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OUR PHYSICAL WORLD 



calms, from the southeast south of it. These constant winds are 

 known as the trade winds. 



The temperature of the air is not the only factor that deter- 

 mines its weight or pressure. If it is carrying a great deal of 

 moisture, it is lighter than when it is dry, because the water 

 vapor displaces air and the latter is heavier than the former. 



FIG. 61. A fire. Note the piece of roofing carried up by the hot air 



The combination of these factors with others produces belts of 

 high pressure from which the trade winds move toward the 

 Equator, and other less steady winds known as "the westerlies" 

 which move toward the poles. The air does not move due north 

 and south toward the poles, but for the reason already indicated 

 these constant winds blow from the southwest in the Northern 

 Hemisphere, and from the northwest or nearly west in the 



