96 Mr.. Kemp's Description of a New Mercurial Air- Pimp. 



receiver becomes so weak as to be incapable of raising the valve of 

 the barrel. 



It is evident that a pump on this construction cannot exhaust 

 to any great extent, because a certain force is necessary for open- 

 ing the valves, and this power is only obtained by the elasticity of 

 the air remaining in the receiver. 



This limit to exhaustion is caused much sooner, too, on account 

 of the quantity of air which necessarily remains between the under 

 surface of the piston and the bottom of the barrel, from the impos- 

 sibility of bringing their surfaces into close contact, along with an- 

 other quantity, insinuated among the leathers of the piston, ex- 

 panding and filling the barrel at every ascent of the piston, as no 

 part of it can be expelled through the piston valve, until it has ar- 

 rived at the density of the atmosphere. 



This portion, then, which, on this construction, cannot be got 

 rid of, always acts as a counter force on the elasticity of the air 

 within the receiver, and completely prevents any farther exhaus- 

 tion. 



Various methods have been proposed for carrying the exhaustion 

 to a greater extent, but that adopted by Cuthbertson in the con- 

 struction of his pump, is by far the most perfect. 



The valves in this construction act independently of the atmo- 

 sphere altogether, and are opened by the working of the punip. 

 In this manner the exhaustion can be carried to a much greater 

 extent, as the elasticity of the air within the receiver has no force 

 to overcome in lifting the valves, but has merely to flow into the 

 barrel till an equilibrium is restored. Another peculiarity of it is, 

 that the atmosphere is completely excluded from the upper surface 

 of the piston, and a vacuum in consequence is formed there. 



When the exhaustion therefore is carried to a high degree, the 

 air contained between the under surface of the piston and bottorii 

 of the barrel, along with that among the stuffing of the leathers, 

 will be of the same density as that portion contained in the upper 

 part of the barrel, which may be four or five hundred times rarer 

 than that of the atmosphere. When the piston, therefore, again 

 ascends, the barrel is filled with air of this high rarity, and it con- 

 sequently admits of a new portion of air to flow in readily from the 

 receiver. Now, in every other construction, the portion of air be- 

 tween the surface of the piston and bottom of the barrel being of 

 the density of the atmosphere, when the piston is raised, it just 

 expands and fills the whole barrel with air of a considerable den- 

 sity, which eflfectually prevents any great degree of rarefaction 

 from taking place. 



The excellence of the principles on which Cuthbertson's air- 

 pump has been constructed, has not been sufficiently appreciated, 

 otherwise we should not see individuals coming forward with their 

 improvements on the air-pump founded on the most exploded and 

 iinphilosophical principles, and asserting that they are capable of 

 producing an equal degree of exhaustion with his. 



