40 FLUID PRESSURE 



course this keeps the tube full, and the water will 

 continue to run as long as the vertical distance be is 

 greater than that from b to x. 



Experiment 37. Dip a long rubber tube into water until it is 

 full. Pinch the end of the tube tightly to close it ; draw this end 

 out of the water, letting it hang over the side of the vessel. 

 When the end is lower than the liquid surface in the vessel let it 

 go, placing a dish to catch the water which runs. Now pinch the 

 tube somewhere along its length, and again remove the pressure ; 

 the stream ceases when the tube is pinched. Does it flow #gain 

 when the pressure is removed ? How can you stop the flow 

 entirely ? 



Siphons are convenient in getting liquids from barrels or 

 other vessels from which they cannot be easily poured. 



QUESTIONS 



1. What is the atmosphere ? Is it matter ? Why does it exert 

 force upon objects on earth? In what directions is that force 

 felt? 



2. How great is atmospheric pressure? Why do we not feel 

 the weight of it ? Why are not some bodies crushed by it ? Is 

 this pressure greater or less upon high land ? Why ? 



3. What is a vacuum? What is a partial vacuum? What is 

 the condition of the air in a partial vacuum ? Name some exam- 

 ples of vacua which you have observed. 



4. How does the atmosphere behave toward a vacuum ? State 

 any familiar examples of this. Why does a liquid run from a 

 jug in spurts? Why does it not run in the same way from 

 a pitcher? 



5. What is a barometer? How is it made? Would a tube 

 serve the purpose if open at its upper end? Why? How do 

 barometer changes indicate changes of weather? 



6. Explain how atmospheric pressure is used in raising water 

 from wells. How high may water be raised by atmospheric 

 pressure ? 



