Mr. R. Gill on an Air-pump without Clearance. 257 



of the valves, but also that of the atmosphere which presses upon 

 them. If the space in the pump-barrel could be diminished 

 indefinitely, it is clear that the air contained in it would acquire 

 a pressure equal to lifting any weight of valve, even if extremely 

 rarefied at the beginning -of the stroke. But this is far from 

 being the case in our air-pumps; it is impossible that the 

 piston should fit so perfectly the sides and the bottom of the 

 cylinder as to fill entirely its capacity ; and consequently a certain 

 quantity of air must remain in the pump-barrel, which, expand- 

 ing as the piston is raised, fills the pump and maintains in it a 

 certain pressure which prevents other air entering from the re- 

 ceiver : the exhausting action must then necessarily cease. 



To avoid this difficulty, Torricellian air-pumps have been con- 

 trived, in which a vacuum is obtained by filling a receiver with 

 mercury and then allowing the mercury to run out through a 

 vertical tube of such a height that the column of mercury con- 

 tained in it overcomes the pressure of the atmosphere. It would 

 be inconvenient to fill the principal receiver itself, so that an ar- 

 rangement has to be made equivalent in action to the pump- 

 barrel ; this renders the apparatus complicated and difficult to 

 manage. Besides, it is well known that, in order to expel the 

 air completely from a receiver, it is not enough simply to fill it 

 with mercury : as this liquid does not wet glass or metals with 

 which it does not amalgamate (and these substances only can be 

 practically used for the construction of such receivers), a stratum 

 of air remains interposed between the liquid metal and the walls 

 of the receiver, which of course prevents a perfect vacuum being 

 formed when the mercury is run off; and this circumstance is 

 demonstrated by the fact that barometer-tubes can be properly 

 filled only by boiling the mercury in them. Moreover it has 

 been proved that the Torricellian vacuum is not perfect, as it 

 contains the vapour of mercury. 



In order to remove these difficulties, the following apparatus 

 has been contrived, in which clearance is totally avoided by fill- 

 ing the interstices between the cylinder and the piston with an 

 inevaporable liquid, and in which all the joints are kept covered 

 with liquid, so that no hiding-place is allowed to the air, which 

 consequently must be expelled entirely at every stroke of the 

 piston. Instead of the ordinary arrangement of two cylinders to 

 balance the atmospheric pressure, I have preferred using only one, 

 provided with a cover which prevents the atmosphere from press- 

 ing upon its piston. The bottom,/(Pl. IV.), of the cylinder is bell- 

 shaped and ascends into the cylinder, leaving between its outer 

 surface and the interior of the cylinder an annular space, b b ; 

 on the upper surface of this bottom is fitted the valve v, which 

 alternately closes and opens the passage, g, to the receiver to be 



Phil. Mag. S. 4. Vol. 32. No. 216. Oct. 1866. S 



