MECHANICAL USES OF WATER AND AIE 131 



of operation, but the ones 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 that 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. 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 perhaps flow out of the spout. When the 

 piston ascends, lifting with it the water 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 it lifts the water, for it 

 does not in any way have a hold on the water below it, yet 

 the water rises. 



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

 water in the 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 the 

 same pressure of 15 pounds per square inch upon the surface 

 of the water within the pipe, and the two forces would ex- 

 actly counterbalance 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 pressure of the atmosphere, there is no down- 

 ward 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 water in the well. The atmospheric 



