Water Column - A water column is a hollow casting built to carry the sight glass and tri-cocks, connected to the boiler both at the top and at the bottom. As long as the pipes do not become plugged, gravity will ensure that the height of the water in the column matches the height of the water in the boiler, with allowance for the engine sitting out of level, or the water sloshing around due to movement of the engine. Some boilers built for service in Cana da were required to have shut-off valves on the connecting pipes - these must be open for the sight glass and tri-cocks to read correctly. It is provided with a drain cock that should be opened every so often to prevent the buildup of sediment.