SCIENCE OF COMMON THINGS. 



The common pump. 



How constructed. 



Pump valves. 



the air is removed from the tube by the ordinary pro- 

 cess of inhaling, when the pressure of the atmosphere 

 compels the liquid to fill the space deserted by the air. 



633 Through how great a length of tube could tve raise a liquid by 

 suction ? 



About thirty-two feet. 



633 Why can we not raise it above thirty-two feet f 



Because the atmospheric pressure will only support 

 or balance a column of water or similar liquid of that 

 height. 



634 How is the common pump constructed ? 



The common pump consists of a 

 hollow tube, the lower part of which, 

 descending into the water, is called 

 the siiction-pipe, and the upper part, b 

 (Fig. 20), the barrel or cylinder of a 

 spout, s, at the top of the cylinder ; of 

 an air-tight piston, which works up 

 and down in the cylinder ; and of two 

 valves, both opening upwards, one of 

 which, g, is placed at the top of the 

 suction-pipe, and the other, p, in the 

 piston. 



635 How does the common pump operate f 



When the piston is raised from the 

 bottom of the cylinder, the air above 

 it is drawn uj?, leaving a vacuum be- 

 low the piston ; the water in the well 

 then rushes up through the valve, g, 

 and fills the cylinder ; the piston is then forced down, 

 shutting the valve, g, and causing the water to rise 

 through the piston-valve, p ; the piston is then raised, 

 closing its valve, and raising the water above it, which 

 flows out of the spout, s. 



636 What is a valve f 



A valve, in general, is a contrivance by which water 

 or other fluid, flowing through a tube or^ aperture, is 

 allowed free passage in one direction, but is stopped in 



Fig. 20. 



