Double inclined plane for moving tub boats from one level to another on a canal.. Boats were lowered on rails and counterbalanced by a tub containing water. Such a plane was constructed on Bude Canal at Hobbacott Down. Inclined planes were used as a cheaper alternative to locks, the building of which could make a canal uneconomic. From A Treatise on the Improvement of Canal Navigation, Robert Fulton, (London, 1796). Engraving.