Information - Built by engineer Adrian Stephens at the Dowlais Ironworks in Merthyr Tydfil. It was a simple safety device to warn engineers when a steam boiler was running out of water which could lead to a rise in pressure and an explosion. It was added to every boiler, railway locomotive and steam ship around the world. Years later, Adrian wrote to his niece, 'Neither in want, nor caring for money at the time, I did not think of taking a patent'.