PNEUMATICS. 



183 



Figa. 174, 175, and 176, represent the various forms of valves used in 

 pumps, water-engines, etc. 



Ft^.. IT''.. 



Fig. 174. 



Fig. 175. 



394. "When it is desired to raise water to a greater height than 34 feet, a 

 modification of the pump, called the forcing-pump, is employed. 



■xvhat is a The Forcing-Pump is an apparatus wliich 

 Forcing-Pump? pgjggg vvater from a reservoir, on the principle of 

 the suction-pump, and then, by the pressure of the piston 

 on the water, elevates it to any required height. 



Fig. 177 represents the principle of the construction 

 of the forcing-pump. There is no valve in the pis- 

 ton c (Fig. 177), but the water raised through the suc- 

 tion-pipe a, and the valve g, by the elevation of the 

 piston, is forced by each depression of the piston up 

 through the pipe e e, which is furnished with a valve to 

 prevent the return of the liquid. 



The forcing-pump, as constructed in Fig. 177, ejects 

 the water only at each stroke of the piston, in the 

 manner of a syringe. TVhen it is desired to make the 



flow of the water continuous 

 as in a fire-engine, an air, 

 chamber is added to the 

 force-pump, as is represented 

 at A, Fig. 178. The water 

 then, instead of immediately 

 passing off through the discharging-pipe, partially 

 fills the air vessel, and by the action of the pLston 

 in the pump, compresses the air contained in it. 

 The elasticity of the air, thus compressed, being in- 

 creased, it reacts upon the water, and forces its ascent 

 in the discharge, or force-pipe. When the air in the 

 chamber is condensed into half its original bulk, it 

 wiU act upon the surface of the water with double 

 the atmospheric pressure, while the water in the 

 force-pipe, being subject to only one atmospheric 

 pressure, there will be an unrestricted force, press- 



FiG. 178. 



