CHAPTER XXXVIII 



AIR 



The instability of the air. We are usually not conscious 

 of the air around us, but sometimes we realize that the air is 

 heavy, while at other times we feel the bracing effect of the 

 atmosphere. We live in an ocean of air as truly as fish inhabit 

 an ocean of water. If you have ever been at the seashore, 

 you knOvV that the ocean is never still for a second. Sometimes 

 the waves surge back and forth in angry fury, at other times the 

 waves glide gently in to the shore and the surface is as smooth 

 as glass; but we know that there is perpetual motion of the 

 water even when the ocean is in its gentlest moods. Generally 

 our atmosphere is quiet, and we are utterly unconscious of it; 

 at other times we are painfully aware of it, because of its furious 

 winds. Then again we are oppressed by it because of the vast 

 quantity of vapor which it holds in the form of fog or mist. 

 The atmosphere around us is as restless and varying as is the 

 water of the sea. The air at the top of a high tower is very 

 different from the air at the base of the tower. Not only does 

 the atmosphere vary greatly at different altitudes, but it varies 

 at the same place from time to time, at one period being heavy 

 and raw, at another being fresh and invigorating. 



Winds, temperature, and humidity all have a share in deter- 

 mining atmospheric conditions, and no one of these plays a 

 small part. 



The character of the air. The atmosphere which envelops 

 us at all times extends more than fifty miles above us, its 

 height being far greater than the greatest depths of the sea. 



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