Organs were used especially in the late Classical Antiquity. We are not quite sure wheter the Romans used Vitruvius' notes, or notes of other mechanics.
The two plungers are moved up and down. If you move a plunger down, air (red) flows through a valve. If you move it up, the valve closes and the air is piped to the hemisphere.
This hemisphere is placed in a reservoir, filled with water. The air is partially compressed, and partially the water is pressed down the hemisphere by the air. The water line in the reservoir rises. One part of the air floats thorugh a pipe on the top of the hemisphere.
There the air floats, passing another valve, in a chamber, whis is connected to the pipes. If a key is pressed, a valve opens. Air floats through a pipe, a tone is created.
The interesting part about this organ is, that water keeps an instant pressure. If air floats out of a pipe, the pressure in the reservoir decreases. The water line sinks and the pressure is restorred to normal.