vin HAIL A VERTED B Y BLOOD 183 



atmosphere near the earth. This layer has more 

 heat than any other, and that for three reasons. 

 One is that all evaporation from the earth, contain 

 ing as it does much dry, glowing matter, is always 

 the hotter, the more recently it has left the ground. 

 The second is that the sun s rays are reflected from 

 the ground and return upon themselves. Their 

 reflection heats up the parts next the ground, which 

 thus have more warmth from getting the sun s heat 

 twice. The third reason is that the upper regions 

 are more subject to wind ; but all places that are 

 sunk are less wind-swept. 



IX 



To the foregoing Democritus explanation falls to be 

 added. Every body receives heat more quickly and 

 retains it longer in proportion to its solidity. For 

 example, if three vessels, of copper, glass, and silver 

 respectively, are set in the sun, the heat will 

 penetrate the copper one soonest and will remain 

 in it longest. The reason why Democritus is of 

 this opinion may also be added. In the bodies, 

 he says, that are harder, more compact, and dense, 

 the openings must of necessity be smaller than in 

 others, and in each of the openings the film of 

 air must be thinner. It follows that just as smaller 

 baths and smaller cylinders are heated more rapidly 

 than others, so these concealed apertures, so small 

 as to elude the eye, both feel the heat more quickly, 

 and by reason of this same smallness of calibre 

 give back more slowly the heat they have received. 



