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the heavenly bodies, which is then driven together by the motion of the
clouds and winds and is expelled by the clouds; or as a result of the
filtering of the finest form of light through the clouds^16 and as a result
of its movement; and by the conflagration of the wind which occurs
because of the vigour of its movement and its extreme compression; 102.
and because the clouds are broken by the winds and the atoms which
produce fire are then expelled and so produce the presentation of the
lightning flash. And it will be easy to see [that it could happen] in a great
many other ways, for him who clings always to the phenomena and who
is able to contemplate together what is similar to the phenomena.
The lightning flash precedes the thunder in this sort of arrangement
of clouds because the configuration which produces the lightning flash
is expelled at the same time as the wind strikes [the cloud] and subse-
quently the wind, being confined, produces this booming noise; and
because although both strike together, the lightning flash moves with a
more vigorous speed towards us, 103. while the thunder comes later, just
as happens with some things which strike blows and are observed from
a distance.
Thunder bolts can occur as a result of repeated gatherings of winds,
and their compression and powerful conflagration, and the fracture of
one part and its very powerful expulsion towards the areas below, the
breakage occurring because the places adjacent to it are more dense owing
to the thickening of the clouds; and [it may occur] just as thunder too
can occur, simply because of the expulsion of the fire, when a great deal
of it is confined and very powerfully struck by the wind and has broken
the cloud because it cannot escape to the adjacent areas since they are
always compacting together.^17 104. And thunderbolts can be produced
in several different ways-just be sure that myths are kept out of it! And
they will be kept out of it if one follows rightly the appearances and
takes them as signs of what is unobservable.
Whirlwinds can occur as a result of a cloud being forced in the form
of a column downwards to regions below, being pushed by a mass of
wind and driven by the power of the wind, while at the same time the
wind outside pushes the cloud to one side; and by the formation of the
wind into a circle when some air presses down on it from above; and as
a result of the compacting of the air around it, when a great flow of
winds takes place and is not able to flow off to the side. 105. And when
the whirlwind is forced down to the earth, tornadoes are produced, in
- Scholiast: "Or clouds were incinerated by the fire and the thunder is produced."
- Scholiast: "It generally [strikes] on a high mountain, on which thunderbolts most
often fall."