Of the changes which go on in the air, they describe winter as the cooling of the air above the earth due to the sun's departure to a distance from the earth ; spring as the right temperature of the air consequent upon his approach to us ; summer as the heating of the air above the earth when he travels to the north ; while autumn they attribute to the receding of the sun from us. As for the winds, they are streams of air, differently named a according to the localities from which they blow. And the cause of their production is the sun through the evaporation of the clouds. The rainbow is explained as the reflection of the sun's rays from watery clouds or, as Posidonius says in his Meteorology, an image of a segment of the sun or moon in a cloud suffused with dew, which is hollow and visible without intermission, the image showing itself as if in a mirror in the form of a circular arch. Comets, bearded stars, and meteors are fires which arise when dense air is carried up to the region of aether. A shooting star is the sudden kindling of a mass of fire in rapid motion through the air, which leaves a trail behind it presenting an appearance of length. Rain is the transformation of cloud into water, when moisture drawn up by the sun from land or sea has been only partially evaporated. If this is cooled down, it is called hoar-frost. Hail is frozen cloud, crumbled by a wind ; while snow is moist matter from a cloud which has congealed : so Posidonius in the eighth book of his Physical Discourse. Lightning is a kindling of clouds from being rubbed together or being rent by wind, as Zeno says in his treatise On the Whole ; thunder the noise these clouds make when they rub against each other or burst. Thunderbolt is the term used when the fire is