-238 ^^^* prince's air-fumf. 



Alteration In ' Another alteration, fince made. Is in the lituatlon of Ih© 



the valve-pump, yalve-pump : the laft mentioned pump not having one fixed 

 to it. In all air-pumps having the tops of llie barrels clofed 

 with plates and collars of leather, as in Nairne's, Cuthbert- 

 fon's, and the American pump (as now altered by removing 

 the middle pipe,) it is neceflary to connect oil boxes with the 

 top.plates to receive the oil which is thrown out of the barrels 

 in working the pump. Cuthbertfon's pump has two, one to 

 each barrel. By removing the pipe from between the barrels, 

 in the Americen pump, a fmall barrel is fcrewed in its place 

 to the crofs-piece, which connedls the top-plates covering the 

 Barrel to anfwer valves. I'he barrel anfwers the purpofe of an oil-box in 

 o'l^box^and'al'fo ^'^"■"^on exhauftions. When greater vacuums are wanted in 

 a valve-pump> the receiver, this barrel anfwers alfo for a valve-pump. On 

 the top of the" crofs-piece is fcrewed a collar of leathers con- 

 taining a pidon and its rod, to work occafionally in the barrel 

 below. At the lower end of the barrel is a valve covered with 

 a cap: by unfcrewing the cap, and pafTing down the pifton,. 

 all the oil in the barrel is expelled through the valve; and 

 afterwards the barrel, and the fpace above the valves on the 

 top-plates of the great barrels, are exhaufted of air, by work- 

 ing this fmall pump. The fmall pifton when drawn up to i(s 

 collar of leathers is above the holes in the crofs-piece leading 

 from the valves. When the fmall barrel is ufed only as an oil- 

 i)ox, the collar of leathers, with the pifton, is removed, and 

 a button, with a fliort pipe in it, fcrewed in its place to give 

 vent to the air when expelled from the barrels : In this valve- 

 ThisnoaJdi- pump there is not fo much work as in Cuthbertfon's two oil- 

 tional expence. boxes ; nor is it an additional expence; for the fyringe, which ' 

 is ufed with the lead weight in llic receiver, is made to fcrew 

 to the crofs-piece for this purpofe; the weight being taken 

 off, and a cap fcrewed on over the valve, when ufed as an 

 oil-box. In the collars of leathers, on the tops of the barrels, 

 are put (wo fmall flat boxes, below one or two rings of the 

 The oil neither leathers, the pifton rods paffing through them. Thefe boxes 

 thickened by contain the oil to keep the leathers moift, and air-tight. In 

 carrieToff from this fituation the oil is not thickened by evaporation, nor carried 

 the leathers. ^p from off the leathers, when the pifton rifes, as in Nairne's 

 pump, and the leathers are better fupplied than by the dirty 

 oil paffmg through tlie pump and relumed to the collars by 

 Cutliberlfon's crooked pipes. The American air-pump, made 

 in this manner, is the fihipieft form of any pump of equal power.* 



M cm cir 



