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.... Chapter 10. On the rainbow and the causes of clouds. 1. The rainbow is so called from its resemblance to a bent bow. Its proper name is Iris and it is called Iris, as it were _aeris_ (of the air), because it comes down through the air to earth. It comes from the radiance of the sun when hollow clouds receive the sun’s ray full in front, and they create the appearance of a bow, and rarified water, bright air, and a misty cloud under the beams of the sun create those varied hues. 2. Rains (_pluviae_) are so called because they flow, as if _fluviae_. They arise by exhalation from earth and sea, and being carried aloft they fall in drops on the lands, being acted upon by the heat of the sun or condensed by strong winds. 13. Shadow (_umbra_) is air that lacks sun, and is so called because it is made when we interpose ourselves in the rays of the sun. It moves and is ill-defined, because of the motion of the sun and the force of the wind. As often as we move in the sun, it seems to move with us, because wherever we encounter the rays of the sun, we take the light from that place, and so the shadow seems to walk with us and to imitate our motions. Chapter 11. On the winds. 2. There are four chief winds. The first of these is from the east, _Subsolanus_, and _Auster_ from the south, _Favonius_ from the west, and from _Septentrio_ (north) a wind of the same name blows. These winds have kindred winds one on each side.