FIG. 99 A FOUNTAIN ] The rise of the water in 4 is due to another fact, namely, that any gas expands when the pressure on it is decreased. When you suck air out of the tube you decrease the pressure on the water in the tube and thereby on the air in the bottle; the air then expands and lifts the water into your mouth. Experiment 74. Great pressure of air. With the apparatus Fig. 98 hold your finger over the lower end of the tube, suck as much air as you can out of the tube, pinch the coupling, and remove your finger under water. Does the atmosphere drive water up the tube very rapidly and with great force? [Illustration: FIG. 100 MAGIC ] Experiment 75. A fountain. With the apparatus Fig. 99 suck as much air as you can out of the bottle, pinch the coupling, and open it under water. Does the atmosphere lift the water into the bottle and produce a beautiful fountain? [Illustration: FIG. 101 MORE MAGIC ] Experiment 76. Magic tumbler. Fill a tumbler with water, cover it with a sheet of paper, hold the paper on with your hand, invert the tumbler, and remove your hand (Fig. 100). Does the atmospheric pressure upward support the paper and water? Experiment 77. Magic lift. Fill a tumbler with water, press your palm down on the top with your fingers pointing downward (Fig. 101), straighten your fingers without admitting air to the tumbler, and then lift your hand. Do you lift the tumbler of water also? There is a partial vacuum between your hand and the water, and the atmospheric pressure upward and downward holds your hand and the tumbler together. [Illustration: FIG. 102 TUMBLER PENDULUM ] Experiment 78. A magic pendulum. Pass a string through a small hole in a piece of cardboard, knot the end of the string, and drop melted candle wax over the hole to make it air tight. Fill a tumbler with water, press the cardboard down on the tumbler with the palm of your hand, and lift the string. Do you also lift the tumbler (Fig. 102)? Swing the tumbler gently as a pendulum. [Illustration: FIG. 103 POULTRY FOUNTAIN ] Experiment 79. A poultry fountain. 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. In a poultry fountain the bottle is supported, as shown, with its mouth under water but above the bottom. [Illustration: FIG. 104 A DRINKING FOUNTAIN (_From Butler’s Household Physics. Published by Whitcomb & Barrows, Boston_) ] [Illustration: FIG. 105 HOMEMADE DRINKING FOUNTAIN ] Experiment 80. A drinking fountain.