x LAYERS OF ATMOSPHERE 61 



place by the earth's breath which is charged with 

 heat, while in addition the sun's rays are reflected 

 from the ground, and as far as the reflection extends 

 it renders the atmosphere kindlier and more genial. 

 Besides, the temperature of the lower air is raised 

 by the warm breath of all animals, trees, and 

 crops, whose life is dependent on heat. Add 

 to this also fires on the earth, not merely the 

 artificial ones about which we know, but also those 

 concealed beneath it, some of which have ere this 

 broken out, and myriads of which are blazing away 

 in the hidden depths incessantly. Add, too, that all 

 the fertile parts of the earth have some degree of 

 heat which is exhaled into the air : heat is a con- 

 dition of generation, the frigid is sterile. So, then, 

 the middle portion of the atmosphere being remote 

 from all these influences abides in its native cold : 

 for air is by nature chilly. 



XI 



SUCH being the divisions of the atmosphere, I r 

 may observe that in its lowest layer it is most 

 variable, unstable, and changeful. It is near the 

 earth that the air is, so to speak, most enterprising 

 and most long-suffering, as it tosses or is tossed. 

 But withal, it is not all affected in the same way, 

 but at different times at different points its different 

 parts are in unrest and turmoil. The reasons of 

 the changefulness and inconstancy are in part 

 derived from the earth : her position turning hither 

 and thither is a potent factor in determining the 

 quality of the atmosphere. Other reasons are due 

 to the heavenly bodies, chiefly the sun, whose course 



