The compartment at the rear of the boiler which houses the fire. The firebox is where the fuel, usually coal, but it can be wood or oil, is burnt to provide the heat to boil the water in the boiler. The firebox consists of two copper or steel enclosures, the outer firebox and the inner firebox. They are connected by 'stays', bolts which keep the inner box rigid within the outer box. Normally, the stays are threaded at each end and are screwed into the steel plates of the firebox. The ends are hammered down as a seal. Copper fireboxes were the normal practice for UK railways but in the US, steel was the usual material. The steel firebox was first tried by Alexander Allan on the Scottish Central Railway in 1860. Boiler water surrounds the firebox sides front and top to allow maximum benefit from the fire for heating. The two side areas are often referred to as "legs", as they take on this appearance in cross section. The outer firebox is really an extension to the boiler. When the boiler is filled, water will enter the outer firebox legs and cover the roof or "crown" of the inner firebox. The boiler's tubes are connected to the front wall of the inner firebox so that the hot gases from the fire pass through them to the smokebox. Inside the firebox a brick arch is positioned over the fire so that the heat from the fire is deflected towards the back of the firebox to ensure the hot gases are distributed towards the tubes more evenly. In their inspection, the lighting- up crew will check that the firebricks are secure and undamaged. The two shells of the firebox are joined at the base by what is known as the 'foundation ring' or 'mud ring'. This name arises from the sludge with tends to collect there during the time between boiler washouts, as it is the lowest point of the boiler where water reaches. Firebox shape has developed over the years. To get the required grate area to heat a large boiler, older fireboxes tended to be long but narrow, as they had to rest between the locomotive's frames. This led to difficulties with manual firing, as the coal had to be thrown towards the rear in spite of a slope being provided. Later designs had the frames lowered at the firebox end to allow a wider firebox with a shorter grate. See also the Belpaire boiler.