Abstract: An automatic cover for a liquid storage container and a method of control corresponding thereto including a cover disposed within the container, a membrane connected to a portion of the storage container and connected along a peripheral portion of the membrane to the cover, the membrane and the cover separating the storage container into a liquid storage compartment and a liquid balancing compartment, a pressurized mechanism connected to the peripheral portion of the membrane for controlling the shape and dimension of the cover and a member for feeding balancing liquid to and for withdrawing balancing liquid from the liquid balancing compartment in such a manner that hydrostatic pressure of the storage container is maintained within any predetermined range.