Windsurfing is a surface water sport that combines elements of surfing and sailing. It consists of a board usually 2 to 3 metres long, with a volume of about 60 to 250 liters, powered by wind on a sail. The rig is connected to the board by a free-rotating universal joint and consists of a mast, 2-sided boom and sail. The sail area generally ranges from 2.5 m to 12 m depending on the conditions, the skill of the sailor and the type of windsurfing being undertaken. Some credit S. Newman Darby with the origination of windsurfing by 1965 on the Susquehanna River, Pennsylvania, USA when he invented the 'sailboard', which, incidentally, he did not patent. In 1964, Darby began selling his sailboards. A promotional article by Darby was published in the August 1965 edition of Popular Science magazine. While Darby's 'sailboard' incorporated a pivoting rig, it was 'square rigged' and suffered all the associated limitations.