Caiiflruition if an Air Pump luith Metallic Valves. 1 29 



fcrewed into the end of Figure 5, thefe will compofe the whole pifton, as reprefented by- 

 Figure 2. H in Figure i reprefents the fame part as H in Figure 2, and is that to which 

 the rack is fixed. If this therefore be drawn upwards, it will make Figure 5 fliut clofe into 

 Figure 4, and drive out the air above it ; and when it is puflied downwards, it will open as far 

 as the {boulders a a Figure 4 will permit, and fuffer the air to pafs through. A A Figure 7, 

 is the receiver plate ; B B is a long fquare piece of glafs fcrewed to the undermoft fide of the 

 plate, through which a hole is drilled, correfponding with that in the centre of the receiver 

 plate, and with tlie three female fcrews b b c. 



Suppofe the pifton to be at the bottom of the barrel, and a receiver to ftand upon the plate, 

 the infide of the barrel, from the top of the pifton to a, is full of air, and the pifton fliut ; 

 T^'hen drawn upward, by the hollow cylindrical wire H, it will in its courfe drive the air be- 

 fore it, through the hole a a, into the oil-veflel R, and out into the atmofphere by the tubeT. 

 The pifton will then be at the top of the barrel at a, and the wire q q will ftand nearly as it 

 is reprefented in the figure, juft raifed from the hole L, and prevented rifing higher by means 

 of the nut 0. While the pifton is moved upwards, the air will expand in the receiver, and be 

 driven along the bent tube m into the infide of the barrel. Thus the barrel will be filled 

 with air, which, as the pifton rifes, will be rarefied, in proportion as the capacity of the re- 

 ceiver pipes and barrel is to the capacity of the barrel alone. "When the pifton is moved 

 downwards again by H, it will force the conical part, Figure 5, out of the hollow part. 

 Figure 4, as far as the fhoulders a a : Figure 2 will reft upon a a Figure 4, which will then 

 be fo far open as to permit the air to pafs freely through it, while at the fame time the end 

 of y J is forced againft the top of the hole, and clofes it in order to prevent any air from 

 returning into the receiver. Thus the pifton, while moved downwards, fuffers the air to pafs 

 out between the fides of Figure 4 and 5, and when it is at the bottom of the barrel will have 

 the column of air above it ; confequently,when drawn upward, it will fliut and drive outthis 

 air, and by opening the hole L give a free paffage to more air from the receiver. This procefs 

 being continued, the air will be exhaufted out of the receiver as far as its expanfive power 

 will permit; for in this machine there are no valves, as in the common air pumps, to be 

 forced open by the air in the receiver, which, when its elafticity is diminiflied, it becomes 

 unable to effeft ; but every thing is contrived to open by the motion of the pifton, and there- 

 is nothing to prevent the air from expanding to its utmoft degree. 



In exhaufting with this machine, no other direftions need be obfevvcd, but fuch as are 

 common to all air pumps ; nor is any peculiar care required to preferve it in order, except 

 that the oil veflel G be always kept about half full of oil. When it has ftood a confiderable 

 time without being ufed, it will be proper to draw a table-fpoonful or two through it, by 

 pouring it into the hole in the middle of the receiver plate when the pifton is at the bottom 

 of the barrel ; then, by moving the winch backward and forward, to raife and deprefs the 

 pifton, the oil will be drawn through all the parts of the machine; and what is more than 

 it neceflary in the infide will be forced out through the tube T into the oil vefiel G. Near 

 the top of the cylindrical wire H is a fquare hole, which is intended to let in fome of the oil 

 from the velTel G, that the oiled leathers through which the wire q q Aides, may always be 

 fufficiently fuppticd with it. 



When the pump is required tocondenfe, cither at the fame time that it exhaufts, or fepa- 

 rately, the pi'-cc which contains the bent tube T muft be taken away, and Figure 8 put into 



Vol. I.— June 1797. S its 



