WATER PRESSURE 121 



the handle, for example, may be added for convenience or ease 

 of operation, but the parts mentioned above are the really 

 essential parts of the pump. 



As to the operation of the pump, we may suppose that at the 

 start the cylinder and pipe to the well are filled with water, 

 and the piston is at the top of the cylinder. If the piston is 

 pushed downward, the valve in it will permit the water in the 

 cylinder to pass through the piston into the space above it 

 as it descends. When the piston reaches the bottom of the 

 cylinder, the water in the cylinder will be above the piston. 

 As the piston ascends it is perfectly clear that the water in 

 the cylinder, which is now above the piston, will be lifted by 

 the piston and may flow out of the spout. When the piston 

 ascends, lifting with it the water that is in the cylinder, it 

 does not leave the lower part of the cylinder empty. The 

 water in the well flows up the pipe into the cylinder and fills 

 it to the bottom of the piston. Since the piston is above this 

 water, it cannot be said that the piston lifts the water, for 

 the piston does not in any way have a hold on the water that 

 is below it, yet the water rises. 



134. Why the water rises in the pump. The rise of the 

 water in the suction pipe of the pump is due to atmospheric 

 pressure. The atmosphere is pressing downward upon the 

 surface of the water in the well with a force of approximately 

 15 pounds to the square inch, and this pressure is transmitted 

 to the water within the pipe as an upward force. If the pipe 

 were open at the top, the atmosphere would exert a pressure 

 of 15 pounds per square inch upon the surface of the water 

 within the pipe also, and the two forces would exactly coun- 

 terbalance each other. In that case the water in the pipe would 

 be at rest at the level of the water in the well. Since the 

 pipe is tightly closed by the piston which supports the pres- 

 sure of the atmosphere, there is no downward pressure exerted 

 upon the water below the piston, but there is an upward force 

 of 15 pounds to the square inch transmitted to it by the 



