbow； mock suns；
and rods； since the same account applies to them all。
We must first describe the phenomena and the circumstances in
which each of them occurs。 The halo often appears as a complete
circle： it is seen round the sun and the moon and bright stars； by
night as well as by day； and at midday or in the afternoon； more
rarely about sunrise or sunset。
The rainbow never forms a full circle； nor any segment greater
than a semicircle。 At sunset and sunrise the circle is smallest and
the segment largest： as the sun rises higher the circle is larger
and the segment smaller。 After the autumn equinox in the shorter
days it is seen at every hour of the day； in the summer not about
midday。 There are never more than two rainbows at one time。 Each of
them is three…coloured； the colours are the same in both and their
number is the same； but in the outer rainbow they are fainter and
their position is reversed。 In the inner rainbow the first and largest
band is red； in the outer rainbow the band that is nearest to this one
and smallest is of the same colour： the other bands correspond on
the same principle。 These are almost the only colours which painters
cannot manufacture： for there are colours which they create by mixing；
but no mixing will give red； green； or purple。 These are the colours
of the rainbow； though between the red and the green an orange
colour is often seen。
Mock suns and rods are always seen by the side of the sun； not above
or below it nor in the opposite quarter of the sky。 They are not
seen at night but always in the neighbourhood of the sun； either as it
is rising or setting but more commonly towards sunset。 They have
scarcely ever appeared when the sun was on the meridian； though this
once happened in Bosporus where two mock suns rose with the sun and
followed it all through the day till sunset。
These are the facts about each of these phenomena： the cause of them
all is the same； for they are all reflections。 But they are
different varieties； and are distinguished by the surface from which
and the way in which the reflection to the sun or some other bright
object takes place。
The rainbow is seen by day； and it was formerly thought that it
never appeared by night as a moon rainbow。 This opinion was due to the
rarity of the occurrence： it was not observed； for though it does
happen it does so rarely。 The reason is that the colours are not so
easy to see in the dark and that many other conditions must
coincide； and all that in a single day in the month。 For if there is
to be one it must be at full moon； and then as the moon is either
rising or setting。 So we have only met with two instances of a moon
rainbow in more than fifty years。
We must accept from the theory of optics the fact that sight is
reflected from air and any object with a smooth surface just as it
is from water； also that in some mirrors the forms of things are
reflected； in others only their colours。 Of the latter kind are
those mirrors which are so small as to be indivisible for sense。 It is
impossible that the figure of a thing should be reflected in them； for
if it is the mirror will be sensibly divisible since divisibility is
involved in the notion of figure。 But since something must be
r