A fusible plug is a brass case containing a core of an alloy that will melt at a temperature a little higher than the heat of the water or steam in the boiler. It is practically impossible for the core to refuse to melt if the boiler runs sufficiently short of water to leave the plug exposed to the fire heat only, though, owing to ignorance, the plug might be placed where the fire could not readily act on it. If deposit inside the boiler covers the plug it may melt before its time. A fusible plug is also an element of safety when there is danger by excessive pressure, for as the pressure increases so does the heat of the water or steam, and when the latter reaches a temperature higher than normal the plug will act. Fusible plugs are, of course, no protection when a boiler is weak or develops defects in structure.