110 THE ATMOSPHERE AND ITS SERVICE TO MAN 



grams, but at the temperature of boiling water it weighs 

 only about .946 grams. So a volume of cold air, being 

 heavier, will exert more pressure at the surface of the 

 earth than an equal volume of hot air. 



As air is a gas whose particles can move freely among 

 themselves we should expect that a heavier column of cold 



air would sink down and 

 distribute itself along 

 the surface under sur- 

 rounding lighter air, 

 just as a column of 

 water falls when its 

 supports are withdrawn 

 and forces up the lighter 

 air which surrounds it. 



A similar action is 

 seen when water is 

 poured upon oil : the 

 water sinks to the bot- 

 tom and forces the oil 



FIGURE 56. HOT-AIR FURNACE to rise. Thus if air is 



Cold air presses in from the outside and heated at any place, WC 

 causes the hot air to rise through the h }d t ^ there 



pipes and registers. 



would be a rising current 



of hot air and a current of colder air creeping in to take 

 its place. The winds of the earth are due to this property 

 of air. It is this tendency of heated air to rise that makes 

 hot-air furnaces useful for heating houses (Figure 56). 

 Valleys are generally colder than the surrounding hill- 

 sides, so that delicate crops can be grown successfully 

 on the hillsides although those in the valley may be frost- 

 bitten. 



