Press the lever and watch the diver, which is partly filled with water, and partly with air. Change the pressure force. Note how the volume of air in the diver has changed. Try to make the diver motionless in the middle of the tube’s length.
How does it work
Two forces act on the diver: gravity in the downward direction and buoyancy upwards. The resultant of those forces (that is their sum) determines the diver’s movement. If the diver’s average density is lower than the density of the liquid, buoyancy dominates and keeps the diver at the top (Archimedes’ law). When we press the lever, we increase the pressure in the whole tube (Pascal’s law); that compresses the air in the diver and lets more liquid pour in, which makes the diver heavier. Gravity, now stronger than buoyancy, drowns the diver. When we release the lever, we see the opposite effect.
Legend has it that Archimedes (a Greek scholar from the third century BC) discovered the law named after him while taking a bath. Overjoyed, he took to the Syracuse streets naked, shouting „Eureka!” (‘I have found it!’).
Submarines dive and surface by lowering or increasing the air volume in their pressurised ballast tanks.