A City Seen From Above; Distant Towers and Hilltops
Bright summits over dim bases, and why far towers look thick at the top and thin at the foot.
Two chapters close the discussion of thick air. In the first, an eye above a city in thick air sees the summits of buildings darker and more distinct than their bases, set off against a bright field of low dense air. The second explains that the lower boundaries of remote things are less perceptible than their upper ones: hilltops read sharply against the flanks of hills behind, while the low-lying thick air blurs the bases. The same makes tall towers seen at a distance look thick at the top and thin at the foot, since thin upper air reveals the side-angles that thick lower air conceals; the text runs on to the merlons of distant fortresses. The page is running text without a diagram.
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A city from above: dark, sharp summits against a bright field
An eye that sees a city beneath it in thick air finds the summits of the buildings darker and more distinct than their lower parts. The summits stand against a bright field because they are seen through the low, dense air.
Hilltops read sharper than their fog-blurred bases
The lower edges of remote things are less perceptible than the upper. Hill summits set clearly against the flanks of the hills behind, while the thick air lying in the low places confuses the boundaries of their bases; the same happens to trees and buildings that rise into the air.
Why far towers look thick at top and thin at the foot
Tall towers at long distance appear thick above and thin below, because the thin upper air does not hide the angles of their sides, whereas the thick lower air does. Where air is whiter it hides dark things from the eye more than azure air, as with the merlons of distant fortresses.