c23() ^^' PRIN'CE's AIR-rUMF. 



Valves fiid to be f y prevent the downward ofcillation, which, Dr. Prince 

 to'prevcnt '^' obferves, could only occur in a fingle-barrelled pump, and 

 downward which he obviated by ufing two barrels, the Encyclopedia 



• cilationj jv,^,^^ ,t valves were put into the pifton ; but as thefe require 

 force to open them, the addition feemed rather to increafe 

 the evil, by rendering the ofcillation more fimultaneous with 

 the ordinary rate of working." If, replies the Do6lor, fuch an 

 evil were produced by the defcent of the pifton, it is dif- 

 ficult to conceive, how putting valves into the pifton could 

 but valves could have increafed it. They could not increafe the evil, unlefs 

 Rot incifeafe its ^^^ increafed the refiftance to the air under the pillon. But 

 refiltance. -' ^ . , , . , 



it mufl be a ftrange adertion, that a pifton with a valve in it 



will give more refiftance to the air than a/olid pifton. 



Saidtobedif- Another objedion is : " it appears of very difficult execu- 



ficaltot esecu- ^j^^^^ j^ j^^^ many long, llender, and crooked paffages, which 



muft be drilled through broad plates of brafs, fome of them 



appearing fcarcely praclicable. It is rare to find plates and 



other pieces of brafs without air-holes, which it would be 



but lefs fo than difficult to find out and clofe, &c. Now the fa6l is, there is 



ethers. j^^j. f^ much pipe and du6l-vvork in the American air-pump, 



by more than one half, as in Mr. Nairne's improved pump of 



Smeaton, againft which no fuch objection is brought. There 



can be no reafon to prefume the brafs work would be more 



defedive, unlefs it were more complex in its form. And it is 



obvioufly far fuperior in point of fimplicity to Cuthbertfon's, 



which the Encyclopedia confiders as " the moft perfed that 



has yet appeared." 



After thefe ftridures on the Encyclopedia Britannica, 

 Dr. Prince gives an account of the improvements he has 

 made on his former attempt. 

 Subfequentim- * The following improvements have been made in the 

 American air-pump, by the inventor, to render it more fimplc 

 and convenient. It has been obferved above, that in all air- 

 pumps, made to condenfe as well as exhauft by the fame 

 barrels and winch, there muft be additional Tf)ipes, du6ls and 

 cocks to command and regulate the operations : But this is 

 not the beft method of conftruding the inftrument for ex- 

 haufting and condenling experiments : for a great ftrain is 

 brought upon the rackwork of the pump when feveral atmof- 

 pheres are throw into the receiver : and the pump may be 

 made with lefs trouble and expenfe by fixing a common con- 

 den ft ng 



rovementst 



