ABOUT THE WEATHER. 113 



from west to east around its axis. If it did not, if it 

 moved more slowly than the earth, we who are chained to 

 its surface and revolutions should necessarily be pressed 

 upon by the air ; it would seem to come against us like a 

 hurricane a fact which, as will subsequently be shown, is 

 of great importance in the theory of the wind. I have 

 called the air a fluid, and it is actually such. It flows 

 from one place to another, and this very stream of air is 

 what we name wind. But, it will be asked, where are 

 these places into which it streams, for the air is evenly 

 distributed; consequently, universal equilibrium must prevail, 

 as in a vessel of water at rest. To elucidate this, I must 

 describe somewhat closely one of the most important 

 peculiarities of air. Heat, it is well known, expands the 

 bodies into which it enters. An iron rod, measured at a 

 red heat, is thicker and longer than when it is perfectly 

 cold. The same holds good of air, it expands, and there- 

 fore becomes lighter, as is shown by the simplest kind of 

 balloon, which, from its discoverer, is called a Montgolfier ; 

 this rises when the common air within it is heated by 

 means of a strong flame placed beneath its open extremity. 

 The air, thus become lighter, rises through the colder air, 

 as oil rises through water, and swims upon it. If cold air 

 lies upon an oblique surface, the warm air flows down over 

 the cold, like water upon a mountain, apparently without 

 mixing with it, when the difference of temperature is great 

 enough. 



But since warm air is thinner than cold, that is, because 

 there is less air in an equal space when it is warm than 

 when it is cold, the cold air flows into that space which is 

 warmed, and, because it is heavier, comes to the bottom. 

 If the door of a heated room is opened in very cold 

 weather, the cold air flows in on the ground, and the warm 



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