66 LIVING IN AN OCEAN OF AIR 



minutes without breathing, but this is the limit of human 

 endurance. Incidentally the air all around us presses 

 upon every square inch of our surface with a force of 

 nearly fifteen pounds to the square inch. This great 

 force is necessary to hold us together because the fluids 

 within us are pressing outward with an equal force. Can 

 you imagine what would happen to a person who was 

 suddenly thrust into a vacuum ? Would he die in a short 

 time because of lack of air for breathing or would some- 

 thing startling happen quickly? Discuss this with your 

 classmates and your teacher. 



SELF-TESTING EXERCISE 



Select from the following list those words which best fill the blank spaces 

 in the sentences below and arrange the words in proper numerical order. 

 A word may be used more than once. 



fan coals piston air atmosphere 



bulb reduce sail mass greater 



into winds less pushed breathing 



cools pressure pump walls vacuum 



refrigerator row more valve volume 



heats tube increase weight space 



Air in motion is useful in many ways. The electric (1) 



cools us on hot days. A fan in the automobile (2) the radiator. 



Natural air movements as (3) may move (4) boats and 



kites. Artificial conditions to make the (5) work for us are 



found in devices which are capable of making a (6) For 



example, squeezing the (7) of a medicine dropper and then 



releasing it produces a (8) If this space containing the 



(9) is open to a liquid, the pressure inside the tube over the 



liquid is (10) than the pressure of the (11) on the liquid 



outside the tube; as a result the liquid is (12) (13) the 



tube. Another common device to (14) the pressure and so 



make a (15) is the cylinder having a (16) which can be 



moved back and forth in it. The bicycle or tire (17) is an 



example of this. Air molecules always are pushing against each 



other and against the (18) of whatever is holding them, so the 



more air we pump into a tire, the (19) the load it can hold 



up. The most important use of air is for (20) 



