214 



GENERAL SCIENCE 



motion in the direction of the greater force. The air 

 pressure at B is the same as the air pressure on the 

 liquid in the vessel, so it is the difference between the 

 downward pressure of the two columns of water in CD 

 and CB that causes the water to flow. The liquid flow- 

 ing from C to B tends to produce a vacuum at C, while 

 the air pressure on the liquid in the vessel forces it up 

 the tube CD; this process keeps the liquid flowing. 



146. The Common Lifting Pump. If one end of a 

 tube containing a piston is placed in water and the piston 

 drawn up quickly, the water will follow 

 the piston up the tube. This occurs 

 because the motion of the piston re- 

 duces the pressure on the water in the 

 tube, and then the pressure of the air 

 on the water outside the tube forces 

 the water up the tube, the same as 

 when we take lemonade through a 

 straw by reducing the air pressure at 

 the end of the straw in the mouth. 

 If the piston in the tube in the illus- AlR PRESSUEE FORCES 

 tration were pushed down, the water THE WATER UP THE 

 also would flow down. This return 

 flow of the water is prevented in the common pump by 

 placing valves in the tube. The valves open when the 

 water flows up but close when it tries to flow down. 



In common lifting pumps there is a valve in the piston 

 and one in the pump tube as shown in the illustrations on 

 page 215. When the piston moves down, the valve in it 

 opens and the air escapes, as in diagram i, while the valve 

 in the pump tube was closed by its own weight and by 

 the force of the air trying to pass through. In diagram 

 2 the piston is rising and the valve in it was closed by the 



