steam locomotive boilers
A very unusual subject for FH Prince of Dzun!
The idea of fusible plugs in the crown of the firebox was, in 20th century British locos, to melt and let the steam into the firebox when the water level dropped below the crown. There it wouldn't have the power to douse the fire but could provide some audible warning to the crew. It didn't always work, in particular if the firebox door was shut at the time - there was a fatal boiler explosion on the French railways from this. The pressure relief valves or blow-off valves (up to four) were provided as well - these operated at different pressures and were on top of the boiler casing and let the steam escape to atmosphere. Whilst these would be used every trip, it was hoped that melting the firebox plugs was a never in a career job!
Now Milt - taxi-ing a Lincoln backwards.........