Arrange a No. 6 tube as in 1, Fig. 97, and suck air out at the top. Does the water run uphill into your mouth?
Hold your finger over the top and lift the tube out of the pail (2). Does the water remain in the tube? Fill a bottle with water to overflowing, insert a No. 2 tube into your one-hole stopper, insert the stopper into the mouth of the bottle (3) without admitting air below the stopper, and try to suck water out of the bottle. Do you find that you cannot do so ?
Fig. 98. Water Driven Up Tube By Atmosphere
Repeat (3) with the bottle half full of air (4). Do you find that you can now suck part of the water out of the bottle, and all of it if you admit air?
The "why" of it 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, side-wise, 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.
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.