Chapter 7: On the air and the clouds

1. Air is emptiness, having more rarity mixed with it than the other elements. Of it Virgil says:

_Longum per inane secutus._

Air (_aer_) is so called from αἴρειν (to raise), because it supports the earth or, it may be, is supported by it. This belongs partly to the substance of heaven, partly to that of the earth. For yonder thin air where windy and gusty blasts cannot come into existence, belongs to the heavenly part; but this more disordered air which takes a corporeal character because of dank exhalations, is assigned to earth, and it has many subdivisions: for being set in motion it makes winds; and being vigorously agitated, lightnings and thunderings; being contracted, clouds; being thickened, rain; when the clouds freeze, snow; when thick clouds freeze in a more disordered way, hail; being spread abroad, it causes fine weather; for it is known that thick air is a cloud and that a cloud that thins and melts away, is air.

2. ... Now the thickening of the air makes clouds. For the winds gather the air together and make a cloud. Whence is the expression: “Atque in nubem cogitur aer.”

Chapter 8. On thunder.

1. Thunder (_tonitruum_) is so called because its sound terrifies (_terreat_), for _tonus_ is sound. And it sometimes shakes everything so severely that it seems to have split the heavens, since when a great gust of the most furious wind suddenly bursts into the clouds, its circular motion becoming stronger and seeking an outlet, it tears asunder with great force the cloud it has hollowed out, and thus comes to our ears with a horrifying noise.

2. One ought not to wonder at this since a vesicle, however small, emits a great sound when it is exploded. Lightning is caused at the same time with the thunder, but the former is seen more quickly because it is bright and the latter comes to our ears more slowly....

Chapter 9. On thunder-bolts.

1. ... Clouds striking together make thunder-bolts: for in all things collision creates fire, as we see in the case of stones, or when wheels rub together, or in the woods. In the same way fire is created in the clouds; whence they are clouds before, lightnings later.

2. It is certain that it is from wind and fire that thunder-bolts are formed in the clouds, and that they are launched by the impulse of the winds; and the fire of a thunder-bolt has greater force in penetrating because it is made of subtler elements than our fire, that is, the fire we make use of....