HOW DO WE USE AIR? 59 



Air can exert pressure because of its (1) At (2) level 



the pressure is 14.7 (3) per (4) inch. (5) was the 



first to measure the (6) of the atmosphere. He did this by 



finding how tall a column of (7) the atmosphere would (8) 



Atmospheric (9) decreases as we go up from the surface of the 



earth. The barometer, by measuring (10) (11) , is useful 



in (12) forecasting, in measuring the (13) of (14) and 



to tell the (15) of an airplane. 



STORY TEST 



ARTHUR TELLS ABOUT THE PRESSURE OF THE ATMOSPHERE 



Read carefully and critically. List all the errors and suggest cor- 

 rections. 



Classmates : I have been reading about the pressure of the 

 atmosphere. Pressure is the force exerted by any body on one 

 unit area. We speak of anything as being "light as air" because 

 air has no weight. However, when I hold my open hand out 

 horizontally, the air or atmosphere is pressing down on my hand 

 with a force of more than 100 pounds. Galileo made the first 

 barometer with a long glass tube and mercury. He found that 

 the air pressure was about 15 pounds on every square foot of area 

 at sea level. As one goes higher into the air, the pressure increases 

 roughly in proportion to the elevation. A barometer can be used 

 to measure the height of a mountain, but an altimeter is used to 

 tell how high an airplane is above sea level. A barometer could 

 be used to tell the altitude of an airplane, but the altimeter could 

 not be used to measure the height of a mountain. When a glass 

 of water is inverted so that the water runs out, the glass has 

 nothing in it and is said to be "empty." A space that contains 

 no matter is a vacuum. When you drink soda water through a 

 straw, you pull the liquid up by suction. The air is denser in 

 Death Valley, which is below sea level, than at sea level, but less 

 dense than on a high mountain. When an automobile moves, it 

 must push the air away to make a space to move into. The air 

 pressure on the front of a moving auto is balanced by an equal air 

 pressure on the rear surfaces. 



PROBLEM III. HOW DO WE USE AIR? 



How Is the Air Used? Most boys and girls would 

 not think very long over this question, but what answers 

 would they give. For breathing, most would say, but not 



