The Air We Breathe 



41 



Press the thumb into a rubber ball filled with air. Notice that 

 a rubber automobile tire flattens a little when the automobile is 

 heavily loaded. 



What property of air do 

 these facts show? 



Light a candle and place it 

 under a glass jar. It will soon 

 cease to burn. What does this 

 show ? 



The principle of air pressure 

 may easily be shown by the use 

 of a medicine dropper and a 

 glass of water. Squeeze the bulb 

 of the dropper after placing the 

 open end in the water ; then re- 

 lease the hold on the bulb. Ac- 

 count for what happens in the ^ 

 dropper. Apply this principle THE SIPHON 



tO the explanation Of the USe Of The air pressure at both levels is about the 



. . . . same, but the weight of water is less in the 



a VaCUUm Cleaner, a bicycle short arm than in the long arm. This 



d. causes water to flow down the long arm and 



rain pipe cleaner, an up the short arm. 



airplane, your breathing. 



Some Practical Uses of Air Pressure. The properties of 

 air that have been illustrated by the experiments in this chapter 

 are very important in doing many things that affect our lives. 

 The less the volume into which a certain amount of air is com- 

 pressed, the greater the pressure it will exert. The more air one 

 pumps into an automobile tire the harder it becomes. If air is 

 compressed to one-fourth of its original space, it will exert four 

 times as much pressure as it did in its normal condition. It 

 immediately regains its volume when the pressure is removed. 



Compressed air is often used in setting brakes on railroad 

 trains, automobiles, and buses. It has added greatly to the safety of 

 travel. Compressed air is used to run machine drills, in tunneling 

 and in mining. It is used to keep out mud and water from the box- 

 like rooms, or caissons, in which men work under water or beneath 

 the beds of streams. Large buildings are ventilated by forcing 



