to the irregularity of the clouds and the hollows that intervene where
their density is interrupted. This then, is thunder, and this its
It usually happens that the exhalation that is ejected is inflamed
and burns with a thin and faint fire: this is what we call
lightning, where we see as it were the exhalation coloured in the
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.