On the exhaujling Power of the Air Pump. t6^ 



pidon. Thus, by only turning the cock one way or the other, the cock is made to exhaufl; 

 with one barrel or both. When it exhaufts with one only, the other is rendered i'ub- 

 fidiary to it by clearing it of air after each ftroke. ' 



To render the pump more eafy in working, there fliould be a hole in the bottom of 

 each barrel, furnifhed with a filk valve, through which the air would enter during the firft 

 ftrokes of the pump. The fituation of thefe holes are fhewn by dotted lines in the figure. 

 The air may be reftored to the receiver by unfcrewing the ftop fcrew h. 



Being foiled in all my attempts to procure a mercurial air pump, I turned my thought* 

 to the improvement of the common double barreled pamp, which, I believe, was invented 

 by Dr. Robert Hooke ; and which appeared to me the moft fimple and convenient form, 

 provided it could be made more accurate in its operation. 



The refult of fomc thought, and various experiments made to this purpofe, was, that 

 in theory I brought that pump to the perfection of rarefying permanent dry air 

 248000000 times, fuppofing the air to have a uniform power of cxpanfion to fuch a 

 degree, and the barrels of the pump to be twelve inches long by two in diameter. The 

 drawings and defcription of this pump are now in the hands of Mr. Nairne, who is 

 making one on that conftruftion, particularly adapted to the performance of chemical 

 experiments on air. I have been purpofely fliort in my defcription of the fcheme here- 

 with fent (which differs in many refpefts from that making by Mr. Nairne) as I purpofe 

 giving a full defcription of the other and its performance (if I find it to equal thofe 

 hitherto contrived) when tlie inftrument comes to my hands. 



Query, does the lower valve of Mr. Cuthberton's air pump increafc the power of that 

 inftrument, and would it not work equally well without it ? 



My reafons for fufpefling the utility of this valve are the following : 

 Firft, there mult always be a quantity of air above the plfton, when drawn up, that 

 cannot be expelled. 



Secondly, this quantity of air will be uniformly the fame, whether at the beginning 

 or end of the exhauftion, as it will take the fame force to open the expulfion valve at one 

 time as at another, and this air muft be as denfe as that of the atmofphere at leaft. 



Thirdly, when the pifton begins to defcend, the large conical valve muft thruft out of 

 its (hell, before that at the bottom of the barrel can poffibly ftiut, for the (hutting of the 

 one depends on the motion which caufes the opening of the other. 



Fourthly, when the pifton begins to afcend, the valve at the bottom of the barrel muft 

 open before that in the pifton can fhut, for the reafon above ftated. 



Now let us fuppofe the pifton at the top of the barrel, when the winch is turned the 

 large "onical valve will be opened, before that in the bottom of the barrel can fliut, at 

 leaft there will be an inftant when both valves may be faid to be half open, the air above 

 the pifton being much denfe'r than that below it, will take advantage of this opportunity to 

 diffufe itfclf equally through the barrel and receiver. If, however^ the air does not all 



M ra 2 : cfcape 



