CHAPTER XI 

 WATER PRESSURE 



131. Importance of water. We are so accustomed to the 

 use of an abundance of water that we do not stop to think 

 how great its importance really is unless we are deprived of 

 it by accident or by residence in an arid country. It is, in 

 fact, one of the absolutely indispensable substances. If de- 

 prived of it, we soon die of thirst. We bathe in it and use it 

 to wash our clothes and to cleanse our homes and our utensils. 

 It floats our ships, turns our water wheels, supplies steam for 

 our engines, moistens the ground for our crops, and takes a 

 part in almost every manufacturing industry. 



Many interesting questions arise in connection with the 

 use of water : Why does an iron ship float ? Does it take a 

 stronger dam to hold back a lake of large area than a small 

 one? If pipes lead down into a house from a tank on the 

 roof, will the pressure be greater directly beneath the tank 

 than it is at a faucet on the same floor but off to one side ? 

 Will the pressure depend on the size of the tank ? How strong 

 do the pipes need to be in order to withstand the pressure ? 

 How does a pump lift water? How does a siphon transfer 

 water from one dish to another ? 



132. The siphon. The siphon may be used to transfer water 

 or other liquid from one vessel to another over an intervening 

 elevation. This can be done only in case the outlet of the 

 siphon is at a lower level than the surface of the liquid which 

 is being transferred. For instance, in the illustration (fig. 63) 

 the tube, which serves as a siphon, is represented as transfer- 

 ring water from the beaker at A to the one. at D, and this 

 latter is at a lower level. In starting a siphon to work it is 



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