an improved Air-Pump , &c. 437 
the machine would be too great, and therefore the pump, if it 
anfwered at all, was not likely to be durable. In confequence 
of this, he conlidered upon fome plan or other which might 
anfwer the fame purpofe in a better way, and foon hit upon a 
method of lifting up the valve at the bottom of the barrel, 
and of (hutting it again at pleafure ; which method being put 
into execution, has been found to anfwer exceedingly well. 
Belides this capital improvement, his air-pump is rendered’ 
altogether more convenient for philofophical experiments, by 
anfwering feveral purpofes, which will be enumerated after its 
defeription. 
Plate I. fhews a perfpedtive view of this pump. Plate Il„ 
exhibits a geometrical delineation of its metal or working parts 
detached from the wooden frame. And plate III. reprefents a 
fedtion of its parts. The letters of reference are made to an- 
fwer to the fame parts in all the plates. 
In plate III. fig. 1. AABI is the barrel of the pump, to the' 
brood or flat ends of which are ferewed, by means of five 
ferews, the pieces CE and K, with leathers between, to render 
the junctures perfectly air-tight *. The upper piece CE con- 
tains the collar of leathers for the handle or axis GG of the 
pifton to go through, and the bafin F, which ferves for a cap 
to ferew down the leathers, at the fame time that it holds the 
oil, which may be put into it, in order to let the collar of lea- 
thers hold very tight ; though it is fhewn, by experience, that 
when the leathers are perfectly foaked, there is not the leaf!; 
occafion to keep any oil in the bafin F. The fame upper piece 
* All the leathers ufed for this pump are foaked in melted hog’s lard; and 
when the parts of the inftruiHent are put together, a little oil is fmeared upon 
thofe furfaces of the brafs pieces which go agaitift the leather, though this oil may- 
be fpared. 
CE 
