OF THE AIR-PUMP. 209 



backwards and forwards, it alternately raises and de- 

 presses the pistons in their barrels ; always raising one 

 whilst it depresses the other. And, as there is a vacuum 

 made in each barrel when its piston is raised, every 

 particle of air in the receiver M pushes out another, by 

 its spring or elasticity, through the hole i, and pipe 

 G G into the barrels ; until at last the air in the receiver 

 comes to be so much dilated, and its spring so far weak- 

 ened, that it can no longer get through the valves ; and 

 then no more can be taken out. Hence, there is no 

 such thing as making a perfect vacuum in the receiver ; 

 for the quantity of air taken out at any one stroke, will 

 always be as the density thereof in the receiver : and 

 therefore it is impossible to take it all out, because, 

 supposing the receiver and barrels of equal capacity, 

 there will be always as much left as was taken out at 

 the last turn of the handle. 



There is a cock k below the pump-plate, which being 

 turned, lets air into the receiver again ; and then the 

 receiver becomes loose, and may be taken off the plate. 

 The barrels are fixed to the frame E e e by two screw- 

 nuts//, which press down the top piece E upon the 

 barrels : and the hollow trunk H (in page 208) is cover- 

 ed by^a box, G H (in page 207.) 



There is a glass tube I m m m n open at both ends, 

 and about 34 inches long ; the upper end communicating 

 with the hole in the pump-plate, and the lower end 

 immersed in quicksilver at n in the vessel N. To this 

 tube is fitted a wooden ruler m m, called the gage, which 

 is divided into inches and parts of an inch, from the 

 bottom at n (where it is even with the surface of the 

 quicksilver) and continued up to the top, a little below 

 I to 30 or 31 inches. 



As the air is pumped out of the receiver M, it is like- 

 wise pumped out of the glass tube / m n, because that 

 tube opens into the receiver through the pump-plate; 

 and as the tube is gradually emptied of air, the quick 

 14. 



