PUMPING MACHINERY 275 



used to pump the water, three to four days' supply should 

 be stored to provide for a calm. If a gasoline engine is 

 used, it will not be necessary to store for so long an in- 

 terval. Two systems of storing water are now in use : the 

 elevated tank and the pneumatic tank. 



391. Storage tanks. The elevated tank may be placed 

 outside on a tower, or in the building upon an upper 

 floor. The objection to placing a tank in a building 

 is the great weight to be supported. It has the advan- 

 tage of being protected from dirt and the weather. 

 The elevated tank on a tower is exposed to freezing in 

 winter and to the heat of the sun in summer. Further- 

 more, a tower and a wooden tank are not very durable. 

 The elevated tank is cheaper than the pneumatic system 

 where a large amount of storage is desired. A reservoir 

 located on a natural prominence, when such a location 

 can be secured, "offers many advantages in the way of 

 capacity and cheapness. 



The pneumatic or air-pressure system has an inclosed 

 tank partly filled with air and partly with water. When 

 filled the air is under pressure, and, being elastic, will 

 give the same kind of pressure to the water as an elevated 

 tank. One of the principal advantages of the air-pressure 

 system is that the tank may be buried in the ground or 

 placed in the cellar in a cool place. The disadvantage is a 

 limited capacity for the cost. 



If water be pumped into a closed tank until the tank 

 is half full, the air contained will give a pressure of about 

 15 pounds a square inch, which is sufficient to force the 

 water to a height of 33 feet. Air in the tank follows the 

 well-known law of gases known as Boyle's law pressure 

 X volume = constant. If the air be pumped to a pressure 

 of 10 pounds before the introduction of the water, the 

 maximum discharge from the tank will be had at the 



