It represents a pump, when it is not working.
How are the valves?
The valves are both closed.
What keeps the water above the lower valve?
The lower valve is air-tight, and, therefore, water-tight, so that it keeps the water above it.
What keeps the water in the tube below the lower valve?
The pressure of the air on the water in the well, forces it up the tube to the lower valve.
What does [Fig. 17] represent?
It represents a pump, when the handle is being raised.
How is the lower valve?