THE SUCTION-PUMP 291 



29. The Suction- Pump. The Forcing -Pump. In the 

 air-pump which has been described, a stop- cock is 

 used for alternately shutting and opening the com- 

 munication between the receiver, the cylinder, and the 

 external air. In many air-pumps, however, and in all 

 pumps used for water, the stop-cock is replaced by 

 valves. A valve may be denned as a contrivance which 

 allows a current of water, or air, or steam, etc., to pass 

 through a tube or aperture in one direction, but not 

 in the other. 



One of the most common form of valves is the c clack- 

 valve,' fig. 193. It is usually constructed by attaching 

 to a plate of metal or wood, /, larger than the aperture, 

 PP, which the valve is intended to stop, a piece of 

 leather, which extends on one side beyond the plate, and 

 oeing flexible, forms the hinge on which the valve plays. 

 Such a valve is usually closed by its own weight, and is 

 leld closed more firmly by the pressure of the fluid 

 vhose return it is intended to obstruct (fig. 193 A); 

 >n the other hand, it is opened by the pressure of the 

 luid which passes through it, as shown at B. Another 

 brm is the c conical valve,' fig. 194. It consists of a 

 ircular metallic plate with conical sides, resting in a 

 lonical seat ; in the middle of the valve there is usually 

 short spindle, which passes through, holes made in 

 irossbars, B B, and this guides the valve in its perpen- 

 icular motion. The action of the conical valve shown 

 fc A and B of the figure corresponds exactly to that of 

 ie clack-valve. If the valve is to open downwards, it 

 tust be kept pressed against its seat by a spring F, 

 iown in fig. 194 C. 



