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to the irregularity of the clouds and the hollows that intervene where

their density is interrupted. This then, is thunder, and this its

cause.

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.

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