4H Ctnftruftion cf the Air Pump. [Book V. 



towards B, and a vacuum made in the barrel from 

 e to b : upon which part of the air in the glafs M 

 (Fig. i.) by its fpring rufhes through the hole i in the 

 brafs plate L L along the pipe G G, which communi- 

 cates with both barrels by the hollow trunk I H K, 

 and pufhing up the valve b (a valve is a bit of leather 

 that covers a hole as the flapper of a bellows, admitting 

 the air in, but fuffering none to go back) the air then 

 raifing the valve enters into the vacuity b e of the bar- 

 rel B K. For the air will naturally prefs into thofe 

 places where it is leaft refifted. All this is done by 

 drawing the handle towards D. Next turning the 

 handle forward the contrary way towards C> the pifton 

 de is depreffed in the barrel, and as the air which had 

 got into the barrel cannot be pulhed back through the 

 valve b, for the valve clofes like the flapper of a bel- 

 lows, and will not let the air back the way it came, 

 the air muft therefore afcend through an hole in the 

 piftonj and efcapes through a valve at d; and is hin- 

 dered by that valve from returning into the barrel 

 when the pifton is again raifed. At the next raifing 

 of the pifton, a vacuum is again made in the-fame 

 manner as before, between b and , upon which more 

 of the air which was left in the glafs receiver M gets 

 cut thence> and runs into the more empty barrel B K 

 through the valve b. The fame thing is effected with 

 regard to the other barrel A I, and as the handle F is 

 turned backwards and forwards, ir alternately raifes 

 and deprefles the piftons in their barrels, always raifing 

 one while it depreffes the other. And as there is a 

 vacuum nude in each barrel when its pifton is raifed, 

 every particle of air irr the receiver M pufiies out ano- 

 ther through the hole i and pipe G G into the barrels, 

 until at Jail the air in the glafs receiver comes to bu 

 fo much rarefied tha: it can no longer get through 



the 



