384 PRACTICAL LESSONS IN SCIENCE. 



sponge holds water, but they are two gases mingled together, 

 each independent and sustained by the same force of heat. When 

 there is mingled with the air as much water vapor as the tem- 

 perature can sustain, the air is said to be saturated with mois- 

 ture, while if it might contain much more it is called dry air. As 

 the temperature of moist air is lowered the water-vapor at length 

 begins to condense into dew, fog or cloud. 



Air in tropical regions, near the sea, contains the most water- 

 vapor. Nature's method of lowering the temperature of the air 

 is by sending it upward or toward the poles. The winds from the 

 equatorial regions are relatively warm and moist winds, and in 

 general are rain-bearing winds. As winds rise over mountain 

 chains their temperature is lowered and the water vapor falls as 

 rain, as they move toward the poles the same thing occurs. The 

 up ward current from the tropical regions rises so high that mois- 

 ture is condensed, forming the copious rains of those regions. 



In South America the easterly winds of the torrid region com- 

 ing from the ocean, leave some moisture on the low mountains 

 near the eastern coast, but the greater part falls as they rise 

 over the giant Andes, giving that region an exceptionally heavy 

 rainfall and one of the most interesting river systems in the 

 world. 



In North America the westerly winds furnish a heavy rainfall 

 for the region west of the mountains and north of San Francisco. 

 And southwesterly winds that come over the regions of Arizona, 

 as upper currents, are the main source of rainfall in the upper 

 Mississippi valley, and westerly winds give Europe an abundant 

 rainfall. 



The eastern plateaus of Asia receive but little moisture on ac- 

 count of their high mountain borders ; their surfaces are barren, 

 and the air is nearly diathermic. During summer they become 

 excessively hot, and the air over them so light that exceptional 

 winds flow in from the east and south, which in rising over the 

 mountains lose their moisture as heavy rainfalls, forming an ex- 

 tensive system of great rivers. 



Africa furnishes a region of exceptional rainfall. The regular 



