The atmosphere which surrounds the earth exerts a pressure of 15 pounds per square inch on everything at the earth’s surface. It exerts this pressure equally downward, sidewise, and upward.
It is this atmospheric pressure on the water in the pail (1) which lifts the water into the tube when you decrease the pressure on the water in the tube by sucking out air and then water.
It is this pressure upward that supports the water in 2.
The water does not rise in 3 because the atmosphere cannot exert pressure downward on the water in the bottle.
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?
