154 AGRICULTURE ON THE PACIFIC SLOPE 



A balloon rises in air because it is lighter than air. 

 When we go up in the basket of a big balloon, we carry 

 a barometer. The higher we go the more the mercury 

 falls, because the air pressure gets smaller. So the 

 barometer tells us just how high we are above the earth. 

 If we could go to where the air ends, the quicksilver in 

 the glass tube would fall until level with that in the cup, 

 because there would be no more air pressure. 



If we look at figure 78, we see on the Pacific Coast 

 the word Low. This means that in this region the air 

 pressure is low, as shown by the barometer. A similar 

 region is seen on the Atlantic Coast. Midway between 

 the pressure is high, as shown by the word HIGH. 

 The wind blows away from the HIGH and toward the 

 Low, as shown by the arrows. This is what we should 

 expect. Air moves, just as water does, from the place 

 of greater pressure to the place of less pressure. 



The map shows us that a storm is entering the coun- 

 try from the west. Its center is at the word Low. 

 In front of this center (to the east and south) rain is 

 falling. Still further ahead of it low rain clouds are 

 seen, and the temperature is rising. Still further east 

 of it we see high feathery clouds, and further still, 

 clear sky. 



Behind it (to the north and west) we see clearing 

 weather. On the Atlantic Coast, behind the storm 

 which is just leaving the country, we also see clearing 

 weather, then clear sky and falling temperature. 



