POLK STEAM-EQUINE 85 



filling also the steam-vessel, with a pressure approach- 

 ing to that in the boiler, and equal to the weight of the 

 pole and pump-rods. A comparatively small supply 

 of steam from the boiler into the steam-vessel brought 

 it up to the boiler pressure, sending the pole and pump- 

 rods upwards with a spring. The steam-valve then 

 closed, allowing the momentum of the great weight of 

 pump-rods, together with the expanding steam, to com- 

 plete the up-stroke. The discharge-valve was then 

 opened for a moment, allowing a blast of steam to 

 escape, reducing the pressure say to one-half. The 

 weight of the rods caused their downward movement, 

 raising the load of water in the plunger-pole pumps, 

 and at the same time compressing the steam from 

 the pole-case into the steam-vessel, equal at the finish 

 of the stroke to the support of the pole and pump- 

 rods. This most simple steam-engine combined in the 

 greatest degree the two elements of expansion and 

 momentum. 



The up-stroke began with a much higher pressure of 

 steam than was necessary to raise the load ; having 

 given momentum to the rods, the supply of steam was 

 cut off, and the stroke was completed by expansion. 

 The down - stroke began with a comparatively low 

 pressure of steam under the pole. The unsupported 

 pump-rods fell downwards, setting in upward motion 

 the column of water in the plunger-pole pumps. The 

 discharge-valve was closed long before the completion 

 of the down-stroke, and the momentum of the moving 

 mass of rods and water compressed the steam driven 

 from the pole-case into the steam- vessel up to a pressure 

 equal to the support of the pole and pump-rods. The 

 pole was, therefore, continually floating or rising and 

 falling in steam of ever- varying pressure. 



