To make the poultry fountain (Fig. 103), fill a bottle with water, hold your thumb over the mouth, invert the bottle over the pan of water, and remove your thumb under water. Does the atmospheric pressure on the water in the pan hold the water in the bottle?
Lift the bottle until the mouth is a little above the water in the pan. Does air enter and water run out until the mouth is again covered with water? This is what happens when the poultry, by drinking, lower the water below the mouth of the bottle.
Fig. 103. Poultry Fountain
In a poultry fountain the bottle is supported, as shown, with its mouth under water but above the bottom.