Pneumatics. 69 



the air-pump, place the glass receiver M upon 

 the leather, so that the hole i in the plate may be 

 within the glass. Then, turning the handle F 

 backward and forward, the air will be pumped 

 out of the receiver; which will then be held 

 down to the plate by the pressure of the external 

 air or atmosphere. For, as the handle F is turn- 

 ed backward, it raises the piston d in the barrel 

 B K, by means of the wheel E and rack D : and, 

 as the piston is leathered so tight as to fit the 

 barrel exactly, no air can get between the piston 

 and barrel ; and therefore all the air above d in 

 the barrel is lifted up towards B, and a vacuum is 

 made in the barrel from b to d, upon which, part 

 of the air in the receiver M, by its spring, rushes 

 through the hole i, in the brass plate L L, along 

 the pipe G, which communicates with both bar- 

 rels by the hollow trunk I H K, and pushing up 

 the valve 6, enters into the vacant jplace b d of 

 the barrel B K. For wherever the resistance or 

 pressure is taken off, the air will run to that place, 

 if it can find a passage. Then, if the handle F 

 is turned forward, the piston d will be depressed in 

 the barrel ; and, as the air which had got into the 

 barrel cannot be pushed back through the valve 

 b 9 it will ascend through a hole in the piston, and 

 escape through a valve at d, and be hindered by 

 that valve from returning into the barrel, when 

 the piston is again raised. At the next raising of 

 the piston, a vacuum is again made, in the same 

 manner as before, between b and d ; upon which 



