504 Dejcriptkn of (tnmu Air-ptmp, 



with the plate at the bottom of the barrel, tliis plate being turned in the lathe upon the piftoit 

 rod, which fitted its focket exaftly, fo that not the leaft fpaee might be left for lodgment of air- 

 under the pifton. The leathers are of the beft buck-fkin, dreffed in the ufual manner, finn 

 in its texture, but not harih ; and, being well dried, were foaked in a mixture of three parts, 

 fuet, melted with one part oil, before tliey, were put together : they were tlien, when cold^. 

 turned in a latlie on the pifton with a^ very fliarp tool *. 



The cylindrical iron rod of tlie pifton is well finifhed, and moves through a collar of leathers.. 

 As it is altogether neceffary that this rod fhould move molt exactly in the middle, or axis, of . 

 the barrel, care was taken that the pofition of the box, and of the focket annexed to it, fhould 

 be fecured, by having a prominent part of the plate of the box of leathers inferted into the 

 pump barrel, and a like projecting part of the lid of the box inferted into the box itfelf ; alfo 

 that the pifton rod fhould molt exactly fill the focket in the lid. Over the collar of leathers, 

 within the box, lies a tinned brafs plate, which is prefled down upon them by the ends of 

 three fcrews 3 3, fcrewed through the lid of the box. 



The pifton is moved up and down by the toothed flat bar, or rack, F, whofc end is fur- 

 niftied with an arm G (to be taken oiF occafionally), which arm is fecured againft a flioulder 

 formed on the end of the pifton rod by a nut H fcrewed on the rod ; tlae rack is moved by a 

 fmall fteel wheel L, of twelve teeth, turned by the handle X on its axis L. This wheel is 

 fuftained by the cheeks |K K, fig. i. and 2. furnilhed with projedling fockets, through which 

 its axis pafles, and thefe cheeks are fcrewed to the iron bar M, which is a part of the frame 

 fupporting the whole machine ; by this bar it is faftened by clamps to whatever table it is 

 placed on. From the middle of the bar, and at right angles with it, extends horizontally an 

 arm N (rivetted, and brazed upon the bar), the under furface of the arm being on a level with 

 the upper one of the bar ; this arm fupports the gage-glafsY, and terminates in a crofs-piece 

 (making together the form of the letter T), to which piece and to the arm the receiver-plate is 

 fcrewed by fcrews underneath, inferted into its thick margin. 



The bar M fupports the upright pieces, or pillars, 0, 0, terminating in the femicircular arms 

 py pi on which the barrel is faftened by four fcrews at py p, fcrewed a little way into its pro- 

 jefting rings. Thefe pillars are made of iron, and very ftrong, being half an inch in thick- 

 nefs ; they are alfo moft firmly fixed, and fhould be brazed in the bar which fupports them j 

 this being neceffary, as all the force of the handle of the wheel, by which the rack is worked, 

 bears againft thefe pillars ; and if they were to be fhaken, the cement, by which the re- 

 ceiver-pipe is connefted with the pump, wohW be broken, which cannot otherwife happen. 

 Thefe pillars fupport alfo the iron cafe, or fheath, P P, extended underneath the pump- 



• If tanned leather were ufed for the pifton, &c. it might be foaked in oil alone ; but, befides that, it is 

 ?pt to grow too hard by compreflion, and alfo to corrode the brafs of the barrel more, from the acid imparted 

 by the bark, with which it is tanned ; it alfo emits a greater quantity of elaftic fluid in vacuo than leather 

 which is net tanned. On thefe accounts, I have ufed buck-ftin leather for the pifton ; but the texture of 

 this being very loofe, oil only would not fill its pores, fo as to make the pifton air-tight, even when the 

 leathers were clofely prefled ; and it was neceflary to fill its grain with the above ftiflfer compofition. Yet 

 this leather will alfo generate air, and is harder to be made air faft ; it alfo moves fo ftiffly in the barrel, that, 

 perhaps, good flioe leather is preferable to it.— L. 



banelj 



