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act of its ejection。 It comes into existence after the collision and
the thunder； though we see it earlier because sight is quicker than
hearing。 The rowing of triremes illustrates this： the oars are going
back again before the sound of their striking the water reaches us。
However； there are some who maintain that there is actually fire
in the clouds。 Empedocles says that it consists of some of the sun's
rays which are intercepted： Anaxagoras that it is part of the upper
ether （which he calls fire） which has descended from above。 Lightning；
then； is the gleam of this fire； and thunder the hissing noise of
its extinction in the cloud。
But this involves the view that lightning actually is prior to
thunder and does not merely appear to be so。 Again； this
intercepting of the fire is impossible on either theory； but
especially it is said to be drawn down from the upper ether。 Some
reason ought to be given why that which naturally ascends should
descend； and why it should not always do so； but only when it is
cloudy。 When the sky is clear there is no lightning： to say that there
is； is altogether wanton。
The view that the heat of the sun's rays intercepted in the clouds
is the cause of these phenomena is equally unattractive： this； too； is
a most careless explanation。 Thunder； lightning； and the rest must
have a separate and determinate cause assigned to them on which they
ensue。 But this theory does nothing of the sort。 It is like
supposing that water； snow； and hail existed all along and were
produced when the time came and not generated at all； as if the
atmosphere brought each to hand out of its stock from time to time。
They are concretions in the same way as thunder and lightning are
discretions； so that if it is true of either that they are not
generated but pre…exist； the same must be true of the other。 Again；
how can any distinction be made about the intercepting between this
case and that of interception in denser substances such as water？
Water； too； is heated by the sun and by fire： yet when it contracts
again and grows cold and freezes no such ejection as they describe
occurs； though it ought on their the。 to take place on a proportionate
scale。 Boiling is due to the exhalation generated by fire： but it is
impossible for it to exist in the water beforehand； and besides they
call the noise 'hissing'； not 'boiling'。 But hissing is really boiling
on a small scale： for when that which is brought into contact with
moisture and is in process of being extinguished gets the better of
it； then it boils and makes the noise in question。 Some…Cleidemus is
one of them…say that lightning is nothing objective but merely an
appearance。 They compare it to what happens when you strike the sea
with a rod by night and the water is seen to shine。 They say that
the moisture in the cloud is beaten about in the same way； and that
lightning is the appearance of brightness that ensues。
This theory is due to ignorance of the theory of reflection； which
is the real cause of that phenomenon。 The water appears to shine
when struck because our sight is reflected from it to some bright
object： hence the phenomenon occurs mainly by night： the appearance is
not seen by day because the daylight is too in； tense and obscures it。
These are the theories of others about thunder and lightning： some
maintaining that lightning is a reflection； the others that
lightning is fire shining through the cloud and thunder its
extinction； the fire not being generated in each case but existing
beforehand。 We say that the same stuff is wind on the earth； and
earthquake under it； and in the clouds thunder。 The essential
constituent of all these phenomena is the same： namely； the dry
exhalation。 If it flows in one direction it is wind； in another it
causes earthquakes； in the clouds； when they are in a process of
change and contract and condense into water； it is ejected and
causes thunder and lightning and the other phenomena of the same
nature。
So much for thunder and lightning。
Book III
1
LET us explain the remaining operations of this secretion in the
same way as we have treated the rest。 When this exhalation is secreted
in small and scattered quantities and frequently； and is transitory；
and its constitution rare； it gives rise to thunder and lightning。 But
if it is secreted in a body and is denser； that is； less rare； we
get a hurricane。 The fact that it issues in body explains its
violence： it is due to the rapidity of the secretion。 Now when this
secretion issues in a great and continuous current the result
corresponds to what we get when the opposite development takes place
and rain and a quantity of water are produced。 As far as the matter
from which they are developed goes both sets of phenomena are the
same。 As soon as a stimulus to the development of either
potentiality appears； that of which there is the greater quantity
present in the cloud is at once secreted from it； and there results
either rain； or； if the other exhalation prevails； a hurricane。
Sometimes the exhalation in the cloud； when it is being secreted；
collides with another under circumstances like those found when a wind
is forced from an open into a narrow space in a gateway or a road。
It often happens in such cases that the first part of the moving
body is deflected because of the resistance due either to the
narrowness or to a contrary current； and so the wind forms a circle
and eddy。 It is prevented from advancing in a straight line： at the
same time it is pushed on from behind； so it is compelled to move
sideways in the direction of least resistance。 The same thing
happens to the next part； and the next； and so on； till the series
becomes one； that is； till a circle is formed： for if a figure is
described by a single motion that figure must itself be one。 This is
how eddies are generated on the earth； and the case is the same in the
clouds as far as the beginning of them goes。 Only here （as in the case
of the hurricane which shakes off the cloud without cessation and
becomes a continuous wind） the cloud follows the exhalation
unbroken； and the exhalation； failing to break away from the cloud
because of its density； first moves in a circle for the reason given
and then descends； because clouds are always densest on the side where
the heat escapes。 This phenomenon is called a whirlwind when it is
colourless； and it is a sort of undigested hurricane。 There is never a
whirlwind when the weather is northerly； nor a hurricane when there is
snow。 The reason is that all these phenomena are 'wind'； and wind is a
dry and warm evaporation。 Now frost and cold prevail over this
principle and quench it at it