Prior to the introduction of the low water cutoff device, how was a Scotch Marine Boiler protected from low water conditions?

The fusible plug serves as a critical safety feature in a Scotch Marine Boiler to protect against low water conditions. It is designed to melt at a specific temperature, which corresponds to the safe operating limits of the boiler. If the water level drops too low, the resulting decrease in water coverage exposes the fusible plug to higher temperatures from the boiler's heat source. Once the temperature exceeds the melting point, the plug melts, creating a vent for steam and thereby preventing a potentially dangerous pressure build-up inside the boiler.

This safety measure was particularly important before the implementation of more modern devices like low water cutoff systems. While other choices like the low water alarm and manual inspection are valid measures for monitoring water levels, they rely on human intervention or alerts rather than providing immediate physical protection against overheating. The pressure relief valve operates independently to release excess pressure but does not directly address low water conditions in the same way as the fusible plug. Therefore, the fusible plug stands out as the appropriate mechanism for low water protection prior to the development of more advanced systems.