 A Cartesian diver is a demonstration of Boyle's law. We have a small amount of air trapped in the diver. The diver is opened to the surrounding water. If we apply a pressure to the water, the pressure is transmitted throughout the water and causes the sample of air to decrease in volume. As the volume of the air decreases, the buoyancy of the diver decreases and the diver sinks. If we release the pressure, the volume of the air inside the diver expands, this increases the buoyancy of the diver and the diver rises. Pressure and volume of a gas are inversely related at constant temperature and constant amount of gas.