Chap. CCCVII.—Why the Air is to appear whiter as it approaches nearer to the Earth.
As the air is thicker nearer the earth, and becomes thinner as it rises, look, when the sun is in the east, towards the west, between the north and south, and you will perceive that the thickest and lowest air will receive more light from the sun than the thinner air, because its beams meet with more resistance.
If the sky terminate low, at the end of a plain, that part of it nearest to the horizon, being seen only through the thick air, will alter and break its natural colour, and will appear whiter than over your head, where the visual ray does not pass through so much of that gross air, corrupted by earthy vapours. But if you turn towards the east, the air will be darker the nearer it approaches the earth; for the air being thicker, does not admit the light of the sun to pass so freely.