PROPERTIES OF GASES 105 



pushes open the valve and enters the cylinder. The pres- 

 sure on the water in the pipe is thus decreased, and water 

 is pushed farther up in the pipe, by the air pressure on 

 the surface of the water in the well. Atmospheric pres- 

 sure, which must be depended on to push the water from 

 the well through the pipe into the cylinder, can sustain a 

 column of water about 30 feet high. The distance, there- 

 fore , between the surface of the water in the well and the 

 bottom of the cylinder must not be more than 30 feet. 



When after several strokes the water has reached the 

 cylinder, if the piston is pushed down, the water presses 

 the valve in the piston open and' passes above it. The water 

 is then lifted to the outlet, by few or many strokes, accord- 

 ing to the length of the cylinder. 



The suction pump or, as it is better named, the lift pump 

 is used, not only to raise water from wells and cisterns, but 

 to remove gasoline and kerosene from barrels and tanks, 

 to pump out sea water which has leaked into the hold of 

 a ship, to pump river and lake water for use in factories, 

 and for many other similar purposes. 



113. The Force Pump. There are two important differ- 

 ences between a lift pump and a force pump. The force 

 pump has no valve in the piston, and it has an opening 

 near the bottom of the cylinder, into a .tube, out of which 

 the water is forced to levels much higher than the pump. 

 Water is raised from a cistern or well to the bottom of the 

 cylinder by atmospheric pressure, just as it is in the lift 

 pump. It is then forced out of the exit tube by pressure 

 of the solid piston in its down stroke. The water is thus 

 sent out in a spurt, through the exit tube, with every 

 stroke of the piston. 



In order to furnish a steady stream and to avoid succes- 

 sive shocks to the pipe, the force pump of a fire engine is 

 provided with an air chamber through which the water 

 passes from the cylinder. Here the water compresses the 



