113 



interposing- between the pump and pipe leading to the receiver a 

 piece figured at Fig. 4, in which is a conical piece of brass, (A) 

 ground into a seat, and having a leather valve, (B) of the ordinary 

 construction, opening towards the barrel. With this addition, it 

 acts like a pump of the ordinary construction. Suppose the pis- 

 ton to be drawn up from the bottom of the barrel, the air above 

 it cannot return into the receiver as before, it must be expelled 

 through the parchment valve, (H) in the upper part of the pump, 

 see Fig. 3 after the piston passes the hole, the space below it is filled 

 from the receiver, and in its descent this portion of air is driven 

 through the lower valve while the vacuum left above it is again fil" 

 led from the receiver, and discharged as before through the upper 

 valve : (H) at last the air in the receiver will be unable to open the 

 valve which is interposed between it and the barrel. If more per- 

 fect exhaustion be required, the conical piece of brass (A) is now 

 to be pushed out of its seat, which is effected by turning a wire 

 that moves through a small collar of leathers, and which carries a 

 pinion playing into the teeth of the little rack (U). It is obvious 

 that the pump is now restored to its original form, and the ex- 

 haustion must proceed sine limite as before. 



It may be said that this produces complication, but if it does it 

 doubles the rapidity of exhaustion, and in this respect may be 

 compared to a pump, with a second barrel piston, &c. and there 

 can be no doubt of its superior simplicity, besides it is of very 

 easy execution, for the little collar of leathers is easily made, and 

 always remains stamich,* and any accidental leakage of the valye 



• As there is so little difficulty in making a collar of leatliers, it may perhaps be worth con- 

 sidering whether one having a rod with a groove cut in halt' its length, might not be substi- 

 tuted for the stopcock of (the air pump, which is always so diliicult to be made and kept 

 staunch. 



