Which type of boiler is referred to as the first internally fired boiler?

The Scotch Marine Boiler is recognized as the first internally fired boiler primarily due to its design where the combustion occurs within the boiler shell itself, as opposed to outside it. This internal firing mechanism allows for a more efficient heat transfer, as the hot gases produced from the fuel combustion directly contact the boiler water.

This type of boiler is characterized by its horizontal design, featuring a large cylindrical shell that contains fire tubes where the hot gases travel through. The structure enhances the heating surface area, making it effective for steam generation. The design also allows for easier maintenance and inspection compared to other types of boilers.

The historical significance of the Scotch Marine Boiler cannot be understated, as it laid the groundwork for future boiler designs by addressing issues such as efficiency and safety in the steam generation process. By contrast, the other choices—packaged boilers, externally-fired boilers, and high-pressure boilers—do not share the same historical importance as the first internally fired boiler, as they represent either advancements in boiler technology or specific applications rather than the foundational design that the Scotch Marine represents.