Buildings and Mountains Seen Through Thick Air
Dense air settles low near earth and water; a column diagram shows a tower fading degree by degree.
Three short chapters develop the low-lying-air theme. The first argues that air grows thicker the nearer it is to water or earth, proved from weight: the heavier a thing, the less it rises, so the lighter air climbs and the denser settles low. The second treats buildings in thick air, with a marginal column diagram: the eye n sees the tower a d less distinct but clearer at every lower degree, and more distinct but less clear at every higher degree. The third states that a dark thing looks clearer the more remote it is, so the near base of a mountain seems farther off than its own summit, which is in fact more distant.
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Air thickens toward water and earth because the heavier sinks
The nearer air lies to water or earth, the thicker it becomes. This is proved by weight: a thing rises less the heavier it is, so the lighter rises more than the heavy, leaving the denser air to settle in the low places.
The tower a d fades degree by degree from foot to top
A part of a building is less evident in thicker air and more distinct in thinner. A marginal column diagram shows that the eye n, viewing the tower a d, sees each lower degree less distinct yet clearer, and each higher degree more distinct yet less clear.
A near mountain base looks farther than its remote summit
A dark thing shows clearer the more remote it is, and darker the nearer it is to the eye. So the lower parts of anything in thick air seem more remote at their feet, and the near base of a mountain appears farther off than the summit, which is in fact the more distant.