HOW DO WE USE AIR? 



65 



You can make a vacuum in the tube, but there is no 



force to push the water up. If you loosen the stopper, 



air can get in and 



with its force of 



fifteen pounds to 



the square inch lifts 



the water out as air 



replaces it in the 



bottle. 



Have you seen 

 anyone pour oil, 



sirUD Or Other Rea <* the paragraph and explain the diagram. 



liquids out of a gallon tin can having a flat top? They 

 usually tip the can so that the opening is at the lowest 

 part of the top surface. Study the diagram. Do you see 

 that air must enter and push upward through the liquid in 

 order to displace it ? The flow is jerky and irregular, often 

 spattering. When the can is lifted up, the liquid runs over 

 the top of the can. How can one prevent the bubbling of 

 air up through the liquid and so make a more even flow ? 

 This is done very simply. Hold the can with the outlet 

 at the highest level in pouring. The liquid will then flow 

 out and the air will pass in above the liquid. Some people 

 prefer to punch a hole through the top at the corner op- 

 posite the outlet and then the liquid passes out without 

 interference just as it does from the evaporated milk can 

 when two holes have been made in the top. If you do 

 not understand why this helps to make an even flow, ask 

 to have it explained. 



Man's Use of the Air. The most important use of the 

 air is for breathing. We may go without food for weeks, 

 without water for days, but we cannot go without air for 

 much more than a minute. Try to see how long you can 

 hold your breath. It is said that divers for pearls have 

 been known to remain under water for two or three 



H. & W. SCI. 1 6 



