THE ATMOSPHERE 



79 



not be clogged by sand and dirt. Such a pump is used 

 for pumping out the water from coffer dams and swamps, 

 and is also much used in city water systems. The pump 

 consists of wheel blades similar to those of a turbine wheel. 

 As the wheel is turned rapidly by some power, the air 

 is removed from the intake pipe and the water is forced 



FIG. 78. Force Pump. 



FIG. 79. Section of Centrifugal Pump. 



up by atmospheric pressure into the housing of the wheel. 

 The blades of the wheel then force the water out through 

 the discharge pipe (Figure 79). 



The Siphon. -- Take a short glass tube about twelve 

 inches long and bend it into the shape of the letter U, 

 with one arm of the tube slightly longer than the other. 

 Fill the tube with water, and then, holding the finger 

 over the long end of the tube, insert the other end in a 

 jar of water (Figure 80), letting the long end hang over 



