Round-topped boiler

In the extreme case, this gave rise to the haycock boiler, where the firebox was raised into a vaulted dome.

In most cases though, the firebox was merely larger by the width of the internal iron framing section.

This led to the development of the "waisted" form (see figure 2), where a narrow firebox flared upwards and outwards to meet the boiler barrel.

This flat surface, with steam pressure on its upper side, requires stays to support it.

As it is not a constant distance from the boiler barrel, unlike the firebox sides, this staying is difficult to arrange.

This has the advantage that a large hole may be cut in the boiler directly above the firebox, to mount the steam dome.

Careful inspection and periodic replacement of them was needed, to avoid the risk of a boiler explosion.