HYDRAULICS. 189 



valve opening upwards. There is also a valve, T, 

 on the top of the suction-pipe ; and it will be of 

 advantage, though not absolutely necessary, to put 

 a valve, L, at the bottom of the rising-pipe. Now, 

 suppose the piston at the bottom of the working- 

 barrel ; when it is drawn up, it tends to compress 

 the air above it, because the valve in the piston 

 remains shut by its own weight. The air, there- 

 fore, is driven through the valve L, into the rising- 

 pipe, and escapes. In the mean time, the air 

 which occupied the small space between the pis- 

 ton and the valve T, expands into the upper part 

 of the working-barrel ; and its elasticity is so much 

 diminished thereby, that the atmosphere presses 

 the water of the cistern into the suction-pipe, 

 where it rises until an equilibrium is again pro- 

 duced. The next stroke of the piston downwards 

 allows the air, which had come from the suction- 

 pipe into the barrel during the ascent of the piston, 

 to get through its valve. Upon drawing up the 

 piston, the air is also drawn off through the rising- 

 pipe. Repeating this process brings the water at 

 last into the working-barrel, and it is then driven 

 along the rising-pipe by the piston. 



This is one of the best forms of a pump. The 

 rarefaction may be very perfect, because the piston 

 can be brought so near to the bottom of the 

 working- barrel; and for forcing water in opposition 

 to great pressures, it appears preferable to the 

 common forcing-pump ; because in that, the piston- 

 rod is compressed and exposed to bending, which 

 greatly hurts the pump, by wearing the piston and 

 barrel on one side. This soon renders it less tight : 

 and much water squirts out by the sides of the 

 piston. But in this pump the piston-rod is always 

 drawn or pulled, which keeps it straight, and rods 



