A new apparatus is presented that is capable of applying a constant fluid pressure at inflow and outflow boundaries. The apparatus can be refilled during operation and does not rely on an overflow mechanism. The device is constructed of two vessels, one that contains the delivered fluid and the other that contains a less dense fluid. By matching the fluid densities and the areas of the vessels, the absolute elevation of the delivered fluid is maintained as the fluid is added to or removed from the system. The history of the development of the device, the underlying physical principles, and photographs of a prototype device during operation are shown. Finally, possible applications of the method are compared with those of the popular Mariotte bottle.