The Summary of Physics Restored

79The middle region of air is not the one in which clouds gather and flashes of lightning and thunder occur; that is the upper part and boundary of the lower region. Rather, the region above the clouds, to which watery nature is prevented by its weight from rising, should be called the middle one. Sulphurous exhalations, however, freed from the weight of vapours, ascend to it and wander through it; there they grow hot through their own motion or another's and catch fire. Such are the fiery meteors of various kinds seen in the middle region. Hence we may conjecture that it abounds in heat and moisture, not watery but fatty moisture of the kind that feeds fire. There is much temperateness and tranquillity in it, because it is not pressed by turbulent winds, and only the lighter excretions of lower nature are carried up into it.