1 84 PHYSICAL SCIENCE BK. iv 



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 con 

 sistency, 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 the air is, and the farther it is withdrawn from the 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. 



XI 



1 BUT contrariwise, certain persons assert that moun 

 tain peaks ought to be warmer in the degree in 

 which they are nearer the sun. Such people seem 

 to me, however, to be astray in supposing that the 

 Apennines and the Alps and other mountains 

 famed for their exceeding height are so greatly 

 elevated that their size should enable them to feel 

 in any special way the sun s proximity. No doubt 

 those are lofty heights so long as the standard of 

 comparison is ourselves. But when one regards 

 the size of the universe, the lowness of them all 

 becomes evident. Compared with one another, 



2 mountains are surpassed or surpass in height. But 



