THE ATMOSPHERE. 45 



air in motion. Its force depends upon the rate at which 

 the air is moving, varying from a slight motion, which 

 gives a gentle breeze, to a velocity of forty or fifty 

 miles per hour, producing a hurricane, a tornado, or a 

 whirlwind. 



Cause of Winds. Winds are always caused by the 

 unequal density or weight of different portions of the 

 atmosphere. This is generally due to differences in tem- 

 perature. It is true of the air, as of other substances, 

 that heat expands it, making a given bulk of it lighter ; 

 and that cold contracts it, making a given bulk heavier. 

 If a quantity of oil is poured into a body of water, the 

 water, which is heavier, will push the lighter oil to the 

 surface. So, if the atmosphere in any locality becomes 

 warmer and lighter than the surrounding atmosphere, 

 the latter will push it into the upper regions, and will 

 rush in to fill the space underneath. 



The high wind that accompanies a thunder-storm at 

 the close of a hot day is due to the fact that the atmos- 

 phere, which has been heated during the day, is now 

 rapidly rising, while the cooler atmosphere around is 

 rushing in. 



The land and sea breezes, which are common on the 

 sea-coast, are due to the unequal heating of the atmos- 

 phere over land and water at different times of day. 



Rain. Rain is an accumulation of the vapor of the 

 atmosphere into drops, which, by their weight, fall to the 

 ground. There is a limit to the quantity of water which 

 the air is capable of absorbing and retaining as an invis- 

 ible vapor. When it contains as much as possible, it is 

 said to be saturated. Now, warm air is able to hold more 

 moisture than cold air ; hence, when air which is satu- 

 rated becomes colder, for any reason, it can no longer 



