The Common Lift Pump
Pumps were used successfully to raise water from wells long before their action was understood. They consist of a cylindrical metal barrel with a tube at the side near the top to act as a spout. At the bottom of the barrel, where it joins a pipe leading to the well, there is a valve B, which consists of a hinged leather flap weighted by a brass disc so that it is normally shut. A plunger carrying a leather cup and fitted with a second valve A is moved up and down inside the barrel by a handle H.
To start the pump working it is first primed by pouring some water on to the top of the plunger. This makes a good seal, ensuring air can not leak past the plunger during the first few strokes needed to fill the pump with water.
When the plunger moves down the valve B closes owing to the force of gravity and the weight of water on it. At the same time water inside the pump passes upwards through the valve A into the space above the plunger.
On the upstroke the valve A closes owing to the force of gravity on it and the weight of water above it. Also as the plunger rises, water is pushed up the pipe through the valve B by atmospheric pressure acting on the surface of the water in the well. At the same time, water above the punger is raised and flows out of the well.
The maximum length of column that can be supported by atmospheric pressure is about 10m, so this is the maximum theoretical height that water can be raised by the common pump. In practice the maximum height is less because air dissolved in the water collects at the top of the water column.