THE DISTRIBUTION OF WATER 21 



good deal of the air above us has moved away, we have 

 what is called low pressure, that is, a low pressure of air 

 on the surface of the earth. But if the air is piled up 

 higher than usual in some places, those places have high 

 pressure. Then what happens? It is then that the winds 

 begin to blow. The air seems to try to get evenly distrib- 

 uted again. It rushes from the high-pressure places to 

 the low-pressure places. The greater the difference in 

 pressure, the faster the air will move. It is like water 

 rushing down a steep slope. It may rush so fast that there 

 is a great storm. So the air is always trying to smooth it- 

 self out (trying to equalize its pressures), and the sun, by 

 heating it unevenly, is always getting it stirred up again. 

 As a result of these two things, the air is always moving 

 and always distributing heat and moisture over the sur- 

 face of the earth. 



You will learn a good deal more about this when we 

 study the weather. But now you have learned enough to 

 understand how it is that water gets distributed. It is the 

 heat of the sun that does it. It is the heat of the sun that 

 starts evaporation, and it is the heat of the sun that keeps 

 the air moving. So it is the sun that keeps both air and 

 water moving. It is the sun, if anything, that is the heart 

 of the world, and the air and water that the sun moves 

 sustain life just as truly as the blood that your heart moves 

 sustains life in your body. 



Gravity. The fourth thing that explains the movements 

 of both air and water is gravity. It is heat that explains 

 why air and water go up. It is gravity that explains why 

 they come down again. We must understand both of 

 these things and see how they work together. 



