No; the upper valve is not air-tight.
Fig. 16.
Fig. 17.
Fig. 18.
Explain the working of a pump.
When the pump-handle is lowered, the water, above the valve or bucket attached to the end of the rod, is raised so that it runs out at the spout; and the vacuum, thus produced by raising the water, is filled by the water in the well, which is forced up the pump-tube by the pressure of the air. When the pump is at rest, the lower valve, which is air-tight, prevents the water from sinking back into the well, so that the pump-tube is always full of water, whether we are pumping or not.
What does [Fig. 16] represent?