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the brafs. and becoming clammy) will render the valve not air- 

 tight, I am therefore perfuaded that no valve will fo perfedly 

 or certainly exclude air as a flop-cock; and this I take to be 

 the chief reafon why Mr. Smcaton's pump, as improved by Meffis. 

 Kaas's and Hurler's contrivance for raifing the valve at the 

 bottom of the barrel, does not in pradice anfwer as well as 

 any other : for if fome air did not return into the receiver by 

 that valve, in pumps on his conftrudion, it would be hard to 

 tell why it fhould not perform as well as a pump of any other-' 

 conftrudion. 



To the bottom of the barrel is fattened, by four fcrews pafled 

 through its prominent rim, the circular plate R, forming, in part,, 

 the fhell of the cock S, and which has the high ridge T on the 

 back of it ; all thefe being only one piece of metal, which is 

 reprefented feparately in fig. 5, and was caft in thap form : the 

 round plate is about ith of an inch thick, but the cavity of 

 the cock intrudes fo far on this that, when the key of the cock 

 is put in, the thicknefs of the plate in the part diredly over- 

 the key, where the duds open into the barrel, is not more 

 than -^th of an inch : the length of the fhell of the cock is 

 three inches ; that of the key -^th of an inch lefs : the dia- 

 meter of the key at the thicker extremity is i vVths of an 

 inch, and at the fmaller end i ith of an inch. It is turned ' 

 by the handle u, fattened on its axis as near to the end of the 

 fliell as may be, viz at the diftance of ith of an inch. Care 



Vol. VI. T t was 



