SUCTION PUMP. 



Fig. 12. 



opening upwards, prevents its return, since the weight of the column 



above it will keep it closed. When the barrel A c becomes filled 



with water, the syringe no longer acts as 



such, but works on the principle of the 



lifting pump, already explained. When 



the piston descends, the valve x is closed 



and the valve v opened, the water passing 



through the piston. When the piston is 



raised, the valve v is closed, and the column 



of water above the piston is projected 



upwards. 



Meanwhile the pressure of the atmo- 

 sphere on the water in the well causes more 

 water to rise in the pump-barrel following 

 the piston. 



The atmospheric pressure is capable of 

 supporting a column of about 34 feet of 

 water.* It is evident, therefore, that such a 

 pump as is here described can only be efficient 

 when the piston is at a height of less than 

 34 feet above the surface of the water in 

 the well, since otherwise the atmospheric 

 pressure would not keep the water in 

 contact with the piston. 



The suction-pump, therefore, as com- 

 pared with the lifting-pump, saves 34 feet 

 length of pump rod ; but otherwise there 

 is no comparative mechanical advantage. 



It might appear at first view that the pressure of the atmosphere 

 sustaining a column of water in the suction-pipe, supplies aid to 

 the power that works the pump, and spares an equivalent amount 

 of that power. 



This, however, is not the case, as will appear from a due 

 consideration of all the forces which are in operation. 



14. Of these forces there are some which are directed down- 

 wards from the top of the column raised by the piston towards the 

 bottom of the well, and others which are directed upwards. Now 

 it is evident that the mechanical power applied to draw the piston 

 up will have to overcome all that excess by which the forces 

 downwards exceed the forces upwards. Let us suppose a column 

 of water resting on the piston, after having passed through 

 the valve v. The upper surface of this column is pressed upon by 

 the weight of the atmosphere ; the piston has, therefore, this 



See Tract on Barometer (10). 



155 



