AIR PRESSURE AND WHAT IT IS 



count the number of times you breathe in one minute, you 

 will notice a distinct increase. Compare this with the 

 forced draft in the engine. Evidently the body needs 

 more air when we exercise than when we sit quietly, and 

 this wonderful human machine responds automatically to 

 its demands. 



Air pressure and what it is. We are told that air presses 

 down on us with a force of fifteen pounds to the square 

 inch. It is easy to understand that water exerts pressure 

 on a vessel which contains it, but it is more difficult to think 

 of air as having weight and so exerting pressure. We may 

 actually see in an experiment whether or not air has Weight 

 by weighing a bottle before and after pumping air out 

 of it. 



Experiment. Does air have weight? 



Materials: A two-liter bottle with one-hole rubber stopper. Glass and 

 rubber tubing. Burette clamp. Mohr clamp. Platform scales. 

 Sand or weights. Air 

 pump for suction or _ 



compression. Vase- ^= ^5^=3 ^ 

 line. 



Method and Observa- 

 tions: Pass the glass 

 tube through the rub- 

 ber stopper and at- 

 tach a rubber tube to 

 it. Place the Mohr 

 clamp on the rub- 

 ber tube. Set the bottle on one arm of the platform scales and 

 fasten the rubber tubing to a clamp on a ring stand leaving the tubing 

 loose enough to move freely up and down with the movement of the 

 balance. Make the stopper tight with vaseline and tie it in if air is 

 to be forced into the bottle. Exactly balance the bottle by pouring 

 sand into a jar or using weights on the other balance arm. 



By means of a force pump attached to the end of the rubber tube force 

 air into the bottle. What is the evidence to show if air has weight? 

 Disconnect the pump to open the tube to the air. Result? 



