Section 6. The department, with the approval of the governor and council, may, upon the application of an agent of the United States, in the name and behalf of the commonwealth, convey to the United States the title of the commonwealth to any tract of land covered by navigable waters and necessary for the purpose of erecting a lighthouse, beacon light, range light or other aid to navigation, or light keeper's dwelling; but such title shall revert to the commonwealth if such land ceases to be used for such purpose.