306 AIR PUMP. 



not- be Co accurately fitted to the lower valve as, when put 

 down upon it, to leave no vacuity between them ; a portion of 

 air will necetfarily be retained in the barrel, which by itspref- 

 f'ure ftill further prevents the opening of the lower valve, and 

 caufes the operator to come to the limit of rarefaction much 

 fooner than he would upon the fuppofition that a perfect va- 

 Improvemcnts cuum were made in the barrel. Several very ingenious con- 

 o{ Cuthbertfon trivances have been invented to remove this imperfection, 

 among which thole of Cudibertfon and Prince are among the 

 lateft, and cannot fail of giving the reader a very high idea of 

 their fagacity and talents for invention. The method ufed by 

 the Rev. Mr. Prince, of removing the lower valve by open- 

 ing the bottom of the barrel into a ciftem which has a com- 

 munication with the receiver, firft gave the hint that it might 

 The air-pump be poffible in fome fimilar way to difpenfe with both the valves, 

 of Prmcc 1m- anc j by this means carry the air pump to a greater degree of 

 perfection. In purfuing this fubject I found that all this might 

 be effected, and in a way that admitted of much (implicity of 

 construction. 

 Defcriptlon of a The principle upon which this pump operates may be feen 

 new air pump. • the f i| owin ^ defcriplion of it. F, {Plate XIV. Fig. 1.) is 

 the pump pjale. O C is the barrel lying in a horizontal pofi- 

 tion underneath the pump plate, and nearly in contact with it. 

 A and B are two ducts leading from the pump plate into the 

 barrel. The pifton P is without a valve, being folid and accu- 

 rately fitted to the barrel. The pifton rod M is cylindrical and 

 moves air tight in the leathern collar O. There is another 

 pifton, N, made like the farmer, but (horter, and acted upon 

 by the fpring S, which is thence termed the fpring pifton. The 

 ends of thefe piftons are very carefully fitted to each other, fo 

 that when they are brought into contact they will form one 

 uniform cylinder without any vacuity betwixt them. H is the 

 winch, with a pinion and rack by which it is worked. The 

 pump is fupportedby a pedeftal upon which :t is firmly fixed. 

 A folid pifton The manner In which it operates is this: Suppofe the re- 

 works in a bar- ceiver placed over the duct A, leaving the duct B open to the 

 pafles through a external air, alfo the fpring pifton in the fituation N, exclud- 

 coliar of leather, ing the external air from the barrel, as reprefented in the figure 



SrbartTferve, and the P' ft ° n P in COnla61 witil lL The P ift ° U P > b > m0vin £ 



one to admit air towards the duct A, forms a vacuum in the barrel. When it 



from the receive £- ;s , t [ ie du6l A. it opens a communication between the 

 and the other to -*X*n *<J * i 



discharge it. , recover 



