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This long preamble leads up to the point we are now examining. All air is the denser the nearer it is to the earth. In water and other liquids the dregs are always at the bottom; in like manner in the atmosphere the thickest portions settle down to the lowest part nearest the earth. But it has already been proved that all things, in proportion as they are denser and more compact in their consistency, guard more faithfully the heat they have received. On the other hand, the more exalted the air is, and the farther it is withdrawn from the pollutions of earth, the less contaminated and the more pure it is; and so it does not retain the sun’s rays, but transmits them as if through a vacuum; hence it is less warmed by them.