36 



AIR COMPRESSORS 



reservoir exerts a resistance equal to the area of the piston 

 times the main-reservoir pressure; this is considerably less 

 than the combined pressures exerted by the steam and air 

 pressure on top of their respective pistons. When the pump 

 mechanism is reversed the action is simply a repetition of that 



just described. It is of little importance whether the high- 

 pressure air piston varies in its stroke or not, since it can 

 neither interfere with the valve-gear travel nor govern the 

 quantity of free air taken into the pump; therefore, its action 

 is immaterial so long as it forces all the air received from the 

 low-pressure cylinder into th<- main reservoir. 



