heated surface area. As these are short tubes of large diameter and the boiler continues to use a relatively low pressure, this is still not considered to be a water-tube boiler. The tubes are tapered, simply to make their installation through the flue easier. Scotch marine boiler The Scotch marine boiler differs dramatically from its predecessors in using a large number of small-diameter tubes. This gives a far greater heating surface area for the volume and weight. The furnace remains a single large-diameter tube with the many small tubes arranged above it. They are connected together through a combustion chamber – an enclosed volume contained entirely within the boiler shell – so that the flow of flue gas through the firetubes is from back to front. An enclosed smokebox covering the front of these tubes leads upwards to the chimney or funnel. Typical Scotch boilers had a pair of furnaces, larger ones had three. Above this size, such as for large steam ships, it was more usual to install multiple boilers. Locomotive boiler A locomotive boiler has three main components: a double-walled firebox; a horizontal, cylindrical "boiler barrel" containing a large number of small flue-tubes; and a smokebox with chimney, for the exhaust gases. The boiler barrel contains larger flue-tubes to carry the superheater elements, where present. Forced draught is provided in the locomotive boiler by injecting exhausted steam back into the exhaust via a blast pipe in the smokebox. Locomotive-type boilers are also used in traction engines, steam rollers, portable engines and some other steam road vehicles. The inherent strength of the boiler means it is used as the basis for the vehicle: all the other components, including the wheels, are mounted on brackets attached to the boiler. It is rare to find superheaters designed into this type of boiler, and they are generally much smaller (and simpler) than railway locomotive types. The locomotive-type boiler is also a characteristic of the overtype steam wagon, the steam-powered fore-runner of the truck. In this case, however, heavy girder frames make up the load-bearing chassis of the vehicle, and the boiler is attached to this. Taper boiler Certain railway locomotive boilers are tapered from a larger diameter at the firebox end to a smaller diameter at the smokebox end. This reduces weight and improves water circulation. Many later Great Western Railway and London, Midland and Scottish Railway locomotives were designed or modified