CHAPTER VII 



AIR 



75. The Instability of the Air. We are usually not con- 

 scious 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 know that the ocean is never still for a sec- 

 ond ; 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. 



76. 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 



CL. GEN. SCI. 6 8l 



