from the earth after them. Islands near the continent really form part
of it: the intervening sea is not enough to make any difference; but
those in the open sea can only be shaken if the whole of the sea
that surrounds them is shaken too.
We have now explained earthquakes, their nature and cause, and the
most important of the circumstances attendant on their appearance.
Let us go on to explain lightning and thunder, and further
whirlwind, fire-wind, and thunderbolts: for the cause of them all is
As we have said, there are two kinds of exhalation, moist and dry,
and the atmosphere contains them both potentially. It, as we have said
before, condenses into cloud, and the density of the clouds is highest
at their upper limit. (For they must be denser and colder on the
side where the heat escapes to the upper region and leaves them.
This explains why hurricanes and thunderbolts and all analogous
phenomena move downwards in spite of the fact that everything hot
has a natural tendency upwards. Just as the pips that we squeeze
between our fingers are heavy but often jump upwards: so these
things are necessarily squeezed out away from the densest part of
the cloud.) Now the heat that escapes disperses to the up region.
But if any of the dry exhalation is caught in the process as the air
cools, it is squeezed out as the clouds contract, and collides in
its rapid course with the neighbouring clouds, and the sound of this
collision is what we call thunder. This collision is analogous, to
compare small with great, to the sound we hear in a flame which men
call the laughter or the threat of Hephaestus or of Hestia. This
occurs when the wood dries and cracks and the exhalation rushes on the
flame in a body. So in the clouds, the exhalation is projected and its
impact on dense clouds causes thunder: the variety of the sound is due