Arrange the apparatus as in Fig. 81, put the arms together in a glass of water, suck a little air out of the top coupling and close it with a glass plug. Do you find that the water rises to the same level in each?
Place the arms in separate tumblers filled with water to the same level and repeat. Does the water rise to the same level?
Add an extra length to one arm and repeat. Are the levels different but are they equal distances above the water in their respective tumblers?
Place the tumblers on the table, make one tube slanting, and repeat the experiment (Fig. 82). Are the levels again the same?
When you suck air out of the tee, you decrease the air pressure in the two tubes, and the atmospheric pressure on the water in the tumblers lifts the water into the tubes.