70 Experimental Philosophy. [Lecture 6. 



more of the air that was left in the receiver M 

 gets out thence by its spring, and runs into the 

 barrel B K, through the valve b. The same thing 

 is to be understood with regard to the other bar- 

 rel A I ; and as the handle F is turned backward 

 and forward, it alternately raises and depresses 

 the pistons in their barrels, always raising one 

 wffile it depresses the other. A vacuum being 

 made in each barrel when its piston is raised, the 

 particles of air in the receiver M push out one 

 another by their spring or elasticity, through the 

 hole i, and pipe G, into the barrels ; until at last 

 the air in the receiver becomes so much dilated, 

 and its spring so far weakened, 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 of it in the receiver: 

 and therefore it is impossible to exhaust it entire- 

 ly, 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 & below the barrels, which 

 being turned, lets the air into the receiver again ; 

 and then the receiver becomes loose, and may be 

 taken off the plate. 



There is also a glass tube m n (fig. 29.) open 

 at both ends, and about thirty-four inches long ; 

 the upper end communicating with a hole in the 



