particles and then the wind is beaten about and so catches fire.
A phenomenon in these islands affords further evidence of the fact
that winds move below the surface of the earth. When a south wind is
going to blow there is a premonitory indication: a sound is heard in
the places from which the eruptions issue. This is because the sea
is being pushed on from a distance and its advance thrusts back into
the earth the wind that was issuing from it. The reason why there is a
noise and no earthquake is that the underground spaces are so
extensive in proportion to the quantity of the air that is being
driven on that the wind slips away into the void beyond.
Again, our theory is supported by the facts that the sun appears
hazy and is darkened in the absence of clouds, and that there is
sometimes calm and sharp frost before earthquakes at sunrise. The
sun is necessarily obscured and darkened when the evaporation which
dissolves and rarefies the air begins to withdraw into the earth.
The calm, too, and the cold towards sunrise and dawn follow from the
theory. The calm we have already explained. There must as a rule be
calm because the wind flows back into the earth: again, it must be
most marked before the more violent earthquakes, for when the wind
is not part outside earth, part inside, but moves in a single body,
its strength must be greater. The cold comes because the evaporation
which is naturally and essentially hot enters the earth. (Wind is
not recognized to be hot, because it sets the air in motion, and
that is full of a quantity of cold vapour. It is the same with the
breath we blow from our mouth: close by it is warm, as it is when we
breathe out through the mouth, but there is so little of it that it is
scarcely noticed, whereas at a distance it is cold for the same reason
as wind.) Well, when this evaporation disappears into the earth the
vaporous exhalation concentrates and causes cold in any place in which
this disappearance occurs.
A sign which sometimes precedes earthquakes can be explained in
the same way. Either by day or a little after sunset, in fine weather,