First mentioned by Hero of Alexandria, the Steam Engine was actually only used for any practical applications beginning in the mid 18th century. It was during this period that James Watt first made a ship that was powered by steam, but he would be far from the last: the 19th century is actually referred to by some historians as "The Age of Steam". Used to propel ships, trains, and factory machines, it was thought in its time to be the ultimate energy source. Indeed, even up to the 21st century, steam was still a major source of power -- albeit now heated not only by coal, but by nuclear fission.