One valve is placed in the lower part of the pump-tube. How near the water must this valve be? It must be within thirty-four feet of the water. Why must the lower valve be within thirty-four feet of the water in the well? Because the air can force water up a tube only to the height of thirty-four feet. Does this valve move about in the pump-tube? No; it remains fixed in one place. Is this valve air-tight? Yes; the lower valve or bucket in a pump must always be air-tight. Where is the upper valve? It is fastened on the end of a rod, which is attached to the pump-handle. Does this valve remain fixed in one place? No; it moves up and down when the pump-handle is moved. Is the upper valve air-tight? No; the upper valve is not air-tight. [Illustration: Fig. 16.] [Illustration: Fig. 17.] [Illustration: 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? 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? The lower valve is closed. Why is the lower valve closed? So as to prevent the water from being forced back again into the well. How is the upper valve? The upper valve is open. Why is the upper valve open? So that the water may pass up through the valve, while the valve is descending. What does Fig. 18 represent? It represents a pump when the handle is being lowered. How is the upper valve? The upper valve is closed. Why is the upper valve closed? So as to lift up the water which is above it. What becomes of the water which is lifted up? It runs out of the spout of the pump. As the upper valve is raised up, what is produced under it? A vacuum is produced under it. How is this vacuum filled? The water below the lower valve is forced up, so as to fill the vacuum. How is the lower valve? The lower valve is open. Why is the lower valve open? So that the water may pass through and fill the vacuum above the valve. Where are pumps mostly used?