260 OUTLINES OF NATURAL PHILOSOPHY. 



piston, and a quantity of it is then lifted up every 

 time the piston is raised. 



This machine is called the Sucking Pump, and the 

 piston is called the Sucker. 



The valve of the piston, when lowest, must be nearer 

 to the surface of the water in the reservoir than 

 S3 feet, otherwise the water can never rise above 



it. 



351. The piston of the sucking pump, when it is 

 raised, sustains the weight of a column of water, 

 of a base equal to the section of the piston, and a 

 height equal to that at which the water in the 

 pump stands above the surface of the reservoir. 



This is the measure of that part of the pressure of 

 the external air that is not counteracted by the 

 elasticity of the air under the piston. 



To work the pump, a force must, therefore, be em- 

 ployed, able to overcome the weight of this column, 

 and of the piston, together with the friction of the 

 piston against the sides of the pump. 



352. In a sucking pump, the height of the 

 lower, or fixed valve, above the surface of the wa- 

 ter = a, the length of the stroke of the piston = b, 

 and the height of a column of water in equili- 

 brium 



