The top is outside the pressure vessel; the bottom is inside. The topmost cylinder is the drive piston; it has an external force applied to it to pressurize the vessel. The six squares are cross-sections through three rings; the topmost (dark gray) one is a hard material, the middle one a softer one, and the bottom one a much softer one. Bridgman originally used hard steel, softer steel, and rubber. The rubber would actually protrude into a groove in the soft steel ring above, to help hold it in place (not shown). The upturned T shape is a horizontal disk on a vertical shaft; it compresses these rings when the pressure vessel is pressurized.