56 Experimental Philosophy. [Lecture 5. 



valve b. The spring of the air being thus weak- 

 ened by this second motion, the pressure of the 

 atmosphere without the pump will cause the 

 water again to ascend within it, we will suppose 

 to F. By the next stroke the air will be almost 

 entirely exhausted, and the water will rise in the 

 body of the pump above the boxll, perhaps as 

 high as B. On forcing down the piston or bucket 

 again, the valve b in the box H will be shut by 

 the pressure as before, and the valve a in the 

 piston G will be opened by the same pressure, 

 and consequently water instead of air will now be 

 raised by the elevation of the piston. When the 

 piston is thus raised, it is evident that a vacuitm 

 will again be produced between the box H and 

 the piston C, which will instantaneously be filled 

 up by the water flowing through the valve b, as 

 before described. Thus, by the continual work- 

 ing of the pump, the water will be raised by the 

 piston into the wider space, and caused to flow 

 through the spout I. Every time the piston or 

 bucket is raised, the valve b is lifted up by the 

 water beneath, and every time the piston or 

 bucket is forced down, the valve a rises, and the 

 valve b is depressed. For the easiness of work- 

 ing in common pumps, the rod D is fixed to a 

 handle, which acts as a lever, and turns on a pin 

 in the body of the pump. 



We have not yet, however, explained the diffi- 

 culty respecting the pump of the duke of Flo- 

 rence ; and you do not yet understand why the 



