> 26 Ccmpnrifcii bttween the FfftBs of the Cuiiiiicn Pump and the Air Ptimp. 



rate experiments in the Pneumatic Chemiftry. The air is exhaufled out of a receiver by 

 this Inftrument, by an aflion nearly fimllar to that of the common fucking pump, as it is called. 

 In the common pump there ii a bucket, which by Itatherlng is made to Aide, water-tiglit, up 

 and down the .pipe ; but Inllead of a bottom, it is provided with a valve or flap opening up- 

 wards. By this contrivance the bucket may be moved downwards through a column of 

 water without producing any progredivc motion in the fluid, becaufe the valve then optns 

 and alTords a free p.lVage ; and, on the contrary, when it is moved upwards, it muft alfo 

 move the whole of the column above the Hap or valve, which is then fhut. Another valve, alfo 

 opening upwards, is fixed in the pipe below the ordinary range of the bucket, and ferves to 

 prevent the return of any fluid which may lie between the two valves during the defcent of 

 tJie bucket itfcJf. 



In working this common and very ufeful engine, it is firft neceflTary to pour a fmall quan- 

 tity of water into the pipe above the bucket ; after which, the handle being moved pro- 

 duces the following eflecl : As the bucket defcends, the fpace between the two valves be- 

 comes dimiiiiflied, and the air contained in that fpace lifts the upper valve, and in part rifes 

 with nolle through the water; after which, as the bucket rifes, and again enlarges the 

 fpace, the remaining air expands by its fpring, which is thus weakened. The air in the pipe 

 beneath the lower valve will therefore a£l nure ftrongly upwards by its entire fpring than 

 the air above does downwards. It will open the valve, and rife up into the upper fpace ; at 

 the fame time that the prelVure of the external air, upon the water in the well, will drive part 

 of the water after it into the lower part of the pipe. By continuing the work for a few ftrokes 

 the included air is thus entirely drawn out and fucceeded by water, provided the height from 

 the furface of the fluid in the well to the upper valve be not fo great as to include a column 

 of upwards of thirty-three feet. For fuch a column would be fuflSciently heavy to counter- 

 poife the whole preflure of the atmofphere, ind would accordingly prevent any farther 

 afcent. 



If this lad-mentioned cafe were to exifl:, or if the lower aperture of the pipe were clofcd, 

 it may without difliculty be underftood that in theory, that is to fay, with a perfe£l inftru- 

 ment, the air beneath the lower valve would become more and more rarefied to a certain 

 point as the procefs was continued. And the progrefs.and extent of this rarefaftion would 

 be computable from the known ratios of the greateft and lead fpaccs between the valves 

 when in their extreme pofitions, together with the artumed conditions that the air, however 

 greatly rarefied, would diiTufe itfclf equally through the containing fpace, and that the 

 lower valve {hall aflord no rcfiftance to the power applied to open it when the bucket is 

 elevated. It will alfo follow, that the fame rarefaction might be produced in any veflcl con- 

 taining air, and communicating only with the pipe of the pump beneath the lower valve. 



Thus it is feen that the co.Timon fucking pump is, in facl, an air pump before it operates 

 as a pump for water; and that nothing more isrcijaired to conflitute the philofophic^d air 

 pump than convcnienci of fize, accuracy of workmandiip, and, that the rcfillance at the 

 valves be either diminilhed or removed. 



It would be to little purpofc to enter into any difcuHion concerning the orij;innl conftruC- 

 tion of the air pump, as invented by Otto Guerickc, and afterw.ards improved and mod ufe- 

 fuUy employed by our eminent countryman Robert Uoyle. It is generally allowed at prc- 

 fent, that fuch air pumps as operated by means of ftoivcocks inftead of the lower valve, are 



not 



